Reversible Martensitic Phase Transformation Research Articles

Electron beam powder bed fusion of Ti-30Ta high-temperature shape memory alloy: microstructure and phase transformation behaviour

ABSTRACT The present study reports on additive manufacturing of a Ti-30Ta (at.%) high-temperature shape memory alloy (HT-SMA) using electron beam powder bed fusion (PBF-EB/M) technique. Detailed microstructure analysis was conducted to reveal the microstructural evolution along the entire process chain, i.e. from gas-atomised powder to post-processed material. PBF-EB/M processed structures with near full density and an isotropic, β-phase stabilised microstructure, i.e. equiaxed β-grains of around 20 µm in diameter with no preferred crystallographic orientation, are reported. As revealed by differential scanning calorimetry, post-process heat-treated Ti-Ta demonstrates a reversible martensitic phase transformation well above 100°C. Although partly unmolten Ta-particles after both gas atomisation and PBF-EB/M remain a challenge towards robust processing, PBF-EB/M appears to show significant potential for fabrication of Ti-Ta HT-SMAs, especially when functional metal parts and components with complex shapes are required, which are difficult to fabricate conventionally.

Hyper‐Elastic Deformation via Martensitic Phase Transformation in Cadmium Telluride

Cadmium telluride (CdTe) is a highly promising material for photovoltaics (PV) and photodetectors due to its light‐absorbing properties. However, efficient design and use of flexible devices require a deep understanding of its atomic‐level deformation mechanism. Herein, uniaxial compression deformation of CdTe monocrystalline with varying crystal orientations is investigated using molecular dynamics (MD) with a newly developed machine‐learning force field (ML‐FF), alongside in‐situ micropillar compression experiments. The findings reveal that CdTe bulk deformation is dominated by reversible martensitic phase transformation, whereas CdTe pillar deformation is primarily driven by dislocation nucleation and movement. CdTe monocrystals possess exceptional super‐recoverable deformation along the <100> orientation due to hyper‐elastic processes induced by martensitic transformation. This discovery not only sheds light on the peculiarities observed in micropillar experimental measurements, but also provides pivotal insights into the fundamental deformation behaviors of CdTe and similar II–VI compounds under various stress conditions. These insights are crucial for the innovative design and enhanced functionality of future flexible electronic devices.

Prototype Progress Report: Elastocaloric Heat Pumps

Abstract Both shape memory and elastocaloric effects are related to the reversible martensitic phase transformation. For the shape memory effect, thermal energy is used to drive the material through the phase transformation and recover its original shape. For the elastocaloric effect, mechanical energy is used to induce phase transformation, and the associated latent heat is used to pump heat for cooling or heating. Heat pumps based on the elastocaloric effect have a low environmental impact and are highly efficient. However, designing a prototype to harvest the full potential of the elastocaloric effect has been challenging. This article summarizes prototyping efforts undertaken by three different research groups.

High thermal stability effect of vanadium on the binary CuAl base alloy for a novel CuAlV high-temperature shape memory alloy

Abstract In this study, two high-temperature shape memory alloys (HTSMAs) of CuAlV with unprecedented chemical compositions were fabricated using the arc melting technique, followed by traditional ice-brine water quenching after the melting process. To characterize the shape memory properties and structure of the alloys, a series of tests including differential calorimetry (DSC and DTA), EDS, optical microscopy, and XRD were conducted. The DSC tests, performed at different heating and cooling rates, demonstrated highly stable reversible martensitic phase transformation peaks at high temperatures, which were also confirmed by the results of DTA tests. Microstructural XRD and optical microscopy tests were conducted at room temperature, revealing the martensitic structure of the alloys in both cases. Based on all the results, the effects of different minor amounts of vanadium additives directly on the CuAlV alloy were discussed.

A phase-field model for interactive evolution of phase transformation and cracking in superelastic shape memory ceramics

This work presents a modified phase-field model for accurate coupling of phase transformation and cracking in shape memory ceramics. The existing phase-field models underestimate the elastic response at the beginning of the mechanical response. We modified the chemical free energy to control the rate of phase transformation and consequently obtain a physical elastic response before initiation of phase transformation. First, the forward and reverse martensitic phase transformation in a superelastic single crystal 3 mol% yttria-stabilized tetragonal zirconia is studied. Then, the interaction between phase transformation and fracture under displacement-controlled loading condition is investigated. The model predicts a realistic mechanical response and the experimentally observed microstructure and crack deflection due to the phase transformation. In addition, the model captures the reverse phase transformation and the stress drop due to the crack propagation.

Degradation of recovery properties after heat treatment in hot-forged NiTiFe alloy

NiTiFe is a kind of low-temperature shape memory alloy (SMA) with great application potential in aerospace, such as pipe joints. Heat treatment can effectively regulate the microstructure and recovery properties (recovery strain and recovery stress) of SMAs. In this study, the microstructure, recovery properties, and transformation temperatures of hot-forged Ni47Ti50Fe3 shape memory alloy (SMA) before and after heat treatment (HT) with different HT temperatures (550–950 ℃) were analyzed. The results show that when the HT temperature is above 650 ℃, the density of high- and low-angle grain boundaries was reduced. The recovery properties of the hot-forged NiTiFe alloy deteriorate with increasing HT temperature. The initial hot-forged samples showed the best recovery strain of 8.32% and recovery stress of 449 MPa, which were reduced to 5.99% and 299 MPa, respectively, after 950 ℃ HT. The R↔B2 phase transformation temperature changes little with the increasing HT temperature. But the reverse martensitic phase transformation finish temperature increased from − 75 ℃ to − 50 ℃ after tensile deformation at − 180 ℃. The EBSD and HRTEM analyses of the microstructure after the free and constrained recovery tests were carried out. The {114}B2 and {112}B2 twins remained after the free recovery process, and more of those twins remained after the constrained recovery process. Although the recrystallization and grain growth can reduce the defects density which are the obstacles to the form of detwinning martensite, it also makes the deformation twins prone to form during the tensile and constrained recovery process, thus leading to poor recovery properties.

Domain memory effect in the organic ferroics

Shape memory alloys have been used extensively in actuators, couplings, medical guide wires, and smart devices, because of their unique shape memory effect and superelasticity triggered by the reversible martensitic phase transformations. For ferroic materials, however, almost no memory effects have been found for their ferroic domains after reversible phase transformations. Here, we present a pair of single-component organic enantiomorphic ferroelectric/ferroelastic crystals, (R)- and (S)-N-3,5-di-tert-butylsalicylidene-1-(1-naphthyl)ethylamine SA-NPh-(R) and SA-NPh-(S). It is notable that not only can their ferroic domain patterns disappear and reappear during reversible thermodynamic phase transformations, but they can also disappear and reappear during reversible light-driven phase transformations induced by enol–keto photoisomerization, both of which are from P1 to P21 polar space groups. Most importantly, the domain patterns are exactly the same in the initial and final states, demonstrating the existence of a memory effect for the ferroic domains in SA-NPh-(R) and SA-NPh-(S). As far as we are aware, the domain memory effect triggered by both thermodynamic and light-driven ferroelectric/ferroelastic phase transformations remains unexplored in ferroic materials. Thermal and optical control of domain memory effect would open up a fresh research field for smart ferroic materials.

Role of point defects in stress-induced martensite transformations in NiTi shape memory alloys: A molecular dynamics study

Shape-memory properties of equiatomic NiTi rely on the thermal- or stress-induced reversible martensitic phase transformation between the $B2$ and $B{19}^{\ensuremath{'}}$ phases. Irradiation defects suppress thermal-induced transformations, but their effects on stress-induced transformations are poorly understood. We use molecular dynamics to investigate the effect of vacancies, vacancy clusters, interstitials, and antisite defects on stress-induced transformations. All defects suppress the transformation, but vacancy clusters do so to the greatest extent, while antisite defects do so to the least extent. The responsible mechanisms are grain boundary pinning and chemical disordering.

In Situ Synchrotron X‐Ray Diffraction during High‐Pressure Torsion Deformation of Ni and NiTi

A down‐sized high‐pressure torsion device is developed to be used in an INSTRON deformation machine available at the High Energy Materials Science beamline at PETRA III. This setup allows to obtain synchrotron diffraction patterns in situ during severe plastic straining. Two different materials are studied: in pure Ni, the dislocation density and coherently scattering domain size are analyzed; in NiTi shape memory alloy, amorphization and a reverse martensitic phase transformation are investigated. The in situ experiments facilitate the characterization of the microstructural evolution of these materials depending on uniaxial loading, hydrostatic pressure, and torsional strain.

Enhancing fatigue life by ductile-transformable multicomponent B2 precipitates in a high-entropy alloy

Catastrophic accidents caused by fatigue failures often occur in engineering structures. Thus, a fundamental understanding of cyclic-deformation and fatigue-failure mechanisms is critical for the development of fatigue-resistant structural materials. Here we report a high-entropy alloy with enhanced fatigue life by ductile-transformable multicomponent B2 precipitates. Its cyclic-deformation mechanisms are revealed by real-time in-situ neutron diffraction, transmission-electron microscopy, crystal-plasticity modeling, and Monte-Carlo simulations. Multiple cyclic-deformation mechanisms, including dislocation slips, precipitation strengthening, deformation twinning, and reversible martensitic phase transformation, are observed in the studied high-entropy alloy. Its improved fatigue performance at low strain amplitudes, i.e., the high fatigue-crack-initiation resistance, is attributed to the high elasticity, plastic deformability, and martensitic transformation of the B2-strengthening phase. This study shows that fatigue-resistant alloys can be developed by incorporating strengthening ductile-transformable multicomponent intermetallic phases.

Influence of annealing temperature on microstructure and shape memory effect in austenite-martensite duplex Ni47Ti44Nb9 rolled sheets

The evolution of phase composition and microstructure such as grain size, dislocation and crystallographic texture in austenite-martensite duplex Ni47Ti44Nb9 rolled sheets during the annealing was systematically investigated. The influence of annealing temperature on shape memory effect was also examined. The strong {001} fiber texture tested in as-rolled sheets is identified as a wrong result, which is mistakenly introduced by X-ray Diffractometer, due to the interference of martensite. Different annealing temperatures result in variation of phase composition and microstructure. For instance, in the case of 600 °C, the phase fraction of austenite increases and the dislocations develop to form tiny sub-grains due to reverse martensitic phase transformation and recovery. The texture of NiTi austenite mainly concentrates on {111}〈110〉. In the case of 950 °C, the sub-grains develop to trigger recrystallization, resulting in larger grains and decreasing of dislocations and {111}〈110〉 texture intensity. In addition, the sheets annealed at 600 °C possess higher recoverability than those of 950 °C, which can be attributed to the combined role of grain size, dislocation and crystallographic texture.

Thermostructural shape memory effect observations of ductile Cu-Al-Mn smart alloy

The Cu-Al-Mn shape memory alloy (SMA) with a new different composition was fabricated by arc melting method. The characteristic shape memory effect (SME) property of Cu-Al based SMA was revealed by performing thermostructural measurements. The differential scanning calorimetry (DSC) tests were taken to observe the reversible martensitic phase transformation peaks of the alloy as evidence of SME property of the alloy. To determine the basic thermodynamical parameters of the alloy, these endothermic and exothermic transformation peaks were analyzed by the tangent differentiation method that was performed automatically by the DSC analyzing program over a manually selected part on the DSC curve and by these analyses the characteristic martensitic transformation temperatures (working temperatures) that found below 100°C and the enthalpy change values of the alloy were directly obtained. The other kinetic transformation parameters of the alloy - the entropy change, hysteresis, and equilibrium temperature - were also determined. The common high-temperature behavior of the Cu-Al based Heusler alloys was detected by differential thermal analysis (DTA) measurement. The XRD and metallography tests that were conducted at room temperature showed the presence of M18R and the dominant 2H martensite structures that formed in the alloy and this dual martensitic structure was also prescribed by determining the theoretical e/a ratio of the alloy. Furthermore, the microhardness tests on the alloy demonstrated the high ductility feature of the alloy. All results demonstrated that the CuAlMn alloy exhibiting a shape memory effect property can be useful in smart alloy applications.

Formation of [formula omitted] contraction twins in titanium through reversible martensitic phase transformation

We report the discovery of a non-conventional {112¯2} twinning mechanism in α-titanium through reversible α→ω→α martensitic phase transformations. Specifically, the parent α-phase first transforms into an intermediate ω-phase, which then quickly transforms into a twin α-phase, leading to the formation of {112¯2} contraction twins. In addition, we prove that the reversible α→ω→α phase transformations follow strict orientation relations between the parent α-, intermediate ω-, and twin α-phases. Finally, we demonstrate that our mechanism agrees with classical twinning theory in the shuffle, shear, and conjugate twinning plane. This study reveals the important role of the intermediate ω-phase in the twinning process, adding critical details to the existing mechanism of {112¯2} twinning.

Effect of off-stoichiometric compositions on microstructures and phase transformation behavior in Ni-Cu-Pd-Ti-Zr-Hf high entropy shape memory alloys

High entropy shape memory alloys (HE-SMAs) show reversible martensitic phase transformations at elevated temperatures. HE-SMAs were derived from binary NiTi, to which the elements Cu, Pd, Zr and Hf are added. They represent ordered complex solid solutions. Their high temperature phase is of B2 type, where the added elements occupy sites in the Ni-(Cu, Pd) and Ti-sub-lattices (Zr, Hf). In the present study, advanced microstructural and thermal characterization methods were used to study the effects of the additional alloy elements on microstructures and phase transformations. The ratios of Ni-equivalent (Ni, Cu, Pd) and Ti-equivalent (Ti, Zr, Hf) elements in HE-SMAs were varied to establish systems that correspond to stoichiometric, under- and over-stoichiometric binary alloys. It is shown that basic microstructural features of cast and heat-treated HE-SMAs are inherited from the nine binary X–Y subsystems (X: Ni, Cu, Pd; Y: Ti, Zr, Hf). The phase transition temperatures that characterize the martensitic forward and reverse transformations depend on the concentrations of all alloy elements. The data obtained demonstrate how martensite start temperatures are affected by deviations from the composition of an ideal stoichiometric B2 phase. The findings are discussed in the light of previous work on the concentration dependence of SMA transformation temperatures, and directions for the development of new shape memory alloy compositions are proposed.

Effect of hydrogen embrittlement towards thermal and mechanical behavior of NiTi shape memory alloy

NiTi arch wires are susceptible to hydrogen embrittlement upon contact with dental brackets in oral cavity during orthodontic treatment. This study investigated the effect of hydrogen absorption and diffusion over time towards the thermal and mechanical properties of NiTi shape memory wire. The hydrogen absorption process was carried out via electrolytic charging at constant current density for 16 hours in 1.0 weight percent (wt.%) sodium sulphate solution. The hydrogen charged wires were aged at room temperature in air for different durations to allow further inward diffusion of the hydrogen into the specimens. The results show that after hydrogen charging, the latent heat of forward and reverse martensitic phase transformation of the NiTi wire changed from 19.96 to 11.98 J/g, and 19.21 to 13.42 J/g, respectively. Further suppression and disappearance of thermal transformation peaks were observed as the charged specimen aged for 7 days. The transformation stress level on tensile deformation increased by almost 90 MPa after hydrogen charging, and exhibited non-flat stress plateau after further aging.

Nucleation of {1012} Twins in Magnesium through Reversible Martensitic Phase Transformation

We report the discovery of a rigorous nucleation mechanism for {101¯2} twins in hexagonal close-packed (hcp) magnesium through reversible hcp-tetragonal-hcp martensitic phase transformations with a metastable tetragonal phase as the intermediate state. Specifically, the parent hcp phase first transforms to a metastable tetragonal phase, which subsequently transforms to a twinned hcp phase. The evanescent nature of the tetragonal phase severely hinders its direct observation, while our carefully designed molecular dynamics simulations rigorously reveal the critical role of this metastable phase in the nucleation of {101¯2} twins in magnesium. Moreover, we prove that the reversible hcp-tetragonal-hcp phase transformations involved in the twinning process follow strict orientation relations between the parent hcp, intermediate tetragonal, and twin hcp phases. This phase transformation-mediated twinning mechanism is naturally compatible with the ultrafast twin growth speed. This work will be important for a better understanding of the twinning mechanism and thus the development of novel strategies for enhancing the ductility of magnesium alloys.

Chemical complexity, microstructure and martensitic transformation in high entropy shape memory alloys

High entropy shape memory alloys (HESMAs) represent a relatively young class of functional materials. They show a reversible martensitic phase transformation which allows to exploit shape memory effects at relatively high temperatures. HESMAs represent ordered complex solid-solutions. Their high temperature phase is of B2 type, and various elements (e.g. Ni, Cu, Ti, Zr, Hf) occupy sites in specific sub-lattices. In the present work, we study the processing and the functional properties of HESMAs. We study effects of chemical complexity on solidification microstructures and martensitic transformations. Binary, ternary, quaternary, quinary and senary model alloys were investigated using advanced microstructural and thermal characterization methods. The results show that element partitioning during solidification results in a redistribution of individual alloy elements in dendritic/interdendritic regions. Surprisingly, the atomic ratios of the two groups of elements which occupy the Ni- (first group: Ni, Cu and Pd) and Ti-sub-lattice (second group: Ti, Zr, Hf) are maintained. This allows the material to form martensite throughout its heterogeneous microstructure. The effect of chemical complexity/composition on martensite start temperatures, MS, is discussed on the basis of valence electron concentrations, cV. Some of the alloys fall into MS(cV)-regimes which are uncommon for classical Ni-Ti-based shape memory alloys. In the present work, a new HESMA of type NiCuPdTiZrHf was identified which has the potential to provide maximum shape memory strains close to 15%.

Thermo-magneto-mechanical coupling dynamics of magnetic shape memory alloys

This paper is to improve our previous phase-transformable material constitutive model (Chen et al., 2014) and implement it in a mass-spring configuration to study the recently discovered dynamic phenomena in (Zhang et al., 2018a, 2018b): the harmonic oscillator of the cyclic magnetic-field-induced deformation in Magnetic Shape Memory Alloys (MSMA) is modulated by a thermo-magneto-mechanical coupling feedback loop where the cyclic field-induced Martensite Reorientation (MR) provides cyclic dissipative deformation whose dissipation heat influencing the material temperature modifies the temperature-dependent MR process and/or triggers the martensite-to-austenite Phase Transformation (PT) to modify the martensite volume fraction so as to change the deformation oscillation amplitude. Such a feedback loop causing the amplitude modulation was ignored in the existing models. This paper develops a dynamic model to include the feedback loop by considering the heat balance (i.e. the interactions among heat generation due to MR, the latent heat release/absorption of PT and the heat transferred to the ambient), by introducing proper kinetics of the temperature–dependent MR and PT processes, and by taking into account the inertial dynamic effect with a simple mass-spring configuration. The simulation based on the model captures all the main features of the experimental phenomena and provides the relations between the macroscopic responses (the deformation amplitude and temperature evolution) and the microscopic physical mechanisms (the kinetics of MR and PT). The study reveals that, with the coupling effects, the MSMA system can be smart to keep its temperature constant by self-organized microstructures under varying external thermo-magneto-mechanical boundary conditions. Moreover, the forward and reverse martensitic phase transformations influenced by the coupling dynamics show little hysteresis, contrasting to the usual hysteretic kinetics of the quasi-static martensitic phase transformation.

Microstructural Characterization and Mechanical Behavior of NiTi Shape Memory Alloys Ultrasonic Joints Using Cu Interlayer.

NiTi shape memory alloys (SMAs) are a class of functional materials which can be significantly deformed and recover their original shape via a reversible martensitic phase transformation. Developing effective joining techniques can expand the application of SMAs in the medical and engineering fields. In this study, ultrasonic spot welding (USW), a solid-state joining technique, was used to join NiTi sheets using a Cu interlayer in between the two joining sheets. The influence of USW process on the microstructural characteristics and mechanical behavior of the NiTi joints was investigated. Compared with conventional fusion welding techniques, no intermetallic compounds formed in the joints, which is extreme importance for this particular class of alloys. The joining mechanisms involve a combination of shear plastic deformation, mechanical interlocking and formation of micro-welds. A better bonding interface was obtained with higher welding energy levels, which contributed to a higher tensile load. An interfacial fracture mode occurred and the fracture surfaces exhibited both brittle and ductile-like characteristics with the existence of tear ridges and dimples. The fracture initiated at the weak region of the joint border and then propagated through it, leading to tearing of Cu foil at the fracture interface.

Size effects of NiTi nanoparticle on thermally induced martensitic phase transformation

Molecular dynamics simulations are performed to study the thermally induced martensitic phase transformation (B2-B19′) of NiTi free-standing nanoparticles. We successfully reproduce the experimental observations that the martensitic transformation temperature decreases with reducing particle size and the martensite may form self-accommodated herringbone morphology. It is found that a core-shell structure with shell thickness of 0.3 nm (approximately equal to one unit cell) exists in NiTi nanoparticles and the B2-B19′ transformation is dominated by the core region. In addition, three detwinning manners are observed in the reverse martensitic phase transformation (B19′-B2) process. It is concluded that the suppression of B2-B19′ transformation in NiTi nanoparticles is due to the lack of nucleation sites and the size dependent martensitic morphology originates from the constraints of surface shell.