Martensitic Transformation Behavior Research Articles

Phase field simulations on the rate‐ and grain‐size‐dependent crack propagation of polycrystalline NiTi shape memory alloy

AbstractA three‐dimensional non‐isothermal phase field model of polycrystalline NiTi shape memory alloy (SMA) was established based on the Ginzburg–Landau's theory, which considered the interaction between crack growth and martensite transformation (MT). Then, the crack propagation of polycrystalline NiTi SMA and its dependence on the system's grain size and loading rate were studied by the phase field simulations. The results show that the proposed phase field model can characterize the MT behavior, temperature field changes, and stress‐displacement response of NiTi SMA during the crack growth with different grain sizes and loading rates. That is the higher the loading rate, the higher the fracture toughness is; the larger the grain size, the higher the fracture toughness is and the more significant the influence of grain boundary on the crack growth path becomes. Meanwhile, as the grain size increases, the fracture mode gradually changes from the transgranular fracture (for the fine‐grained alloy with a grain size ≈ 15 nm) to the intergranular one (for the coarse‐grained alloy with a grain size ≈ 100 nm).

Effects of laser welding on the martensitic transformation of a shape-memory Ti-Ni alloy

The objective of this study is to investigate the martensitic transformation behavior of a Ti-Ni alloy subjected to a laser welding process. Materials characterization techniques were used to analyze the influences of this welding procedure on the phase transformation behavior. This study can provide valuable insights for the improvement of Ti-Ni alloy welding processes used for technological applications, such as in the aerospace industry. In this study, samples were prepared under three conditions: thermal treatment at 400°C, thermal treatment at 500°C, and as-received. The characterization techniques used were: Optical Microscopy (OM), Differential Scanning Calorimetry (DSC), Electron Dispersion Spectroscopy (EDS), X-Ray Diffraction (XRD), and Vickers Microhardness (HV). The results allow for analysis of thermoelastic martensitic transformation and variations in the compositions of the alloy elements. The research, in general, aims to repurpose welding waste in Ti-Ni alloys for the production of mini sensors. The enthalpy was higher for the alloy treated at 500°C and welded. The research findings led to the observation of martensitic phase transformation in the laser-welded Ti-Ni alloy, which suggests that it can be used as a sensor/actuator.

Martensitic Transformation Behavior of Fe–Ni–C Alloys Monitored by <i>In-situ</i> Neutron Diffraction during Cryogenic Cooling

Martensitic Transformation Behavior of Fe–Ni–C Alloys Monitored by <i>In-situ</i> Neutron Diffraction during Cryogenic Cooling

Effects of Mean Normal Stress on Strain-Hardening, Strain-Induced Martensite Transformation, and Void-Formation Behaviors in High-Strength TRIP-Aided Steels

To analyze various types of cold formability in TRIP-aided polygonal ferrite (TPF), annealed martensite (TAM), and bainitic ferrite (TBF) steels, the effects of the mean normal stress on the strain-hardening, strain-induced martensite transformation, and void-formation behaviors were investigated. The strain-hardening behavior was influenced by positive mean normal stress and was hardly influenced by zero and negative mean normal stresses in all steels. Positive mean normal stress promoted the strain-induced martensitic transformation behavior, especially in TBF steel due to the high mechanical stability of the retained austenite. The void-formation behavior was also promoted by positive mean normal stress, especially in TPF steel. These behaviors were also related to the microstructural properties, such as the matrix structure, retained austenite characteristics, and second phase.

Deformation-induced martensitic transformation behavior at cryogenic temperatures in an austenitic stainless steel studied by <i>in situ</i> neutron diffraction experiments

Deformation-induced martensitic transformation behavior at cryogenic temperatures in an austenitic stainless steel studied by <i>in situ</i> neutron diffraction experiments

Thermal stability of Ag-doped Ni-Mn-Ga high-temperature shape memory alloy

In the present study, the microstructure features and martensitic transformation behaviour of a Ni-Mn-Ga alloy were tailored by adjusting the Ag content. In addition, the effect of Ag content on the thermal stability was also explored. The results revealed that the non-modulated martensite phase and Ag particles coexisted in the Ni-Mn-Ga-Ag alloys. Different martensite variants were in the {112} type I twinning relationship. With increasing Ag content, not only all transformation temperatures but also the thermal hysteresis decreased continuously in a quasilinear manner. This was attributed to the introduction of small and soft Ag particles, which were applied to release the elastic energy during the martensitic transformation. The decreasing trend of elastic energy was highly nonlinear. The alloy with 4 at% Ag addition was the percolation threshold. In addition, the relationship of energy dissipation against Ag is not monotonic. While the energy dissipation of Ni48Mn25Ga19Ag8 is almost zero, the others were much higher. Furthermore, unlike others, Ni48Mn25Ga19Ag8 featured good martensitic transformation stability upon thermal cycling.

Enhancing the thermal stability and recoverability of ZrCu-based shape memory alloys via interstitial doping

ZrCu-based shape memory alloys (SMAs) are regarded as emerging smart materials with high phase transformation temperatures. However, unfavorable thermal stability and poor strain recovery rate hinder commercial applications. Herein, these issues were addressed by doping interstitial B atoms, and the impacts of B content on microstructure, martensitic transformation behavior and properties of ZrCu-based SMAs were systematically investigated. It was shown that (Zr50Cu25Ni7.5Co17.5)99.98B0.02 SMAs possessed excellent thermal stability and shape memory effect. The change of martensitic transformation temperature for this alloy was 15.7 °C after several cycles. The shape recovery rate reached 92.5 % at 8 % compressive pre-strain, which was attributed to the de-twinning of (0 0 1) compound twins and the appearance of (1 1 1) type I twins. Furthermore, it was elucidated for the first time by first-principles calculations that B atoms were covalently bonded with Zr and Cu atoms occupying the octahedral interstitial positions in the B19′ and Cm phases. This study offers an available pathway to exploit efficient and advanced products.

Unexpected sluggish martensitic transformation in a strong and super-ductile high-entropy alloy of ultralow stacking fault energy

Displacive martensitic transformation usually prevails in face-centered cubic (FCC) steels and high-entropy alloys (HEAs) of low stacking fault energy (SFE; e.g., <15 mJ/m2) upon deformation. Here, we present and rationalize an unexpectedly sluggish and suppressed martensitic transformation behavior in a newly developed non-equiatomic HEA (Fe30Ni20Co20Cr20Si10, at.%) with an ultralow SFE, i.e., ∼7 mJ/m2 measured by weak-beam dark-field scanning transmission electron microscopy (STEM) at room temperature. The recrystallized FCC HEA with an average grain size of ∼14.9 µm shows an ultimate tensile strength of 903 MPa and a fracture elongation of ∼83.1 %. Plastic deformation of this alloy with ultralow SFE is characterized by the intensive formation of stacking faults (SFs), whereas the volume fraction of hexagonal close-packed (HCP) martensite is very low even at cold-rolled state with an equivalent strain of 1.85, i.e., < 8.3 % according to neutron diffraction analysis. Atomic-scale TEM observation demonstrates that chemical short-range order (CSRO) domains are present in the FCC solid solution. Apart from contributing to the high strength, the CSRO domains and atomic size misfit (2.36 %) promoted by the high content of Si are regarded to enhance the local barrier for the successive accumulation of SFs, hindering the formation of HCP martensite and mechanical twin. The uniform formation of SFs in the FCC matrix upon deformation effectively alleviates strain localization, contributing to the sustained strain-hardening ability and super-ductility. These insights suggest a strategy to design low-SFE alloys with excellent mechanical properties via manipulating the rate of martensitic transformation by CSROs and atomic size misfit.

A deep insight into the mechanical properties of Mg-Sc alloys through DFT

Mg-Sc alloy is the lightest shape-memory alloy (SMA) discovered so far, offering a new material platform for lightweight SMAs. The main challenge for this system is to reveal its micro-mechanism and tune its performance for practical applications. In this work, we conducted the first-principles calculations to investigate the mechanical properties of Mg-Sc alloys. The effect of Sc content on the martensitic transformation behavior by combining dynamical stability, phonon dispersion and Fermi surfaces analyses were examined. We found that the Sc content should be neither below 15.625 at.% nor above 21.875 at.% to ensure the electronic structure and lattice dynamics stability. The effects of Sc content and rare-earth elements doping on the hardness of Mg-Sc alloys were also explored. We demonstrated that Gd doping can enhance the hardness of Mg-Sc alloys due to the strong interatomic interactions. Our results provide a solid basis for designing high-hardness Mg-Sc-based lightweight SMAs.

High-strength porous NiTi shape memory alloys with stable cyclic recovery properties fabricated using elemental powders

Based on previous research regarding the high-vacuum sintering of elemental powder (EP) NiTi alloys, this study investigates the preparation of high-strength, highly cyclically stable porous NiTi shape memory alloys. The microstructures, martensitic transformation behaviors, compressive and compressive recovery properties, and cyclic stabilities of the sintered and aged porous NiTi alloys were compared and analyzed. The porosity and average pore size increased with the addition of NaCl, with the oxygen content increasing slightly. The sintered alloy displayed an inhomogeneous distribution of the Ni4Ti3 precipitate, which exhibited a dispersed and homogeneous distribution after aging. The sintered porous NiTi alloy exhibited a multistage phase transformation. The martensitic transformation was suppressed after aging owing to changes in the size and distribution of the precipitates, and only R-phase transformation was observed. Porosity and aging precipitation influenced the dual yield points and recovery properties of porous NiTi. The pinning effect of aging precipitation increased the yield point and reduced the residual strain in the recovery. However, at a high porosity, the volume fraction of NiTi matrix decreased, resulting in a small strengthening contribution of precipitation. The disruption of porous structure was the dominant causes of irreversible strain. As the porosity increased, the porous structure became increasingly susceptible to damage during cyclic loading. Overall, the EP porous NiTi alloy exhibited a higher compressive strength, enhanced compression recovery properties and cyclic stability compared to those reported in previous studies. This is because of the optimization of high-temperature homogenization via sintering at 1250 °C and enhancement effects of aging precipitation. This study provides a novel approach for developing porous EP NiTi alloys.

Effects of Thermal Cycling and Porosity on Phase Transformation of Porous Nanocrystalline NiTi Shape Memory Alloy: An Atomistic Simulation

Herein, the effects of thermal cycling and porosity on the martensitic phase transformation behavior of porous nanocrystalline (NC) NiTi shape memory alloys (SMAs) at the atomic scale are investigated by molecular dynamics simulation. The simulation results show that thermal cycling causes the NC NiTi SMAs to repeatedly undergo martensitic phase transformation and inverse phase transformation processes. The martensitic phase transformation capability is suppressed with increasing porosity and number of cycles, but the suppression effect of thermal cycling is weakened with increasing porosity. Moreover, the start temperature of martensitic transformation, residual martensitic phase, and the fraction of interstitial atoms increase with increasing porosity. Thermal cycling causes the disordered structures and shear plastic deformation to accumulate as cycling increases. Recoverable shear deformation is generated within the grain, but shear plastic deformation is mainly concentrated at both grain boundaries and pore surfaces. Residual martensitic phase increases after thermal cycling. These results provide important explanations and references for a deeper understanding of phase transformation behavior and pore effects caused by thermal cycling.

Molecular dynamics study on martensitic transformation behavior of SLM-NiTi alloy induced by temperature and stress

Selective laser melting (SLM) technology is currently one of the most promising additive manufacturing technologies for complex metal components. NiTi alloy has been highly regarded in advanced applications due to its excellent shape memory and good biocompatibility. However, as a new material, SLM-NiTi alloy is far from being applied in actual advanced fields. In the actual processing, such as grinding, turning, polishing, electrical discharge machining, all involve changes in temperature and stress, Therefore, it is very important to study the martensitic phase transition caused by temperature and stress changes in the precision machining process of SLM-NiTi alloy. However, it is difficult to observe the martensitic phase transition changes directly in the actual processing, so the method of molecular dynamics is adopted in this paper. Moreover, in the process of preparing NiTi alloy by selective laser melting, the ratio of Ni to Ti is very important, which determines the final forming quality. Therefore, this paper studied the martensitic transformation behavior induced by temperature and stress under different nickel proportions, different initial temperatures and different model sizes, and expounded the variation laws of stress–strain, potential energy, volume and dislocation. The microstructure and shear strain were demonstrated on the atomic scale. The results show that temperature plays an important role in the martensite transformation of SLM-NiTi alloy, low temperature will largely inhibit martensite transformation, and high temperature will promote martensite transformation. The stress induced martensite reorientation in SLM-NiTi alloy is accomplished by the migration of the interface between different martensite variants. When the nickel content is 52% and 55%, there is no inflection point between volume and potential energy with the change of temperature, when the nickel content is 50.8%, there is an obvious jump between volume and potential energy. The research in this paper is helpful to guide the processing technology of SLM-NiTi alloy, and also broadens the application of additive manufacturing materials.

Effect of heterogeneous microstructure on martensitic transformation behavior and mechanical properties of a cold rolling medium Mn steel

A heterostructured medium Mn steel with duplex phases (ferrite and austenite) and multiple degrees of grain sizes is fabricated by a cold rolling process combined with repeated annealing. The obtained sample exhibits excellent mechanical properties with the ultimate tensile strength above 1110 MPa and the total elongation of 34.4%, the mechanical properties being higher than those of the sample produced by conventional cold rolling. The microstructural evolution and the strain partitioning behavior of the material during deformation are investigated. It is indicated that the great strain hardening ability rely on the combination of the transformation induced plasticity (TRIP) effect and the hetero-deformation induced (HDI) hardening. The sample with heterostructure induces more sufficient martensitic transformation. Meanwhile, the generation of large grains intensifies the mechanical incompatibility and makes it possible to accommodate larger amount of geometrically necessary dislocations (GNDs), thus inducing more effective HDI strengthening and HDI hardening, so that the comprehensive mechanical properties have been improved.

Role of Zr and thermomechanical treatment on phase transformation and functional properties of NiTi-based shape memory alloys

A systematic investigation was conducted to evaluate the role of Zr addition on the martensitic phase transformation behavior and properties of NiTi-based shape memory alloys. Equiatomic binary NiTi alloy and ternary Ni50Ti45Zr5 and Ni50Ti40Zr10 alloys (at.%) were processed by a simple thermomechanical treatment including cold rolling followed by post-deformation annealing at 450 °C for 30 and 60 min. The microstructural investigation and thermal analyses reveal that martensitic phase transformation is suppressed by adding Zr. Solid solution strengthening due to Zr addition, strain hardening due to cold rolling and formation of (Ti,Zr)2Ni precipitates in a dual-phase microstructure during annealing leads to a significant increase in hardness in the thermomechanically treated alloys (e.g., up to 550 Hv for Ni50Ti45Zr5 alloy). The stress-strain curve of martensitic Ni50Ti50 alloy reveals the essential characteristics of shape memory effect, however, Ni50Ti45Zr5 alloy shows typical features of pseudoelasticity with significantly high stress plateau (∼600 MPa) and remarkable recovered strain of 3.5% during unloading.

Unconventional precipitation and martensitic transformation behaviour of Ni-rich NiTi alloy fabricated via laser-directed energy deposition

ABSTRACT In this study, we report an unconventional precipitation and martensitic transformation behaviour of directly aged Ni-rich NiTi alloys fabricated via laser-directed energy deposition (LDED). Ni4Ti3 particles precipitate uniformly under all ageing conditions and no traditional multiple-step martensitic transformations are observed. We conclude this unique behaviour to the intrinsic characteristics of the LDED technique, which are metastable microstructures and high residual stresses. On the one hand, these features make grain boundaries no longer a fevered location for precipitation and, on the other hand, significantly suppress the martensitic transformation when ageing at low temperatures (300°C/400°C). As the aging temperature increase (500°C), residual stresses release significantly, accompanied by the growth of Ni4Ti3 precipitates from several nanometres to 452 ± 181 nm with increased interparticle spacing. At the same time, reverse martensitic transformations change from two-step (B19′ → R → B2) to single-step (B19′ → B2).

Effects of layer thickness on deformation-induced martensite transformation and tensile behaviors in a multilayer laminate

Multilayer laminates with a 304 stainless steel as surface layers and with a low C steel and a medium-Mn steel as alternating central layers have been developed in the present study. The number of interfaces and the layer thickness have been varied, while maintaining the similar microstructures for each layer. The uniform elongation is observed to increase from 10.1% to 37.8%, and the product of strength and elongation is found to increase from 13.6 GPa·% to 36.8 GPa·% monotonically with decreasing layer thickness in multilayer laminates, while the yield stress remains almost constant. Firstly, deformation-induced martensite transformation is significantly promoted with decreasing layer thickness. Secondly, the more interfaces can induce the accumulation of higher density of geometrically necessary dislocations, resulting in better mechanical properties. Lastly, the main cracks nucleate and propagate at the interfaces of low C steel layers and medium Mn steel layers, thus the samples with smaller layer thickness have more interfaces and require more energy consumption during the micro-fracture process, resulting in better tensile performance.

Whole martensitic transformation process in Fe–Mn–Si–Cr shape memory alloy by improved characterization of volume resistivity

Since Fe–Mn–Si–Cr shape memory alloy (Fe-SMA) has been discovered, an improvement of the shape memory effect (SME) is challenged. Numerous attempts have been made thus far to improve SME. To effectively control such performance, it becomes imperative to comprehend the real-time behavior of the stress-induced martensitic transformation (SIMT), which is strongly related to SME, during deformation. In the past, the measurement of volume resistivity has been proposed and applied to capture the forward SIMT behavior during the tensile loading process at various strain rate. However, it is quite hard to find investigations of the reverse martensitic transformation after the loading, which is more essential for understanding SME. Therefore, a comprehensive evaluation of the whole SIMT process including the loading process at various strain rate is strongly required. In this study, an improved measurement of volume resistivity is established by introducing both the four-probe and active-dummy techniques with a development of a diameter measurement at high temperature. Then, the volume resistivity of Fe-SMA is measured during the whole deformation including loading under tension at various strain rate, unloading, and heating processes. Besides, the microstructures of Fe-SMA after unloading and heating processes are observed, respectively. Finally, the real-time whole martensitic transformation in Fe-SMA is first investigated with the novel optimization method. The negative strain rate of change in volume resistivity during whole processes can be found. More amount of austenite can be obtained at lower strain rate, resulting in a larger SME.

Microstructural, crystallographic, and mechanical characteristics in Ni-Mn-Ga alloys directionally solidified under a transverse magnetic field

• Magnetic field-assisted directional solidification (MFADS) can control structure. • Macro-/micro-segregation and orientation can be adjusted by MFADS. • Microsegregation-induced microstructure influences mechanical properties. • Stress-induced crystallographic evolution is traced by in-situ EBSD. Abstract In this study, the effect of transverse magnetic field-assisted directional solidification (MFADS) on the microstructures in Ni-Mn-Ga alloys has been investigated. The results show that the magnetic field is capable of inducing transversal macrosegregation perpendicular to the magnetic field, causing the emergence of martensite clusters in the austenite matrix. Moreover, the magnetic field alleviates the microsegregation on a dendritic scale and promotes the preferred growth of austenite dendrites. On the basis of the above investigation, several special samples are designed using the MFADS to study the crystallographic evolution and mechanical behavior during thermal/stress-induced martensite transformation. The martensite cluster in the austenite matrix is used to investigate the martensite transformation and growth under cooling-heating cycles. The crystallographic relationship and phase boundary microstructure between martensite and austenite have been characterized. In addition, the microsegregation on a dendritic scale can significantly influence the martensite variant distribution, corresponding to the performance during compressive circles based on the analysis of the deformation gradient tensor. The stress-induced superelasticity is closely dependent on orientation, well explained from the perspective of different resolved shear stress factors and correspondence variant pair formation transformation strain. The crystallographic evolution has been characterized during in-situ stress-induced transformation. The findings not only deepen the understanding of martensite transformation and mechanical behavior under a thermal/stress field in Ni-Mn-Ga alloys but also propose a promising strategy to obtain microstructure-controllable functional alloys by MFADS.

Model for deformation-induced martensitic transformation in irradiated materials

Deformation-induced martensitic transformation (DIMT) is commonly reported to be easily activated in irradiated materials, but the existing transformation kinetics models of DIMT hardly capture the irradiation effect. In this work, we develop a transformation kinetics model considering the effect of irradiation hardening, shear band evolution and the role of irradiation-induced voids. A crystal plasticity framework incorporating the newly developed transformation kinetics model is established. We show that the framework is capable of capturing the accelerated evolution of martensite volume fraction in both cases with and without irradiation-induced voids. Moreover, the framework successfully simulates two post-irradiation phenomena observed in experiments, i.e. the localized distribution of martensite fraction and the evolution of the dominant transformation pathway. The present work is one of the first attempts at the theoretical modelling of DIMT in irradiated materials, and has the potential to benefit the application of irradiation in tailoring the martensitic transformation behaviour.

Improving the functional stability of TiNi-based shape memory alloy by multi-principal element design

This study investigated the evolutions of the superelasticity and elastocaloric effect of a multi-principal element Ti40Zr10Ni40Co5Cu5 strain glass alloy during 3001 cyclic compressions. The results were compared with a conventional solution-treated Ti49Ni51 shape memory alloy. The Ti40Zr10Ni40Co5Cu5 strain glass alloy exhibited a higher hardness, lower residual strain, and lower dislocation density due to multi-principal element solid solution strengthening and strong lattice distortion. Furthermore, it showed stable critical stress and elastocaloric cooling temperature drop during the cyclic test, and the values were maintained at 90.9 and 83.3 % after 3001 cycles, respectively. Digital image correlation analyses showed that the Ti40Zr10Ni40Co5Cu5 strain glass alloy exhibited a more homogeneous martensitic transformation behavior instead of the localized Lüders-type one. This feature originated from the high density of atomic defects, which in turn improved the temperature uniformity of the material during elastocaloric cooling. Experimental results demonstrated that the multi-principal element design method effectively suppressed dislocation slip and thus improved the functional stability and transformation homogeneity of the shape memory alloy, so such alloys are potential candidates for solid-state refrigerants.