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Chemical phase equilibria and phase diagrams are well established and have served as indispensable guides for designing materials through chemical tuning. However, a general thermodynamic theory for strain phase equilibria remains elusive despite extensive studies which have revealed dramatic impacts of mechanical strain on the stability of phases and domain states in solid-state materials. Here, we establish the thermodynamic theory of strain equilibria and a general framework for efficiently constructing multidomain and multiphase diagrams of arbitrarily strained solids under incoherent conditions. As examples, we obtain temperature-strain phase diagrams of ferroelectric PbTiO 3 , strongly correlated VO 2 , and unconventional ferroelectric Hf 0.5 Zr 0.5 O 2. We reveal the analogs of the Gibbs phase rules, multiple- and multi-critical points, common-tangent construction, and level rule in strain equilibria to those in the familiar chemical phase equilibria. Our strain equilibria theory offers a powerful framework for predicting the thermodynamic stability of structural phases and domains in strained solids and for guiding the strain engineering of their functional properties. [Display omitted] [ABSTRACT FROM AUTHOR]

The effects of Ni additions on microstructure and microhardness of near-eutectic Al-9Ce-xNi-0.75Mn-0.18Sc-0.12Zr (x = 2.5, 3.2, and 3.9 wt.%) alloys are investigated for various cooling rates. The as-cast microstructure consists of fine, micron-scale Al 11 Ce 3 and Ni-rich phases formed during eutectic solidification. Two Ni-rich phases are observed: (i) Al 27 Ce 3 Ni 6 at higher Ni contents and slower solidification rates, and (ii) Al 9 (Ni,Mn,Fe) 2 at lower Ni contents and faster solidification rates. While these Ni-containing alloys have higher microhardness than a Ni-free control alloy, varying the Ni concentration in the 2.5–3.9 wt.% range does not significantly affect the microhardness, indicative of competing strengthening and weakening effects from adding Ni. The alloy with intermediate Ni content (3.2 wt.%) is selected for further microstructural and mechanical characterization. It contains four coarsening-resistant strengthening constituents: (i) micron-scale Al 11 Ce 3 and (ii) Al 27 Ce 3 Ni 6 or Al 9 (Ni,Mn,Fe) 2 platelets, all formed during eutectic solidification, (iii) L1 2 -Al 3 (Sc,Zr) nanoprecipitates formed on aging, and (iv) Mn atoms in solid solution in the α-Al matrix. The combined strengthening mechanisms impart high strength at ambient and elevated temperatures, as measured by microhardness (as a function of aging time), by compression and tensile experiments, and by compressive creep measurements. Alloys previously reported in the literature - with various combinations of one, two or three of these four strengthening phases - show lower hardness and creep resistance, indicating that cumulative strengthening can be achieved when combining mechanisms; the present alloy containing all four phases shows a remarkably high creep threshold stress of 62 MPa at 300 °C. [Display omitted] [ABSTRACT FROM AUTHOR]

Models for grain size dependence of flow stress typically ignore contributions from geometrically necessary dislocations (GNDs) in the grain boundary regions. Ashby had proposed the following equation for flow stress (τ) as a function of grain size (d) and effective plastic strain (ε ¯): τ = τ 0 + C ′ G b ε ¯ d using an unknown empirical coefficient, C' , In this study, the coefficient, C' , and equation for the grain size dependence of flow stress were investigated in Mg-4Al using nanoindentation. C' ranges from 1.4 to 7.11 that is related to the hardness difference (∆H 0) between the adjacent grains as C ′ = 0.62 Δ H 0 . Ashby equation was used to predict uniaxial stress-strain curves in grain sizes of 55 µm, 187 µm, and 333 µm. The results showed lower flow stress and lower hardening rate compared to experimentally measured tensile stress-strain response. The discrepancy seems to be a result of hardening effect of SSDs. We propose a modified Ashby equation that takes into account the hardening effect of SSDs and GNDs separately, resulting in a better agreement between calculated and experimentally measured values. This analysis demonstrates estimation of uniaxial stress-strain response using nanoindentation measurements that captures effects of grain size, and strain hardening from SSDs and GNDs. The combination of the revised equation and nanoindentation method will enable rapid exploration of stress-strain response of a diverse range of single-phase polycrystalline alloys. [Display omitted] [ABSTRACT FROM AUTHOR]

A 2D bicrystal atomistic model of dislocation transmission through a Σ11<101>{131} symmetric-tilt grain boundary reveals details of Hall-Petch breakdown in the single dislocation regime in Al, Ni, and Cu. Partly based on a previous study [1] , this research determines the stress required for a single dislocation to be transmitted through the Σ11 boundary and finds that single dislocation transmission typically deviates from Hall-Petch behavior because the leading partial dislocation becomes trapped in the boundary and emits glissile grain boundary disconnections that cause boundary sliding deformation at stresses well below boundary transmission stresses. Thus, mechanisms of inverse Hall-Petch, namely grain boundary shear and sliding, are shown to operate in lieu of grain boundary transmission in the single dislocation regime. However, by controlling the applied stresses, inverse Hall-Petch can be reversed and typical Hall-Petch grain boundary transmission regained. This, in turn, allows the direct computation of Hall-Petch coefficients. A second dislocation on the same slip plane preserves typical Hall-Petch behavior for Al and Ni and does not lead to boundary sliding events, thus implying a critical grain size for Hall-Petch breakdown based on the pileup model. [Display omitted] [ABSTRACT FROM AUTHOR]

Metallic/intermetallic materials with BCC structures hold an intrinsic instability due to phonon softening along [110] direction, causing BCC to lower-symmetry phases transformation when the BCC structures are thermally or mechanically stressed. Fe 50 Rh 50 binary system is one of the exceptional BCC structures (ordered-B2) that has not been yet showing such transformation upon application of thermal stress, although mechanical deformation results in B2 to disordered FCC (γ) and L1 0 phases transformation. Here, a comprehensive transmission electron microscopy (TEM) study is conducted on thermally-stressed samples of Fe 50 Rh 50 induced by quenching in water and liquid nitrogen from 1150 °C and 1250 °C. We demonstrated that samples quenched from 1150 °C into water and liquid nitrogen show the presence of 1/4{110} and 1/2{110} satellite reflections, the latter of which is expected from phonon dispersion curves obtained by density functional theory calculation. Therefore, it is proposed that Fe 50 Rh 50 maintains the B2 structure that is in premartensite state. Once Fe 50 Rh 50 is quenched from 1250 °C into liquid nitrogen, formation of two short-range ordered tetragonal phases with various c / a ratios (∼1.15 and 1.4) is observed in line with phases formed from mechanically deformed (30 %) sample. According to our observations, an accurate atomistic shear model ({110}〈 1 1 ¯ 0 〉) is presented that describes the martensitic transformation of B2 to these tetragonal phases. [Display omitted] [ABSTRACT FROM AUTHOR]

The present work proposes a new elementary deformation mechanism which governs high temperature and low stress creep of single crystal superalloys (SXs), where the misfit between the γ- and the γ′-phase plays a central role. In the coherent two phase SX microstructure, there is a tendency to minimize the overall elastic strain energy. This is accomplished by the formation of dislocation networks in the γ-phase close to the γ/γ'-interfaces. The stress fields of the network dislocations and misfit stresses accommodate each other to keep the overall strain energy of the system at a minimum. Previous work has shown that dynamic recovery is associated with knitting-out reactions, where dislocations from the network shear the γ'-phase and annihilate with dislocations of opposite sign on the other side of the γ'-phase region. Due to the presence of the misfit this results in an increase of elastic strain energy which is counteracted by coupled knitting-in reactions where newly arriving γ-channel dislocations re-establish the minimum energy configuration. Here we provide microstructural evidence for knitting-out and knitting-in reactions and show that while these reactions clearly occur, dislocation network spacings stay constant. The misfit between the γ- and the γ′-phase accounts for a constant network spacing. Dislocation networks are not static but represent dynamic steady state equilibrium structures in an evolving microstructure. Knitting regular networks requires climb processes which are suggested to be rate controlling. This new view of high temperature and low stress creep mechanism of SXs allows to rationalize previous results published in the literature. [Display omitted] [ABSTRACT FROM AUTHOR]

We have investigated the influence of the deformation temperature from 25 °C (RT) to -196 °C on the dislocation structures associated with strain localization phenomena in an austenitic Fe-30Mn-6.5Al-0.3C (wt.%) low-density steel by electron channeling contrast imaging (ECCI), electron backscatter diffraction (EBSD), and bright-field transmitted forescattered electron imaging ((BF) t-FSEI) techniques. The characteristics of the dislocation structures were evaluated on the main texture components, i.e. <111>//tensile axis, <112>//tensile axis, and <001>//tensile axis directions. The inhomogeneous character of the plastic behavior is promoted upon cryogenic deformation due to the formation of dislocation structures associated with strain localization, namely microbands (MBs) and deformation bands (DBs). ECCI analysis of the dislocation structure reveals that the deformation temperature has a strong influence on the thermal-assisted dislocation processes controlling the dislocation configurations and MB formation mechanisms. The MB nucleation mechanism evolves from a cross-slip-assisted mechanism at RT deformation conditions to a slip band-assisted mechanism at cryogenic deformation temperatures. This effect has a profound effect on the grain orientation dependence of the MB structure but not on its crystallographic alignment. On the other hand, cryogenic deformation temperatures (-196 °C) enhance the material's mechanical strength and ductility due to the activation of deformation twinning, which is associated with the reduction of the stacking fault energy. We find that MBs have a small contribution to strain-hardening and ductility due to the small mechanical resistance of these dislocation structures against the advance of deformation twins and dense dislocation layers, and the comparatively small plastic strain accommodated by them, respectively. These findings provide new insights into the microband-induced plasticity (MBIP) effect. [Display omitted] [ABSTRACT FROM AUTHOR]

The paper presents a full-field strain analysis of the appearance/disappearance of twinned martensite in a Cu–Al–Ni single crystal subjected to thermal cycles. Localized Spectrum Analysis (LSA) was employed to process images of a checkerboard pattern preprinted on the sample's surface, enabling us to obtain strain maps with a suitable trade-off between strain resolution (2 × 1 0 − 4 ) and spatial resolution (0.26 mm). During the cooling of the initially austenitic sample, habit plane variants (HPVs) were identified from the in-plane components of their mean right strain tensor. The match between measured data and theoretical expectations enabled us to measure the stretch parameters associated with the cubic-to-orthorhombic transformation occurring in the material. Various deformation mechanisms were revealed by an analysis in the strain space. In particular, the process of martensite appearance during cooling appeared to be based on the coordinated activation of HPVs pairs whose strain levels globally compensate each other. Elastic strain gradients were also evidenced, highlighting the need for additional deformation to achieve kinematic compatibility at microstructure interfaces. Lastly, several thermal cycles were applied to the previously-trained sample, enabling us to quantify the level of spatio-temporal repeatability of the phase transformation. [Display omitted] [ABSTRACT FROM AUTHOR]

We report on phase and strain changes in Ti 1-x Al x N (0 ≤ x ≤ 0.61) coatings on cutting tools during turning recorded in operando by high-energy x-ray diffractometry. Orthogonal cutting of AISI 4140 steel was performed with cutting speeds of 360–370 m/min. Four positions along the tool rake face were investigated as a function of time in cut. Formation of γ-Fe in the chip reveals that the temperature exceeds 727 °C between the tool edge and the middle of the contact area when the feed rate is 0.06 mm/rev. Spinodal decomposition and formation of wurtzite AlN occurs at the positions of the tool with the highest temperature for the x ≥ 0.48 coatings. The strain evolution in the chip reveals that the mechanical stress is largest closest to the tool edge and that it decreases with time in cut for all analyzed positions on the rake face. The strain evolution in the coating varies between coatings and position on the rake face of the tool and is affected by thermal stress as well as the applied mechanical stress. Amongst others, the strain evolution is influenced by defect annihilation and, for the coatings with highest Al-content (x ≥ 0.48), phase changes. [Display omitted] [ABSTRACT FROM AUTHOR]

Elemental powders of Al, Ti, Sc, and Zr are blended to create binary Al-0.5Ti and Al-0.25Ti-0.25Zr (at%) alloys via laser powder-bed fusion (L-PBF), with Al-1Ti and Al-0.25Sc-0.25Zr alloys produced for comparison. The Al-Ti alloys formed primary D0 23 -Al 3 Ti precipitates upon solidification, whereas the Al-(Sc/Ti)-Zr alloys formed primary L1 2 -Al 3 (Sc,Ti,Zr) precipitates which nucleated bands of fine grains. Upon aging, the Al-Ti alloys do not harden (consistent with most Ti remaining in solid solution), unlike the Al-(Sc,Ti)-Zr alloys, as expected from precipitation of secondary L1 2 -Al 3 (Sc,Ti,Zr) nanoprecipitates. Upon compressive creep testing at 300–400 °C, Al-0.25Ti-0.25Zr exhibits steady-state strain rates comparable to those of Al-0.5Zr at low stresses (up to ∼14 MPa), but about an order of magnitude faster at higher stresses. The effects of bands of fine grains (formed by L1 2 -Al 3 (Sc,Ti,Zr) primary precipitates at the bottom of the melt pools) on hardness and creep resistance is investigated via finite element modeling of simple slab models, with varying geometries approximating experimental melt pool geometries. The degree of connectivity of coarse- and fine-grain regions is a greater factor in the resulting creep strain rates than the volume fraction or spatial orientations of these two regions. This suggests that reducing the concentration of Zr (in Al-0.5Zr), or partially substituting with a less expensive element such as Ti (as for Al-0.25Ti-0.25Zr), may maintain the added l -PBF processability benefits of Zr and reduce the volume fraction of fine grains without sacrificing creep resistance, allowing for further tailoring of custom microstructures. [Display omitted] [ABSTRACT FROM AUTHOR]

Predicting the mechanical failure of parts in service requires understanding their deformation behavior, and associated dynamic microstructural evolution up to the near-atomic scale. Solutes are known to interact with defects generated by plastic deformation, thereby affecting their displacement throughout the microstructure and hence the material's mechanical response to solicitation. This effect is studied here in a polycrystalline Ni-based superalloy with two different Nb contents that lead to a significant change in their creep lifetime. Creep testing at 750°C and 600 MPa shows that the high-Nb alloy performs better in terms of creep strain rate. Considering the similar initial microstructures, the difference in mechanical behavior is attributed to a phase transformation that occurs along planar faults, controlled by the different types of stacking faults and alloy composition. Electron channeling contrast imaging reveals the presence of stacking faults in both alloys. Microtwinning is observed only in the low-Nb alloy, rationalizing in part the higher creep strain rate. In the high-Nb alloy, atom probe tomography evidences two different types of stacking faults based on their partitioning behavior. Superlattice intrinsic stacking faults were found enriched in Nb, Co, Cr and Mo while only Nb and Co was segregated at superlattice extrinsic stacking faults. Based on their composition, a local phase transformation occurring along the faults is suggested, resulting in slower creep strain rate in the high-Nb alloy. In comparison, mainly superlattice intrinsic stacking faults enriched in Co, Cr, Nb and Mo were found in the low-Nb alloy. Following the results presented here, and those available in the literature, an atomic-scale driven alloy design approach that controls and promotes local phase transformation along planar faults at 750°C is proposed, aiming to design superalloys with enhanced creep resistance. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Determination of strain path across martensitic transformation in materials with two transformation orientation relationships (ORs) is challenging due to the temporal and spatial limitation of the modern techniques. In this work, an analyzing strategy, i.e. , the determination of transformation path via the variant organization feature of martensite, was suggested and applied for the Ni–Mn-based alloys as an example, based on the consideration that the different crystallographic symmetries of transformation systems will result in distinct organizations of martensite variants. For the selected Ni–Mn-based alloys, the orientation examination revealed that both the K−S and the Pitsch OR are respected. Further analyses in terms of the strain and stress compatibility condition and the minimum energy criteria showed that, theoretically, the K−S path produces 2 variants as a self-accommodated variant group, whereas the Pitsch path produces 4 variants as a self-accommodated variant group. Compared with the experimental results, the Pitsch path is the energetically favorable transformation path and actually occurs in stress-free austenite of the Ni−Mn based alloys. The significance of this work is multi-fold. It first resolves the long puzzling issue of transformation strain path of Ni–Mn-based alloys. Moreover, the analyzing scheme can be generalized to determine the transformation characters for martensitic transformation in other systems with multiple possible transformation orientation relationships. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Using a partitioned-energy thermodynamic framework which assigns energy to that of atomic configurational stored energy of cold work and kinetic-vibrational, we derive an important constraint on the Taylor-Quinney coefficient, which quantifies the fraction of plastic work that is converted into heat during plastic deformation. Associated with the two energy contributions are two separate temperatures – the ordinary temperature for the thermal energy and the effective temperature for the configurational energy. We show that the Taylor-Quinney coefficient is a function of the thermodynamically defined effective temperature that measures the atomic configurational disorder in the material. Finite-element analysis of recently published experiments on the aluminum alloy 6016-T4 [1], using the thermodynamic dislocation theory (TDT), shows good agreement between theory and experiment for both stress-strain behavior and temporal evolution of the temperature. The simulations include both conductive and convective thermal energy loss during the experiments, and significant thermal gradients exist within the simulation results. Computed values of the differential Taylor-Quinney coefficient are also presented and suggest a value which differs between materials and increases with increasing strain. [ABSTRACT FROM AUTHOR]

Triple junctions, intersections of three or more grains in polycrystalline solids, are known locations of potential stress concentration and strain localization. In this work, intragranular lattice curvatures and elastic strains are measured at triple junctions via a novel zoom-in style combination of synchrotron X-ray techniques. Additionally, this work revisits the impact of including elastic strain gradients while calculating the dislocation density within embedded grains via the Nye dislocation tensor. Without elastic strain gradients, the dislocation density is underestimated at a hotspot by 27% of the maximum dislocation density value. Spatial regions near triple junctions are found to contain both the highest values of the intragranular misorientations metrics and generally higher values than grain boundaries (between two grains). A heterogenous distribution of intragranular misorientation, elastic strain, and dislocation density is measured within a grain along a single triple junction line, and the triple junction line end points (quad points) exhibit substantially different micromechanical fields. This work provides a unique 3D view of triple junctions within individual grains and highlights their spatially heterogenous intragranular micromechanical response along triple junction lines, especially when compared to relatively less complex behavior along grain boundaries. While a grain averaged example is provided of intergranular strain across grains, several examples of intragranular strain were visualized near a triple junction. The combined analysis of intragranular lattice curvature (associated with strain metrics) and elastic strain (associated with stress) provides a more complete characterization of the 3D sub-grain fields than previously reported. [Display omitted] [ABSTRACT FROM AUTHOR]

Polar domain walls in centrosymmetric ferroelastics (e.g., BiVO 4 films) give rise to inhomogeneity and offer promises for solar energy conversion applications via expediting charge-carrier separation. However, the origin of polar domain walls and the relationship between them and spontaneous strain from structural phase transition remain elusive at the mesoscale in BiVO 4 films. Here, from the perspective of phenomenological thermodynamics, the relationship between four variants of monoclinic (M) phase and the corresponding lattice parameters during the tetragonal-monoclinic (T-M) phase transition is revealed. Using phase-field simulations, it suggests that the formation of four variants of ferroelastic twin domains arises from spontaneous strain from T-M phase transition. Moreover, it is demonstrated that (i) polar domain walls in BiVO 4 films originate from the flexoelectric effect induced by strain gradient across the domain walls during phase transition process; (ii) shear strain gradient dominates the contribution to the polarization vectors at the domain walls; (iii) 180° head-to-head or tail-to-tail in-plane polarization configurations emerge at the adjacent domain walls. The stability of several types of ferroelastic domain walls is also analyzed from the point of view of total free energy. These results not only provide a fundamental understanding of the origin of polar domain walls and the formation mechanisms of ferroelastic twin patterns, but also stimulate potential applications of polar domain walls of non-polar BiVO 4 films in energy conversion. [Display omitted] [ABSTRACT FROM AUTHOR]

The migration behaviour of different grain boundaries with misorientations close to the Σ3 CSL orientation relationship in high purity Al bicrystals under the capillary driving force and applied mechanical stress was investigated. The experiments were performed by an in-situ technique to observe and measure the boundary migration with a scanning electron microscope. The ability of the nearly Σ3 60° 〈 111 〉 incoherent {110} and {112} boundaries to move under capillary driving force was found to depend critically on the initial boundary inclination. While boundaries with inclinations near {112} can easily assume a curved shape and migrate, boundaries with an initial {110} plane remain stationary or form non-mobile facets. This is attributed to the essential anisotropy of the inclination dependence of the energy γ (ψ) of 60° 〈 111 〉 tilt boundaries with differently high torque d γ / d ψ around {112} and {110} inclinations, as revealed by atomistic simulations of the respective boundaries. The Σ3 70.5° 〈 110 〉 tilt boundary with the geometry corresponding to the coherent {111} twin boundary, was found to be immobile under both driving forces applied. The 59.2° 〈 111 〉 tilt grain boundary with geometry near the Σ3 {110} incoherent twin boundary was found to be quite mobile under applied shear stress. The measured migration activation enthalpy H = 0.45 eV for this boundary is the lowest among the values obtained in previous experiments for any other stress driven grain boundary in Al bicrystals of the same purity. Moreover, this boundary migrated with a zero coupling factor, i.e. without producing any measurable shear parallel to the boundary plane. [Display omitted] [ABSTRACT FROM AUTHOR]

An investigation of the effect of solutes on the migration of a single { 10 1 ¯ 2 } Gd decorated twin boundary in a Mg-4wt.% Gd binary alloy in terms of the corresponding stress-strain response and the defect structure in the wake of (de)twinning was undertaken in this work. The results were benchmarked against those of a pure Mg sample that was tested under same conditions. We established that the critical stresses for both twinning and detwinning increased by 29% and 10%, respectively, due to the addition of Gd. We showed that the latter is mediated by the pinning effect of Gd atoms that decorated the pre-existing twin boundaries in the alloy. Conversely, a 68% decrease in the detwinning stress was recorded in pure Mg. We attributed the decrease to the nucleation-free nature of detwinning along with the absence of solutes atoms at the pre-existing twin boundaries. Unlike previous investigations, the work does not just highlight how twinning and detwinning affects the concurrent slip activity in pure Mg and Mg alloys, it establishes a direct link between these activities and the magnitude of the observed mechanical response. [Display omitted] [ABSTRACT FROM AUTHOR]

The study is focused to understand the micro-mechanisms that govern the deformation behavior of a Ni-rich NiTi alloy at the sub-micron scale. This alloy undergoes reversible stress induced martensitic transformation leading to pseudoelasticity. Correspondingly, significant strain recoverability is noted. In this investigation, nanoindentation is utilized as the primary experimental tool. Various parameters including the indenter tip configuration, size and applied load levels are varied systematically. It is observed that compared to the most commonly used sharp Berkovich indenter tip, blunt spherical tip imposes reduced amount of strain and strain-gradient within the indentation volume. This opens up the window for systematically varying the sequential deformation modes in NiTi. Further in-depth analysis revealed that optimum combination of indenter tip configuration, size and applied load level is prerequisite to appreciate the pseudoelastic mechanism in NiTi. Most importantly, a tailored and simplified protocol is formulated for converting nanoindentation load-displacement response to corresponding indentation-stress-strain behavior for pseudoelastic alloy. These indentation-stress-strain curves featuring the signature deformation characteristics of NiTi alloy are realized without utilizing any specialized experimental modes. In a nutshell, this study highlights the importance of choosing adequate nanoindentation parameters for assessing the functional characteristics of pseudoelastic NiTi system at sub-micron scale. Accordingly, overall localized deformation behavior of this unique alloy is investigated. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Image, graphical abstract Polycrystalline alloys are used pervasively across structural applications contingent upon extensive experimental testing. A statistically representative number of tests are necessary to expose the variability in the material's performance, as a result of non-uniform microstructures and associated micromechanical fields. In a more direct means of capturing this pertinent information, multi-modal experimental techniques are presented to measure and track the complete micromechanical state, evolving during loading, of each and every grain within the regions of interest. Specifically, a combination of high-energy X-ray diffraction microscopy and digital image correlation coupled with electron backscatter diffraction are conducted on a specimen for each of the alloys, Haynes 282 and Ti7Al. The results of the multi-modal analysis definitively demonstrate that the degree of heterogeneity increases with deformation level and is used to assess the number of grains necessary for a representative volume element description of the stress state for each of these materials. Moreover, higher resolution imaging is used for identification of the slip system activity and subsequently used to study slip transmission events. An accurate knowledge of the resolved shear stress in adjacent grains (grain interactions) is demonstrated to be a key descriptor of the slip transmission events. [ABSTRACT FROM AUTHOR]

Nickel (Ni)-based superalloys are used for safety-critical components in the aerospace industry because of their high strength at elevated temperatures. These materials are subjected to complex thermal and mechanical cycling due to service conditions, which leads to thermo-mechanical fatigue (TMF) failure. The failure mechanisms associated with TMF are dependent on microstructural characteristics and loading parameters, such as temperature, strain range, and strain-temperature phasing, requiring a large set of experimental test matrices to study TMF failure. One means to reduce the necessity of extensive and exhaustive testing for material qualification is with model-based approaches. In this work, to study the complexities of micromechanical TMF damage, a temperature-dependent, dislocation density-based crystal plasticity model for a Ni-based superalloy is generated with uncertainty quantification. The capabilities of the model's temperature dependency are examined via direct instantiation and comparison to a high-energy X-ray diffraction microscopy (HEDM) experiment under coupled thermal and mechanical loads. Unique loading states throughout the experiment are investigated with both crystal plasticity predictions and HEDM results to study early indicators of TMF damage mechanisms at the grain scale. Via a crystal plasticity-based failure metric that can predict likely sites for crack initiation, regions of extreme values indicate the spatial location of microstructural damage, while showing correlation with high strain gradients. The results also indicate that the extreme value of thermo-mechanical damage accumulation is influenced by the surrounding grains characteristics including slip system activity, lattice curvature, and neighboring grain compatibility interactions, quantified via a grain interaction strength-to-stiffness ratio orthogonal to the grain boundary. [Display omitted] [ABSTRACT FROM AUTHOR]

Heterogeneous microstructural design has been proven to be an effective strategy in breaking the strength–ductility dilemma in nanostructured metals. However, the precise control of heterogeneous microstructures to achieve strength–ductility synergy remains challenging. Here, we demonstrate a novel powder metallurgy approach for creating three-dimensional (3D) core–shell nanostructures with highly tunable shell thickness and grain size distributions. These 3D nanostructures enable superior strength–ductility synergy in pure copper, pushing the boundary of the Ashby map to unchartered territory. A combination of microstructural characterization, atomistic simulations and crystal plasticity modeling reveals that the generation and accumulation of geometrically necessary dislocations near the core–shell interface play a pivotal role in accommodating the strain gradient and sustaining a high strain-hardening rate during plastic deformation. Our work provides a viable approach for designing bulk nanostructured materials with 3D heterogeneous ingredients and demonstrates a promising pathway for the development of strong and ductile materials. [Display omitted] [ABSTRACT FROM AUTHOR]

Interfaces play a crucial role in mechanical behaviors of laminated materials. In this study, a series of hard/soft nanotwinned Cu laminates with varying interface spacing from 200 to 33 μm are prepared by means of direct current electrodeposition. Simultaneous improvement of strength and work hardening with decreasing interface spacing is found in tensile tests. Extra strengthening and geometrically necessary dislocations (GNDs), but without any strain concentration, are found in the vicinity of interfaces. An obvious strain discrepancy appears across the interface and decreases with decreasing interface spacing. Most importantly, a highest plastic strain gradient appearing in the vicinity of interfaces, regardless of interface spacing, indicates a significant deformation compatibility across the interfaces. The spatially-distributed strengthening mediated by interface contributes for increasing strength and decreasing strain discrepancy by decreasing the interface spacing. [Display omitted] [ABSTRACT FROM AUTHOR]

The superior strength and toughness balance prevails in the face-centered-cubic-structured high/medium-entropy alloys (H/MEAs) of low strength, while a gain in yield strength is normally accompanied by a sacrifice in toughness, leading to the strength and toughness trade-off particularly when yield strength increases to the gigapascal levels. Here, we showed the superior fracture toughness by heterostructuring an N-doped CrCoNi MEA having different levels of yield strength higher than 1 GPa. The fracture toughness was 91 MPa · m 1 / 2 at yield strength of 1.3 GPa in the heterogeneous lamella structure and 168 MPa · m 1 / 2 at yield strength of 1.0 GPa in the heterogeneous grain structure. The fracture toughness was attributed to forest hardening plus an extra hetero-deformation induced hardening. The pile-ups of geometrically necessary dislocations were observed to be formed at the domain boundaries to accommodate strain gradient at the plastic zone of the crack tip. The chemical short-range orders were found for the enhanced strain hardening near the crack tip by the interaction with dislocations. Moreover, a new parameter was proposed to characterize the work hardening capacity at the crack tip by the integral of hardness increment in the plastic zone which shows a linear relation-ship with the J -integral value during the crack initiation. [Display omitted] [ABSTRACT FROM AUTHOR]

Modeling the full-range deformation behaviors of materials under complex loading and materials conditions is a significant challenge for constitutive relations (CRs) modeling. We propose a general encoder-decoder deep learning framework that can model high-dimensional stress-strain data and complex loading histories with robustness and universal capability. The framework employs an encoder to project high-dimensional input information (e.g., loading history, loading conditions, and materials information) to a lower-dimensional hidden space and a decoder to map the hidden representation to the stress of interest. We evaluated various encoder architectures, including gated recurrent unit (GRU), GRU with attention, temporal convolutional network (TCN), and the Transformer encoder, on two complex stress-strain datasets that were designed to include a wide range of complex loading histories and loading conditions. All architectures achieved excellent test results with an root-mean-square error (RMSE) below 1 MPa. Additionally, we analyzed the capability of the different architectures to make predictions on out-of-domain applications, with an uncertainty estimation based on deep ensembles. The proposed approach provides a robust alternative to empirical/semi-empirical models for CRs modeling, offering the potential for more accurate and efficient materials design and optimization. [Display omitted] [ABSTRACT FROM AUTHOR]

Abstract Shear coupling implies that all grain boundary (GB) migration necessarily creates mechanical stresses/strains and is a key component to the evolution of all polycrystalline microstructures. We present MD simulation data and theoretical analyses that demonstrate the GB shear coupling is not an intrinsic GB property, but rather strongly depends on the type and magnitude of the driving force for migration and temperature. We resolve this apparent paradox by proposing a microscopic theory for GB migration that is based upon a statistical ensemble of line defects (disconnections) that are constrained to lie in the GB. Comparison with the MD results for several GBs provides quantitative validation of the theory of shear coupling factor as a function of stress, chemical potential jump and temperature. [ABSTRACT FROM AUTHOR]

Abstract Large-scale molecular dynamics simulations are used to investigate shock-induced damage and fracture in 3C SiC single crystals at an elevated initial temperature of 2000 K and a high tensile strain rate of ∼1010 s−1. Three crystal orientations have been evaluated: [001], [110] and [111]. A comprehensive comparison has been made between cases at 2000 K and at 300 K to address the effects of high temperature on the mechanical performance of SiC under shock loading. Results show that for shock compression, the high temperature decreases the longitude elastic wave speeds as well as the shock stresses. The shock-induced plasticity is mainly in the form of deformation twinning at 300 K, but twinning is absent at 2000 K. The high temperature decreases the structural phase transition threshold pressure in SiC from ∼90 GPa at 300 K (for all three orientations) to ∼75 GPa in [001], ∼57 GPa in [110] and ∼64 GPa in [111] at 2000 K, with corresponding particle velocities of 2.75 km/s, 2.0 km/s, and 2.25 km/s, respectively, in agreement with trends observed in recent experiments. The spall fracture behavior reveals that high temperature reduces the spall strength with an average spall strength of ∼20.7 GPa in [001], ∼21.4 GPa in [110] and ∼22.5 GPa in [111] at 2000 K in the classical spall regime, which are about 33% lower than strengths measured at 300 K. However, in the micro-spall regime the spall strengths are very similar at both temperatures. The corresponding thresholds of particle velocity to trigger spall decrease at elevated temperature except for [001] loading, as well as the thresholds for generating overdriven phase transition waves. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]

Abstract High Entropy Alloys are inherently complex and span a vast composition space, making their research and discovery challenging. Developing quantitative predictions of their phase selection requires a large quantity of consistently determined experimental data. Here, we use combinatorial methods to fabricate and characterize 2478 quinary alloys based on Al and transition metals. All data are publicly available at http://materialsatlasproject.org/. Phase selection can be predicted for considered alloys when combining the content of FCC/BCC elements and the constituents' atomic size difference. Mining our data reveals that High Entropy Alloys with increasing atomic size difference prefer BCC structure over FCC. This preference is typically overshadowed by other selection motifs, which dominate during close-to-equilibrium processing. Not suggested by the Hume-Rothery rules, this preference originates from the ability of the BCC structure to accommodate a large atomic size difference with lower strain energy penalty which can be practically only realized in High Entropy Alloys. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]

Abstract Domain behavior of (111)- oriented perovskite ferroelectric films is significantly different from (001)-/(101)- oriented ones, resulting in enhancing property responses such as a superior susceptibility and a reduced coercive field. However, the domain structures and evolutions with the thickness of (111)-oriented ferroelectric films, which are crucial to further understand the distinctive properties, are still obscure. In this study, the ferroelectric domains of (111)- oriented PbTiO 3 films are investigated by transmission electronic microscopy (TEM) and piezoresponse force microscopy (PFM). We identify the domain evolution with the film thicknesses under anisotropic strains imposed by the orthorhombic GdScO 3 (101) O substrates. Contrast analysis and electron diffraction patterns reveal that only four ferroelectric variants evolve in PTO films: d 2 +, d 2 -, d 3 + and d 3 -, with the polarization directions along [010], [0 1 ¯ 0], [001] and [00 1 ¯ ], respectively. Two kinds of domain walls are formed: "inclined" (011) domain walls for d 2 -/d 3 + (d 2 +/d 3 -) domains, "normal" (01 1 ¯) domain walls for d 2 -/d 3 + (d 2 +/d 3 -) domains. The width of periodically distributed d 2 +/d 3 + (d 2 -/d 3 -) domains increases with film thickness following the square root rule. Aberration-corrected scanning transmission electronic microscopy demonstrates the lattice characteristics of domains in (111)- oriented PbTiO 3 films are consistent with tetragonal ferroelectric domains. PFM studies reveal that both the out-of-plane and in-plane polarization components of d 2 -/d 3 + (d 2 +/d 3 -) domains are non-identical, whereas the d 2 +/d 3 + (d 2 -/d 3 -) domains possess uniform out-of-plane polarization component and non-uniform in-plane polarization component. This study discloses the domain structure in (111)- oriented tetragonal ferroelectric films under anisotropic strains and the association with the ferroelectric properties. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]

Abstract A modification to the empirical Olson-Cohen strain-induced austenite to martensite transformation kinetic model is proposed. The proposed kinetic model accounts for the stress state at the grain level and the crystallography of the transformation mechanism. Two transformation mechanisms sensitive to the local stress state are incorporated in the model. First, the resolved shear stress on a slip plane in the direction perpendicular to the Burgers vector determines the stacking fault width (SFW) which in turn determines the potential nucleation sites. Second, the stress triaxiality governs the probability of the structural α ′-martensite formation at a nucleation site. The kinetic model is implemented in the elasto-plastic self-consistent (EPSC) crystal plasticity model to study the stress state and texture dependence of the strain-induced α ′-martensite transformation and the mechanical response of metastable austenitic steels. The simulations are compared with experimental mechanical and phase fraction data from different austenitic steels subjected to simple tension, plane strain tension, equibiaxial tension, simple compression, and torsion. It is demonstrated that the appropriate modeling of α ′-martensite phase fractions allows capturing the experimentally measured mechanical response. The implementation and insights from these predictions, including the role of texture evolution on martensite transformation, are discussed in this paper. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]

Abstract Dealloyed nanoporous metals are emerging as a new class of structural and functional materials for a wide range of applications. Nevertheless, the dealloying process usually leads to significant volume shrinkage, large internal stresses, surface oxidation, stress-corrosion cracking and thereby extensive fabrication flaws. This is particularly serious for nanoporous non-noble metals because of their high affinity with oxygen and resulting serious oxidation. Therefore, dealloyed nanoporous metals usually have poor mechanical performances, particularly, under tension which is highly sensitive to flaws. Consequently, high tensile strength has not been achieved from technically-important and economic nanoporous transition metals, such as Ni and Cu. In this study we report flaw-free nanoporous Ni fabricated by utilizing an ultrafine grained precursor alloy, high-temperature dealloying and post-dealloying annealing. The resulting nanoporous Ni shows an excellent tensile strength, which is one order of magnitude higher than all reported tensile strengths of dealloyed nanoporous metals. The strong and ductile nanoporous Ni developed in this study can be scaled up for large-scale structural and functional applications where tensile properties are required. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]

Abstract The effects of strain on the ionic conductivity of rare-earth substituted CeO 2 have been extensively studied, but the results have been inconsistent and focused upon the 'optimised' conductors such as Gd or Sm substituted CeO 2 where defect association is minimised. By thermally annealing epitaxial films deposited by pulsed laser deposition, we varied the strain systematically, whilst avoiding any influence from interfacial or grain boundary effects. The activation energy of the in-plane conductivity was found to increase with increasing compressive biaxial strain, which was quantitatively in excellent agreement with previous computational and experimental studies. These results provide a much needed quantitative consensus on the effects of lattice strain on ionic transport. Furthermore, we demonstrate that the change in the activation energy for Yb-substituted CeO 2 is around three times that for Gd or La substitutions for the same applied strain, indicating the important role played by defect association. These results have significant implications for ionic transport at reduced or ambient temperatures, where changes in conductivity due to strain may be several orders of magnitude larger for 'non-optimised' conductors compared with 'optimised' conductors. We rationalise our results by considering the defect-defect interactions in these materials and through force-field calculations. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]

Abstract By using classical molecular dynamics simulations and a novel technique to identify defects based on the calculation of atomic strain, we have elucidated the detailed mechanisms leading to the anomalous generation and growth of {001} loops found after ultra-fast laser annealing of ion-implanted Si. We show that the building block of the {001} loops is the very stable Arai tetra-interstitial [N. Arai, S. Takeda, M. Kohyama, Phys. Rev. Lett. 78, 4265 (1997)], but their growth is kinetically prevented within conventional Ostwald ripening mechanisms under standard processing conditions. However, our simulations predict that at temperatures close to the Si melting point, Arai tetra-interstitials directly nucleate at the boundaries of fast diffusing self-interstitial agglomerates, which merge by a coalescence mechanism reaching large sizes in the nanosecond timescale. We demonstrate that the crystallization of such agglomerates into {001} loops and their subsequent growth is mediated by the tensile and compressive strain fields that develop concurrently around the loops. We also show that further annealing produces the unfaulting of {001} loops into perfect dislocations. Besides, from the simulations we have fully characterized the {001} loops, determining their atomic structure, interstitial density and formation energy. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]

Abstract Low-energy structures of bimetal interfaces commonly occur in nature, yet higher energy forms, made by deviations in the interface plane, are also likely. While these variants may occur less frequently, they can still play an important role in the response of the material under deformation, by acting as preferential sites for defect formation and boundary motion. Here, using atomic-scale simulation and interface defect theory and considering two bimetal systems, Cu/Ag and Cu/Nb, we show that high-energy interfaces can achieve a local, low-energy state by forming atomic-scale serrations and that the predicted size and location of the serrations are consistent with experimental observation. For several distinct strain states, we reveal that interfaces with atomic-scale serrations bear both higher barriers for dislocation nucleation and higher resistances to interfacial shear compared to their planar interface variants. This desirable combination is not a characteristic of the more commonly studied low-energy interfaces, which typically possess a high nucleation barrier and low sliding resistance. We explain, through an analysis of the misfit dislocation structure, that the serrations alter the number of dislocations emitted, change the favorable slip systems, and alleviate the stress concentrations generated by misfit dislocations. An interface engineering strategy is then proposed for designing atomically serrated interfaces to improve mechanical strength and hinder localization and creep of metallic nanomaterials. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]

Abstract Dry sliding wear tests of steel disks of various compositions were performed with consideration of the effects of microstructure, shear strain and shear strain rate using a ball-on-disk tester with a tungsten carbide ball under ambient atmosphere and room temperature. We found a severe–mild wear transition phenomenon, which we called self-lubrication, and categorized friction behavior into two regimes: a high-friction regime and a low-friction regime. However, self-lubrication occurs only at specific strain levels and rates related to materials with martensite and pearlite structures. We characterized the microstructures of the worn surface, subsurface and matrix and found a thin self-lubricating layer composed of nano-oxide particles with sizes in the range 6–20 nm attached to the surface. The formation of these nano-oxide particles is controlled by lamellar structure (martensite or pearlite) nano-lamination, oxidation and solid-state amorphization during dry sliding. The high density of geometrically necessary dislocations and defects induced by plastic deformation under wear shear strain levels and rates is important in the formation of nano-lamellar microstructures and amorphous oxide. These results both clarify nano-lamellar microstructure solid-state amorphization and oxidation behavior and reveal the mechanism of self-lubricating layer formation observed in this work. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]

Abstract Current alloy development efforts in High Entropy Alloys call for a better understanding of solution hardening in high-concentration chemically-complex alloys. Here we propose a general scheme for assessing the overall solute-dislocation interaction, independent of concentration, stress, and temperature. While significant progress has been made in including solute-dislocation interactions in FCC metals (Leyson et al) there are open questions as to how to model more complex (solute-solute-dislocation) chemical interactions that arise in high concentration solid solutions. A method similar to Olmsted et al , is developed to quantify representative energy landscapes as a dislocation percolates through a field of solutes. This approach uses molecular dynamics under stress in order to estimate the strengthening parameters for a system over a wide range of solute concentrations and a moderate range of temperatures. While MD introduces additional complications to the simulation, due to coupling of temperature with the potential energy of the system, it also provides significant flexibility in terms of avoiding local minimum, assessing the effects of dislocation bowing and the effects of stress and temperature. Also, by avoiding the direct assessment of solute-solute bonding very complex systems can be evaluated efficiently. As proof of concept the method is applied to Al solutes in FCC Ni, where the system is known to be dominated by strong Al solute-solute interactions at high solute concentrations. Self-consistency of the method is shown by computing various strengthening parameters near critical percolation stress states at various system sizes, concentrations, and temperatures. We propose an extension to current solute-dislocation hardening models (i.e. Leyson et al.) to include solute-solute bonding and compare the scaling prediction and temperature dependence of solid solution models to the molecular dynamics results. Graphical abstract Interaction energy profile of an edge-dislocation with a solute obtained using lattice statics (LS) (solid black line) at zero temperature and no applied stress, compared to the estimates obtained using molecular dynamics (MD) at a finite temperature (1 K) under various applied stresses (colors) and various thermostat relaxation times used during the simulation (symbols). Image 1 [ABSTRACT FROM AUTHOR]

Abstract Alloying Mg with rare-earth elements, such as Nd, Gd, or Y, enables hardening from the precipitation of metastable coherent phases during aging at low temperatures. While the aging potential of binary Mg-Nd alloys is relatively limited due to the nucleation of coarse β 1 and β precipitates at the expense of the strengthening β‴ precipitates, binary Mg-Y alloys exhibit exceptional stability of the strengthening coherent β S ′ phase when aged at 200 °C. Through combination of high-resolution characterization, density functional theory calculations, and finite-element elasticity studies, we demonstrate that the strengthening β S ' phase is thermodynamically stable at low temperatures (as opposed to metastable as in other Mg-RE binaries) and that misfit strains play a key role in not only controlling precipitate structure and morphology but also their unusual evolution into an interconnected network. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]

Abstract Cyclic stresses generally lead to fatigue damage and failure with important implications for material and component design, safety, performance and lifetime costs in major structural applications. Here we present unique results for copper to demonstrate that a thin superficial layer of graded surface nanostructure over a coarse-grained core suppresses strain localization and surface roughening, thereby imparting unprecedented resistance to both low-cycle and high-cycle fatigue without compromising ductility. Progressive homogenization of the surface-graded copper is shown to be superior in fatigue properties compared to that of any of its homogeneous counterparts with micro-, submicro- or nano-grained structures. Since the findings here for enhancing resistance to fatigue are broadly applicable to a wide spectrum of engineering metals and alloys, the present results offer unique pathways to mitigate fatigue damage using a broad variety of processing routes, geometric design considerations, and structural parameters in many practical applications. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]

Abstract The distribution of strain in hard mode oriented lamellar stacks of the two-phase γ-TiAl/α 2 -Ti 3 Al alloy Ti-45Al-2Nb-2Mn (at.%)-0.8 vol% TiB 2 was measured at several temperatures up to 633 °C by in situ micropillar compression, complemented by electron backscatter diffraction orientation mapping and digital image correlation strain mapping of a thermally stable surface Pt speckle pattern. Post-mortem transmission electron microscopy further identified the finest scale deformation structures. It was found that slip and twinning transverse to the lamellae operates within discreet bands that zigzag across the lamellar structure. The shear strain within each band is approximately constant across the pillar width. This is inconsistent with current energetic models for transverse twin formation in γ-TiAl, which assume independent, non-interacting twins. This is explained using a mathematical formulation for the stress required to operate this transverse mechanical twinning as a function of strain. This study has elucidated how the multi-scale combination of several transverse twinning systems on different {111} planes in γ-TiAl lamellae can relieve the elastic stresses generated at a lamellar interface by the primary (highest Schmid factor) twinning system. It is thought that the facilitation of this mechanism will promote the ductilisation of lamellar γ-TiAl alloys. This is crucial for an increased damage tolerance and ease of component manufacture, leading to a more widespread use of γ-TiAl alloys. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]

Abstract We applied a hot rolling direct quenching and partitioning (HDQ&P) process to a low-C low-Si Al-added steel and obtained a Q&P steel containing 40 vol % of ferrite. Microstructure characterization was performed by means of SEM, EBSD, TEM and XRD. Atomic-scale characterization of carbon partitioning among the phases was carried out by atom probe tomography (APT). The carbon distribution in the retained austenite and near the martensite/retained austenite interfaces was quantitatively analyzed to study its partitioning behavior. The macroscopic strain distribution evolution across the tensile sample surface was investigated using macro digital image correlation (DIC) analysis. Combining these results with joint micro-DIC and EBSD analysis during quasi in-situ tensile testing, we investigated the strain partitioning among the different phases and the TRIP effect. Coupling of these results enabled us to reveal the relation among carbon partitioning, strain partitioning and the TRIP effect. The large blocky retained austenite with a side length of about 300–600 nm located near the ferrite/martensite (F/M) interfaces has low stability and transforms to martensite during the early deformation stages, i.e. at average strain below 21%. The retained austenite films in the centers of the martensite regions are more stable. The carbon distribution in both, the martensite and the retained austenite are inhomogeneous, with 0.5–2.0 at. % in the martensite and 4.0–7.5 at. % in the retained austenite. Strong carbon concentration gradients of up to 1.1 at. %/nm were observed near the martensite/retained austenite interfaces. The large blocky retained austenite (300–600 nm in side length) near the F/M interfaces has 1.5–2.0 at. % lower carbon content than that in the narrow retained austenite films (20–150 nm in thickness). The ferrite is soft and deforms prior to the martensite. The strain distribution in ferrite and martensite is inhomogeneous, varying by up to 20% within the same phase at an average strain of about 20%. Ferrite deformation is the main origin of ductility of the material. The balance between ferrite fraction and martensite morphology controls the TRIP effect and its efficiency in reaching a suited combination of strength and ductility. Reducing the ferrite volume fraction and softening the martensite by coarsening and polygonization can enhance the strain carried by the martensite, thus promoting more retained austenite in the martensite regions enabling a TRIP effect. The enhancement of the TRIP effect and the decrease of the strain contrast between ferrite and martensite jointly optimize the micromechanical deformation compatibility of the adjacent phases, thus improving the material's ductility. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]

Abstract Recent studies on hydrogen enhancement of the strain-induced generation of vacancies, i.e. the HESIV mechanism, and its role in hydrogen embrittlement (HE) of steels are overviewed. A high density of vacancies and their clustering have been detected by newly developed low temperature hydrogen thermal desorption spectroscopy and positron lifetime measurements. The promotion of crack nucleation and growth is ascribed to the evolution of damage associated with intense strain localization, and characteristic features of HE such as dependence on the microstructures of steels and test conditions are consistent with the HESIV mechanism. The predominant and supplementary roles of vacancies and hydrogen, respectively, in premature fracture are demonstrated by experiments devised to separate each contribution. A constitutive relation for porous materials is applicable to describe mechanistic effects of the HESIV mechanism on crack growth resistance and strain localization. The sequential void nucleation, growth and link mechanism in nanoscale are suggested. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]

Abstract Variant selection of Ni 2 (Mo,Cr) precipitates (γ′) in a low coefficient of thermal expansion Ni-Mo-Cr-W alloy has been investigated as a function of precipitate size and applied stress. The lenticular-shaped, coherent γ′ precipitates form with a {110} M /(010) p habit plane. During conventional aging, all 6 variants of the Ni 2 (Mo,Cr) precipitates form and coarsen with aging time. With the application of external loading below the yield stress during aging, variants with {110} M habit planes that experience the largest dilations resulting from the externally applied stress are favored. Phase field modeling, coupled with elasticity theory, has been employed to simulate the development of variant selection of γ′ precipitates with and without applied external loading. Similar to the experimental results, the γ′ precipitates tend to form parallel to each other during coarsening and display variant selection when external stress is applied during aging. The mechanism is believed to be the reduction of elastic strain energy caused by the precipitate/matrix lattice mismatch. Thus, the nature of the selection indicates that the sign of the misfit is positive along [010] p. Because the misfit strain energy increases with increasing precipitate size, variant selection occurs during coarsening. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]

Abstract Cross-sectional observations of the deformation bands formed during in situ tensile observation at the free surface of both ultralow- and low-carbon lath martensitic steels were conducted. The cross-sectional observations clearly revealed the formation of steps along substructure boundaries of the ultralow-carbon lath martensite during deformation. In contrast, severe undulation indicating strain accumulation was observed in the vicinity of high-angle boundaries for the low-carbon lath martensite, and no clear steps were formed along substructure boundaries, even though small filmlike ferrites were found. The "greasy-film" mechanism proposed by Maresca [Maresca et al., Modelling and Simulation in Materials Science and Engineering 24(2016)025006, Maresca and Curtin, Acta Materialia 134(2017)302] as well as the plastic instability theory proposed by Dao and Asaro [Dao and Asaro: Mech. Mater., 23(1996)71] was examined considering the non-Schmid effects and the anisotropic activation of slip systems in lath martensite. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]

Abstract The mechanical response of a copper-2 wt% beryllium alloy in either a solution treated or aged condition has been investigated within the shock loading regime. In terms of Hugoniot Elastic Limit, shear strength and spall (dynamic tensile) strength, the material in the aged condition has been observed to be significantly stronger than when in the solution treated state. Whilst these trends are the same as when tested under quasi-static conditions, the level of strengthening due to shock loading is less than during quasi-static loading. It is believed that the distribution of non-shearable particles within the microstructure (caused by aging) restrict the ability of the material to deform via dislocation motion and generation, which in turn reduces strain-rate sensitivity. Thus the relative differences in strength between aged and solution treated materials will be less under shock loading conditions compared to quasi-static conditions. In contrast, strong similarities between the shear strengths of the solution treated material and pure copper were noted. It has been suggested that twinning in the alloy may be responsible (due to the small contribution of twins to shock induced strengthening), although comparison of partial dislocation separation between the two materials suggest otherwise. [ABSTRACT FROM AUTHOR]

Abstract The creation of structural defects in low-mismatched GaAs/GaAs 0.92 Sb 0.08 /GaAs(001) heterostructures and their evolution during strain relaxation have been studied using transmission electron microscopy as well as high-resolution x-ray diffraction and atomic force microscopy. These GaAsSb films had thicknesses in the range of 50–4000 nm with 50-nm-thick capping layers and were grown using molecular beam epitaxy. The strain relaxation had three distinct phases as the film thickness was increased, whereas the thin GaAs capping layers exhibited only the initial sluggish stage of relaxation in heterostructures with thick GaAsSb films. The character of the misfit dislocations at the two interfaces was determined using g.b analysis, and atomic-scale structural information was obtained using aberration-corrected electron microscopy. Stage-I relaxation took place primarily by glide of dissociated 60° dislocations. Although the films were mostly free of threading dislocations, many curved dislocations extended into the substrate side for heterostructures that had undergone Stage-II and Stage-III relaxation. Investigation of dislocation density evolution at the cap/film interface and morphological evolution of the growth surface revealed a strong correlation. The smoother growth surface in the heterostructure with 4000-nm-thick film resulted in a reduced areal density of surface troughs that acted as nucleation sites for dislocations, which explained the decreased dislocation density at the cap/film interface. Overall, these results prove that heterogeneously nucleated surface half-loops are the primary source of threading dislocations in low-mismatched heterostructures. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]

Abstract Recent studies have demonstrated that many metal nanowires (NWs) with low stacking-fault energies display ultrahigh strength and can accommodate large plastic strains by spreading mechanical twins throughout the entire volume of the NWs. This previous observation on the plasticity, however, is largely different from that exhibited by Al NWs. In situ tensile tests performed on the <110> Al NW revealed that the NW exhibited ultrahigh strength (∼2.7 GPa) and superelasticity (∼4.8%), while contrary to expectations, it failed quickly once plastic flow initiates and displayed a limited plasticity (∼1.2%). This low plasticity was attributed to the formation of thin-layered twins with a zigzag configuration. Upon further deformation, these twins self-locked with each other, which prevented the NWs from carrying further plastic strains. Here, by employing the in situ micro-mechanical test and the atomic simulations, we performed quantitative and comprehensive analyses to explore why Al NWs display ultrahigh strength and how twins with a zigzag configuration are formed in the Al NWs. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]

Abstract A coherent core-shell structure in 0.24BiFeO 3 -0.56K 0.5 Bi 0.5 TiO 3 -0.20PbTiO 3 ceramics, with T m around 385 °C, was revealed using a combination of characterisation methods including scanning and transmission electron microscopy. The core regions consist of a cubic matrix with minor monoclinic phase with limited coherence length, while the shell regions are a tetragonal phase with characteristic long-range ordered ferroelectric domain structures. The electric field-induced strains for both core and shell regions were investigated by conducting an in-situ poling study using high energy synchrotron X-ray diffraction (XRD). A methodology is developed to evaluate both intrinsic (lattice strain) and extrinsic (domain switching) contributions for each phase based on diffraction peak profile fitting for multiple reflections. It is shown that there were no significant variations in the cubic and tetragonal phase fractions under an electric field in the range from −6 to +6 kV mm−1. An irreversible strain ∼0.04% was attributed mainly to ferroelectric domain switching in the tetragonal shell regions, while a reversible lattice strain ∼0.12% was identified in the pseudo-cubic core, leading to significant levels of intra-granular stress. Strain anisotropy, dependent on the crystallographic orientation of different grain families, was also evident. Macroscopic strain measurements, conducted using a novel digital image correlation (DIC) method, demonstrated the formation of typical 'butterfly'-type strain-electric field loops and were consistent with the total strain values calculated from the XRD results. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]

When metallic microparticles impact substrates at high enough velocity, they bond cohesively. It has been widely argued that this critical adhesion velocity is associated with the impact velocity required to induce adiabatic shear instability. Here, we argue that the large interfacial strain needed to achieve bonding does not necessarily require adiabatic shear instability to trigger. Instead, we suggest that the interaction of strong pressure waves with the free surface at the particle edges—a natural dynamic effect of a sufficiently rapid impact—can cause hydrodynamic plasticity that effects bonding, without requiring shear instability. We proceed on this basis to postulate and confirm a proportionality between critical velocity and the bulk speed of sound, which supports the viewpoint that shear instability is not the mechanism of adhesion in cold spray. [ABSTRACT FROM AUTHOR]

Superelastic behavior and elastocaloric effect were investigated in a Ti-44Ni-5Cu-1Al (at%) alloy subjected to various thermomechanical treatments. The specimen heat-treated at 673 K for 5 min after hot rolling and subsequent cold rolling exhibited excellent superelastic strain of 4.9% with a small stress hysteresis of 90 MPa when the maximum tensile stress was 500 MPa. This specimen also exhibited a large elastocaloric effect with a temperature decrease of 17 K when the stress of 500 MPa was removed adiabatically. No remarkable deterioration was observed for the superelastic strain and elastocaloric effect up to 5000 mechanical cycles. The maximum superelastic strain obtained was 6.8% under a tensile stress of 750 MPa. Transmission electron microscope observation and in-situ X-ray diffraction analysis under tensile stress revealed that the average grain size of the specimen is about 40 nm, and the specimen exhibits a successive B2-B19-B19’ transformation. [ABSTRACT FROM AUTHOR]

Lead zirconate titanate (PZT) is the base compound for the highest performing piezoelectric compositions. When doped with Nb, PZT has superior electrostrain and piezoelectric properties. However, the origin of that electrostrain involves both intrinsic and extrinsic contributions which have been challenging to deconvolute. In the present work, we utilize high-energy, synchrotron X-ray diffraction (XRD) in combination with an area detector to measure the response of 1% Nb-doped PbZr x Ti 1- x O 3 (PZT, 0.50 ≤  x  ≤ 0.56) piezoceramics to electric fields. Using analysis involving micromechanics-based calculations and pair distribution functions (PDFs), it is found that both the intrinsic and extrinsic contributions are important for realization of high electrostrain. In the compositions nearest the morphotropic phase boundary (MPB), the relative contributions of the intrinsic response increase. The interdependence of crystal symmetry (tetragonal and rhombohedral), spontaneous strain, and the extent of non-180° domain switching are also elucidated. An orientation dependence in the field-induced lattice strain is observed and attributed to extrinsic effects, i.e. , the intergranular interaction between domain switching and lattice strain. Finally, the PDFs suggest that a continuous rotation of the polarization vector occurs in the tetragonal phase samples due to piezoelectric distortion, being most obvious in the compositions near the MPB, but is not observed in the rhombohedral phase samples. [ABSTRACT FROM AUTHOR]

A mean-field model based on the elasticity theory of point defects has been developed to investigate the role of uniform stress fields on the long-range ordering of carbon atoms in bct-iron. From an analysis of the thermodynamic equilibria, composition – temperature – stress state diagrams are derived. We demonstrate that ferrite, martensite and supercritical iron are various instances of the same bct-iron phase region. A coherent mapping of the phase transitions is drawn, identifying (i) continuous transitions such as ferrite ordering, martensite enhanced ordering and ferrite – martensite transformation, and (ii) discontinuous transitions such as temperature-induced martensite and stress-induced martensite. Our analysis is supported by rigid-lattice Monte Carlo simulations. Recently published experimental results on highly-drawn perlitic wires are re-interpreted in terms of supercritical iron, rather than strain-induced martensite. Novel low-temperature thermomechanical treatments of supersaturated ferrite are suggested, for improved nanostructure design of martensitic steels. [ABSTRACT FROM AUTHOR]