Martensitic Transformation Research Articles

Exceptional strength-ductility synergy in the novel metastable FeCoCrNiVSi high-entropy alloys via tuning the grain size dependency of the transformation-induced plasticity effect

In-depth knowledge of the coupling between grain refinement and the transformation-induced plasticity (TRIP) effect in metastable alloys is a viable approach for the improvement of strength-ductility synergy, which needs systematic research with consideration of commercial austenitic stainless steels and novel high-entropy alloys (HEAs). Accordingly, in the present work, two Si-containing metastable HEAs in the Fe47Co30Cr10Ni5V8-xSix system (x = 3 and 6 at.%) were designed, and the TRIP-assisted AISI 304L stainless steel was also considered for comparison. The alloys were processed by cold rolling and annealing to obtain different grain sizes. Reducing the stacking fault energy (SFE) through adjusting chemical composition contributes to minimizing the detrimental effect of grain refinement on the ductility of TRIP alloys, while extremely low SFE must be avoided owing to the fast kinetics of deformation-induced martensitic phase transformation, which leads to the deterioration of ductility. In contrast to AISI 304L stainless steel, a strong TRIP effect was maintained upon grain refinement in the Fe47Co30Cr10Ni5V2Si6 HEA due to the remaining apparent SFE in the appropriate TRIP range. The tuned kinetics of martensitic transformation was found to be responsible for the exceptional ductility (∼65%) of Fe47Co30Cr10Ni5V2Si6 HEA at an ultrahigh tensile strength of 1250 MPa. Therefore, considering the identical trend of SFE with grain size, an appropriate initial SFE value is important for tuning the grain size dependency of the TRIP effect. Moreover, the ultrahigh strength was attributed to the high volume fraction of α΄-martensite as well as the high strength of the martensite phase due to the high Si content. Accordingly, for achieving strong-yet-ductile HEAs, a high Si content is recommended to benefit from solid solution strengthening in the martensite phase, a specially-designed chemical composition is needed for attaining a high volume fraction of α΄-martensite, and SFE should be in a desirable range to tune the kinetics of martensitic phase transformation.

Large and reversible elastocaloric effect induced by low stress in a Ga-doped Ni-Mn-Ti alloy

The Solid-state cooling based on caloric effects is considered a potential alternative to conventional refrigeration technology, which uses ozone-depleting gases. Several shape memory alloys have attracted attention for solid-state cooling since they present a high reversibility of the caloric effect, which depends mainly on thermal hysteresis and sensitivity to the applied field. In the present work, a study substitution of Mn by Ga in the Ni50Mn34Ti16 alloy led to diminished thermal hysteresis in the martensitic transformation by 6 K. The elastocaloric effect, thermal and microstructure properties of a polycrystalline Ni50Mn32Ti16Ga2 alloy have been characterized. The elastocaloric effect was obtained indirectly from the length change as a function of temperature at constant stress. An isothermal entropy change (ΔSISO) of 23.0 J kg−1 K−1 during heating and 22.0 J kg−1 K−1 during cooling was observed for applied stress of 160 MPa. In addition, the ΔSISO is reversible for a temperature span between 287 and 319 K, reaching a maximum of 20.5 J kg−1 K−1 at 299 K. The thermal hysteresis changed slightly while the applied stress increased up to 160 MPa since the sensitivity of the martensitic transformation temperatures to stress was 0.150 K M/MPa during cooling and 0.160 K/MPa during heating. The X-ray diffraction analysis revealed a mixture of B2-type cubic austenite, 5 M modulated martensite, and a second intermetallic phase identified as Ni3Ti. All these results were obtained around room temperature.

Effect of Structural Configurations on Mechanical and Shape Recovery Properties of NiTi Triply Periodic Minimal Surface Porous Structures

Based on the advantages of triply periodic minimal surface (TPMS) porous structures, extensive research on NiTi shape memory alloy TPMS scaffolds has been conducted. However, the current reports about TPMS porous structures highly rely on the implicit equation, which limited the design flexibility. In this work, novel shell-based TPMS structures were designed and fabricated by laser powder bed fusion. The comparisons of manufacturability, mechanical properties, and shape recovery responses between traditional solid-based and novel shell-based TPMS structures were evaluated. Results indicated that the shell-based TPMS porous structures possessed larger Young’s moduli and higher compressive strengths. Specifically, Diamond shell structure possessed the highest Young’s moduli of 605.8±24.5 MPa, while Gyroid shell structure possessed the highest compressive strength of 43.90±3.32 MPa. In addition, because of the larger specific surface area, higher critical stress to induce martensite transformation, and lower austenite finish temperature, the Diamond shell porous structure exhibited much higher shape recovery performance (only 0.1% residual strain left at pre-strains of 6%) than other porous structures. These results substantially uncover the effects of structural topology on the mechanical properties and shape recovery responses of NiTi shape memory alloy scaffolds, and confirm the effectiveness of this novel structural design method. This research can provide guidance for the structural design application of NiTi porous scaffolds in bone implants.

Influence of β-Stabilizing Nb on Phase Stability and Phase Transformation in Ti-Zr Shape Memory Alloys: From the Viewpoint of the First-Principles Calculation

In the present study, the effect of the Nb element on the lattice parameters, phase stability and martensitic transformation behaviors of Ti-Zr-based shape memory alloys was extensively investigated using the first-principles calculation. The lattice parameters of both the β parent phase and α′ martensite phase gradually decreased with Nb content increasing. For the α″ martensite phase, the lattice constant (a) gradually increased with the increase in Nb content, whereas the lattice constants (b and c) continuously decreased due to the addition of Nb. Based on the formation energy and density of state, β→α′ martensitic transformation occurred, as the Nb content was not more than 12.5 at.%. However, the Ti-Zr-Nb shape memory alloys with a Nb content higher than 12.5 at.% possessed the β→α″ martensitic transformation. However, both the largest transformation strain and sensitivity of critical stress to temperature (dσ/dT) can be optimized by controlling 12.5 at.% Nb in the Ti-Zr-Nb shape memory alloy, which was favorable to obtaining the largest elastocaloric effect.

Influence of a Tensile Stress on the Thermodynamics of Martensitic Transformations: Frictional Work and Stored Elastic Energy Calculations in Polycrystalline Alloys

Influence of a Tensile Stress on the Thermodynamics of Martensitic Transformations: Frictional Work and Stored Elastic Energy Calculations in Polycrystalline Alloys

Van der Waals semiconductor InSe plastifies by martensitic transformation.

Inorganic semiconductor materials are crucial for modern technologies, but their brittleness and limited processability hinder the development of flexible, wearable, and miniaturized electronics. The recent discovery of room-temperature plasticity in some inorganic semiconductors offers a promising solution, but the deformation mechanisms remain controversial. Here, we investigate the deformation of indium selenide, a two-dimensional van der Waals semiconductor with substantial plasticity. By developing a machine-learned deep potential, we perform atomistic simulations that capture the deformation features of hexagonal indium selenide upon out-of-plane compression. Unexpectedly, we find that indium selenide plastifies through a martensitic transformation; that is, the layered hexagonal structure is converted to a tetragonal lattice with specific orientation relationship. This observation is corroborated by high-resolution experimental observations and theory. It suggests a change of paradigm, where the design of new plastically deformable inorganic semiconductors can focus on compositions and structures that facilitate phase transformations, going beyond the conventional dislocation slip.

Dual effects of pre-deformation and shape recovery process on martensitic transformation behavior in NiTi-Nb composite wire

The martensitic transformation behavior of NiTi-Nb composite wire following the pre-deformation and shape recovery (PD&SR) process was investigated. The wire drawing process significantly decreased the martensitic transformation start temperature (Ms) of NiTiNb alloy. Interestingly, the PD&SR process exhibited dual effects on the initial Ms temperature, depending on the annealing temperature. When the composite wire was annealed at higher temperature, the PD&SR process decreased the Ms temperature. In contrast, when the composite wire was annealed at lower temperature, the PD&SR process raised the Ms temperature. These dual effects can be attributed to the competition between the hindering effect of deformation dislocation and the promoting effect of stress coupling near the interface between the β-Nb phase and the NiTi matrix. At lower annealing temperature, the stress coupling generated by the β-Nb nanowires and the NiTi matrix is stronger, leading to an increase in the Ms temperature. Furthermore, the pre-deformation strain and shape recovery temperature influenced the dislocation density and the strength of stress coupling, thereby regulating the Ms temperature within a certain range.

Constitutive modeling of the slip-twinning transition in martensitic transformations

Martensite is the result of a diffusionless displacive phase transition. In Fe-based alloys it transforms the FCC to the BCC, BCT, or HCP structures and occurs at high strain rates. It has two typical morphologies, known as lath and plate. A quantitative constitutive description of the slip-twinning transition in the martensitic transformation is presented. It is based on the temperature and strain-rate sensitivities of slip, which are much higher than those for twinning. Thus, twinning becomes a favored deformation mechanism at low temperatures and high strain rates. The Hall-Petch coefficient, for the inclusion of grain size effects, is two times larger for twinning than slip. Constitutive relationships for slip and twinning are presented and applied to the martensitic transformation in steels; the lath-to-plate morphology change observed with increasing carbon content is successfully predicted as a function of grain size by calculations incorporating the two modes of deformation. A simple calculation of the strain rates during martensitic transformation is also provided. This methodology is applied to the Fe–C system and can be extended to the Fe–Ni–C system and to thermoelastic martensites, where twinning is favored over slip to enhance reversibility.

Effect of Partition Temperature on Microstructure and Mechanical Properties of Cold‐Rolled Medium‐Manganese Steel

The influence of different heat‐treatment conditions on the microstructure evolution and mechanical properties of cold‐rolled Fe–7.69Mn–2.76Al–0.12C (wt%) steel is investigated using various techniques, including scanning electron microscopy, transmission electron microscopy (TEM), and X‐ray diffraction. In the results, it is shown that the stability of austenite is significantly affected by different partitioning temperatures. It is found that factors such as dislocation density, temperature, and grain size collectively influence partitioning behavior. In the observations, it is revealed that when the partitioning temperature is raised from 120 to 180 °C, the dislocation density of the face centered cubic phase within the test steel decreases from 10.38 × 1016 to 7.67 × 1016 m−2. Additionally, within specimens exhibiting higher dislocation densities, carbon element diffusion is more uniform. During the experiment, the poor stability of the austenite is found to be susceptible to stress‐inducing the phase of the martensitic transformation, and the type of the martensite transformation affects the deformation process and the performance of submission behavior. In the observations under TEM, a phenomenon where variations in dislocation density within individual austenite grains may lead to differing stability across grain regions is revealed, thereby triggering localized martensitic phase transformations.

Mechanical behavior of shape memory alloys considering the effects of body fluids corrosion for biomedical applications

Abstract Shape memory alloys (SMAs) are widely used in biomedical engineering, including cardiovascular stents, artificial skeletons, and orthodontic implants. For the above applications, the body fluids corrosion processes will inevitably cause deterioration in the mechanical properties of the SMAs actuator during its service life, which will threaten the safety of human health. To analyze such problems, experimental measurements have been carried out to investigate the influence of body fluid corrosion on the mechanical properties of SMAs. Changes in the mechanical properties, such as Young’s modulus and phase transformation temperatures of SMAs under body fluids corrosion were tested firstly in the simulated body environment with the 0.9 wt. % NaCl solution at 37. With an increase of the immersion time, the results show that the Ti (titanium) percentage, austenitic (reverse) transformation start temperature, austenitic (reverse) transformation finish temperature, and maximum residual strain all increase, the Ni (nickel) percentage, martensitic transformation finish temperature, tensile strength, and yield strength decrease, and the martensitic transformation start temperature first decreases and then increases. The research in this work can provide an experimental basis for further study of the SMAs materials in biomedicine applications.

Mechanisms of laser non-uniform shock peening on the mechanical properties of SUS 304 stainless steel weld joints

The technique of Laser Non-Uniform Shock Peening (LNUSP) is proposed based on the residual stress distribution in welded joints and the shock wave peak pressure of SUS 304 stainless steel. The study investigated the effects of LNUSP on the mechanical properties of laser beam oscillation welded (LWW) joints of SUS 304 stainless steel. The results indicate that LNUSP induced residual compressive stress with an affected layer depth exceeding 0.9 mm, leading to a more uniform residual stress distribution. LNUSP also induced martensitic phase transformation, altered the grain structure orientation, formed parallel dislocation arrays that refined the grains, increased dislocation density leading to dislocation tangles, and induced mechanical twins of 20–70 nm. Furthermore, LNUSP significantly enhanced microhardness and promoted a more uniform hardness distribution, favoring a better balance between strength and ductility. LNUSP notably increased the yield strength (YS) and ultimate tensile strength (UTS) by 49.4% and 5.1%, respectively, compared to the original samples. The LNUSP-treated samples demonstrated superior strength and plasticity, attributed to the uniformity of internal plastic deformation.

Strengthening mechanism and martensite transformation behavior in grain-refined low-Ni austenitic stainless steel

Low-Ni (less than 3 wt%) austenitic stainless steels with different C contents were designed and heat treated to utilize both the grain refinement and carbide strengthening, and their effects were systematically investigated to elucidate the mechanism of tensile property improvement. Grain refinement to 2–3 μm with homogeneously distributed Cr23C6 carbides resulted in excellent combination of yield strength (YS) and tensile elongation (YS × Tensile elongation of nearly 30,000 MPa %) without yield point elongation. The strengthening due to grain refinement was complemented by precipitation hardening. Grain refinement inhibited deformation-induced martensite transformation (DIMT) to decrease the strain hardening rate, while fine Cr23C6 carbides uniformly distributed within austenite grains promoted DIMT and increased strain hardening rate by depleting Cr and C concentrations. Two competing effects of grain refinement and Cr23C6 carbide formation on DIMT resulted in the increase of strain hardening rate as the latter was more dominant on austenite stability. Strengthening mechanisms were clearly explained by quantifying the contributions from solid solution, grain refinement and precipitation hardening. Stacking fault energy and driving force for γ → α’ varied with C contents and annealing treatment, indicating that the optimum combination of C addition and thermomechanical treatment can yield the high performance low-Ni austenitic stainless steels.

The Effect of Fe Content on the Shape Memory Effect of Ni-Mn-Ga-Fe Shape Memory Alloy Microwires after Ordering Heat Treatment

The shape memory capabilities of Heusler alloy microwires with two different contents of Fe element instead of Ga element following step-by-step ordering heat treatment were explored based on the stoichiometric ratio of Ni2MnGa. The melt-drawing technique was used to create the polycrystalline microwires, and the two microwires had Fe atomic contents of 4.7 at.% and 5.5 at.%, respectively. The field emission scanning electron microscope was used to analyze the microwire’s surface condition as well as the microscopic tensile fracture morphology. Using an X-ray diffractometer, the microwires’ crystal structure was identified for phase analysis. Differential scanning calorimetry was used to examine the microwires’ behavior during martensitic transformation. Using a dynamic mechanical stretcher, the elongation and recovery rate of microwires’ one- and two-way shape memory behavior were examined. The findings demonstrated that the microwire phase structure, martensitic transformation behavior, and shape memory capabilities all displayed good properties after the heat treatment was ordered.

Preparation and Characterization of Ni-Mn-Ga-Cu Shape Memory Alloy with Micron-Scale Pores

Porous Ni-Mn-Ga shape memory alloys (SMAs) were prepared by powder metallurgy using NaCl as a pore-forming agent with an average pore size of 20–30 μm. The microstructure, phase transformation, superelasticity, and elastocaloric properties of the porous alloys were investigated. The prepared porous alloy had a uniform pore distribution and interconnected microchannels were formed. Cu doping can effectively improve the toughness of a porous alloy, thus improving the superelasticity. It was found that porous Ni-Mn-Ga-Cu SMAs have a flat stress plateau, which exhibits a maximum elongation of 5% with partially recoverable strain and a critical stress for martensite transformation as low as about 160 MPa. In addition, an adiabatic temperature change of 0.6 K was obtained for the prepared porous alloy at a strain of 1.2% at about 150 MPa. This work confirms that the introduction of porous structures into polycrystalline Ni-Mn-Ga SMAs is an effective way to reduce costs and improve performance, and provides opportunities for engineering applications.

Laser Cutting of Titanium Alloy Plates: A Review of Processing, Microstructure, and Mechanical Properties

The growing use of titanium alloys has led to the gradual replacement of traditional processing methods by laser cutting technology, making it the preferred method for processing titanium alloy plates due to its high efficiency, precision, and adaptability. In this review, the characteristics of laser cutting technology and its application in titanium alloy plate processing are summarized, outlining several aspects of the cutting process, microstructure, and mechanical properties of the material after cutting, along with simulation predictions. Previous research categorized laser-cutting input parameters into beam parameters and process parameters, with the commonly used parameters being the laser power, cutting speed, and gas pressure. Various parameter combinations can achieve different cutting qualities, and seven indices can be used to evaluate the cutting process, with the surface roughness and slit width serving as the most common indices. Different auxiliary gases have shown a significant impact on the laser cutting quality, with commonly used gases consisting of nitrogen, argon, and air. Argon-assisted cutting generally results in better surface quality. Due to the rapid temperature change, the titanium alloy microstructure will undergo a non-diffusive martensitic phase transformation during laser cutting, producing a heat-affected zone. Experimental studies and simulations of the mechanical properties have shown that the occurrence of a martensitic phase transformation increases the hardness and residual tensile stress of the material, which reduces the fatigue strength and static tensile properties. In addition, studies have found that the more streaks appear on the cut surface, the lower the fatigue strength is, with fatigue cracks arising from the stripes. Hence, the established analytical solution model and three-dimensional finite element model can effectively predict the temperature distribution and residual stress during the cutting process. This can provide a better understanding of the high residual stress characteristics of the cutting edge and the stripe formation mechanism, allowing researchers to better explore the mechanism of laser cutting.

Dendrite growth kinetics and microstructure evolution mechanism of Ni2TiAl intermetallic compound within substantially undercooled liquid Ni64Al19Ti17 alloy

The dendrite growth kinetics and microstructure evolution mechanism of the Ni2TiAl intermetallic compound within substantially undercooled liquid Ni64Al19Ti17 alloy were investigated using electrostatic levitation (ESL) and drop tube (DT) techniques. The growth velocity of primary Ni2TiAl dendrites displayed a power-law relation with liquid undercooling, achieving a maximum of 80 mm/s at the undercooling of 254 K (0.16 TL). The solidification pathway transformed from primary Ni2TiAl growth plus subsequent (Ni3Ti+Ni2TiAl) eutectic growth into the successive growth of Ni2TiAl and Ni3Ti phases as the alloy undercooling increased sufficiently. The high cooling rates of freely falling alloy droplets effectively suppressed the martensitic transformation observed for primary Ni2TiAl intermetallic compound under typical ESL radiative cooling condition. Both nanoindentation and Vickers hardness performances of the Ni2TiAl phase formed under these two conditions exhibited conspicuous enhancement due to grain refinement effects. The microstructures with martensitic transformation under ESL condition showed higher hardness than those under DT condition despite their larger grain size.

Enhanced cryogenic mechanical properties of heterostructured CrCoNi multicomponent alloy: Insights from in-situ neutron diffraction

Heterostructured materials (HSMs) has been shown to improve the strength-ductility trade-off of conventional alloys but their cryogenic performance has not been studied during real-time deformation. We investigated heterostructured CrCoNi medium-entropy alloy by in-situ neutron diffraction at cryogenic (77 K) and room (293 K) temperatures. The significant mechanical mismatch at interfaces between fine and coarse grains, due to pronounced grain size disparity, resulted in exceptional yield strength of 918 MPa at 293 K. The yield strength further increased to 1244 MPa at 77 K with an excellent uniform elongation of 34 %. The exceptional strength–ductility combination at 77 K can be attributed to enhanced geometrically necessary dislocation pile-up density boosted from high-mechanical mismatch interfaces, as well as higher planar faults, and martensitic phase transformation. Comparison with homogenous counterparts demonstrates the potential of HSMs as a new strategy to improve the mechanical performance of different materials, including medium-/high-entropy alloys for cryogenic applications.

Wrinkled Shrinkage on Surface Monophasic Structure of Floating Refractory Alloy Droplets Solidified in Space Environment

AbstractThe surface‐wrinkled shrinkage structure of spheres is very common and also quite intriguing to explore and reveal the analogous droplet solidification kinetics in the space environment. Here, a liquid‐to‐solid phase transition experiment of floating refractory alloy droplets is designed to study the surface wrinkled shrinkage structures aboard China Space Station with a 10−5 gravity level. The experimental Ti‐Ni‐V alloy droplets undergo an extremely high overheating temperature beyond 1800 K and then achieve a supercooling of 307 K, displaying a strong thermodynamic metastability. The spherical surface deformation structures of this alloy under microgravity coupled with extreme metastable solidification are explored. Moreover, through the active control of metastable rapid solidification, a surface monophasic structure and a B2 structure capable of stress‐induced martensitic transformation are facilitated by wrinkled shrinkage dynamics under microgravity. Their findings shed further light on the surface deformation structures during the microgravity solidification process.

Thermodynamic–kinetic calculations and dilatometric verification of the martensitic and bainitic transformations in 4% medium-Mn steel

AbstractThe present study investigated the theoretical and experimental phase transitions phenomena during continuous cooling and isothermal holding above and below Ms temperature in 4 mass. % Mn medium-Mn steel. The thermodynamic–kinetic calculations were performed using JMatPro software, and phase transformations were recorded using a BÄHR high-resolution DIL805A/D dilatometer. The research covered continuous cooling rates from 60 to 0.05 °C s–1 and isothermal holding temperatures in a range between 420 °C and 230 °C. The issues related to both modelling and dilatometric methodology were discussed. The CCT and TTT diagrams were prepared on the basis of dilatometry and compared with the results of light microscopy and hardness tests. The alloy containing about 4 mass.% Mn and 0.22% Mo exhibited very high hardenability as only continuous cooling with rates lower than 1 °C s–1 allowed the bainitic transformation to be initiated. The bainitic transformation is accelerated after passing the Ms temperature. Both the incubation time and the time needed to complete the transformation were significantly reduced (the incubation time from 100 s to below 1 s and the completion time from over 4000 s to below 1000 s). The obtained microstructures were homogeneous and refined.

Assessment of the mechanical and functional properties of nitinol alloys fabricated by laser powder bed fusion: Effect of strain rates

Laser Powder Bed Fusion-fabricated (LPBFed) nitinol shape memory alloys (SMAs), which undergo solid-solid phase transitions between austenite and martensite, are increasingly utilized in various technical fields and are subjected to a wide range of strain rates during service. This study pioneers the investigation of a comprehensive range of strain rates (3.16 × 10−6-10°/s), encompassing the strain rates used in quasi-static tensile tests reported in the existing literature, to understand their effects on the deformation mechanisms of LPBFed nitinol SMAs. Utilizing multi-scale characterization, including macroscopic mechanical evaluation, mesoscale analysis of deformation surfaces, and microscale examination of microstructure, we reveal distinct deformation behaviors at different strain rates. At lower strain rates, the mechanical behavior exhibits little sensitivity to changes in the strain rates, with stress-induced martensite transformation or martensite detwinning and variant reorientation being the primary deformation mechanisms. However, at higher strain rates, we observe significant variability in mechanical response, dominated by dislocation-induced slip and reduced occurrence of stress-induced martensitic transformation. This study provides the first comprehensive database for the engineering applications of LPBFed Nitinol SMAs and offers critical insights into optimizing mechanical and functional assessments for these advanced materials.