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Microstructural changes induced by a hydrogen permeation into the NiTi superelastic alloy were investigated in-situ using the X-ray synchrotron diffraction. A new design of an electrochemical cell enabled to uncover time and position dependent processes under a flat alloy surface exposed to the cathodic hydrogen. The diffraction data supported by thermo-elastic FEM calculations helped to quantify an evolution of compressive stresses in the B2 austenitic phase hosting hydrogen atoms. The compressive stress state initiates a formation of martensitic phases starting from the exposed surface layer and advancing into the alloy volume with increasing time of hydrogen charging. We have performed the ab-initio DFT study in order to rationalize volumetric changes associated with variations in the B2 austenite and B19′ martensite lattice parameters. The numerical results also contributed to the identification of a new hydride phase with orthorhombic crystal structure and lattice parameters a = 0.8505 nm, b = 0.7366 nm and c = 0.4722 nm. [Display omitted] [ABSTRACT FROM AUTHOR]

In the NiTi-Hf alloy system, coherent H-phase (orthorhombic) nanoprecipitates have been shown to have strong impact on the B2 (cubic) to B19' (monoclinic) martensitic transformation (MT) and the corresponding stress-strain behavior. Two typical martensite-precipitate microstructures have been observed: intra-martensite precipitates (IMP) and inter-precipitate martensites (IPM). In the former, high density small H-phase nanoprecipitates are embedded in individual domains of different variants of the B19' martensite that form a herringbone structure, while in the latter, finer twinning B19' martensites are confined in between larger H-phase nanoprecipitates. In this study we investigate the mechanisms underlying the formation of these two distinct microstructures through computer simulations using the phase field method. In particular, we study temperature- and stress-induced MTs in the presence of H-phase nanoprecipitates, concentration variations and stress field in the B2 matrix created by the precipitation reaction, and the corresponding thermal and stress hystereses and superelasticity for different alloy compositions and aging treatments. The simulation results indicate that densely populated nanoprecipitates 7∼20 nm in length and 3∼8 nm in width lack the strength to impede the growth, coalescence and coarsening of martensitic domains, resulting in the formation of IMP. In contrast, plate-like nanoprecipitates with diameters of ∼100 nm and thickness of ∼20 nm effectively hinder the growth, coalescence and coarsening of martensitic domains, giving rise to IPM. The IPM microstructure is accompanied by an almost linear volume fraction vs. temperature curve with less than 1 °C hysteresis during thermal cycling and a slim stress-strain hysteresis as compared to that of the IMP microstructure. Therefore, thoughtful choices in alloy composition and aging treatment enable the attainment of distinctly varied precipitate and martensitic microstructures, each characterized by unique pseudoelastic or superelastic properties. [Display omitted] [ABSTRACT FROM AUTHOR]

The work clarifies several key questions in shape memory research that have eluded previous studies. The findings show that dislocation slip emanates at austenite-martensite interfaces during unloading and aligns with the internal twin boundary interface of martensite. It was observed that the type II internal twins of the martensite become parallel dislocations in the austenite. During reloading, these dislocations act as nucleation sites for the martensitic twins, reducing the nucleation barrier and the transformation stress. The precipitates facilitate martensite nucleation but also act as an obstacle to martensite front motion, restrict detwinning, and pin the interfacial dislocations during unloading, thereby contributing to residual strains and martensite stabilization. Martensite nucleation is not suppressed by the size of the thin film, which is of the order of 85 to 105 nanometers thick, and repeated transformation occurred cycle after cycle. Single crystals deformed in the <101> LD exhibited the best recoverability of up to 5.5 % and tensile stresses of up to 1.4 GPa. It was demonstrated for the first time that, when favorably oriented, Ni 4 Ti 3 precipitates undergo a reversible phase transformation to R-phase and can accommodate up to 4 % reversible strains. [Display omitted] [ABSTRACT FROM AUTHOR]

To shed light on the origin of the loss of functional properties of NiTi with temperature increasing above 100 °C, we have investigated stress induced martensitic transformations in nanocrystalline NiTi shape memory wire by thermomechanical tensile testing supplemented with post-mortem reconstruction of martensite variant microstructures in grains by nanoscale orientation mapping in TEM. The stress induced martensitic transformation generating recoverable transformation strain as well as plastic strain is not completed at the end of the upper stress plateau. The higher is the test temperature, the larger is the volume fraction of retained austenite as well as the plastic strain. The martensite variant microstructures in NiTi wire deformed up to the end of the stress plateau at 120 °C contain partially detwinned single domains of (001) compound twin laminate filling entire grains. It is proposed that the stress induced martensitic transformation proceeds via habit plane interface between austenite and second order laminate of (001) compound twins and that the martensite promptly reorients and deforms plastically by dislocation glide in the [100](001) slip system. When the wire is loaded further beyond the end of the stress plateau, the stress induced martensitic transformation continues and the martensite deforms plastically. It is concluded that the observed gradual loss of superelastic functionality of NiTi with increasing temperature does not originate from the plastic deformation of austenite, as widely assumed in the literature, but that it derives from the loss of resistance of the stress induced martensite to the plastic deformation under increasing stress. [Display omitted] [ABSTRACT FROM AUTHOR]

Twin intersections have been reported to significantly impact on macroscopic properties by relaxing localized stress concentration and accommodating strain. Here the atomic-scale structural characterization of the twin intersection region in a deformed high Mn twinning-induced plasticity (TWIP) steel was conducted using high-resolution transmission electron microscopy. Detailed microstructural features were revealed at the twin intersection region, including twin intersection rotation, low angle grain boundary and ε-martensite. The twin intersections were observed to preserve the face-centered cubic structure, which exhibits deviation angles of ∼0–15o with respect to the barrier twin depending on the localized stress concentration. Interestingly, the ε-martensite was observed in the vicinity of the twin intersection, which shows a wedge shape, distinct from the usual plate-like deformation-induced martensite. Therefore, we identified here a novel stress relaxation mechanism at the twin intersection region. Based on the detailed microstructural characterization of the twin interaction region at the atomic scale, the fundamental dislocation mechanisms of twin transmission and interaction were discussed. The research thus advances the understanding of twin intersection rotation and ε-martensite transformation by twin-twin interactions. [Display omitted] [ABSTRACT FROM AUTHOR]

The pseudoelastic and pseudoplastic properties of NiTi alloys result from the closeness of the structures between the B2 cubic austenite and the B19' monoclinic martensite, and from the facility to transform one into each other. The paths followed by the atoms during the B2 → B19' transformation are usually imagined as combinations of simple shears and shuffles. Here, we propose a simplified hard-sphere atomistic model of phase transformation decomposed into three distinct types of rotational movements. The inputs are the Ti and Ni atomic diameters and the monoclinic angle β. The outputs are the lattice parameters and atomic positions of the B19' phase. The results are remarkably close to those reported in the literature. The model permits a better understanding of the B19' structure and the way it is inherited from its parent B2. The value of the monoclinic angle corresponds to the highest molar volume; which suggests that it could be a consequence of a maximization of the vibrational entropy. The hard-sphere model also successfully predicts the B19 structures reported in the binary alloys AuTi, PdTi, and AuCd. The agreement is less satisfying for the B19 structure observed in the ternary NiTiCu alloys. It is concluded that the B19' phase in NiTi alloys, and the B19 phase in AuTi, PdTi, and AuCd alloys can be considered as "simple" hard-sphere structures. [Display omitted] [ABSTRACT FROM AUTHOR]

The cubic B2 to monoclinic B19' martensitic transformation in NiTi shape memory alloys proceeds by the nucleation and propagation of habit plane interfaces that remain undistorted and unrotated during the transformation. Due to incompatibility of austenite and martensite lattices in NiTi, habit plane interfaces are assumed to form between austenite and twinned martensite as described by Phenomenological Theory of Martensite Crystallography (PTMC). This view is currently widely spread in the literature in spite of the fact that it contradicts experimental observations of interfaces between austenite and single martensite lattice by TEM and EBSD methods frequently reported in the literature. On this account, we propose a different solution for the formation of strain compatible habit planes propagating during stress induced martensitic transformation by considering the effect of elastic deformation of both lattices caused by the external stress. Using modified PTMC theory, we evaluate the magnitudes of the uniaxial stress in tension and compression along a general austenite crystal direction, for which strain compatible habit plane interfaces can be established between austenite and single variant of martensite. The stress calculated in such a way is, however, too high. Nevertheless, considering the pre-transformation softening of austenite, the critical stress decreases to 500 MPa commonly observed in experiments. Three scenarios are simulated assuming softening of elastic constants of austenite. Orientation dependence and tension-compression asymmetry of predicted habit planes of stress induced martensitic transformation is compared with available experimental results. • PTMC was adapted to consider the effect of elastic strains. • Elastic softening under stress make austenite and martensite lattices compatible. • Softening of C 44 elastic constant of austenite provides most effective compatibility. • Habit planes under stress depend on magnitude and orientation of the uniaxial stress. [Display omitted] [ABSTRACT FROM AUTHOR]

Functional fatigue—changes to the material response during cyclic loading—is a major barrier to the cycle lifetime demands of shape memory alloy technologies. Functional fatigue is caused by permanent changes to the microstructure such as the generation of dislocations during the forward and reverse martensitic phase transformation. In this work, far-field and near-field high-energy diffraction microscopy (ff- and nf-HEDM) are used to characterize the local accumulation of geometrically necessary dislocation (GND) density in the austenite phase in situ and in 3D across a bulk Ni 49.9 Ti 50.1 polycrystalline shape memory alloy during load-biased thermal cycling. A custom nf-HEDM data analysis procedure is used to reconstruct spatially-resolved intragranular misorientation maps that are then converted to spatially-resolved GND density maps. In this way, GND density is tracked in individual grains across cycles. We find that neither Schmid factor nor the maximum transformation work correlates strongly with GND density evolution during load-biased thermal cycling. The results show that the spatially-resolved GND density is distributed heterogeneously, but GND density increases faster near grain boundaries, in grains at the sample surface, and in grains with large volumes, indicating that these regions/types of grains will undergo different functional fatigue behaviors. Finally, the effect of grain neighborhood and grain boundaries on GND evolution are investigated, highlighting the role played by grain boundaries and the grain neighborhood in the evolution of GND density. This work demonstrates the utility of nf-HEDM for understanding the evolution of subgrain-scale plastic deformation, including materials that undergo a martensitic phase transformation. [Display omitted] [ABSTRACT FROM AUTHOR]

Martensitic crystallography plays a vital role in the texture evolution and mechanical properties in Nickel-Titanium (NiTi) shape memory alloys when subjected to deformation. However, their microstructural changes during deformation are not well known and understood. In this study, we systematically investigate the stress-induced change of the microstructure of NiTi shape memory alloy under compression using the electron backscattered diffraction (EBSD) technique, and more specifically the variant reorientation, the evolution of textures, and the formation of twins. The EBSD maps reveal that upon strain the martensite morphologies shift from needle to block and their orientations are strongly reinforced along the loading axis. To quantify these microstructural changes, we used the Interaction Work (IW) associated with the lattice distortion of single variant, which provides a good fit with the experimental observation. A strong dependence of reorientation on the pre-textures of the parent B2 grains was also put in evidence. Other plasticity sources were confirmed such as dislocations and deformation twins. The calculations indicate that, for some specific orientations of parent grains, the active deformation twins could be easier for {011} M twin than that of (20 1 ¯) M deformation twin, whereas other orientations only favor the (20 1 ¯) M deformation twin. [Display omitted] [ABSTRACT FROM AUTHOR]

Materials with unusual large negative and positive thermal expansions are attractive for applications in high-precision actuation and thermal expansion compensation. So far, such unusual property has been reported in a variety of framework materials and polymers but it remains challenging to obtain in metals. Here, we report a colossal linear thermal expansion in single-phase nanocrystalline NiTi ferroelastic alloy under uniaxial compressive stress. By microscale isothermal severe plastic deformation, we fabricated fully austenitic and martensitic NiTi nanocrystalline micropillars that respectively possess high yield stresses of 2.65 GPa and 2.23 GPa and strong negative, and positive temperature dependence of Young′s modulus (d E /d T). We demonstrate that due to the large d E /d T , the coefficient of linear thermal expansion (CTE) of the austenitic and martensitic NiTi micropillars can be tailored to values of +106×10−6 K–1 and -88×10−6 K–1 by applying 2.5 GPa and 2.0 GPa compressive stress. Such tailored CTEs are approximately 10 times larger than the typical value (∼10×10−6 K–1) of metallic materials. Our work provides a mechanical route to obtain colossal positive and negative thermal expansion in single-phase nanostructured ferroelastic alloys at microscale. [Display omitted] [ABSTRACT FROM AUTHOR]

Additive manufacturing (AM) of composition-sensitive NiTi shape memory alloys (SMAs) is hindered by uncontrollable non-equilibrium microstructures, leading to functional performance degradation and limitations in high-throughput net-shaping applications. To address these challenges, we propose a novel approach combining annular intra-laser directed energy deposition (AIL-DED) with a tailored gradient post-treatment process. This enables the fabrication of nanocomposite microstructures in NiTi SMAs with precise control over the homogenization of Ti 2 Ni nanoprecipitates. In-situ monitoring and characterization of thermal history, microstrain, and constitutional phases, were utilized to uncover the relationship between multi-scale structural features and mechanical responses. The interplay between laser reflectivity, energy input, and lattice thermal vibrations affects forming quality in AIL-DED. Dynamic temperature variations were studied before reaching thermal equilibrium, revealing insights into thermal and mechanical interactions during the process. These findings contribute to understanding challenges in fabricating high-quality NiTi SMAs. Examining phase transformation behavior and its correlation with processing parameters deepens understanding of microstructural evolution and provides insights for tailoring material properties. The stress relaxation and dislocation evacuation at elevated temperatures caused the expansion of lattice and interplanar spacing in the age-treated SMA's B2 matrix phase, exhibiting a stable two-step phase transformation behavior (B19′↔R↔B2). AIL-DED combined with two-stage heat treatment strategy demonstrated enhanced homogeneity of non-equilibrium microstructures, resulting in the formation of high-density, well-dispersed Ti 2 Ni nanoprecipitates, with a semi-coherent interface to B2 matrix. The combination of load-bearing strengthening through well-dispersed Ti 2 Ni nanoprecipitates and Orowan strengthening at the semi-coherent interface yielded simultaneous enhancements in nanohardness, ultimate compressive strength, compressive fracture strain, and deformation recovery, following the AIL-DED combined with two-stage heat treatment process. The fracture mechanism transitioned from a mixture of brittle-ductile behavior to predominantly ductile characteristics. This study provides valuable insights that advance additive manufacturing and enhance the functional performance of SMAs in critical applications. [Display omitted] [ABSTRACT FROM AUTHOR]

To explore the possibility of customising the functional behaviour of NiTi shape memory alloy via controlling texture, binary Ni 55 Ti 45 (wt.%) alloys were manufactured in as cast and hot swaged conditions, presenting contrasting initial texture and macroscopic performance. In situ time-of-flight neutron diffraction technique was employed to study the texture effect on the microstructural evolution during Shape Memory Effect (SME), and a range of properties were evaluated. It was found that (i) hot swaging process leads to change in grain morphology and increase in microstrain; (ii) thermal expansion coefficients of martensite and austenite variants were weakly affected by the texture and phase transformation constraint; (iii) significant texture effect on the elastic properties at both macro- and micro-scale was quantified by Elasto-Plastic Self-Consistent (EPSC) modelling approach, while the anisotropic elastic moduli lie within the range of single crystal state and twinned structure; (iv) texture evolution during SME is weakly related to the initial microstructure; (v) martensite reoriented so that the <010> axis became aligned parallel to the loading direction, and retained this orientation upon unloading, revealing the underlying correlation between texture evolution and detwinning. Based on the experimental results, a multi-variant model was proposed to quantify the lattice strain evolution during SME. Validity of the conceptually simple and parametrically parsimonious model was confirmed by validation against experimental data. Image, graphical abstract [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]

This study addresses the selectivity of self-accommodation microstructures in the initial stage of martensitic transformation in B19' TiNi. It is known that specific habit plane variant (HPV) pairs, including { 11 1 ¯ } m type I twin planes that are called initial stage microstructure (ISM) in this study, are preferentially formed in the initial stage of martensitic transformations. To elucidate on the selectivity of the ISM, the energy barrier at the formation of the twin plane, which is the candidate for the starting point of HPV pair formation, and the geometry of the HPV pair are theoretically analyzed in this paper. Using geometrical non-linear theory analysis, the compatible twin plane which has exact twin orientation relationship can be formed in the specific five groups of HPV pair. Using ab-initio simulations and Eshelby theory analysis results, { 11 1 ¯ } m type I twin is determined to have a relatively small energy barrier among the twins in TiNi. Therefore, this paper shows that the HPV pair having a twin plane with a small energy barrier of formation and a structure that can effectively reduce the elastic strain is preferentially formed as the ISM. Image, graphical abstract [ABSTRACT FROM AUTHOR]

It is shown that the broadenings of the forward and reverse martensitic transformations are proportional to the second derivatives of the elastic energy by the transformed martensite volume fraction. The shifts of the transitions, beside the effect of chemical contribution to the equilibrium transformation temperature, T o , can depend on the change of the elastic and dissipative energies as well. The effect of mechanical stabilization (by making the surfaces roughened) in Ni 49 Fe 18 Ga 27 Co 6 single crystal, where the transformation too place by single interface motion, can be well treated in thermoelastic approximation. In Ni 50 Mn 28.5 Ga 21.5 single crystal (where the usual multivariant martensite structure developed) not only the dissipative energy, but the slope of the second derivative of the elastic energy for cooling has also been altered after mechanical stabilization and there can exist an elastic energy contribution to the width of the hysteresis too. Symmetry conforming short range ordering under uniaxial stress (SIM-aging) in Ni 53 Mn 25 Ga 22 and Ni 51 Fe 18 Ga 27 Co 4 single crystals caused an asymmetric change in the second derivatives of the elastic energy: the sharpening of the corresponding branches of the hysteresis was stronger for heating than cooling. The thermoelastic balance condition did not fulfill for the SIM-aged samples and the elastic energy contributions to the shift of the transformation temperatures could even compensate the increase of T o (caused by the stabilization by symmetry conforming short range ordering). It is also illustrated that the dissipated energy per one cycle could decrease even if the area of the hysteresis has been increased. Image, graphical abstract [ABSTRACT FROM AUTHOR]

CuZr-based alloys are being considered as potential shape memory alloys for use in high-temperature applications. We have conducted a study on the effects of several alloying elements on the shape memory properties of these alloys using polycrystalline thin-film samples. Here we report on the explosive formation of martensite in supercooled CuZr, CuZrNi and CuZrCo samples. This explosive transformation behavior is characterized by the following observations: 1) The high-temperature austenitic phase can be supercooled below the martensite finish temperature M f. At a critical temperature below M f , austenite transforms to martensite across the entire sample in less than a microsecond. 2) The critical temperature has a narrow distribution and decreases slightly with higher cooling rate. 3) Observation of supercooling and explosive transformation behavior depends on the temperature history above the austenite finish temperature A f. If a sample is quenched immediately after heating above A f , martensite forms gradually on cooling below M s ; if a sample is allowed to dwell a few seconds above A f , the martensite forms explosively. We suggest that the gradual transformation proceeds by martensite growth on defects that accumulate during successive transformation cycles. If the sample is allowed to dwell at a temperature above A f , however, these defects are annihilated and the transformation is nucleation-limited. Nucleation of martensite then requires significant supercooling. The defect annihilation process is highly sensitive to temperature and has an apparent activation energy of 326 kJ/mol, which is too large for a simple diffusion-limited process. Transmission electron microscopy of CuZrCo samples suggests that the defects may be related to the presence of residual martensite. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Much progress has been made over the past few decades in resolving the mechanism for the transformation from ordered body-centered-cubic (B2) austenite to monoclinic martensite (B19′) in NiTi shape memory alloy, as well as the twinning mechanisms in martensite. However, so far no lattice correspondence analyses on the atomic scale have been conducted, as a result, these mechanisms have not been completely understood in terms of the pathways for reversible phase transition and reversible twinning in martensite. In this work, atomistic simulations were performed to investigate how B2 austenite transforms to B19′ martensite and how twins are formed during phase transformation. In particular, lattice correspondence in the structural evolutions was carefully analyzed to reveal the martensitic transformation mechanism and twinning mechanism with much better clarity. The simulation results show that, when martensite grains were nucleated in austenite, they already formed a twin relationship as a natural result of the crystal structure. Thus, self-accommodation is achieved naturally. Coalescence occurred subsequently and some martensite grains grew at the expense of their neighbors, indicating that the interfaces are elastically mobile and their migration is reversible. Eventually twinned martensite variants were created. It is shown that, in the B2 → B19′ transformation, the B19′ monoclinic structure can be treated as a distorted hexagonal close-packed (HCP) structure. With this treatment, it is demonstrated that the martensitic transformation is essentially similar to BCC ↔ HCP transformation which only involves atomic shuffles and is reversible, and twinning in martensite is essentially similar to { 10 1 ¯ 2 } twinning in HCP metals which only involves atomic shuffles and is reversible as well. Another significant discovery is that the B19′ martensite exhibits a very special dual-lattice structure: the relatively weak Ti-Ti bonding of the monoclinic unit cell permits easy reorientation of lattice units. As a result, two differently oriented lattice units co-exist in the same martensite grain which loses long range periodicity, providing an extra degree of freedom for accommodating external and internal strains. These results explain well why B19′ martensite is extremely adaptive to straining and why deformation is reversible. Lattice transformation pathways between B2, B19′ and B19′ twin in NiTi. If the atoms of B2 austenite shuffle along the black arrows, the B2 transforms to B19′ martensite variant; if the atoms of B2 austenite shuffle along the dark brown arrows, the B2 austenite transforms to another B19′ martensite variant (B19′ twin). The two martensite variants are already in {011} twin relationship. The purple arrows in the B19′ indicate the shuffles that are needed to transform the B19′ to the B19′ twin. All these lattice transformations only involve atomic shuffles, rendering them all reversible, which is necessary for shape memory effect. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Ti-Nb based alloys exhibit superelastic and shape memory properties over a wide range of temperatures, making them attractive for industrial applications. However, within the literature there are significant variations in the reported transformation behaviour for a given composition that remain unexplained. These variations are problematic to engineering design and limit the industrial uptake of these alloys. To investigate the source of these discrepancies, the transformation behaviour of Ti-24Nb (at.%) in two significantly different microstructural conditions, following cold rolling (CR) and following a subsequent solution heat treatment (ST), has been studied using in situ synchrotron diffraction data whilst undergoing a three step thermal cycle. When cooling from 350 to -196˚C, the CR condition material underwent both the β to αʺ and β to ω transformations, whilst only the β to ω transformation was observed in the ST condition material. These results show that the transformation behaviour of Ti-Nb alloys, in particular the β to αʺ transformation, is significantly influenced by prior processing and that theories of mutual exclusivity between αʺ and ω phase formation need revisiting. In addition, several features within the datasets were inconsistent with a solely thermally driven martensite formation mechanism and an alternative description that includes other factors is discussed. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Stress-induced martensitic transformations enable metastable alloys to exhibit enhanced strain hardening capacity, leading to improved formability and toughness. As is well-known from transformation-induced plasticity (TRIP) steels, however, the resulting martensite can limit ductility and fatigue life due to its intrinsic brittleness. In this work, we explore an alloy design strategy that utilizes stress-induced martensitic transformations but does not retain the martensite phase. This strategy is based on the introduction of superelastic nano-precipitates, which exhibit reverse transformation after initial stress-induced forward transformation. To this end, utilizing ab-initio simulations and thermodynamic calculations we designed and produced a V 45 Ti 30 Ni 25 (at%) alloy. In this alloy, TiNi is present as nano-precipitates uniformly distributed within a ductile V-rich base-centered cubic (bcc) β matrix, as well as being present as a larger matrix phase. We characterized the microstructure of the produced alloy using various scanning electron microscopy (SEM) and transmission electron microscopy (TEM) methods. The bulk mechanical properties of the alloy are demonstrated through tensile tests, and the reversible transformation in each of the TiNi morphologies were confirmed by in-situ TEM micro-pillar compression experiments, in-situ high-energy diffraction synchrotron cyclic tensile tests, indentation experiments, and differential scanning calorimetry experiments. The observed transformation pathways and variables impacting phase stability are critically discussed. Image, graphical abstract [ABSTRACT FROM AUTHOR]

The grain size (GS) dependence of fatigue crack growth in nanocrystalline superelastic NiTi shape memory alloys (SMAs) with the average GS of 10, 30, 60 and 235 nm is investigated. Synchronized measurements of the fatigue crack growth, load-displacement response and temperature field of the compact tension specimens are performed. It is found that the fatigue crack growth rate (d a /d N) is monotonically decreased by 10 times when the GS is increased from 10 to 235 nm. The significant GS effect on the fatigue crack growth is also confirmed by the direct observation of fatigue striation spacing on the postmortem fracture surface. It is shown that the increase of the GS significantly enhances the phase transition and plasticity of the material and therefore enhances the crack-tip shielding (K sh) which reduces the effective driving force (Δ K eff) of the fatigue crack growth. Subtracting K sh due to phase transition and plastic deformation from the external applied K app , the effective crack-tip fatigue driving force Δ K eff is obtained. The d a /d N -Δ K eff curves for different GS specimens then almost collapse onto a single curve which may represent an intrinsic fatigue property of the NiTi in the absence of the shielding effects. Therefore the observed apparent GS dependence of the fatigue crack growth resistance in the superelastic NiTi SMAs is mainly caused by the shielding effect from the GS dependent phase transition and plastic deformation. Image, graphical abstract [ABSTRACT FROM AUTHOR]

This study investigated the mechanisms and temperature dependence of the R phase variant reorientation deformation in a nanocrystalline NiTi wire by means of in-situ synchrotron high-energy X-ray diffraction technique during tensile deformation. The NiTi wire exhibited a single stage B2 → R transformation upon cooling with the R → B19′ martensitic transformation completely suppressed down to the liquid nitrogen temperature. The R phase variant reorientation deformation in tension was revealed to proceed in a Lüders-manner, as evidenced by sudden changes of diffraction peak intensity and increases of lattice elastic strains of the R phase during the deformation. The variant reorientation obeys the principles to yield maximum crystallographic strain in the direction of the applied load. Thus, the variants with larger d -spacing values aligned to the axis of tension grew at the expense of those of lower d -spacing values. The increases of lattice elastic strains are attributed to the internal load transfer among the different crystallographic variants during the R phase reorientation. The crystallographic strain of R phase reorientation was found to increase with decreasing the deformation temperature. This corresponds directly to the continued evolution of the structure of the R phase of increased structural distortion relative the B2 phase with decreasing the temperature. This reflects the second order nature of the R phase. Image, graphical abstract [ABSTRACT FROM AUTHOR]

The low symmetry martensite phase in Nickel-Titanium (NiTi) shape memory alloy (SMA) has a hierarchical microstructure with micro-scale crystallites, which themselves consist of nano-laminates with an internal twin structure. The martensite phase deforms inelastically though reorientation of the twin structure. Furthermore, structural features in the specimens such as cracks or notches interact with the microstructure, influencing the deformation of martensite. In this work, using multi-scale experiments and macro-scale modeling, we demonstrate that the deformation of martensite in NiTi at the macro scale in a notched specimen is largely determined by the structural features, and the hierarchical nature of the microstructure plays a smaller role. At the micro-scale, in-situ measurements indicate that the martensite microstructure continuously evolves during tensile loading. However, the nano-scale twin laminate structure persists despite the large imposed deformation and the presence of stress concentrating features such as a notch. This observation is counter to the intuition of martensite twins reorienting to large domains of the most favored crystallographic variant at larger loads. This persistence of a nano-laminate structure rather than coarsening into large domains of individual variants indicates that the deformation field around cracks/notches in martensite may be modeled for fracture-related phenomena with adequate accuracy using coarse-grained models, without a compelling need for martensite single-variant-scale models. [ABSTRACT FROM AUTHOR]

A superelastic metastable β Ti-24Nb-4Zr-8Sn (wt.%) alloy is analyzed in the solution treated state after different applied strains in order to reveal evolution of microstructures and explore plastic deformation mechanisms. Deformation bands are observed by optical microscopy, EBSD and TEM. After 3% of strain, only few thin {332}<113> β twins are observed, while after 5% of strain, complex hierarchically β twinning structures composed of primary and secondary bands are observed by TEM. Indeed, very thin secondary {332}<113> β twins are observed (about 50 nm in average) and their massive occurrence leads to the loss of the initial {332}<113> β twinning relationship between matrix and primary bands which is transformed passively to an unusual type II <531> β twinning relationship. Similarly, the orientation relationship between matrix and secondary twins tends to another abnormal type I {541} β twinning relationship. As in situ synchrotron XRD experiments show that almost all the β phase is transformed into α" martensite under loading condition, a crystallographic reconstructing method is then used to determine the real α″ orientation before stress releasing. The relaxed {332}<113> β twins is found to correspond perfectly to the reversion of {130}<310> α″ martensitic twins occurring under loading condition. The unusual twinning relationships between matrix and primary/secondary twins are shown to correspond to type II <512> α" and no twinning relationships in α" martensite, respectively. Schmid factor analyses show that the activation of primary twins obeys the Schmid law. However, the activation of secondary twins disobeys the Schmid law due to the variant selection of α" martensite occurring during primary twinning. Image, graphical abstract [ABSTRACT FROM AUTHOR]

We present a systematic study of the magnetostructural and magnetic transitions in the prototype metamagnetic shape memory alloy Ni 50 Mn 34.5 In 15.5 under hydrostatic pressure and combined pressure and magnetic field. Pressure extends the area of stability of the antiferromagnetic martensitic phase. At low magnetic field the pressure derivatives of the Curie temperatures of austenite, T CA , and martensite, T CM , show opposite signs. This fact is described in the framework of the Landau thermodynamic model as arising from a weak long-range antiferromagnetic state of martensite. Two volume magnetoelastic constants were estimated using the experimental values of the pressure derivatives of T CA and T CM. A correlation between the signs of the pressure shifts of T CA , and T CM and the distance between Mn-Mn nearest neighbours is established, which matches the empirical Castelliz-Kanomata diagram. The entropy change at martensitic transformation (MT), Δ S MT , grows up when the MT temperature, T M , is approaching T CA under the influence of pressure, but under constant non-zero pressure this dependence is inverse. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Solid-state refrigeration based on the caloric effects has been conceived to be a high-efficient and environmental-friendly alternative to replace the vapor-compression refrigeration technique. The implementation of solid-state refrigeration requires that the refrigerants should possess not only remarkable caloric effect but also wide working temperature region. In this work, we demonstrate that various caloric effects can be achieved successively in a multiferroic Ni 50 Mn 35 In 15 meta-magnetic shape memory alloy prepared by directional solidification, including inverse magnetocaloric effect around inverse martensitic transformation, conventional magnetocaloric effect around Curie transition and elastocaloric effect above Curie transition. Among them, the elastocaloric effect is particular striking, where a giant adiabatic temperature variation up to –19.7 K is achieved on removing a moderate stress of 350 MPa due to the elimination of negative magnetic contribution, with the specific adiabatic temperature change of 56 K GPa–1. Furthermore, through the combination of these successive caloric effects, a broad refrigeration temperature region covering the temperature range from 270 K to 380 K can be achieved. It is demonstrated that the combination of various caloric effects could be a promising way to extend the working temperature range of solid state refrigeration. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Mg-Sc shape memory alloys are attractive for a wide range of applications due to their low density. Unfortunately, the use of these alloys is hindered by a low martensitic transformation temperature (173 K). We used density functional theory to characterize the energetics associated with the martensitic transformation in a Mg-Sc (19.44 at.% Sc) alloy from a disordered body centered cubic (BCC) austenite to a disordered orthorhombic martensite. The simulations predict lattice parameters and diffraction patterns in good agreement with experiments and the martensite phase to be 11 ∓ 1 meV/atom lower in energy than austenite at zero temperature, consistent with the low martensitic transformation temperature. A local ordering analysis of various structures revealed the origin of stacking faults in the HCP ordering in the martensite phase. In addition, we explore the effect of epitaxial strain on the relative energy between the two phases with the objective of increasing the martensitic transformation temperature. Compressive strain along [100] and tensile strain along [ 0 1 ¯ 1 ] on the closest packed plane (011) stabilize the martensite phase with respect to austenite. Bi-axial strain between 5 and 7% increases the zero-temperature energy difference between the phases by over 60%. Similar stabilization of the martensite phase can be achieved by the addition of pure Mg as a coherent second phase. Superlattices with 50 at.% Mg result in an energy difference of 18.1 meV/atom between the two phases at zero temperature. These results indicate that coherency strains can be used to increase the martensitic transformation and operation temperature of Mg-Sc alloys to room temperature. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Additive manufacturing is being increasingly used in the fabrication of Ti-6Al-4V parts to combine excellent mechanical properties and biocompatibility with high precision. Unfortunately, due to the build-up of thermal residual stresses and the formation of martensitic structure across a wide range of typical processing conditions, it is generally necessary to use a post-thermal treatment to achieve superior mechanical performance. This investigation aims to obtain a deeper understanding of the micro/nanostructural evolution (α ′ martensite phase decomposition), accounting for the kinetics of phase transformation during the heat treatment of 3D-printed Ti-6Al-4V alloy. As the mechanism of phase transformation and stress relaxation is still ambiguous, in this study the changes in crystal lattice, phase, composition and lattice strain were investigated up to 1000°C using both in situ high temperature X-ray diffraction (XRD) and transmission electron microscopy (TEM). Based on the result a mechanism of phase transformation is proposed, via the accommodation/substitution of Al, V and Ti atoms in the crystal lattice. The proposed mechanism is supported based on elemental concentration changes during heat treatment, in combination with changes in crystal structure observed using the high temperature XRD and TEM measurements. This study provides a deeper understanding on the mechanism of phase transformation through martensitic decomposition, as well as a deeper understanding of the influence of post-thermal treatment conditions on the alloy's crystal structure. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Solid-state cooling based on the caloric effect of phase transformation materials has attracted considerable interest with the increased demand for energy-efficient and environmentally friendly cooling technologies. Here, we have systematically studied the microstructural evolution, martensitic transformation (MT) behaviors, and elastocaloric effect (eCE) of directionally solidified Ni 35.5 Co 14.5 Mn 35 Ti 15 all- d -metal Heusler metamagnetic shape memory alloys. Electron backscatter diffraction (EBSD) analysis revealed the coexistence of Heusler-type 〈001〉-oriented dendritic crystal and Ti-poor interdendritic phase for as-solidified alloys. Upon homogenization annealing, the sample presents a columnar-like morphology with 〈105〉 textured five-layer-modulated (5 M) martensite along the growth direction. Besides, the multi-modulated martensite with 7 M and 8 M structures was identified by transmission electron microscopy (TEM). Using a unique technique of in situ digital image correlation (DIC) with the combination of infrared thermography, MT and eCE behaviors were studied. Ni 35.5 Co 14.5 Mn 35 Ti 15 alloy yielded a large adiabatic temperature change (Δ T ad) of 11.5 K with a low critical stress (σ cr) of 38 MPa and a moderate stress hysteresis (Δ σ hy) of 54 MPa when subjected to uniaxial stress, resulting in the largest value of |Δ T ad / σ cr | = 0.31 K MPa−1. This improved eCE is attributed to the enhanced compatibility in the oriented polycrystalline alloy and the high mobility of the low-energy twin boundary in the multi-modulated martensite. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Ferromagnetic shape-memory alloys can realize smart functional sensors and actuators due to their distinctive capabilities of field-induced strain, which have attracted extensive interests. However, their response and transformation kinetics under the stimulus of Joule heating are still lack, which limits their extensive application into electrical-related sensors. In this work, we firstly report the dynamic structural transformation and magnetic domain evolution of smart shape memory alloy induced by Joule heating in-situ TEM with thermoelectric holder by using Ni 2 MnGa as experimental model. The stimulus of low-power Joule heating induces the reversible structural and magnetic domain transformations of the smart Ni 2 MnGa shape memory alloy. Whilst, high-power Joule heating induces an irreversible process of structural transformation and magnetic domain evolution. Our work should be significant to expand the smart shape memory alloy into electrical-related applications. Image, graphical abstract [ABSTRACT FROM AUTHOR]

The mechanistic origin of fatigue in Shape Memory Alloys (SMAs) is addressed using atomistic simulations. A causal explanation is proposed for the known agreement between the fatigue-activated slip system and the martensitic twinning system. As a model system, the Type II twin boundary (TB) in NiTi B 19′ martensite phase is analyzed. TEM-based models have established the presence of disconnections on the TB. Topological models establish the TB migration to depend on the motion of twinning partials on these disconnections. A disconnection is setup within a Molecular Statics (MS) framework. A twinning partial is positioned on it by enforcing continuum displacement fields external to a prescribed core of atoms which is subsequently relaxed under governance of the interatomic potential. The displacement fields are calculated from the anisotropic Eshelby-Stroh formalism and enforced in a non-Cauchy-Born adherent manner to obtain the right core structulre. TB migration is simulated as a motion of this disconnection under applied load. In the presence of a barrier to this motion, a dislocation reaction occurs where a stacking fault emits at the TB while returning a residual negated partial. The emissary fault partial is proposed as a precursor to the resulting slip observed in reverse-transformed austenite. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Cuboidal micropillars of nanocrystalline superelastic NiTi shape memory alloys with an average grain size of 65 nm were fabricated by focused ion beam and then subjected to cyclic compression. It is found that the micropillars have maintained superelasticity for over 106 full-transformation cycles under a maximum compressive stress of 1.2 GPa. Functional degradation of the micropillars mainly occurs in the first 104 cycles where hysteresis loop area and forward transformation stress rapidly decrease from initial 11 MPa (MJ/m3) and 586 MPa to 6 MPa and 271 MPa. In the 104 ~ 106 cycles, stress-strain responses of the micropillars show asymptotic stabilization. Residual strain is accumulated to 3.3% and multiple ~50 nm wide extrusions are found at the surface of the micropillars after 106 cycles. SEM and TEM studies indicate that cyclic phase transformation results in formation and glide of transformation-induced dislocations that create surface steps and the extrusions. The dislocations inhibit reverse transformation and result in residual martensite and residual stresses. The dislocations and the residual martensite lead to the functional degradation. The role of the residual martensite in the functional degradation is further verified by 21% recovery of the residual strain and an increase of 278 MPa in the forward transformation stress after heating up the cyclically deformed micropillars to 100 °C. The recorded over 106 phase transformation cycles under a maximum stress of 1.2 GPa of the NiTi shape memory alloys at microscale open up new avenues for applications of the material in microscale devices and engineering. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Thermo-mechanical treatment is an effective way to tune the microstructure and enhance the output strain of NiMn-based magnetic shape memory alloys through field driven variant reorientation or structural transformation. Comprehensive knowledge on the microstructural feature and related transformation crystallography is of great importance for the microstructure control and property optimization. In this work, we demonstrate the crossing twin of seven-layered modulated (7 M) martensite assembled in herringbone shape induced by thermo-mechanical treatment in a directionally solidified Ni 50 Mn 30 Ga 20 alloy. Based on the electron backscatter diffraction (EBSD) measurements, it is found that the crossing twin is composed of four 7 M martensite variants with two mutually crossing systems of twin, i.e. , type I twin and compound twin. The type I twin in the crossing twin, being with {1 –2 10} 7 M as the twinning plane, is quite different from that of the self-accommodated martensite (i.e. , {1 –2 –10} 7 M as the twinning plane). A novel transformation orientation relationship between austenite and 7 M martensite in crossing twin is resolved to be {1 0 1} A //{1 –2 10} 7 M and <1 0 –1> A //<10 10 1> 7 M , in contrast to that of self-accommodated case, i.e. , {1 0 1} A //{1 –2 –10} 7 M and <1 0 –1> A //<–10 –10 1> 7 M. By using the experimentally determined orientation relationship between two phases to construct the deformation gradient matrix, the underlying mechanism for the selection of twinning system is further discussed. Image, graphical abstract [ABSTRACT FROM AUTHOR]

We present a computational-experimental study on the ternary alloying effect of CuZr-based shape memory alloy. The transformation behavior, including crystallization, martensite-austenite transformation temperature and hysteresis of Cu-Zr-X (X = Ni, Co, Hf) thin-film samples were investigated by nanocalorimetry. We used ab initio simulations to determine the B2-Cm transformation pathway, evaluate the lattice parameters, the relative phase stability, and the twin boundary energy as a function of composition. Experimental results show that alloying with Ni or Co reduces the hysteresis of the martensitic transformation, while Hf increases it. These observations are in agreement with the trend of the middle eigenvalue of the martensitic transformation matrix. The energy difference between the pure phases obtained from simulations suggests that both Co and Ni stabilize martensite against austenite. However, experiments show that Co decreases transformation temperature, while Ni increases it. We attribute this observation to the larger twin boundary energy and strain energy in the Co-containing alloy. Our results indicate that ab initio simulations are a helpful tool in the development of new shape memory alloys, provided the energy terms associated with the fine twin structure of the martensite are taken into account. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Superelastic NiTi wires were deformed in tension up to gradually increasing total strains at various temperatures, recoverable strains were evaluated and lattice defects left in the microstructure of deformed wires were analyzed by TEM. The recoverable strains evaluated in tensile tests: i) are surprisingly large - exceed 10% strain at low temperatures T ≤ 20 °C, ii) are not significantly restricted by large plastic deformation up to 55% at low temperatures, iii) display local maxima in dependence on total strain in tensile tests at low temperatures, iv) decrease with increasing test temperatures and iv) increase with increasing total strain in tensile tests at high temperatures T ≥ 50 °C. Besides slip dislocations, key lattice defects created by the tensile deformation beyond the martensite yield point are deformation bands containing {114} austenite twins or R-phase. No other austenite twins were found statistically relevant. It is concluded that the plastic deformation of NiTi wires is initiated by deformation twinning in oriented martensite, accompanied by dislocation slip in austenite and/or martensite in extent depending on the test temperature and total strain. The deformation twinning is found to play an ambiguous role in NiTi deformation. When it proceeds at low temperatures (T ≤ 20 °C) beyond the yield point, it raises recoverable strain up to 13%, while at high temperatures (T ≥ 50 °C) when it proceeds already within the plateau range as intermediate step of the stress induced B2=>B19'=>B2T martensitic transformation, it restricts the recoverability of transformation strains. The repeated deformation twinning in oriented martensite enables low temperature deformation of NiTi wires with specific microstructure up to ∼50% strain and causes severe microstructure refinement during the mechanical and thermomechanical processing of NiTi. Image 1 [ABSTRACT FROM AUTHOR]

In this work, we measure the microstructure response of the austenite phase during martensitic phase transformation tens of micrometers beneath the surface of a bulk single crystal nickel-titanium shape memory alloy. Using an emerging dark-field X-ray microscopy (DFXM) technique, the austenite phase fraction, relative misorientation, and elastic lattice plane strain are measured in the interior of the microstructure with a spatial resolution of 108 nm. The results show that some defects consistently induce forward transformation and delay reverse transformation, while other defects consistently impede the propagation of both forward and reverse transformation fronts. We also show that the austenite undergoes an orientation splitting wherein the austenite near the transformation front is constrained from rotating and the austenite far from the transformation front is free to rotate. Finally, we measure interfacial strain fields at the transformation front that extend tens of micrometers into the material. We use an analytical model to show how these strain fields can be explained by a lack of kinematic compatibility between the austenite and martensite phases at the austenite-martensite interface. Image 1 [ABSTRACT FROM AUTHOR]

Martensite variant microstructures in grains of nanocrystalline NiTi shape memory wire deformed in tensile test at room temperature up to 15% strain were analyzed in transmission electron microscope (TEM) with the aim to reveal the mechanism of plastic deformation of B19' monoclinic martensite which created these microstructures. The analysed microstructures consisted of parallel and/or wedge arranged deformation bands within the martensite matrix oriented in a common <010> low index zone. High resolution TEM (HRTEM) analysis of 35 interfaces within a single grain was performed. The observed interfaces were reasonably planar on mesoscale of individual grains but highly irregular on atomic scale. Majority of analysed interfaces displayed interface planes and lattice misorientations similar to (100), (20-1) and (10-1) martensite twins, but not exactly, many interfaces deviated from the exact twin planes and misorientations. Some interfaces displayed crystal lattices only rotated within the <010> low index zone, i.e. lacking the mirror symmetry of adjoining lattices characteristic for twins. On the other hand, traces of (001) crystal planes were mirror symmetric across all observed interfaces. These experimental observations were rationalized by assuming that the analysed martensite variant microstructures were created by the plastic deformation of B19' martensite by kwinking deformation proceeding via coordinated [100](001) dislocation slip based kinking combined with (100) twinning. [Display omitted] [ABSTRACT FROM AUTHOR]

NiTi shape memory wire was deformed in martensite state at low temperature −125 °C up to 750 MPa stress and subsequently heated far above the A f temperature while the stress 750 MPa was held constant. Under such conditions, the heated B19' martensite in the wire was driven towards its thermomechanical stability limits and eventually transformed into plastically deformed austenite. In order to evaluate the mechanism by which such highly constrained reverse martensitic transformation proceeds, martensite variant microstructures and lattice defects evolving in the heated wire were analyzed by TEM and HRTEM. It is found that, the oriented B19' monoclinic martensite deformed plastically via kwinking deformation involving coordinated dislocation slip and twinning and simultaneously adopted a long period modulated crystal structure, before it transformed to austenite upon heating under applied stress. It is proposed that the modulated martensite structure forms from the B19' structure via coordinated slip of partial dislocations with Burgers vector b = a / 2 [100] on (001) crystal planes. Coordinated slip was enforced by the kwinking deformation and partial dislocations started to glide because the martensite was exposed to elevated temperatures. Plastic deformation of oriented B19' martensite enabling the reverse martensitic transformation upon the constrained heating is discussed in view of its impact on NiTi functionality in engineering applications. [Display omitted] [ABSTRACT FROM AUTHOR]

Ti–Ni based shape memory alloys are promising elastocaloric materials for solid-state refrigeration. In this paper, we made a comprehensive study on elastocaloric effect of a nanocrystalline Ti–44Ni–5Cu–1Al (at%) alloy exhibiting successive B2–B19–B19′ transformation. The maximum adiabatic temperature decrease by stress removal from 600 MPa is Δ T adi ∼25 K with a small temperature distribution of ∼0.5 K. The value of Δ T adi is consistent with that calculated from strain-temperature relation. The effective working temperature window is ∼55 K, resulting in a high refrigeration capacity of RC = 4.2 kJ/kg. Material coefficient of performance reaches COP ∼9.6 when an Otto cycle is considered. The middle eigenvalue of the transformation matrix is λ 2 ∼ 0.99, implying high lattice compatibility between the parent and martensite phases. These properties are related to the diffuse nature of successive B2–B19–B19' martensitic transformation of this alloy. Image 1 [ABSTRACT FROM AUTHOR]

The factors that affect the fatigue performance of shape memory alloys (SMAs), including fatigue crack growth (FCG) response, is far from being well-understood. In this study, we point to a mechanism that degrades the FCG performance considerably. We introduce the notion of FCG being affected by shielding and deshielding mechanisms, the former enhancing the resistance while the latter reducing the materials' resistance. We show that the deshielding mechanism creates additional driving forces (positive K contribution) of both Mode II and Mode I types (as much as 5–10 MPa m1/2) which accelerates the crack advance. The origin of the positive K component is associated with the localized martensite variant formation that is highly asymmetric with respect to the crack tip. We derive a resultant ΔK in excellent agreement with that measured based on experimental displacement measurements. Overall, this study represents an advancement of our understanding in FCG of SMAs by quantifying the deshielding mechanism. Image 1 [ABSTRACT FROM AUTHOR]

Doping point defects into shape memory alloys (SMAs) influences their transformation behaviors and mechanical properties. We propose a general Landau free energy model to study doping effects, which only assumes that point defects produce local dilatational stresses coupled to the non-order parameter volumetric strain. Different dopants can be represented by their range of interaction and potency of dilatational stress. Time-dependent simulations based on our model successfully reproduce experimentally observed doping effects in SMAs, including the elevation or suppression of the transformation temperature, the modification of mechanical properties, the appearance of a cross-hatched tweed structure and the emergence of a frozen glassy state with local strain order. We predict that the temperature range for superelasticity will be enhanced in the crossover region between martensite and strain glass. In addition, an Elinvar effect appears most likely in alloys with dopants tending to increase the transformation temperature, which needs to be verified experimentally. Moreover, the two parameters in the Landau model, the interaction range and potency of the dilatational stress, inspire us to identify three material descriptors with which we can construct an empirical machine learning model. The model predicts the transformation temperature, and the slope of the change in transformation temperature as a function of doping composition, enabling an effective search for doped SMAs with targeted properties via machine learning. Image 1 [ABSTRACT FROM AUTHOR]

We present three-dimensional phase-field simulations of martensite microstructure formation in low-carbon steel. In this study, a full set of 24 Kurdjumov-Sachs symmetry variants of martensite is considered. Three different carbon compositions are investigated in order to reveal the effect of carbon content on the martensite microstructure formation. The simulations are performed using the finite strain framework which allows considering real martensite transformation strains. Using Neuber elasto-plastic approximation to the mechanical equilibrium solution, realistic stresses and strains can be obtained during martensite formation resulting in realistic mechanical driving forces for the transformation. The simulated microstructures are compared to experimental results for three carbon compositions. Good agreement between simulated and experimental results is achieved. Image 1 [ABSTRACT FROM AUTHOR]

It is well known that interfaces play an important role in determining the mechanical response of materials. This paper focuses on the transforming shape memory alloy NiTi and is aimed towards a better understanding of austenite-martensite interface structure (steps and dislocation arrays) and the determination of transformation stress corresponding to the translation of this interface. In the present work, we characterize the defect content at the cubic-monoclinic interfaces via the Topological Model. The defect-induced displacement fields are generated within the framework of the Eshelby-Stroh formalism and further improved with Molecular Statics simulations accounting for interactions at the atomic level. The resulting defect core disregistry fields are employed as input to a modified Peierls-Nabarro framework for evaluating the transformation stress. We applied the proposed methodology to the particular case of NiTi alloy single crystals of specific orientations and predicted the transformation stress levels in close agreement with experiments. Moreover, the short-range interactions of dislocation core disregistry fields are shown to be responsible for the experimentally observed non-Schmid behavior of transformation stress levels. Overall, the paper represents an effort to improve our understanding of shape memory materials considering theory, computer simulation and experiment. Image 1 [ABSTRACT FROM AUTHOR]

Shape memory alloys, especially the newly developed Ni–Mn-based heusler-type intermetallic compounds, exhibit specific mechanical responses to mechanical loading. Although the deformation behaviors have been studied for reducing the number of martensite variants, the mechanisms are not fully revealed. Thus in this work the compression process of twin-related 7M modulated martensite of Ni–Mn-Ga intermetallic compound was studied by in-situ neutron diffraction at macroscopic scale and by interrupted in-situ EBSD at microscopic scale. It is revealed that the mechanical response of the 7M martensite is featured by three states: a linear elastic-plastic state, a steady plastic state, and a second linear plastic state. The plastic deformation is initiated by the detwinning of the existing variants in the first linear state. It proceeds to the steady state by intensive detwinning of the these variants and by twinning of the remaining variants that result in the disappearance of the existing variants and the appearance of new variants, then by intermartensitic transformation to form non-modulated martensite (NM). These three shear processes are highly coordinated and compatible with the annihilation of the local incompatible strains by reverse intermartensitic transformation, which allows a steady progress of deformation and a continuous reorientation of the variants. The reorientation produces new twins with unfavorable orientations and limited deformation capacity, leading to a stress increase for further deformation. The present work provides comprehensive information on deformation mechanisms of Ni–Mn-Ga 7M martensite at each characteristic deformation step that is useful for mechanical simulation of deformation behaviors of intermetallic compounds. Image 1 [ABSTRACT FROM AUTHOR]

This work addresses the fundamentals of inter-critical austenite reversion in a Ti-stabilized 12Cr-6Ni-2Mo (at.%) supermartensitic stainless steel, combining thermodynamic and experimental assessments. The calculation of the temperature and composition at which ferrite and austenite phases have the same free energy, i.e. T 0 and C 0 (T), respectively, is discussed as a methodology to understand the austenite reversion and stabilization mechanisms. An ultra-fast heating rate of 500 °C s−1 provided isothermal austenite nucleation and growth from a fully solubilized martensite, allowing direct comparison with the compositional tie-lines and the transformation paths described by the free energy calculations. Isothermal transformation temperatures below and above T 0 (625 °C) were used. Below T 0 , massive reversion was suppressed since it would imply a free energy increase. The opposite occurred above T 0 , since the critical Ni concentration for austenite reversion was lower than for the solubilized case. Transmission electron microscopy and atom probe tomography evidenced that, in all cases, lath growth occurred by local equilibrium partitioning of Ni, along with co-segregation of ferrite-stabilizing elements (Cr and Mo) at the advancing interface. The complex interaction between Cr, Ni and Mo on the energy gain upon nucleation of austenite revealed that Cr segregation can be beneficial while the adverse effect of Mo can be quickly outbalanced by Ni. The most stable reverted laths were obtained for transformation temperatures at least 15 °C below T 0 with average austenite/martensite Ni partitioning factors higher than 2.0. Image 1 [ABSTRACT FROM AUTHOR]

The temperature-dependent elasticity of the B19′ NiTi is unknown today. To gain insights into the lattice-level temperature-dependent elasticity of the B19′ crystal, we present results of in-situ neutron diffraction experiments performed on polycrystalline martensitic specimens in the temperature range of 300 down to 50 K. The experimental results are compared with the density functional theory molecular dynamics (DFT-MD) and Quasi Harmonic Approximation (QHA) calculations. The results confirm that the temperature-dependent Young's modulus (TDYM) of the B19′ crystal is strongly anisotropic. For different crystallographic orientations, the change in Young's modulus over the temperature range of 300–50 K (Δ E (h k l) = E (h k l) 50 K − E (h k l) 300 K ), ranges from Δ E (10 2 ¯) = 2.8 ± 3.5 GPa (extremely weak dependence) to Δ E (103) = 59.6 ± 9.1 GPa (strong dependence). Moreover, it is found that the orientation-specific TDYM and thermal expansion (TE) of the B19′ crystal are correlated. The crystallographic orientations with weak and negative TE responses exhibit a weaker TDYM than the orientations with positive TE. The DFT-MD and QHA results capture qualitatively the above experimental observations and further show that there are orientations in a B19′ crystal exhibiting elastic softening (Δ E (h k l) <0) and ideally no change in Young's modulus (Δ E (h k l) = 0) with cooling. This is found to originate from the strong negative temperature dependence of c 35 stiffness constant. The experimental results along with the first-principles calculations confirm that the Elinvar and Invar are two confluent properties in NiTi SMAs and can be tailored by texturing B19′ crystallographic orientations. Image 1 [ABSTRACT FROM AUTHOR]

Abstract The degradation of the superelastic properties of a commercial NiTi alloy is studied during uniaxial, biaxial and load-path-change cycling performed in-situ with synchrotron X-ray diffraction. Careful examination of the diffraction pattern during uniaxial loading shows the R-phase as a transition between the austenite and the B19′ martensite. Degradation of the superelasticity is found to depend strongly on the loading and unloading path followed, and it is discussed in terms the B19′ martensitic variant selection, the accumulation of dislocations, and the residual R-Phase and B19' martensite. Cycling biaxially leads to faster degradation than uniaxially due to a larger accumulation of dislocations. If the deformation cycle contains a load path change, dislocation accumulation increases further and more martensite is retained. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]

Abstract With a small gap in the density of states and a substantially semiconducting behavior half Heusler alloys have drawn attention as thermoelectric materials. For this study we have selected Hf-free compounds, Ti 0.5 Zr 0.5 NiSn, Ti 0.5 Zr 0.5 NiSn (with a densification aid (DA)) and Ti 0.5 Zr 0.5 NiSn 0.98 Sb 0.02 as well their parent alloys TiNiSn and ZrNiSn as cheap thermoelectrics. Electrical resistivity, thermal conductivity and specific heat were evaluated below room temperature (4.2–300 K) in order to get insight into the mechanism of transport properties. SEM and TEM investigations as well as DFT (density functional theory) calculations accompany this research. The fine-grained epitaxial microstructure with a large number of dislocations warrants a low thermal conductivity at ultralow values (∼30 mW/cmK at 300 K) at a narrow band gap with a sufficiently high density of states at the Femi level. High order of components mixing strongly affects the stability of the solid solutions by the configuration entropy term, which causes a shrinkage of the miscibility gap. For the electronic density of states (DOS) the split Zr band and impurity Ni band induce a significant reduction of the effective energy gap and thus explain n-type of conductivity of the compounds and solid solutions studied. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]

Abstract The NiTiCu alloys belong to a class of materials with excellent shape memory properties. The limitations in shape memory properties arise due to onset of slip at interfaces and also in austenite domains. As slip mediated plasticity is a source of irreversibility, it is imperative to understand the glide resistance of austenite which can be rather complex. In this paper, we develop a model to precisely derive the CRSS for slip substantiated with the uniaxial loading experiments on single crystals in a wide range of orientations employing Digital Image Correlation. We illustrate the core spreading of {011}<001> dislocations and evaluate the role of non-Schmid stress components which introduces profound anisotropy in CRSS levels. The model matches the experimental findings in single crystals with excellent agreement. The theory and experiments show significant crystal orientation dependence of plasticity which must be taken into account when designing with these alloys. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]