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Ferromagnetic shape memory alloys attracted increasing interest as multicaloric materials for solid state refrigeration. The most effective field coupling is achieved through the combination of the magnetic and mechanical induction of thermoelastic martensitic transformation. In the present work, we present an experimental investigation on NiMnGaCu polycrystalline cast alloy, by means of an experimental setup for compression tests in isothermal conditions under a longitudinal magnetic field. The setup has been ad hoc designed and developed specifically for this purpose. In this way, we can measure the elastocaloric and magnetocaloric effect at the same time. Moreover, the evolution of the entropy changes ΔS and the functional caloric parameters vs the magnetic field is evaluated. The application of magnetic field seems to act like a supplementary mechanical longitudinal stress. These preliminary results are fundamental to achieve a comprehensive understanding and modeling of coupled multicaloric phenomena. [ABSTRACT FROM AUTHOR]

In practical applications, TiNi shape memory alloys (SMAs) exhibit behavior that can pose a challenge with current constitutive models and their implementations in finite element method (FEM) software. TiNi SMA devices typically operate in the forward or reverse martensitic transformation regime, which is known as subloop loading. During such cyclic loading–unloading, the hysteresis stress–strain loop changes because of material damage, which can be considered the fatigue of TiNi SMAs. During both the loading and unloading processes, the stress plateau decreases. At the same time, the accumulated (residual) martensitic transformation strain increases. In this study, the experimental investigation results and observations of the aforementioned phenomena are presented. Next, the phase-field damage model is employed, along with a modified Lagoudas constitutive model, to simulate the change in stress–strain hysteresis. Furthermore, a fatigue function is used to simulate the accumulation of martensitic transformation strain. The experimental stress–strain response is compared with the simulation results, and good quantitative and qualitative agreement is obtained. The damage and martensitic volume fraction with respect to strain are discussed for full-loop and subloop loading. The observations and conclusions, as well as open questions, are presented. Possible directions for future research are provided. [ABSTRACT FROM AUTHOR]

The occurrence of adiabatic shear bands, as an instability phenomenon, is viewed as a precursor to failure caused by instability at high strain rates. Metastable β titanium alloys are extensively utilized due to their excellent mechanical properties, which are often subjected to high strain rate loads in service conditions. Understanding and studying their adiabatic shear instability behavior is thus crucial for preventing catastrophic failure and enhancing material performance. In this study via detailed microstructural analyses in the adiabatic shear region of a Ti-10V-2Fe-3Al alloy subjected to high strain rates, it was observed that α″ martensitic transformation and nano-twinning plus β-to-α phase transformation with α″ martensite as an intermediate phase occurred, in addition to substantial fine grains. The grain refinement mechanisms were mainly related to dynamic recovery dominated by dislocation migration alongside severe plastic deformation. [ABSTRACT FROM AUTHOR]

This study reveals the impact of the formation mechanism of a two-phase (β + γ) structure during heat treatment on thermoelastic L21(B2)-10M/14M-L10 martensitic transformations and elastocaloric parameters of polycrystalline Ni54Fe19Ga27 alloy. It is experimentally shown that annealing of the as-cast Ni54Fe19Ga27 alloy in the temperature range 1173–1463 K for 0.5 h followed by water quenching leads to the precipitation of the γ phase at grain boundaries and inside grains. As the annealing temperature increases from 1173 to 1463 K, the thickness of the γ-phase layer at the grain boundaries doubles, particles inside the grains coarsen, and their volume distribution becomes nonuniform. Simultaneously, the martensitic transformation temperatures increase by 31–69 K. The nonuniform distribution of the γ-phase particles and the morphological features of martensite (refinement of its twinned structure) lead to a 5–6-fold widening of the martensitic transformation intervals in crystals annealed at 1448 K compared to the as-cast alloy. After cyclic superelastic tests with 20 to 100 loading/unloading cycles, two-phase (β + γ) polycrystals demonstrate the stable adiabatic cooling temperature ∆Tad (2.7–3.0 K) and do not crack along grain boundaries, unlike those in the as-cast state. Significant fatigue strength and a high coefficient of performance (up to 18.3) make (β + γ) Ni54Fe19Ga27 polycrystals promising for practical use in solid-state cooling. [ABSTRACT FROM AUTHOR]

In the present study, the capacity of two commercial NiTi and NiTiCu shape memory alloy (SMA) wires to develop work-generating (WG) and constrained-recovery (CR) shape memory effects (SMEs), as well as the capacity of a commercial NiTiFe super-elastic wire to act as cold-shape restoring element, have been investigated. Using differential scanning calorimetry (DSC), the reversible martensitic transformation to austenite of the three NiTi-based wires under study was emphasized by means of an endothermic minimum of the heat flow variation with temperature. NiTi and NiTiCu wire fragments were further tested for both WG-SME and CR-SME developed during the heating, from room temperature (RT) to different maximum temperatures selected from the DSC thermograms. The former tests revealed the capacity to repetitively lift various loads during repetitive heating, while the latter tests disclosed the repetitive development of shrinkage stresses during the repetitive heating of elongated wires. The tensile behavior of the three NiTi-based SMA wires was analyzed by failure and loading–unloading tests. The study disclosed the actuation capacity of NiTi and NiTiCu shape memory wires, which were able to develop work while being heated, as well as the resetting capacity of NiTiFe super-elastic wires, which can restore the initial undeformed shape of shape memory wires which soften while being cooled down. These features enable the design of a robotic gripper based on the development of NiTi-based actuators with repetitive action. [ABSTRACT FROM AUTHOR]

Al2O3 and Ga2O3 exhibit numerous crystal phases with distinct stabilities and material properties. However, the phase transitions among those materials are typically undesirable in industrial applications, making it imperative to elucidate the transition mechanisms between these phases. The configurational similarities between Al2O3 and Ga2O3 allow for the replication of phase transition pathways between these materials. In this study, we investigate the potential phase transition pathway of alumina from the θ-phase to the α-phase using stochastic surface walking global optimization based on global neural network potentials, while extending an existing Ga2O3 phase transition path. Through this exploration, we identify a novel single-atom migration pseudomartensitic mechanism, which combines martensitic transformation with single-atom diffusion. This discovery offers valuable insights for experimental endeavors aimed at stabilizing alumina in transitional phases. [ABSTRACT FROM AUTHOR]

The impact of Co-addition (x = 0, 2, 4, and 6 at. %) in the as-cast and annealed Ni50Mn37.5Sn12.5 Heusler alloy at 900 °C for 24 h on the microstructure, magnetic properties, and the martensitic transition was studied using X-ray diffraction (XRD), scanning electron microscopy, vibrating sample magnetometry, and differential scanning calorimetry. The crystal structure of as-cast samples consists of a 14M modulated martensite structure, a face-centered (FCC) γ phase, and a face-centered tetragonal (FCT) MnNi-type phase L10. The as-cast samples show a dendritic microstructure with different contrasts and non-uniform distribution. The annealed samples exhibit dual 14M and γ phases for the Co0 and Co2, but 14M + γ + MnNi for the Co4 and Co6. The appearance of the martensitic transformation in the annealed Co0 and Co2 samples can be due to the disappearance of the dendritic microstructure. The characteristic temperatures (martensite start, Ms; martensite finish, Mf; austenite start, As; and austenite finish, Af) decrease with Co addition. A ferromagnetic-like order exists at a lower temperature of 1.8 K for the as-cast and annealed samples and decreases at 300 K. The annealing increases the fraction of the AFM contributions at 300 K. The exchange bias values of the Co0, An-Co2, and An-Co6 are 146.7 Oe, 24 O2, and 32.6 Oe, respectively, at 300 K. [ABSTRACT FROM AUTHOR]

The subject of the research is the possibility of using laser surface hardening to modify the tribological properties of ADI castings. ADI cast irons are a very progressive material; these cast irons find their application (due to their characteristic properties) in demanding applications. In these applications, the tribology of their surface is an essential parameter. This research focused on the change in tribological properties due to laser hardening of the surface layers. ADI cast iron samples processed at different isothermal holding temperatures were selected for the experiment. This is because these temperatures have a major influence on the initial structure (before laser exposure), which also affects the laser hardening process. To analyze the structural changes, metallographic examinations were performed. The microhardness was also measured on each sample in relation to the distance from the surface. The samples were also subjected to a tribological test (linear reciprocating tribometer) during which the coefficient of friction was recorded. The surface conditions after the tribological experiment were evaluated using an electron microscope. Tribological experiments revealed very different behavior of laser-hardened ADI cast iron surfaces depending on their isothermal holding temperature. At the same time, a homogeneous martensitic layer on the surface (a consequence of the relatively uniform distribution of carbon in the initial ausferritic structure) was achieved by laser, which contributed to a significant increase in hardness and wear resistance. A trend of decreasing friction coefficient as a function of isothermal holding temperature was observed for non-laser-hardened surfaces. This phenomenon supports the theory of a possible martensitic transformation of the high-carbon austenite contained in the structure of ADI castings. [ABSTRACT FROM AUTHOR]

In the present study, effect of ultrasonic impact treatment (UIT) on the microstructural characterization and mechanical properties of 316L stainless steel (hereinafter referred to as 316L) was investigated experimentally. The fatigue fracture mechanism of 316L before and after UIT was revealed. The experimental results indicated that the martensitic grain size induced at the impact edge was about 2.00 Å. The surface modified 316L formed a gradient nanostructure and induced a martensitic phase transformation. The hardness of the surface layer of the modified 316L was twice the hardness of its matrix. The tensile strengths of 316L before and after UIT were 576 MPa and 703 MPa, respectively. The stretching stripes of 316L were more disordered after UIT. The fatigue strengths of 316L before and after UIT were 267 MPa and 327 MPa, respectively. The fatigue cracking of 316L started from the austenite grain boundaries. The fatigue fracture surface was relatively rough. The fatigue crack sources of the modified 316L came from internal inclusions. The inclusions were oxides dominated by SiO2. As the stress range increased, the crack initiation site migrated to the interior and the fatigue fracture surface became flatter. [ABSTRACT FROM AUTHOR]

In this study, we conducted in-situ tensile deformation tests using synchrotron X-ray and neutron diffraction techniques on Nb-bearing AISI 201 austenitic stainless steel specimens with varying grain sizes, including ultrafine (UFG, 0.28 μm), fine (FG, 1.75 μm), and coarse (CG, 110 μm) grains. The primary objective was to investigate the effect of grain size on the evolution of lattice strains and deformation mechanisms. Our findings reveal that grain size significantly impacts the evolution of lattice strains, peak broadening, and the distribution of load between the austenite and martensite phases. Notably, the average lattice strain and peak broadening were found to be higher in the CG specimen compared to the UFG and FG counterparts. While the lattice strains in the UFG and FG steels are all compressive, a tensile manner can be seen for the CG steel as deformation proceeds. The strain-induced martensite transformation kinetics for the reversion-annealed UFG and FG steels was higher than that of the solution-annealed CG one. A soft behavior from reflection (200) was found compared to other reflections in all UFG, FG, and CG specimens, showing anisotropic elastic properties. The analysis of lattice strain and peak broadening evolutions indicated that dislocation slip predominantly governs the deformation mechanism in the CG steel, while strain-induced martensitic transformation and twinning are more prominent in the UFG and FG specimens. [ABSTRACT FROM AUTHOR]

Incorporating various elements into host shape memory alloys (SMAs) has proven to be an effective strategy for optimizing their functional properties. However, modeling the complex multi-doping effect is challenging. In the present study, we introduced a phenomenological model based on Ginzburg–Landau theory, wherein each doping element is conceptualized as an internal dilatational stress. This internal stress is represented as a spatial Gaussian distribution characterized by two influential parameters: potency (h) and range (σ). The interaction between doping elements arises from the superposition of these stresses. Utilizing a time-dependent Ginzburg–Landau simulation based on our proposed model, diverse combinations of h and σ replicate the varied experimental outcomes associated with multi-doping effects. This model offers insight into the understanding of the doping impact on martensitic transformation and may contribute to the development of SMAs with tailored properties. [ABSTRACT FROM AUTHOR]

Elasticity, featured by a recoverable strain, refers to the ability that materials can return to their original shapes after deformation. Typically, the elastic strains of most metals are well‐known 0.2%. In shape memory alloys and high entropy alloys, the elastic strains can be several percent, as called superelasticity, which are all triggered by external stresses. A superelasticity induced by magnetic field, termed as magneto‐superelasticity, is extremely important for contactless work of materials and for developing brand‐new large stroke actuators and high efficiency energy transducers. In magnetic shape memory alloys, the twin boundary motion driven by magnetic field can output a strain of several percent. However, this strain is unrecoverable when removing the magnetic field and hence it is not magneto‐superelasticity. Here, a giant magneto‐superelasticity of 5% in a Ni34Co8Cu8Mn36Ga14 single crystal is reported by introducing arrays of ordered dislocations to form preferentially oriented martensitic variants during the magnetically induced reverse martensitic transformation. This work provides an opportunity to achieve high performance in functional materials by defect engineering. [ABSTRACT FROM AUTHOR]

The second formation stage of a surface martensite (SM) crystal is associated with a layer-by-layer extension of its faces. A dynamic pattern of every layer formation is similar to a rapid crystal growth in the first stage. The role of dislocation nucleation centers belongs to the dislocations framing the crystal-face surfaces. By the example of the (10-1) face it is shown that the proposed dynamic mechanism is compatible with the observed polar growth of the SM crystal trace on the (100) surface of the sample and the range of its visually observed growth. [ABSTRACT FROM AUTHOR]

One of the most important challenges in endodontics is to have files that have excellent flexibility, toughness, and high fatigue life. Superelastic NiTi alloys have been a breakthrough and the new R-phase NiTi alloys promise to further optimize the good properties of NiTi alloys. In this work, two austenitic phase endodontic files with superelastic properties (Protaper and F6) and two austenitic phase files with the R-phase (M-wire and Reciproc) have been studied. The transformation temperatures were studied by calorimetry. Molds reproducing root canals at different angles (30, 45, and 70°) were obtained with cooling and loads simulating those used in the clinic. Mechanical cycles of different files were realized to fracture. Transformation temperatures were determined at different number of cycles. The different files were heat treated at 300 and 500 °C as the aging process, and the transformation temperatures were also determined. Scanning and transmission electron microscopy was used to observe the fractography and precipitates of the files. The results show that files with the R-phase have higher fracture cycles than files with only the austenitic phase. The fracture cycles depend on the angle of insertion in the root canal, with the angle of 70° being the one with the lowest fracture cycles in all cases. The R-Phase transformation increases the energy absorbed by the NiTi to produce the austenitic to R-phase and to produce the martensitic transformation causing the increase in the fracture cycles. Mechanical cycling leads to significant increases in the transformation temperatures Ms and Af as well as Rs and Rf. No changes in the transformation temperatures were observed for aging at 300 °C, but the appearance of Ni4Ti3 precipitates was observed in the aging treatments to the Nickel-rich files that correspond to those with the R transition. These results should be considered by endodontists to optimize the type of files for clinical therapy. [ABSTRACT FROM AUTHOR]

In this article, an attempt was made to join DP600 steel and Ti6Al4V titanium alloy sheets by resistance spot-welding (RSW) using an interlayer in the form of Cu and Au layers fabricated through the cold-spraying process. The welded joints obtained by RSW without an interlayer were also considered. The influence of Cu and Au as an interlayer on the resulting microstructure as well as mechanical properties (shear force and microhardness) of the joints were determined. A typical type of failure of Ti6Al4V/DP600 joints produced without the use of an interlayer is brittle fracture. The microstructure of the resulting joint consisted mainly of the intermetallic phases FeTi and Fe2Ti. The microstructure of the Ti6Al4V/Au/DP600 joint contained the intermetallic phases Ti3Au, TiAu, and TiAu4. The intermetallic phases TiCu and FeCu were found in the microstructure of the Ti6Al4V/Cu/DP600 joint. The maximum tensile/shear stress was 109.46 MPa, which is more than three times higher than for a welded joint fabricated without the use of Cu or Au interlayers. It has been observed that some alloying elements, such as Fe, can lower the martensitic transformation temperature, and some, such as Au, can increase the martensitic transformation temperature. [ABSTRACT FROM AUTHOR]

The mechanical properties of ausferritic ductile iron are strongly influenced by the ability of the austenite to undergo martensite formation. Strain-induced martensite formation occurring under the right circumstances results in transformation induced plasticity (TRIP) that improves ductility and strength. TRIP has been shown to depend on the austenite's stacking fault energy (SFE), which describes the crystals micromechanical behaviour. In austenitic FeMnAlSi-TRIP steels, TRIP as a reaction to mechanical load only occurs for SFE <20 mJ/m2. For carbon-stabilised austenite as in ADI, the relationship between stacking faults, mechanical properties and martensitic transformation has not yet been established. To investigate the TRIP-effect in ADI, unalloyed ADI with 3.43 wt% C, 2.52 wt% Si and 0.21 wt% Mn was ausferritised and subjected to tensile tests at temperatures between −180 °C and 200 °C. The amount of martensite produced by thermal and mechanical activation, crystalline microstrain and stacking fault density were measured on deformed and undeformed regions of the specimen by XRD and the resulting SFE calculated. Between −70 and 20 °C, the elongation at fracture exceeded 10 % compared to below 2 % in the temperature range of −130 to −180 °C. At temperatures above 80 °C, elongation at fracture gradually decreased to 4.5 % at 200 °C. High sample ductility was associated with lower SFEs as low as 35 mJ/m2, indicating a correlation of stacking fault formation and strain-induced martensite formation in ADI. [ABSTRACT FROM AUTHOR]

Iron-based shape memory alloy (Fe-SMA) is an emerging class of material having a great potential for civil engineering, especially seismic engineering applications. However, the behavior of Fe-SMA strongly depends on temperature-sensitive martensitic transformation, and thus the influence of temperature on the behavior of Fe-SMA should receive particular attention. The current study experimentally investigates how temperature variation affects the mechanical properties, including monotonic tensile strength, low cycle fatigue (LCF), and impact toughness, of Fe-SMA (Fe-17Mn-5Si-10Cr-5Ni in wt%), with a particular focus on its low-temperature behavior. Specifically, the considered temperatures used for conducting monotonic tensile and LCF tests were −40°C , −10°C , and 20°C, whereas the range of temperature for impact toughness tests was from −100°C to 20°C. Additional tests were performed on S30408 stainless steel and conventional Q355 structural steel for comparison purposes. The experimental results suggest that low temperature leads to increased strength and reduced ductility of Fe-SMA. The impact toughness of Fe-SMA is comparable with that of S30408 steel, exhibiting ductile fracture characteristic observed even at −100°C. Fe-SMA outperforms conventional steels with an evidently greater LCF life given the considered temperature variation; although, its LCF life decreases at lower temperatures due to the inhibited reversible martensite transformation. This study also provides a set of practical parameters that facilitate constitutive modeling and strain-based fatigue life prediction of Fe-SMA under various temperatures. [ABSTRACT FROM AUTHOR]

This study systematically investigates the microstructure, martensitic phase transformation, crystal structure, and mechanical properties of (Ni43Mn47Sn9Gd1)100−xBx (x = 0, 0.8, 1.5 and 3 at%) shape memory alloys. Experimental results reveal that these alloys consist of a matrix phase and precipitated phases. The introduction of Gd elements leads to the formation of milky-white particles dispersed along grain boundaries, with the composition identified as GdNiSn. When the B element content reaches 1.5 at%, bright-white particles form and are uniformly distributed within the matrix. Their concentration increases with higher levels of B doping, and they are characterized as Mn2B. Simultaneously, the initially present Gd-rich milky-white particles distributed along grain boundaries, exhibit a diminishing trend with increasing B doping. B doping elevates the alloy's phase transition temperature, and the compressive strength of the alloy approximately follows a linear trend with increasing B content. At a B doping level of 3%, the annealed alloy demonstrates a compressive strength of up to 1313 MPa with a compressive fracture strain of 11.6%, marking a 110% improvement. For the as-cast alloy, a compressive strength of 1652 MPa is achieved, accompanied by a compressive fracture strain of 12.3%, representing a 130% enhancement. Transmission electron microscopy reveals pronounced twinning features on the alloy surface, resulting in the formation of numerous fine lines in the as-cast state, that are magnified into voids after heat treatment. This phenomenon is detrimental to the alloy's mechanical performance; hence, the as-cast compressive strength is favored over the annealed state. [ABSTRACT FROM AUTHOR]

Nickel-Titanium alloys or Nitinol shape memory alloys have applications in various fields like biomedical, agriculture, pharmaceutical, civil, and mechanical engineering. Ferrous-based shape memory alloys (Fe-SMAs) do not possess excellent properties like that of Nitinol. In recent times, Fe-SMAs have been under consideration in the research field to enhance their shape memory properties because of their cost-effectiveness, corrosion resistance, biocompatibility, and high mechanical strength. Fe-SMAs are limited in their applications in the human body due to high austenite start temperature (As ) even though they are biocompatible and being considered for temporary biodegradable implant applications. Several factors affect the phase transformation temperature of Fe-SMA, like alloy composition and heat treatment conditions. Some of the techniques that are useful to reduce the As are quenching, solid solution treatment, ternary element addition, alloying, and thermal-mechanical treatment. In this study, the quenching with brine solution is used to reduce As of Fe-15Mn-10 Cr8Ni-4Si (wt. %). The phase transformation is observed using Differential Scanning Calorimetry (DSC). The best result from experimentation of all the samples collected as As reduced from 93oC to 39.02oC which is very close to the temperature of the human body. The results of this study exhibit significant potential for advancement in the application of Fe-SMAs in the biomedical field for permanent implants in cardiovascular and orthopaedic applications. [ABSTRACT FROM AUTHOR]

Investigation on the effects of Zr and quenching and tempering processes on the microstructure and properties of high-strength Ti-Mo microalloyed steel using SEM, TEM, mechanical property testing, and First-principles calculations. The results show that within the tempering temperature range of 550–650 °C, the microstructures of Ti-Mo microalloyed steel and Ti-Zr-Mo microalloyed steel consist of ultra-low carbon martensite and carbides. Specifically, the strength of Ti-Mo microalloyed steel initially increases and then decreases with the increase in tempering temperature, while the strength of Ti-Zr-Mo microalloyed steel increases with the increase in tempering temperature. During tempering, the addition of Zr element affects the diffusion of C element, delaying the martensitic tempering transformation process, and slowing down the nucleation, growth, and coarsening processes of alloy carbides. First-principles calculation results indicate that ZrC preferentially forms during tempering, resulting in a certain sequential gradient of precipitated microalloy carbides, thereby enhancing the tempering stability of microalloyed steel. Compared to Ti-Mo steel, Ti-Zr-Mo steel exhibits narrower lath martensite width and finer dispersed precipitation phases after tempering at 600 °C and 650 °C. [ABSTRACT FROM AUTHOR]

In materials that undergo martensitic phase transformation, macroscopic loading often leads to the creation and/or rearrangement of elastic domains. This paper considers an example involving a single-crystal slab made from two martensite variants. When the slab is made to bend, the two variants form a characteristic microstructure that we like to call "twinning with variable volume fraction." Two 1996 papers by Chopra et al. explored this example using bars made from InTl, providing considerable detail about the microstructures they observed. Here we offer an energy-minimization-based model that is motivated by their account. It uses geometrically linear elasticity, and treats the phase boundaries as sharp interfaces. For simplicity, rather than model the experimental forces and boundary conditions exactly, we consider certain Dirichlet or Neumann boundary conditions whose effect is to require bending. This leads to certain nonlinear (and nonconvex) variational problems that represent the minimization of elastic plus surface energy (and the work done by the load, in the case of a Neumann boundary condition). Our results identify how the minimum value of each variational problem scales with respect to the surface energy density. The results are established by proving upper and lower bounds that scale the same way. The upper bounds are ansatz-based, providing full details about some (nearly) optimal microstructures. The lower bounds are ansatz-free, so they explain why no other arrangement of the two phases could be significantly better. [ABSTRACT FROM AUTHOR]

A Ti–25Nb shape memory alloy (SMA) exhibits shape memory effect associated with stress-induced martensitic transformation from β to α″ phase. Addition of oxygen stabilizes the β phase and changes stress–strain response. Oxygen-added Ti–25Nb SMAs show a more distinct superelastic behavior. In this work, digital image correlation (DIC) was applied to investigate for the first time full-field deformation of Ti–25Nb, Ti–25Nb–0.3O and Ti–25Nb–0.7O (at. pct) SMAs. The specimens were subjected to loading–unloading tensile tests to study local and global mechanical characteristics related to activity of particular deformation mechanisms of the SMAs. Strain and strain rate fields were quantitatively compared at selected stages of each SMA's deformation. It was found that the Ti–25Nb SMA exhibits a macroscopically localized Lüders-type deformation associated with the stress-induced phase transformation, whereas Ti–25Nb–0.3O and Ti–25Nb–0.7O SMAs show more discrete types of deformation related to activity of interstitial oxygen atoms. As a consequence, at particular stages of deformation, local values of strain rate of Ti–25Nb SMA were significantly higher than those of average strain rate. The results obtained in this paper provide a better understanding of the deformation mechanism in the oxygen-added Ti–25Nb based SMAs. [ABSTRACT FROM AUTHOR]

Shape memory ceramics (SMCs) are regarded as stimulus-responsive intelligent materials, capable of a martensitic phase transformation under the stimuli of temperature and/or stress fields. Research efforts spanning more than 40 years since the 1970 s, have endowed these ceramics with tunable transformation properties, whilst the strain recovery and reversibility are hindered by their significant brittleness. This review first provides a brief introduction and the fundamentals on SMCs. General principles are then proposed to tackle the outstanding issue of brittleness by regulating the transformation-induced lattice strain. Specifically, the intrinsic approaches accommodate the transformation-induced lattice strain by lattice engineering or compositing, and extrinsic approaches release the transformation-induced lattice strain by materials miniaturization and architecting. Following the general principles, the above four state-of-the-art design strategies and associated implementations are critically reviewed. Towards deliberate designability and applications in sensing, actuation, and energy dissipation, this review points out the challenges and future directions in developing new-generation SMCs. [ABSTRACT FROM AUTHOR]

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Extremely low cycle fatigue tests, up to a total axial strain amplitude of 10%, were conducted on Fe-15Mn-10Cr-8Ni-4Si bidirectional transformation-induced plasticity (B-TRIP) steel. The fatigue life was approximately five times longer than that of SUS316 when the total strain amplitude was 4% or higher. The improved fatigue life of Fe-15Mn-10Cr-8Ni-4Si was attributed to reversible bidirectional γ ↔ ε transformation during fatigue deformation, which might mitigate fatigue damage. In contrast, the fatigue life tended to decrease with increasing strain rate when the strain rate was varied from 0.1 to 2.5%/s with a total strain amplitude of 10%. Fractography revealed that the fracture surface varied significantly with strain rate. At low strain rates, crystallographic fracture surfaces characterized by facets and secondary cracks were observed, whereas these features were not observed at high strain rates. Electron backscatter diffraction measurements of the postmortem microstructure showed that frequent ε-martensite formation occurred at low strain rates, whereas martensitic transformation was suppressed at high strain rates. The change in the specimen surface temperature was evaluated in terms of the Gibbs free energy difference between γ-austenite and ε-martensite (i.e.,ΔGγ↔ε), and the effect of strain rate on the extremely low cycle fatigue was discussed from the viewpoint of the deformation mechanism. At a low strain rate, the condition for B-TRIP to work effectively, that is, ΔGγ↔ε is negative but close to zero, was maintained over the entire life span. At a high strain rate, the deformation mechanism changed to one in which γ-austenite was dominant because of the increase in ΔGγ↔ε caused by self-heating; the fatigue damage mitigation mechanism provided by B-TRIP was less likely to occur at high strain rates, which reduced the fatigue life. [ABSTRACT FROM AUTHOR]

Methods of transmission and scanning electron microscopy are used to study premartenstic states and their relation to martensitic transformations in the alloys Cu–38 wt % Zn and Cu–39.5 wt % Zn with shape memory effect. Analysis of the observed diffusion scattering of electrons is carried out, including in situ experiments at heating and cooling and the defect condition of the internal substructure of austenite and martensite. The crystallographic models of martensitic transitions β2 → , β2 → , and β2 → are proposed based on the crystallographic data obtained in the premartensitic state. [ABSTRACT FROM AUTHOR]

A systematic study was conducted on the influence of silicon on the microstructure, stress distribution, and martensitic nucleation and transformation of 301 metastable austenitic stainless steel during cold-rolling deformation. When the deformation amount of conventional 301 stainless steel is ≤20%, the amount of martensite transformation is very small. When the deformation amount is ≥30%, the amount of martensite transformation significantly increases. The introduction of Si significantly improves the amount of martensite transformation and the uniformity of deformation. 301Si-H has a significantly higher amount of martensite in the same deformation microstructure than conventional 301Si-L with a lower silicon content. Increasing the Si content decreases the stacking fault energy of 301 stainless steel. During deformation, Si tends to cluster at the grain boundaries, reducing stacking fault width and increasing dislocation density, creating sites for shear martensite nucleation at the grain boundaries. Simultaneously, significant deformation encourages the formation of deformation twins and facilitates martensite nucleation. [ABSTRACT FROM AUTHOR]

S32750 dual-phase stainless steel (DSS) wires were prepared by cold drawing with a strain of ε = 0~3.6. The mechanical behavior and microstructural evolution of these DSS wires at different strains were investigated. Specifically, the yield strength and ultimate tensile strength of a S32750 DSS wire at a strain of ε = 3.6 reached 1771 MPa and 1952 MPa, respectively. The microstructure of the wire was transformed into a heterogeneous microstructure, which consisted of ferrite fiber grains and a nanofibrous grain structure consisting of austenite and strain-induced martensite nanofiber grains. A sub-grain structure was observed inside the ferrite fiber. The austenitic phase followed the evolutionary steps of stacking faults, twinning, ε-martensite, α-martensite, and, finally, austenite, before transitioning into a nanofibrous grain structure. This nanofibrous grain structure significantly contributed to the strength compared with the relatively coarse ferrite phase. [ABSTRACT FROM AUTHOR]

The present study investigated the microstructure, phase transformation behavior, and functional characteristics of the multicomponent Ti–Ni–Cu–Al–V shape memory alloys with the different annealing treatments. The results indicated that the multicomponent Ti–Ni–Cu–Al–V alloy annealed at 673 K/5 min was primarily composed of the B2 parent phase and Ti2(Ni,Cu) type precipitates distributing along the grain boundaries. As the annealing temperature increased and the annealing time extended, the chemical composition of the matrix changed slightly due to the precipitation of the Ti2(Ni,Cu) phase. Consequently, the martensitic phase (B19) gradually appeared and the volume friction of the martensite phase gradually increased. The phase constituents of the present Ti–Ni–Cu–Al–V shape memory alloy evolved from a B2 austenite phase to a B19 martensite phase with the annealing temperature/time increasing. Additionally, as the annealing temperature and time increased, the grain size also increased. The increment in annealing temperature and the prolongation of annealing time resulted in an increase of martensitic transformation temperatures as a result of the comprehensive effect of chemical composition, grain size, defects' density, etc. Both yield strength and fracture strength decreased, while the elongation significantly increased (reaching 28% at 1123 K/60 min) with the annealing temperature rising and annealing time prolonging. Under the successively applied prestrain to 8% condition, the recoverable strain decreased from 4.2% to 1.7% for the annealed Ti–Ni–Cu–Al–V shape memory alloy with the annealing temperature/time increasing. [ABSTRACT FROM AUTHOR]

The quenching and partitioning (Q&P) process has been shown to be quite promising in the development of a third generation of Advanced High-Strength Steels. As in other metallurgical processes, the austenitic grain size (AGS) is crucial to obtain a microstructure that provides a great mechanical behavior. Hence, this work evaluated the effect of the austenitizing temperature on the AGS and, consequently, on the constituents formed under continuous cooling at different rates. Then, a Q&P modeling for a commercial CMnSi steel, predicting the microstructural evolution and the final phase fractions as a function of the AGS and other parameters, was performed. The AGS increased exponentially with the austenitizing temperature, in the way that the smaller the AGS the finer the martensitic structure formed after cooling. The decrease in the continuous cooling rate leaded to the formation of diffusional constituents prior to the martensitic transformation, which promoted the increase in Ms temperature for larger AGS. In conclusion it is possible to state that an austenitizing condition that provides the smallest possible AGS, followed by an optimized Q&P cycle, has the potential to generate an advanced steel with a final microstructure assisted by the TRIP effect due to the retained austenite content. [ABSTRACT FROM AUTHOR]

The mechanical characteristics, phase composition, residual macroscopic stresses, and surface topography of Co-Cr-Mo-W alloy fabricated using the additive (selective laser melting of powder--SLM) and casting (CT) technologies and modified by ultrasonic impact treatment (UIT) are investigated. A single-contact normal impact-loading mode of UIT is employed. As demonstrated, the macrodefects, including defects of incomplete melting, pores, significant surface roughness, and high levels of residual tensile stresses, which are inherent consequences of 3D printing, are effectively eliminated by short-term UIT in an inert environment. A correlation between the mechanical properties of modified surface layers of the SLM and CT samples and their structural--phase state after UIT exposure for 50 s is established. The strengthening effect of the surface layer of the additively manufactured Co-Cr-Mo-W alloy (by 2 times) is due to the formation of compressive stresses of the first kind (-600 MPa) and martensitic transformation. The Co-Cr-Mo-W alloy obtained by casting technology has lower hardness both in the initial state and after UIT. The strengthening effect does not exceed 1.7 times and is achieved due to a higher level of compressive stresses (-900 MPa) and carbide component refinement. [ABSTRACT FROM AUTHOR]

Excellent strength–ductility balance in metastable Fe–Cr–Ni austenitic alloys stems from phase transformation from austenite (fcc structure) to αʹ martensite (bcc structure) during deformation, namely deformation-induced αʹ martensitic transformation (DIMT). Here, DIMT in a metastable Fe–17Cr–7Ni austenitic alloy was detected in situ and characterized in three dimensions (3D) by employing synchrotron radiation X-ray microtomography. This technique utilizes refraction contrast, which is attributable to the presence of phase boundaries between the parent austenite and the newly formed αʹ martensite phase. By combining microtomography and position-sensitive X-ray diffraction, we succeeded in crystallographically identifying multiple αʹ martensite phases continuously transformed in four groups from a single parent austenitic phase. [ABSTRACT FROM AUTHOR]

In this study, we explore the evolution of martensitic structures during deformation and their correlation with solution temperature in a metastable β-type Ti–10V–2Fe–3Al alloy. After a solution treatment at 760 °C, microstructures consist of equiaxed β grains and primary α particles. Increasing the solution temperature to 780 °C minimally affects equiaxed β grain size (~4.1–4.7 μm) but significantly reduces the α phase volume fraction. The α particles become scarce and completely absent following solution treatments at 800 and 820 °C, where the β grain size grows to ~109.1 and ~151.3 μm, respectively. Thermally-induced martensite (TIM) α″ laths are formed in specimens solution-treated at 770 °C or higher, predominantly populating the β grains of the specific specimens solution-treated at 800 and 820 °C. All solution-treated specimens exhibit a double yielding phenomenon. For specimens solution-treated at 760–780 °C, this results primarily from the stress-induced martensitic transformation (SIMT) process, while for those solution-treated at 800 and 820 °C, it mainly arises from the martensite reorientation (MR) process. The triggering stress for the SIMT/MR process increases with increasing solution temperature. In early deformation stages, TIM α″ laths initially undergo the MR process, transforming into a favored α″ variant, and the SIMT process can occur concurrently. As deformation progresses, the α″ phase changes orientation continuously through deformation twinning, transforming into the favored α″ variant dictated by the evolving stress conditions. This study provides insights into martensitic structure evolution and its influence on mechanical properties in metastable β titanium alloys. [ABSTRACT FROM AUTHOR]

Laser surface texturing is a promising technique for improving the surface properties of metals. In this study, a nanosecond pulsed laser was used to perform surface texturing on a metastable austenitic stainless steel AISI 301LN. The effects of the number of laser passes (from 1 to 1000 passes) on the surface morphology, microstructure, and mechanical properties were investigated. The results show that the depth of laser tracks increases with the number of passes due to thermal accumulation. Increasing the number of laser passes produces a rougher surface, but beyond a certain value, a smoother texture is observed. The amount of α'-martensite phase induced by the laser increases with the number of passes, producing augments in hardness. However, again, this trend changes after a certain number of laser passes, and hardening saturates and even decreases. Therefore, selecting the appropriate laser processing parameters and number of passes is essential to control the dimensions of laser tracks and thus the desired surface texture and also to achieve the level of hardening. [ABSTRACT FROM AUTHOR]

To explore a new method to improve the wear resistance of TiNi shape memory alloy (SMA), Ti-50.8Ni alloy was treated by the method of ultrasonic surface shot peening. The microstructure evolution, hardness, and tribological behaviors have been further investigated to evaluate the effect of ultrasonic surface shot peening (USSP). The surface microstructure can be refined to some extent while the basic phase composition has little change. USSP can facilitate the martensitic transformation in the surface layer, which benefits improving the surface hardness. Additionally, the hardness of Ti-50.8Ni alloy increases first and then decreases with the increase of applied load, but the USSP-treated alloy tends to be more sensitive to load. USSP treatment can improve the wear resistance and reduce the coefficient of friction (COF) in case of a low sliding wear speed of 5 mm/s. However, the tribological properties of USSP-treated alloy are reversely worse in the case of 10 mm/s. This is mainly attributed to the combined effect of stress-induced martensite transformation and degeneration resulting from the frictional heating during the dry sliding wear process. [ABSTRACT FROM AUTHOR]

In this work, the tensile deformation mechanisms of the Fe55Co17.5Cr12.5Ni10Mo5−xCx-based medium-entropy alloy at room temperature (R.T.), 77 K, and 4.2 K are studied. The formation of micro-defects and martensitic transformation to delay the cryogenic fracture are observed. The results show that FeCoCrNiMo5−xCx-based alloys exhibit outstanding mechanical properties under cryogenic conditions. Under an R.T. condition, the primary contributing mechanism of strain hardening is twinning-induced plasticity (TWIP), whereas at 77 K and 4.2 K, the activation of martensitic transformation-induced plasticity (TRIP) becomes the main strengthening mechanism during cryogenic tensile deformation. Additionally, the carbide precipitation along with increased dislocation density can significantly improve yield and tensile strength. Furthermore, the marked reduction in stacking fault energy (SFE) at cryogenic temperatures can promote mechanisms such as twinning and martensitic transformations, which are pivotal for enhancing ductility under extreme conditions. The Mo4C1 alloy obtains the optimal strength–ductility combination at cryogenic-to-room temperatures. The tensile strength and elongation of the Mo4C1 alloy are 776 MPa and 50.5% at R.T., 1418 MPa and 71.2% in liquid nitrogen 77 K, 1670 MPa and 80.0% in liquid helium 4.2 K, respectively. [ABSTRACT FROM AUTHOR]

Due to their high levels of formability and crashworthiness, the new quenching and partitioning (QP) steels are increasingly being used in car‐body structures. Using a unique experimental technique for tensile testing in a large strain‐rate range of 10−4–1000 s−1, an anomalous work‐hardening behavior is found in a QP1180 steel that may affect its formability and crashworthiness. At the lowest strain rate of 10−4 s−1, the work‐hardening rate θ decreases linearly with increasing flow stress. With an increasing strain rate, however, an increasingly severe nonlinearity in the work‐hardening rate is evident, as has been observed in metastable austenitic steels when the deformation‐induced martensitic transformation occurs at lower temperatures. This work‐hardening‐rate peak reaches its maximum at 10 s−1. Above this value, the peak decreases again, and at more than 500 s−1, it behaves like the quasi‐static test. The corresponding material characteristics, such as elongation and strength, change similarly. [ABSTRACT FROM AUTHOR]

We study the thermal properties of a bulk Ni 55 Fe 19 Ga 26 Heusler alloy in a conduction calorimeter. At slow heating and cooling rates ( ∼ 1 K h - 1 ), we compare as-cast and annealed samples. We report a smaller thermal hysteresis after the thermal treatment due to the stabilization of the 14 M modulated structure in the martensite phase. In ultraslow experiments ( 40 mK h - 1 ), we detect and analyze the calorimetric avalanches associated with the direct and reverse martensitic transformation from cubic to 14 M phase. This reveals a distribution of events characterized by a power law with exponential cutoff p (u) ∝ u - ε exp (- u / ξ) where ε ∼ 2 and damping energies ξ = 370 μ J (direct) and ξ = 27 μ J (reverse) that characterize the asymmetry of the transformation. [ABSTRACT FROM AUTHOR]

Achieving 3D-printed Ti6Al4V alloy with customized microstructures and mechanical characteristics remains challenging, wherein the processing efficiency mainly depends on the laser energy, mass deposition rate, and duration. Based on these factors, a simple and eco-friendly direct laser metal deposition approach was followed to get 3D-printed Ti6Al4V alloys at various laser powers (300–500 W). Herein, a 1.5-kW continuous fiber laser with a wavelength of 1080 nm was used to create a stable and dense alloy. The obtained 3D-printed specimens were characterized to assess the laser power–dependent microstructures, compositions, microhardness, grain sizes, color filling, and dimensional stability in terms of height/width. FESEM micrographs of the obtained alloys revealed the existence of porous spherical grains of mean size in the range of 50–81 μm. The alloy deposited at 300 W and 0.495 mm/s scan speed displayed the maximum hardness (excellent bong strength) value of 859.2 HV0.5 devoid of any crack and porosity. XRD patterns of the alloy revealed the existence of α + β martensitic phase transformation which is responsible for the marginal increase of hardness. It is asserted that the proposed 3D-printed Ti6Al4V alloy can be beneficial for the development of efficient structural parts desired for diverse applications. [ABSTRACT FROM AUTHOR]

Ion implantation is widely used for doping semiconductors or electroceramic materials and probing material behaviors in extreme radiation environments. However, implanted ions can induce compressive stresses into the host material, which can induce plasticity and mesoscopic deformation. However, these phenomena have almost exclusively been observed in brittle ionic and/or covalently bonded materials. Here, we present transmission electron microscopy observations of unusual implantation-induced plasticity in two metallic alloys. First, Fe2+ ions induce dislocation plasticity below the implanted layer in a model Fe-P alloy. Next, He+ ions form pressurized cavities which activate the fcc-to-hcp strain-induced martensitic transformation in Alloy 625. In both cases, the plasticity can be explained by a combination of implanted ions being incorporated into the lattice and the creation of irradiation defects. These findings have significant implications for mechanical testing of ion-implanted layers, while also opening pathways for using ion implantation to tune stress distributions in metallic alloys. [ABSTRACT FROM AUTHOR]

Multi-element alloys (e.g., non-equiatomic FeMnCoCr alloys) have attracted extensive attention from researchers due to the breaking of the strengthen-ductility tradeoff relationship. Plenty of work has been conducted to investigate the ingredient-dependent deformation mechanism in these alloys in experiments. However, the atomic simulations on such parameter-related mechanisms are greatly limited with the lack of the related interatomic potentials. In this work, two interatomic potentials are developed within the embedded atom method (EAM) framework for Fe80−xMnxCo10Cr10 high-entropy alloys. The tunability of the cohesive energy-related martensitic transformation (MT) mechanism was comprehensively investigated using molecular dynamics (MD) through a series of unilateral crack configurations with different twin boundary spacings (TBs). It is noted that the main deformation mechanism around the crack tip is transformed from a martensitic transformation to dislocation activities (dislocation or twin) with the variation of different cohesive energies between face-centered cubic (fcc) and hexagonal close-packed (hcp) phases. Additionally, the introduction of twin boundaries significantly enhances the strength and toughness of the alloys. The newly developed interatomic potentials are expected to provide theoretical support for the related simulations, focusing the martensitic transformation mechanism on high-entropy alloys. [ABSTRACT FROM AUTHOR]

The stress-induced martensitic transformations and superelasticity behavior in the NiTi alloy with a single crystal model and a twist grain boundary bicrystal model at different temperatures are studied using molecular dynamics simulations. An atomic tracing method is proposed to identify specific numbers of B19′ martensite variants. Under uniaxial compressive loading, the stress-induced martensitic transformation takes place accompanied by the formation of <011>M type II twins, and the deformation process can be divided into three distinct stages based on microstructure evolution and average atomic total energy. It is found that the twist grain boundary induces an increase in the martensite start temperature, which is consistent with the experimental results. There is no residual B19′ martensite at the end of the unloading process, and the irrecoverable strain mainly results from plastic deformation at the grain boundary through the analysis of atomic local shear strains and has hardly changed with increasing deformation temperature. Remarkably, the grain boundary brings about the acceleration of martensite nucleation and an earlier occurrence of stress plateau. Further simulation results manifest that the presence of the twist grain boundary leads to weakened temperature dependence of martensitic transformation stress and a reduction in the hysteresis loop area. [ABSTRACT FROM AUTHOR]

A multi-stimuli cooling cycle can be used to increase the cyclic caloric performance of multicaloric materials like Ni–Mn–In Heusler alloys. However, the use of uniaxial compressive stress as an additional external stimulus to a magnetic field requires good mechanical stability. Improvement in mechanical stability and strength by doping has been shown in several studies. However, doping is always accompanied by grain refinement and a change in transition temperature. This raises the question of the extent to which mechanical strength is related to grain refinement, transition temperature, or precipitates. This study shows a direct comparison between a single-phase Ni–Mn–In and a two-phase Gd-doped Ni–Mn–In alloy with the same transition temperature and grain size. It is shown that the excellent magnetocaloric properties of the Ni–Mn–In matrix are maintained with doping. The isothermal entropy change and adiabatic temperature change are reduced by only 15% in the two-phase Ni–Mn–In Heusler alloy compared to the single-phase alloy, which results from a slight increase in thermal hysteresis and the width of the transition. Due to the same grain size and transition temperature, this effect can be directly related to the precipitates. The introduction of Gd precipitates leads to a 100% improvement in mechanical strength, which is significantly lower than the improvement observed for Ni–Mn–In alloys with grain refinement and Gd precipitates. This reveals that a significant contribution to the improved mechanical stability in Gd-doped Heusler alloys is related to grain refinement. [ABSTRACT FROM AUTHOR]

Using the first-principles exact muffin-tin orbital method in combination with the coherent potential approximation, the crystal structure and site preference, magnetic and elastic properties, and martensitic transformation (MT) are systematically investigated with the three groups of Heusler alloys: (Co 2 − x M x)VGa (M1 x), Co 2 (V 1 − x M x)Ga (M2 x), and Co 2 V(Ga 1 − x M x) (M3 x , M = Ni and Fe, 0 ≤ x ≤ 1.0). It is shown that instead of the L 2 1 and X A structures, the fcc one is energetically preferred in the cubic M3x (x ≥ 0.8) alloys. In L 2 1 -Ni2x (x ≤ 0.6) and fcc-Ni3x (x = 0.8), Ni atoms even prefer the Ga and Co anti-sites, respectively, and the replaced atoms move to the sublattices of the deficient ones. Their total magnetic moment is dominated by the magnetic exchange interactions corresponding to the pairs of two Co atoms on the different sublattices in M = Ni and Fe1x, Co and Fe in Fe2x and Fe3x (x < 0.8), and Fe and Fe atoms in Fe3x (x ≥ 0.8) alloys, respectively. These Ni1x, Ni2x, and Fe3x with x ≥ 0.4 as well as Ni3x with x ≥ 0.2 alloys are predicted having the MT behavior and also the better mechanical property relative to Co 2 VGa. A lower shear modulus (C ′ = (C 11 − C 12) / 2) generally corresponds to a higher MT temperature, and these alloys, which can undergo the MT are further evaluated with C ′ < 36.50 GPa. Both considerable magnetocaloric and magnetovolume effects can be also expected during the MT of these Fe3x alloys (x = 0.4 and 0.6). In the remaining Fe1x and Fe2x alloys, the Fe doping disfavors the MT and also improves their brittleness. The structural preference of these cubic alloys and also their stability relative to the tetragonal martensite can be mainly attributed to the number of their minority density of states at the Fermi level: the smaller they are, the more stable their system tends to be. [ABSTRACT FROM AUTHOR]