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The Mg-2Zn-1.5Mn alloy (ZM21) sheets were hot-rolled in a single pass at 150 °C with different reductions through on-line heating rolling. Microstructures and edge cracking behavior of the rolled sheets were investigated. The in-situ tensile tests at 150 °C were also carried out, the crack initiation and propagation were compared with the edge cracking behavior of ZM21 sheets prepared by on-line heating rolling. The results reveal that the edge cracks are most likely to originate in the rolling direction and normal direction (RD-ND) plane due to the secondary tensile stress along RD. Edge cracking becomes more severe with an increasing reduction. The edge cracks mainly initiate and propagate in the fine recrystallized grain areas and the junction of recrystallized grains and sub-grains with hard orientation. The in-situ tensile test indicates that micro-cracks mainly initiate at the triple junction of grain boundaries where grains have hard orientation with low basal Schmid factor (SF). Meanwhile, those cracks are more likely to propagate along the grain boundaries with maximum difference in basal Schmid factor. Besides, the crack initiation and propagation during the in-situ tensile deformation at 150 °C are found not to be associated with the recrystallization.

In this work, the microstructure of titania coating fabricated on the surface of hydrostatically extruded titanium grade 4 with the use of the micro-arc oxidation method was studied. The surface topography and microstructure investigations performed with atomic force microscopy and scanning and transmission electron microscopy revealed that, by using an Na2HPO4 electrolyte, a well-adherent porous coating is produced on the top surface and side walls of the extruded rod. The distribution of chemical elements was analyzed by using energy dispersive X-ray spectroscopy. The chemical elements dissolved in the electrolyte i.e. (Na, P and O) incorporated into the coating. Sodium locates preferentially in the outer part of the coating, while phosphorus and oxygen are distributed throughout the whole coating. The most relevant finding shows that a grain refinement caused by a hydrostatic extrusion provoked an increase in density of high-angle grain boundaries (HAGB), which in turn secured the formation of a continuous amorphous layer close to the substrate. The presence of this layer compensates for the effect of anisotropic substrate, producing a comparable and homogenous microstructure with a large number of micropores.

Abstract Zirconia-based shape memory ceramics (SMCs) exhibit anisotropic mechanical response when undergoing elastic deformations as well as during austenite–martensite phase transformation. This behavior results in different types of strain incompatibility at grain boundaries, which we study here using a micromechanical model. A single-crystal model is implemented to provide a full mechanistic three-dimensional description of the anisotropic elastic as well as martensitic transformation stress–strain response, including non-Schmid behavior caused by the significant volume change during martensitic transformation. This model was calibrated to and validated against compression tests of single-crystal zirconia micro-pillars conducted previously, and then used to model bi-crystals. Upon the introduction of a grain boundary, the simulation provides detailed information on the nucleation and evolution of martensite variants and stress distribution at grain boundaries. We identify bi-crystal configurations which result in very large stress concentrations at very low deformations due to elastic incompatibility, as well as others where the elastic incompatibility is relatively low and stress concentrations only occur at large transformation strains. We also show how this approach can be used to explore the misorientation space for quantifying the level of elastic and transformation incompatibility at SMCs grain boundaries. Graphical abstract Micromechanics models provide insights on grain boundary elastic and phase transformation strain incompatibility in shape memory zirconia

The thermal stability and mechanical properties of a gradient-nanograined structure (GNS) CoCrNi medium entropy alloy (MEA) processed by ultrasonic surface rolling were studied by using isothermal/isochronal annealing tests combined with quasi-in-situ electron backscatter diffraction (EBSD) characterization and Vickers micro-hardness (HV) measurements. A layer by layer high-throughput investigation method was used to quantitatively study the grain growth kinetics and grain boundary evolution with different initial grain sizes, which could effectively save specimen and time costs. The grain nucleation and growth, as well as shrink and disappearance process through Σ3 coincidence site lattice boundary migration with slightly lattice rotation during annealing were directly revealed. The layer by layer grain growth kinetics and calculated activation energy indicate that the thermal stability of nano-grained top surface layer is relatively higher than that of nano-twined subsurface layer for the gradient CoCrNi MEA processed by ultrasonic surface rolling. Further analysis show that the grain boundary relaxation and dynamic recrystallization of the topmost nano-grains led to the decrease of grain boundary energy, thus improving their thermal stability. The present work provided theoretical basis for the application of CoCrNi MEA at high temperatures. Moreover, the high-throughput method on the investigation of grain stability by using gradient structure can be easily extended to other materials and it is of great significance for understanding the microstructural evolution of gradient materials.

Usually the improved coercivity of rare earth (RE) based 2:14:1-type permanent magnets via RE-rich intergranular additives is achieved at the cost of more corrosion channels and deteriorated corrosion resistance, which remains a challenging hurdle in the RE-Fe-B community. Distinctly, here we report the concurrent improvements of corrosion resistance and coercivity in 40 wt.% Ce-substituted Nd-Ce-Fe-B sintered magnets through engineering the intergranular phase using simple (Nd, Pr)Hx additive. The dehydrogenated Nd/Pr changes the RE concentration gradients between 2:14:1 matrix and intergranular phases during sintering and enlarges the fraction of corrosion-resistant REFe2 phase, rather than the conventionally assumed Nd/Pr-rich intergranular phase with high chemical vulnerability. The spontaneous formation of REFe2 intergranular phase after (Nd, Pr)Hx addition generates the uniquely enhanced corrosion resistance against the hot/humid and acidic environments, and counts as one peculiar feature of Nd-Ce-Fe-B magnets at high Ce substitution level, being distinct from previously reported Ce-free/lean RE-Fe-B. Simultaneously, the formation of continuous grain boundaries enhances the coercivity from 8.7 to 12.5 kOe with trace addition of (Nd, Pr)Hx. Above findings may spur progress towards developing a high-performance Nd-Ce-Fe-B permanent magnet.

The strength of polycrystalline metals increases with decreasing grain size, following the classical Hall-Petch relationship. However, this relationship fails when softening occurs at very small grain sizes (typically less than 10 to 20 nm), which limits the development of ultrahigh-strength materials. In this work, using columnar-grained nanocrystalline Cu-Ag ‘samples’, molecular dynamics simulations were performed to investigate the softening mechanism and explore the strengthening strategies (e.g., formation of solid solution or grain boundary (GB) segregation) in extremely fine nanograined metals. Accordingly, the softening of pure metals is induced by atomic sliding in the GB layer, rather than dislocation activities in the grain interior, although both occur during deformation. The solid solution lowers the stacking fault energy and increases the GB energy, which leads to the softening of NC metals. GB segregation stabilizes GB structures, which causes a notable improvement in strength, and this improvement can be further enhanced by optimizing the solute concentration and GB excess. This work deepens the understanding of the softening mechanism due to atomic sliding in the GB layer and the strengthening mechanism arising from tailoring the GB stability of immiscible alloys and provides insights into the design of ultrahigh-strength materials.

Significantly enhanced varistor properties via tailoring interface states were obtained in Ca1–2x/3YxCu3Ti4O12-SrCu3Ti4O12 composite ceramics. The breakdown field was improved to 35.8 kV•cm−1 and the nonlinear coefficient in 0.1–1 mA cm−2 was enhanced to 14.6 for Ca0.67Y0.5Cu3Ti4O12-SrCu3Ti4O12. Noticeably, the withstand voltage of single grain boundary reached up to 24 V while the reported ones were constant to about 3 V. Greatly improved properties were attributed to the formation of superior grain boundary rather than the reduced grain size. Surprisingly, with distinct discrepancy of nonlinear performance in the composites, the resistance and activation energy of grain boundary exhibited little differences. Based on the double Schottky barrier at grain boundary and the field-assisted thermal emission model, it was found that the excellent electrical nonlinearity arose from the formation of deeper and broader interface states at grain boundary. In this case, interface states were not easily entirely filled and the barrier could maintain its height under applied voltage. This work provides a novel routine for enhancing the varistor properties of CaCu3Ti4O12 based ceramics by manipulating interface states at grain boundary.

Sintering additives and micro-powder injection molding offer an effective method to densify boron carbide (B4C) and make B4C components with complex shapes. By adjusting the proportion of three kinds of powders (SiC, Al2O3 and Y2O3), four kinds of sintering additives were prepared. The feedstock uniformity, debinding behavior, phase composition and microstructure of micro injection molded B4C components with different sintering additives were studied. The results showed that the defects such as lattice distortion and vacancy were introduced into ball milling, which increase the surface energy and benefit subsequent sintering densification. The feedstock had good uniformity and rheology that met the requirements of micro powder injection molding. After debinding, B4C components had enough strength and showed certain sintering characteristics. The addition of sintering additives was beneficial to densification and sintering temperature reduction. The addition of sintering additives formed a second phase in the B4C crystal and at the grain boundary of B4C crystal, which changed the fracture mode from transgranular fracture to mixed fracture mechanism with transgranular fracture.

Twinning stress is known to be a critical factor for the actuating performance of magnetic shape memory alloys because of the harmful deterioration of their magnetic field-induced strain effect. However, the intrinsic origin of the high twinning stress is still in debate. In this work, we firstly fill this gap by precisely probing the reorientation behaviors of A-C and A-B two common macro-twin interfaces under the stimulus of uniaxial compression in-situ transmission electron microscope. The grain boundary is proved to be the main reason for large twinning stress. The twinning stress of the A-C and A-B type interfaces quantitatively are ∼0.69 and 1.27 MPa within the plate respectively. The A-C type interface evidently has smaller twinning stress and larger deformation variable than the A-B interface. Under the action of compression, not only the orientations of the crystals have changed, but also the roles of the major and minor lamellae have changed for both interfaces due to the movements of twinning dislocations. Combining in-situ and quasi in-situ electron diffraction data, the reorientation process is clearly and intuitively shown by the stereographic projection. Atomic models and the theory of dislocation motion are proposed to phenomenologically clarify the intrinsic mechanism. This work is believed to not only provide a deeper understanding of the deformation mechanism of magnetic shape memory alloys under uniaxial compression testing, but also discover that compression training is not the mechanical training way to decrease the twinning stress of non-modulated martensite in single crystal shape memory alloys.

A criterion based on solidification microstructure was proposed to precisely predict the hot tearing behavior of cast alloys, which takes into account the effects of both mechanical and nonmechanical factors. This criterion focuses on the events occurring at the grain boundary, which are determined by the thermal contraction, solidification shrinkage, grain growing and liquid feeding. This criterion responds to a series of factors that affect hot tearing, such as alloy composition, mold design, casting process and microstructure. Its credibility has been validated by studying the hot tearing behavior of Mg-Ce alloys. In conformity with the experimental results, this criterion predicted decrease in the number of rods occurring hot tearing with increasing cerium content. A simplified criterion was derived and validated by Mg-Ce (equiaxed grain) and Mg-Al (columnar grain) alloy systems, which is suitable for the case where the eutectic liquid fraction is low and the liquid feeding can be ignored. In addition, a hot tearing index for equiaxed grains was proposed, that is, | d T / d ( f s 1 / 3 ) | near ( f s ) 1 / 3 = 1 , and its prediction results were consistent with the hot tearing susceptibility calculated from the experimental results.

Post-sinter annealing process plays an important role in the microstructures and magnetic properties of the Nd-Fe-B sintered magnets. In this paper, systematically investigated are the magnetic properties and microstructures of the as-sintered and post-sinter annealed Nd-Fe-B magnets with Pr-Fe-Ga boundary addition. Two choice consecutive annealing methods are adopted at high and low temperatures, namely the 1st annealing at 880 oC for 2 h and then the 2nd annealing at 440 oC for 3 h. It is exceptional to find out that both the remanence and coercivity are improved after 2nd annealing process for this type of magnet. The coercivity is hugely increased from 10.09 kOe for the as-sintered sample to 17.19 kOe for the 2nd annealed magnet, with a significant increment of 70.37% in coercivity. The extraordinary magnetic properties of Br=14.44 kGs, Hcj=17.19 kOe and (BH)max=51.08 MGOe are obtained for the designated Nd-Fe-B sintered magnets without heavy rare earth (HRE) elements manufactured by dual alloy method. The Curie temperature is monotonically decreased from 634 K to 602 K while the c-axis alignment degree is optimized after annealing. Microstructural observation and analysis indicate that the elemental distribution patterns are altered after the 2nd annealing. The diffusion of the aggregate (Pr,Nd,Cu,Ga)-rich phase from triple junctions into the grain boundary regions is ascribed to the formation of thin and continuous grain boundary layer, which is critical to improve the microstructures and magnetic properties.

The effect of aging treatment on the microstructure and corrosion behavior of a Fe-18Cr-15Mn-0.66N high-nitrogen stainless steel (HNSS) in 3.5 wt.% NaCl solution was investigated using a series of electrochemical tests, scanning electronic microscopy (SEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The results showed that the aging treatment led to the precipitation of Cr2N particles along the grain boundaries and their morphologies changed from dispersive particles to continuous network as the aging time increased up to 60 min. Aging time had minor effects on the corrosion potential and corrosion current density, but resulted in the sharp decrease in the pitting corrosion potential. The passive film behaved as a n-type semiconductor, and the donor density of the passive film increased with the aging time. Meanwhile, the fraction of stable oxide (Cr2O3) in the passive film decreased with the aging time. It demonstrates that the aging treatment deteriorated the protectiveness of the passive film, hence weakened the corrosion resistance of HNSS.

Al-Mn-Fe-Si strips were fabricated via both the twin-roll casting (TRC) and the more conventional route, direct-chill casting (DC). The two types of strips prepared were subjected to thermal exposure at a series of temperatures. Uniaxial tensile tests after the thermal exposure showed that while the DC strip presented a ∼74% decrease in the yield strength and ∼35% decrease in the ultimate tensile strength (UTS) after being exposed to 350 °C for 12 h, the TRC strip, in contrast, maintained its strength at temperatures up to ∼460 °C for the same duration. Systematic microstructure characterization revealed that the different thermal stability in the strength of the two types of strips arised from their distinct evolution in grain morphology and second phase particles during the thermal exposure. The calculation based on Cahn-Lucke-Stuwe (CLS) model suggested that due to the highly supersaturated solute atoms, at the beginning of the thermal exposure, the TRC strip experienced a strong solute drag which reduced the grain boundary migrating velocity to a value that is orders of magnitude smaller than that in the DC strip. With the progress of the thermal exposure, the solute atoms precipitated out, forming densely distributed second phase particles. For one thing, these particles stabilized the grain structure by inducing Zener pinning pressure which could be ten times higher than that in the DC strip, depending on the temperature. For another, they acted as dislocation obstacles and compensated for the strength loss owing to decreasing solution hardening. Both effects contributed to the TRC strip's fairly stable strength regarding thermal exposure below 460 °C. The present work could guide the direct application of the TRC strips in the industry. The results should also be helpful for the development of a fundamental framework for designing advanced TRC Al strips with improved mechanical properties at elevated temperatures.

NiCoCr-based multi-component alloys have drawn much attention due to their exceptional ductility and strain hardening capacity. However, insufficient strength-ductility synergy of NiCoCr alloy has always been an issue that prevents it from extensive applications. According to our previous research, the precipitation of γ'' phase can significantly improve the strength-ductility synergy of this alloy system at room temperature. In this study, the effects of V addition on γ'' phase stability and high temperature mechanical properties have been explicitly investigated. The results indicate that V addition can stabilize metastable γ'' phase in this alloy system and prevent it from transforming into stable δ phase at grain boundaries upon 650 °C aging, resulting in improved mechanical properties at elevated temperature. The specific strength of γ''-strengthened multi-component NiCoCr-based alloy can reach up to 86.2 MPa/g•cm−3 at 650 °C, which is higher than those of Ni-based superalloys, IN 939 and Waspaloy. This work provides theoretical guidance for the novel design of γ''-strengthened alloy for high temperature applications.

In this study, the inhomogeneous dealloying phenomenon during the liquid metal dealloying (LMD) was investigated using Fe50Ni50+Mg and (FeCo)50Ni50+Mg systems. For the Fe50Ni50+Mg system, the inhomogeneous dealloying and wetting of Mg melt occurred along triple junction (TJ) and grain boundary (GB). Temperature increase enhances the inhomogeneous dealloying kinetics and leads to the formation of the plate-shaped abnormal ligaments at the GB region. The energy banlance between a GB energy (γGB) and solid-liquid interface energies (γsl) is the key factor governing the inhomogeneous dealloying and wetting. Particularly, the low-energy twin boundaries were unaffected by the inhomogeneous dealloying. Therefore, precursor microstructure is an important factor determining the final morphology of dealloyed material as well as its physical properties. In the case of the (FeCo)50Ni50 precursor, all TJ and GB were stable against the preferred penetration of Mg melt from 600°C to 800°C. It was concluded that a minor addition of alloying elements (V or Cr) changes GB characteristics as well as γsl of the precursor alloy. Consequently, this significantly influences dealloying mechanisms and final morphology of the dealloyed material. The current findings demonstrate the importance of GB engineering in the precursor materials for the technological application of liquid metal dealloying for the synthesis of advanced structural and functional materials.

Aero-engine turbine blades may suffer overheating during service, which can result in severe microstructural and mechanical degradation within tens of seconds. In this study, the thermal cycling creep under (950°C/15 min+1100°C/1 min)-100 MPa was performed on a directionally solidified superalloy, DZ125. The effects of overheating and thermal cycling on the creep properties were evaluated in terms of creep behavior and microstructural evolution against isothermally crept specimens under 950°C/100 MPa, 950°C/270 MPa, and 1100°C/100 MPa. The results indicated that the thermal cycling creep life was reduced dramatically compared to the isothermal creep under 950°C/100 MPa. The plastic creep deformation mainly occurred during the overheating stage during the thermal cycling creep. The thermal cycling creep curve exhibited three stages, similar to the 1100°C isothermal creep, but its minimum creep rate occurred at a lower creep strain. The overheating events caused severe microstructural degradation, such as substantial dissolution of γ' phase, earlier formation of rafted γ' microstructure, widening of the γ channels, and instability of the interfacial dislocation networks. This microstructural degradation was the main reason for the dramatic decrease in thermal cycling creep life, as the thermal cycling promoted more dislocations to cut into γ' phase and more cracks to initiate at grain boundaries, carbides, and residual eutectic pools. This study underlines the importance of evaluating the thermal cycling creep properties of superalloys to be used as turbine blades and provides insights into the effect of thermal cycling on directionally solidified superalloys for component design.

TC21 titanium alloy, as an important metal to fabricate the aircraft structural components, has attracted great attentions recently. A TC21 titanium alloy with widmanstatten structure was isothermally compressed. Based on the microstructure observation, the evolution of initial β grain, Grain Boundary α phase (αGB), lamellar α and interlayered β was systematically investigated. The results showed that, with the increasing of height reduction, the αGB underwent an evolution process from bending/kinking to breaking inducing the corresponding blurring of initial coarse β grain outline. Meanwhile, a significant phase transformation from α to β took place at the terminations of broken αGB. The evolution of lamellar α and interlayered β in the colony was closely related to their deformation compatibility. In the α colony, the interlayered β experienced a larger deformation amount than lamellar α. The higher distortion energy promoted the occurrence of Dynamic Recovery (DRV) and Dynamic Recrystallization (DRX) to generate many Low Angle Boundaries (LABs) and High Angle Boundaries (HABs) in interlayered β, which induced an apparent grain refinement of β phase. On the contrary, the lower distortion energy and low deformation temperature suppressed the occurrence of DRV/DRX and restrained the globularization of lamellar α. Furthermore, the microstructure observation clearly revealed that the shearing separation mechanism dominated the evolution of the α phase from lamellar to short bar-like morphology.

A theoretical treatment on the oxide-controlled dwell fatigue crack growth of a γ’ strengthened nickel-based superalloys is presented. In particular, this study investigates the influence of an externally applied load and variations in the γ’ dispersion on the grain boundary oxide growth kinetics. A dislocation-based viscoplastic constitutive description for high temperature deformation is used to simulate the stress state evolution in the vicinity of a crack at elevated temperature. The viscoplastic model explicitly accounts for multimodal γ’ particle size distributions. A multicomponent mass transport formulation is used to simulate the formation/evolution of an oxide wedge ahead of the crack tip, where stress-assisted vacancy diffusion is assumed to operate. The resulting set of constitutive and mass transport equations have been implemented within a finite element scheme. Comparison of predicted compositional fields across the matrix/oxide interface are compared with experiments and shown to be in good agreement. Simulations indicate that the presence of a fine γ’ size distribution has a strong influence on the predicted ow stress of the material and consequently on the relaxation in the vicinity of the crack-tip/oxide wedge. It is shown that a unimodal dispersion leads to reduced oxide growth rates (parabolic behavior) when compared to a bimodal one. Stability conditions for oxide formation are investigated and is associated with the prediction of compressive stresses within the oxide layer just ahead of the crack tip, which become progressively negative as the oxide wedge develops. However, mechanical equilibrium requirements induce tensile stresses at the tip of the oxide wedge, where failure of the oxide is predicted. The time taken to reach this critical stress for oxide failure has been calculated, from which dwell crack growth rates are computationally derived. The predicted rates are shown to be in good agreement with available experimental data.

Thermally grown oxides (TGOs) at the ceramic top-coat/metallic bond-coat interface are a pressing challenge in advanced thermal barrier coating (TBC) systems as they can affect the performance and service lifetime of TBCs. Thus, developing novel TBC materials with ultralow oxygen ion diffusivity is very urgent. In this study, we reported the diffusive properties of oxygen ions in a novel pyrochlore-type La2(Zr0.7Ce0.3)2O7 (LZ7C3) material. The measured ionic conductivity and atomistic simulation revealed that the oxygen ion diffusivity in LZ7C3 grains is two orders of magnitude lower than that in conventional 8 wt.% yttria-stabilized zirconia (8YSZ) grains. This is due to the relatively high energy barrier for oxygen hopping in LZ7C3. In addition, it was found that enhancing the order distribution of cations is a strategy to reduce the intrinsic oxygen diffusion of pyrochlore-type oxides. On the other hand, we observed that La3+ cations segregate at the grain boundaries (GBs) of LZ7C3, which results in the electrostatic potential at GBs being comparable to that in the bulk. Furthermore, we found that the oxygen ion diffusion is facilitated at the GBs of LZ7C3 due to the stretched O–Zr/Ce bond and the low coordination at GBs. However, the segregations of Y3+ cations and the increase in the number of oxygen vacancies resulted in the formation of an electrostatic layer at the GBs of 8YSZ, which shielded the oxygen ion diffusion. Despite this, the oxygen ion diffusivity in LZ7C3 was still considerably less than that in conventional 8YSZ. This study offers a stepping stone toward utilizing pyrochlore-type LZ7C3 materials as advanced TBCs at high temperatures.

To overcome the dimension limits of immiscible alloys produced by traditional techniques and enhance their mechanical properties, bulk Cu-Fe-based immiscible alloy with abundant nanotwins and stacking faults was successfully produced by selective laser melting (SLM). The SLM-produced bulk immiscible alloy displays a heterogeneous microstructure characterized by micro-scaled γ-Fe particles dispersed in fine e-Cu matrix with a high fraction (∼92%) of high-angle grain boundaries. Interestingly, abundant nanotwins and stacking faults are generated in the interior of nano-scaled γ-Fe particles embedded within e-Cu matrix. The heterogeneous interface of soft domains (e-Cu) and hard domains (γ-Fe) not only induces the geometrically necessary dislocations (GNDs) but also affects the dislocation propagation during plastic deformation. Therefore, the bimodal heterogeneous interface, and the resistance of nanotwins and stacking faults to the propagation of partial dislocation make the bulk immiscible alloy exhibit an enhanced strength of ∼590 MPa and a good ductility of ∼8.9%.

To improve the strength and ductility of TiAl alloys by second phase, Ti46Al4Nb1Mo alloys doped with different B content (0.4%, 0.8%, 1.2%, 1.6% and 2.0%, atomic percent, hereafter in at.%, referred to as TNM-xB) were prepared. Macro/microstructure evolution, mechanical properties and deformation mechanisms of the alloys were studied systematically. Results showed that the microstructure of TNM-0.4B and TNM-0.8B alloy remained columnar dendrites, and the secondary dendritic arms of columnar grains were more obvious. When the content of B is 1.2%, the columnar dendrites transformed to equiaxed grains, and the α2/γ lamellae colony size was further refined in TNM-1.6B and TNM-2.0B alloy. The morphologies and kinds of the borides were changed with increasing B content, XRD results showed that TiB phase appeared with 1.6%B content, and both TiB and TiB2 phase formed in TNM-2.0B alloy. There were straight and curved TiB phases located around grain boundaries in TNM-0.4B and TNM-0.8B alloy, and when the content of B increased to 1.2%, the curved TiB phases were reduced, while the tiny and straight TiB phases increased. With further increasing B content to 1.6% and 2.0%, the tiny and straight TiB phases were coarser. Compressive testing results showed that the mechanical properties of the TNM alloy were enhanced with increasing B content. The maximum strength and strain of TNM alloy were 2339MPa and 33.7% with 1.6% B addition. The compressive strength and strain were mainly enhanced via refinement of lamellar colony and formation of TiB, and it is found that pile-up of dislocations and deformed twins promoted by TiB are predominant in improving the mechanical properties of TNM alloys with higher strength and strain.

In a material under stress, grain boundaries may give rise to stress discontinuities. Stress localization is crucial to materials' behavior such as segregation, precipitation, and void nucleation. Here, the stress state at a grain boundary perpendicular to a uniaxial external stress is studied systematically. The grain boundary with the most extreme stress discontinuity is determined for cubic materials within the elastic limit for a bicrystal model. Additionally, grain boundaries with negligible stress discontinuity are identified. The influence of the elastic tensor components, C11, C12, and C44, and grain orientation is studied quantitatively.

Hall–Petch slope (k), an important parameter in Hall–Petch relation, describes the efficiency of strengthening effect by grain boundaries. Previously, a highly texture dependent k for Mg alloys is frequently reported, but, in the present study, we report a weak texture dependence of k in a rare-earth containing Mg-2Zn-1Gd plate with two peaks of (0002) poles inclining approximately ± 30° away from the ND toward the TD. Although there is a strong mechanical anisotropy between tension along the TD and RD, the k for TD-tension (280 MPa μm1/2) is quite similar to that for RD-tension (276 MPa μm1/2). Here, RD, TD and ND refer to the rolling direction, transverse direction and normal direction of the plate, respectively. The weak texture dependence of k is well predicted by the compound use of the activation stress difference between neighboring grains (ΔStress) and the geometric compatibility factor ( m ′ ). By analyzing how the texture affects the values for ΔStress and m ′ , the mechanism for this texture independence of k is ascribed to the activation of a high fraction of additional deformation mode, besides the predominant one for both RD-tension and TD-tension, namely, prismatic slip accompanied by a high fraction of basal slip for RD-tension and basal slip accompanied by a high fraction of prismatic slip for TD-tension. This will lead to multiple deformation transfer modes and, consequently, the effect of texture on the ease of deformation transfer across grain boundaries is weakened. As a result, there is a similar k for TD-tension and RD-tension.

The investigate about the effect of the microstructure of Ti6Al4V alloy on its cavitation erosion and corrosion properties in marine can provide the key basis for the application. On the basis of as-received Ti6Al4V(TC4) alloy, FC-TC4 and AC-TC4 alloys were prepared by heat treatment with the cooling method of a furnace and atmospheric environment, respectively. Then the microstructure evolutions of three samples were scrutinized and the effect of microstructure on their cavitation erosion and corrosion resistance was explored. The results showed that more recrystallized grains formed as well as its content of α grains and high-angle grain boundaries increased in AC-TC4 alloy. To FC-TC4 alloy, there was obvious grain growth apart from recrystallization. Moreover, many nanotwins of TiV and TiAl3 were formed separately in FC-TC4 and AC-TC4 alloys due to the dislocation migration during heat treatment. The microstructure evolution led the hardness and elastic modulus of AC-TC4 alloy were the best, followed by FC-TC4 alloy, that of TC4 were the worst. Similarly, passivating ability of AC-TC4 alloy was the best among three samples because of its microstructure. Although cracks extended along the grain boundaries under the action of continual cavitation erosion, the passivation film formed by TiO2 and Al2O3 would enhance their resistance to further corrosion and cavitation erosion in artificial seawater.

ZrC is a promising candidate for the application in ultra-high temperature regime due to its unique combination of excellent properties, such as high melting point, good chemical inertness and high temperature stability. The rapid decrease of strength at high temperatures, however, is one of the obstacles that impedes its practical services. Strengthening of grain boundaries by solute segregation is believed to be an effective way to improve its high temperature performance. Therefore, the segregation tendency of ten solid solute atoms, including Sc, Ti, V, Cr, Y, Nb, Mo, Hf, Ta, W, in ZrC grain boundaries, and the strengthening/weakening effects on grain boundaries due to segregation are investigated by first-principles calculations. The segregation tendency is found dominated by the size effect, which is confirmed by both a qualitative analysis and a quantitative approach based on support vector regression. It means that big atoms tend to segregate to grain boundary sites with local expansions, while small atoms tend to segregate to grain boundary sites with local compressions. Simulations on stress-strain responses indicate that segregation of small atoms (Ti, V, Cr, Nb, Ta, Mo, W) can usually improve grain boundary strengths by inducing compression strains to grain boundaries, even though there is also an exception. In contrast, segregation of Sc and Y will soften grain boundaries. The results reveal that strengthening of grain boundaries by solute segregation is a valuable avenue to enhance high temperature mechanical properties of ZrC, providing guidelines for further design of ZrC based materials.

CrMnFeCoNi high-entropy alloys (HEAs) exhibit an excellent combination of tensile strength and ductility at cryogenic temperatures. This study led to the introduction of a new method for the development of high-performance CrMnFeCoNi HEAs at cryogenic temperatures by jointly utilizing additive manufacturing (AM) and the addition of interstitial atoms. The interstitial oxygen present in the powder feedstock was transformed into beneficial nano-sized oxides during AM processing. The HEA nanocomposite fabricated using laser powder bed fusion (L-PBF) not only contains heterogeneous grains and substructures but also a high number density of nano-sized oxides. The tensile results revealed that the L-PBF HEA nanocomposite has superior yield strengths of 0.77 GPa and 1.15 GPa, and tensile strengths of 0.92 GPa and 1.45 GPa at 298 K and 77 K, respectively. In addition, the Charpy impact energies of the samples tested at 298 K and 77 K were measured as 176.2 J and 103.7 J, respectively. These results indicate that the L-PBF HEA nanocomposite successfully overcomes the well-known strength-toughness trade-off. The tensile deformation microstructure contained a relatively large number of deformation twins (DTs) at cryogenic temperature, a possible consequence of the decrease in the stacking fault energy with decreasing temperature. On the other hand, cracks were found to propagate along the grain boundaries at room temperature, whereas a transgranular crack was observed at cryogenic temperature in the specimens fractured as a result of the Charpy impact.

The influence of surface roughness on the interfacial dynamic recrystallization kinetics and mechanical properties of Ti-6Al-3Nb-2Zr-1Mo hot-compression bonding joints was systematically investigated. It is found that for the bonding interface of rough surfaces, elongated fine grains are formed at the bonding interface due to shear deformation of the interfacial area. As the surface roughness increases, the proportion of elongated grains drastically decreases as they further reorient to form equiaxed grains along the bonding interface of rougher surfaces resulting from severe incompatible deformation of the interface area. Meanwhile, high-density geometrically necessary dislocations accumulate around the interfacial recrystallization area to accommodate the incompatible strain and lattice rotation. A rotational dynamic recrystallization mechanism is thereby proposed to rationalize the formation of fine interfacial recrystallization grains during bonding of rough surfaces. In contrast to that of rough surfaces, bonding interface of polished surfaces exists in the form of straight interface grain boundaries without fine grains under the same deformation conditions. While with the increase of deformation strain, small grain nuclei form along the bonding interface, which is associated with discontinuous dynamic recrystallization assisted by strain-induced boundary migration of interface grain boundaries. Moreover, the bonding joints of rough surfaces show lower elongation compared with that of polished surfaces. This is because the formation of heterogeneous fine grains with low Schmid factor along the bonding interface of rough surfaces, leading to worse compatible deformation capability and thereby poor ductility of bonding joints.

From the perspective of high-temperature applications, materials with excellent high-temperature mechanical properties are always desirable. The present work demonstrates that the binder-free nanopolycrystalline WC ceramic with an average grain size of 103 nm obtained by high-pressure and high-temperature sintering exhibits excellent mechanical properties at both room temperature and high temperature up to 1000 °C. Specifically, the binder-free nanopolycrystalline WC ceramic still maintains a considerably high Vicker hardness HV of 23.4 GPa at 1000 °C, which is only 22% lower than the room temperature HV. This outstanding thermo-mechanical stability is superior to that of typical technical ceramics, e.g. SiC, Si3N4, Al2O3, etc. Nanocrystalline grains with many dislocations, numerous low-energy, highly stable Σ2 grain boundaries, and a relatively low thermal expansion coefficient, are responsible for the observed outstanding high-temperature mechanical properties.

Correlation between mechanical properties and texture characteristics was investigated in a hot-rolled Ti-7333 alloy. After rolling deformation, two strong fiber-textures with 〈100〉 parallel to rolling direction (RD) and 〈110〉 parallel to RD were determined. The mechanism of stress-induced boundary migration mainly controls the nucleation of recrystallized grains holding the similar orientation with the adjacent deformed grains on the bulged grain boundaries. It was revealed that the 〈110〉 fiber-texture with higher Schmid factor (SF) possesses the high deformation ability along RD.

Cube texture contains two easy magnetization directions 〈001〉 parallel to rolling and transverse direction, respectively, which is the most ideal magnetic texture suitable not only for transformers but also for rotating machines. In this study, a strong cube texture with ODF density of 50.73 mrd was successfully obtained by regulating the recrystallized grain orientation using cross-rolling and pulsed electric current, compared to conventional thermal annealing (the average cube texture intensity is ~10 mrd in lots of latest studies). Cross cold rolling process intentionally “created” metastable deformed cube orientation in oriented silicon steel and the specific recrystallization texture rotation path was identified under pulsed electric current in 5 min: {114} →{114} →{114} →{001} →{001} . The cube-oriented grains were induced by pulsed electric current (800 °C) and rapid heating (51.9 °C/s, 750 °C), while the cube grains were observed in the annealed samples at the high temperature (1060 °C). Recrystallized grain size of pulsed samples is about twice that of the annealed sample. This phenomenon is considered that the concurrent effects of electron wind force and Joule heating affected the nucleation, growth and rotation of cube grains by reducing the nuclear barrier, producing higher grain boundary mobility and structural evolution towards a state with lower electrical resistance. This idea of current-controlled texture is worthy of popularization in more materials and the realization of an electromagnetic field to crystal orientation selection is an interesting topic.

Fabrication of thin-wall components using the laser powder bed fusion (LPBF) additive manufacturing (AM) technology was investigated for two “hard-to-weld” high gamma prime Ni-based superalloys RENE 65 (R65) and RENE 108 (R108). Simple block parts with wall thicknesses of 0.25 mm, 1.00 mm, and 5.00 mm are printed using a bidirectional laser scanning strategy without layer-wise rotation. Parts with walls thinner than 5 mm fail before reaching the designated build height. Results indicate that reduction of limiting build height (LBH) corresponds to the reduction of part thickness and is unaffected by alloy composition. On the contrary, the number of internal micro-cracks along columnar grain boundaries in the build direction (BD) increases with part thickness and is significantly higher in R108 than R65. These findings suggest that reduced LBH in parts with thinner walls is not caused by internal micro-crack formation. Fractography and finite element analysis (FEA) of the in-process thermal stresses show that the LBH trend is not explained by the conventional cracking mechanism. Simulations suggest that part thickness affects stress distribution leading to more substantial distortion and consequent failure to add layers for continued fabrication of thinner parts.

The preparation of the compact and full-coverage AgSbS2 thin films is firstly reported using the pyrolysis of the Ag-butyldithiocarbamate and Sb-butyldithiocarbamate complex solution in DMF. The influence of the preparation temperature on the crystal phase, optical absorption, morphology of the AgSbS2 thin films is systematically investigated. The AgSbS2 thin films at 150 °C for 30 min are composed of the individual nanoparticles with the particle sizes of 30–60 nm. When the AgSbS2 thin films are further heated at 350 °C for 2 min, the individual AgSbS2 nanoparticles are melted together and the grain boundary between the AgSbS2 nanoparticles disappear. The solar cells with the architecture of FTO/compact TiO2/AgSbS2/spiro-OMeTAD/Au achieve the photoelectric conversion efficiency (PCE) of 2.09%, along with the open-circuit voltage of 0.44 V, the short-circuit photocurrent density of 10.49 mA cm−2, the fill factor of 0.45. The PCE of 2.09% is the highest value for the AgSbS2 solar cells.

In this study, the dislocation behavior of a polycrystalline Mg-5Y alloy during tensile deformation was quantitatively studied by an in-situ tensile test, visco-plastic self-consistent (VPSC) modeling, and transmission electron microscopy (TEM). The results of the in-situ tensile test show that dislocations contribute to most of the deformation, while a small fraction of dislocations are also activated near grain boundaries (GBs). The critical resolved shear stresses (CRSSs) of different dislocation slip systems were estimated. The CRSS ratio between prismatic and basal dislocation slip in the Mg-Y alloy (~13) is lower than that of pure Mg (~80), which is considered as a major reason for the high ductility of the alloy. TEM study shows that the dislocations in the alloy have high mobility, which also helps to accommodate the deformation near GBs.

Purpose The purpose of this study is to investigate the microstructural evolution of high-strength 2024 Al alloy prepared by the laser powder bed fusion (L-PBF) additive manufacturing (AM) route. The high-strength wrought Al alloy has typically been unsuitable for AM due to its particular solidification characteristics such as hot cracking, porosity and columnar grain growth. Design/methodology/approach In this research work, samples were fabricated using L-PBF under various laser energy densities by varying laser power and scan speed. The microstructural features that developed during the solidification are correlated with operating laser parameters. In addition, finite element modelling (FEM) was performed to understand the experimentally observed results. Findings Microstructure evolution and defect formation have been assessed, quantified and correlated with operating laser parameters. Thermal behaviour of samples was predicted using FEM to support experimental observations. An optimised combination of intermediate laser power and scan speed produced the least defects. Higher energy density increased hot tearing along the columnar grain boundaries, while lower energy density promoted void formation. From the quantitative results, it is evident that with increasing energy density, both the top surface and side wall roughness initially reduced till a minimum and then increased. Hardness and compressive strength were found to decrease with increasing power density due to stress relaxation from hot tearing. Originality/value This research work examined how L-PBF processing conditions influence the microstructure, defects, surface roughness and mechanical properties. The results indicates that complete elimination of solidification cracks can be only achieved by combining process optimisation and possible grain refining strategies.

A fine granular area, FGA, is a typical phenomenon observed at the very high cycle fatigue fracture crack origin with a subsurface defect in the material. The FGA has been widely investigated, and different mechanisms have been proposed. In this paper, the formation of FGA in a non-defect matrix of one austenitic steel during very high cycle fatigue was studied using a progressive stepwise load-increasing method and electron scanning microscopy/electron channeling contrast imaging (ECCI) technique. A nano rough surface area or FGA at the fatigue crack origin has been observed in the subsurface matrix without any defect. It is a new phenomenon. A mechanism was proposed using the dislocation plasticity theory. The formation of FGA in a non-defect matrix is a localized plasticity exhausting process by strain localization, grain fragmentation, stress concentration and nano crack initiation and propagation along low-angle grain boundaries.

The thermal conductivities of doped polysilicon layers depend on grain size and on the concentration and type of dopant atoms. Previous studies showed that layer processing conditions strongly influence the thermal conductivity, but the effects of grain size and dopant concentration were not investigated in detail. The current study provides thermal conductivity measurements for low-pressure chemical-vapor deposition (LPCVD) polysilicon layers of thickness near 1 /spl mu/m doped with boron and phosphorus at concentrations between 2.0/spl times/10/sup 18/ cm/sup -3/ and 4.1/spl times/10/sup 19/ cm/sup -3/ for temperatures from 20 K to 320 K. The data show strongly reduced thermal conductivity values at all temperatures compared to similarly doped single-crystal silicon layers, which indicates that grain boundary scattering dominates the thermal resistance. A thermal conductivity model based on the Boltzmann transport equation reveals that phonon transmission through the grains is high, which accounts for the large phonon mean free paths at low temperatures. Algebraic expressions relating thermal conductivity to grain size and dopant concentration are provided for room temperature. The present results are important for the design of MEMS devices in which heat transfer in polysilicon is important.

This study aims to analyze the effect of thermo mechanical coupling damage in the presence of a phase change (austenite/martensite) in 35NCD16 steel. The impact of increasing mechanical traction load, accompanied by a martensitic transformation on the scale of a single grain with boundary has been studied. The prediction transformation of induced plasticity (TRIP) was evaluated by taking into account the following parameters: twenty shear directions of the martensitic plates, two values of the shear deformation of the martensitic plates, energetic and thermodynamics criteria for getting in order the transformation of the martensitic plates, elastoplastic behavior of the two areas in the first case (martensitic plate and grain boundary) and elastic behavior for the grain boundary in the second case. The numerical calculation is carried out using the finite element method (FEM), implemented in the Zebulon calculation code. The developed approach is validated using the available experimental results reported in the literature. The numerical results showed that the estimation of TRIP given by the energetics criteria with the values of the shear deformation (γ0 = 0.16) are closer to the experiment results.

This investigation aims to make clear the relationships among geometric features, macrostructures, microstructures, and mechanical performances of the weld bead in the gas metal arc welding (GMAW) process for ASTM A606 type IV weathering steel. The results indicate that with the increase of welding heat input, the geometry of the top reinforcement and penetration increases proportionally until reaching the peak value, and then declines with the further increase of welding heat input; while the values of bottom reinforcement increase continuously with the increase of welding heat input. The average ultimate tensile strength (F), maximum displacement (L), and failure energy (E) of the welded joint produced by the moderate welding heat input are 10.27 kN, 1.88 mm, and 13.55 J, respectively, which are obviously larger than those obtained by the insufficient welding heat input (F: 8.33 kN, L: 1.20 mm, and E: 7.10 J). Although the mechanical properties of the welds achieved by the overlarge welding heat inputs present comparable levels (F: 9.73 kN, L: 1.86 mm, and E: 13.51 J), their standard deviation is much higher. Acicular ferrite, grain boundary ferrite, side plate ferrite, Widmanstatten ferrite, and pearlite are noticed in the fusion zone (FZ), while the coarse grain heat-affected zone (CGHAZ) is composed of upper and lower bainite, grain boundary ferrite, Widmanstӓtten ferrite, polygonal ferrite, and pearlite. As the welding heat input increases, the pre-austenite grain width in the FZ and CGHAZ decreases. The descending order of the microhardness distributed in the welded joints is fusion line, CGHAZ, FZ, fine grain heat-affected zone (FGHAZ), and base metal. In addition, the microhardness values of FZ and CGHAZ decline with the rise of the welding heat input. Nonetheless, no apparent links are found between the microhardness levels of the FGHAZ and welding heat input.

The presence of grain boundaries (GBs) has a great impact on the coefficient of thermal expansion (CTE) of polycrystals. However, direct measurement of local expansion of GBs remains challenging for conventional methods due to the lack of spatial resolution. In this work, we utilized the valence electron energy loss spectroscopy (EELS) in a scanning transmission electron microscope (STEM) to directly measure the CTE of Σ5 and 45°GBs of SrTiO3 at a temperature range between 373 and 973 K. A CTE that was about 3 times larger was observed in Σ5 GB along the direction normal to GB plane, while only a 1.4 time enhancement was found in the 45° GB. Our result provides direct evidence that GBs contribute to the enhancement of CTE in polycrystals. Also, this work has revealed how thermodynamic properties are varied in different GB structures and demonstrated the potential of EELS for probing local thermal properties with nanometer-scale resolution.

Aircrafts damages caused by lightning strikes have been known since the early days of aviation. However, the physical effects on the aircraft structure are still being investigated. This work seeks to evaluate the lightning strike effects in the aluminum alloy 7075-T6. Samples were submitted to lightning strike simulation in laboratory and the damages evaluated through characterization techniques. Ultrasound and profilometry tests have shown material loss to 0.272 mm depth in the damaged region. In addition, it was detected the material accumulation occurrence in the damage vicinity of the region. Below the damage, it was found a region where metallurgical changes were identified. The tensile and microhardness tests results have shown reduction in the percentage elongation and hardness increasing in the material affected by lightning. These results are corroborated by the X-Ray Diffraction (XRD) and Rietveld Method (red line) that indicated an increasing in dislocation density and micro-deformation in the material matrix. Optical microscopy results have shown the presence of microcracks on the normal and cross-section surface of the samples damaged. The Energy Dispersive X-Ray Spectroscopy (EDXS) and Electron Backscattered Diffraction Test (EBSD) found coarse intermetallic phases and precipitates compounds with dimensions greater than 1 μm in length. They were responsible for nucleation of the microcracks that propagate along the material grain boundaries.

Despite being strong with many outstanding physical properties, tungsten is inherently brittle at room temperature, restricting its structural and functional applications at small scales. Here, a facile strategy has been adopted, to introduce high-density dislocations while reducing grain boundaries, through electron backscatter diffraction (EBSD)-guided microfabrication of cold-drawn bulk tungsten wires. The designed tungsten microwire attains an ultralarge uniform tensile elongation of ~10.6%, while retains a high yield strength of ~2.4 GPa. in situ TEM tensile testing reveals that the large uniform elongation of tungsten microwires originates from the motion of pre-existing high-density dislocations, while the subsequent ductile fracture is attributed to crack-tip plasticity and the inhibition of grain boundary cracking. This work demonstrates the application potential of tungsten microcomponents with superior ductility and workability for micro/nanoscale mechanical, electronic, and energy systems.

The capture efficiency describes the capability of a sink, such as a grain boundary (GB), dislocation, and void, to absorb point defects (PDs). The bias defines the difference in capture efficiency between the absorption of a vacancy and dumbbell at a sink. Complete kinetic information on PDs, including diffusion barriers and diffusion orientations, as well as accurate saddle points, are needed to determine the capture efficiency and bias at a sink accurately, which is computationally demanding. In the present study, the Self-Evolving Atomistic Kinetic Monte Carlo (SEAKMC) method was used to investigate the defect dynamics of PDs near different types of grain boundaries (GBs) (with both 〈100〉 and 〈110〉 families) accurately in body-centered cubic (BCC) iron (Fe). The capture efficiency, sink strength, and bias factor of different types of GBs were determined in Fe, which, different from traditional rate theory estimation, showed a distinct capture efficiency, sink strength, and bias in different GBs. The results demonstrate a strong positive correlation between the capture efficiency and the GB strain width, instead of the GB misorientation, GB energy, or GB-PD binding energy, which have been investigated previously. This work provides valuable insight into the radiation-induced microstructural evolution of GBs.

This research studied the mechanisms of Ca and Zn microalloying on the enhancement of ductility of extruded Mg-Gd sheet by combing electron backscattered diffraction and slip trace analysis. The ductility and microstructure of extruded Mg-0.6Gd and Mg-0.6Gd-0.3Ca-0.2Zn (wt%) sheets were investigated. Basal slip was the main deformation mode under investigation. Ca and Zn microalloying increased the frequency of grain boundaries (GBs) with misorientation angles (θs)

Degradation of a metallic film under harsh thermal-mechanical-electrical coupling field conditions determines its service temperature and lifetime. In this work, the self-heating degradation behaviors of Pt thin films above 1000 °C were studied in situ by TEM at the nanoscale. The Pt films degraded mainly through void nucleation and growth on the Pt-SiNx interface. Voids preferentially formed at the grain boundary and triple junction intersections with the interface. At temperatures above 1040 °C, the voids nucleated at both the grain boundaries and inside the Pt grains. A stress simulation of the suspended membrane suggests the existence of local tensile stress in the Pt film, which promotes the nucleation of voids at the Pt-SiNx interface. The grain-boundary-dominated mass transportation renders the voids grow preferentially at GBs and triple junctions in a Pt film. Additionally, under the influence of an applied current, the voids that nucleated inside Pt grains grew to a large size and accelerated the degradation of the Pt film.

In this work, we used the selective laser melting (SLM) fabricated Co-Cr alloy with prominent residual strain, extremely non-equilibrium microstructures, and low stacking fault energy as a precursor to fabricate materials with the optimal grain boundary character distribution. The grain boundary engineering (GBE) of the Co-Cr alloy was achieved by a simple heat treatment of the SLM-fabricated Co-Cr alloy. The obtained GBE Co-Cr alloy exhibited 81.47% of special grain boundaries (Σ3n n = 1, 2, 3)), while it substantially disrupted the connectivity of the random high-angle boundaries, successfully reducing the propensity of intergranular degradation. Slow strain rate tests (SSRTs) showed that the GBE Co-Cr alloy possessed lower stress corrosion cracking (SCC) susceptibility and higher ductility in the corrosive environment (0.9% NaCl solution) than in the air. The high fraction of special boundaries, coupled with the stress-induced martensitic transformation (SIMT) in the GBE Co-Cr alloy yielded these results, which unique and rarely simultaneously satisfied for common structural materials. The current “SLM induced GBE strategy” offers a novel approach towards customized GBE materials with high SCC resistance and ductility in the corrosive environment, shedding new light on developing high-performance structural materials.