Temperature-controlled martensitic phase transformations in a model NiAl alloy

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.

The temperature memory effect of shape memory alloy (SMA) induced by an incomplete martensitic inverse phase transformation has the potential values in various engineering fields. It is of theoretical and practical interests to establish a phase transformation model which overcomes the limitation that previous phase transformation models of SMA fail to take into account the temperature memory effect. In this paper, the curves of heat flow versus temperature of SMA during the martensitic phase transformation and martensitic inverse phase transformation are described as the quadratic functions of temperature. A new phase transformation model of SMA is developed based on the differential relationship between martensitic volume fraction and phase transformation free energy. Numerical results show that the new model well predicts the phase transformation behaviors of SMA associated with the temperature memory effect.

Unveiling the mechanism of the carbon ordering and martensite tetragonality in Fe–C alloys studied via deep-potential molecular dynamics simulations

A reverse martensitic phase transformation was observed in Nb-enriched Nb-Co multilayers induced by room temperature 200 ke V xenon ion mixing. Further experiments revealed that this bcc-fcc transition proceeds in two steps, i.e., bcc-hcp and hcp-fcc. A crystallographic model is proposed to explain the two-step transition through shearing and sliding, which are mediated by irradiation-induced defects and strain in the films. In addition, the existence of the hcp and fcc metastable states in the Nb-Co system was confirmed by high-temperature solid state interdiffusion of the corresponding multilayers.

Atomistic study on the phase transformation in NiTi under thermal cycling

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

Characterization of a Metastable Austenitic Stainless Steel with Severe Plastic Distortions

In recent years, a lot of investigations have been focused on ferromagnetic shape memory alloys (FSMAS), such as Ni2MnGa, FePd, CoNi and FeNiCoTi [1–4], due to their large magnetic-field induced strain (MFIS) and high response frequency. Of the above alloys Ni Mn Ga alloy is the only known Heusler alloy exhibiting thermal martensitic phase transformation. It is reported up to now that a maximum 6% of MFIS has been obtained in Ni49.8Mn28.5Ga21.7 single crystal by applying a constant 2 MPa compression stress [5]. However, few results have been reported about the temperature dependence of MFIS in NiMnGa alloy. The purpose of the present paper is to measure the MFIS along the [110] direction of Ni50.5Mn26.2Ga23.4 single crystal at different temperatures. It is interesting to find that the MFIS between the temperature of Ms and Mf is larger than the MFIS below Mf. The specimen used in this study was cut from a bulk single crystal with composition of Ni50.5Mn26.2Ga23.4 (at%). The single crystal was grown in the [110] direction according to the cubic parent phase using a Czochralski instrument with a cold crucible system. This single crystal was annealed at 850 ◦C for 12 h, then quenched into ice water to obtain high L21 ordering [6]. An orientation along the specimen length direction was confirmed to be [110] by Laue X-ray diffraction. The dimensions of specimens for MFIS measurements were 2 mm × 4 mm × 6 mm. The magnetic field induced strain was measured by a strain-gauge attached to the sample which was placed in a furnace between the electromagnets. The external magnetic field was applied parallel to the [110] direction of the sample. The sample was cooled from high temperature to a temperature far below Mf. Then the MFIS was measured at the temperatures of interest. The phase transformation temperatures were determined by a low field ac susceptility method. Fig. 1 shows the χ–T curve of the experimental alloy. It can be seen that, the martensitic transformation start temperature (Ms) and finish temperature (Mf) are 279 K and 274 K respectively. And the reverse transformation temperature (TA) is 283 K. The Curie temperature (Tc) is about 370 K higher than the martensitic transformation and reverse transformation temperatures indicating that both autensite and martensite are ferromagnetic in this alloy. It is seen that the value of Ms–Mf is very small which means that thermal martensitic transformation happens quickly. Fig. 2 shows a typical curve of the MFIS measured along the [110] direction in a reverse magnetic field at 244 K below Mf. It was found that the sample shrinks in the external magnetic field. A small MFIS, about −600 ppm, was obtained with an axial magnetic field up to 2 T. Further, it can been seen the MFIS is reversible and reaches saturation at about 0.8 T. All strain recovers after returning the field to zero. Similar MFIS curves are obtained irrespective of the direction of the applied magnetic field. Fig. 3 depicts the magnetic field induced strains as a function of magnetic field in the temperature range between 244 K and 276 K. According to the different phase states at various temperatures, the experimental results can be divided into two temperature stages: Stage I. Full martensite state which is below Mf. For the specimens tested at 247 K, 255 K and 263 K, the saturated MFIS is only−600 ppm. And with the increasing of temperature only an increase of 100 ppm is observed. In this stage, all the MFIS is attributed to the motion of martensite twin boundaries caused by the magnetic field because only martensite phase exists. It is known that [110] is the hard axis of magnetization for NiMnGa alloy. According to R.C. O’Handley’s model [6] the experimental Ni Mn Ga alloy has weak anisotropy. This means that only a weak driving force could be provided on the twin boundaries regardless of the strength of applied field, which leads to very a small MFIS in the full martensite phase. Stage II. Parent phase and martensite co-exist which is between Ms and Mf. For the specimens tested at 275 K, the strain produced is as large as −900 ppm, meanwhile, of the total strain about −300 ppm strain does not recover when the magnetic field is removed. At 276 K, the saturated MFIS jumps abruptly to −1700 ppm, which is nearly three times as large as that of the specimen measured below Mf, and a greater irreversible MFIS, nearly −1200 ppm, is observed. Clearly, the strain caused by thermal phase transformation plays an important role in the increase of this MFIS. Based on the analysis of experimental results, the temperature dependences of saturated MFIS and the strain recovery

We present molecular dynamics (MD) simulations of a load-induced martensitic phase transformation in pseudo-elastic Lennard-Jones crystals. The model material exhibits martensitic transformations between cubic and hexagonal lattice structures in 2D which represent austenite and martensite. Under axial loading two martensite variants are favoured out of four generic variants possible with this model. In nucleation-and-growth processes the formation of martensite domains are observed in MD simulations and the reverse process upon unloading. Two possible re-transformation mechanisms are identified, a reversible and a reconstructive type. Reversible re-transformations conserve the reference unit cells, while the reconstructive mechanism involves the generation of dislocation motions which destroy the reference unit cells by slip. Both types re-establish the square lattice. While the reversible type represents the predominant reverse transformation mechanism, the reconstructive type is of special importance because it produces lattice defects, and plastic deformation which change both the evolution of subsequent transformation cycles and the magnitude of the phase transformation load.

Nano-twinned Fe-Mn alloy prepared by reverse martensitic phase transformation

Employing ab initio calculations, the relative stabilities of the possible crystalline structures at compositions of CoTa 3 , Co 3 Ta and CoTa were predicted and based on thermodynamic calculations, the possible structural phase transitions were predicted over a broad composition range for the metastable crystalline Co–Ta alloys. To confirm the relevance of the calculations, ion-beam-mixing experiments were conducted using Co–Ta multilayered films with various overall compositions. It turned out that the experimentally observed formation of metastable crystalline Co–Ta phases agreed reasonably well with the theoretical predictions and that the glass-forming range of the Co–Ta system was determined to be within 25–70 at. % of Ta, matching fairly well with that estimated from thermodynamic calculations. Moreover, a reverse martensitic phase transformation was observed in the CoTa 2 multilayers upon ion irradiation. In addition, steady-sate thermal annealing of the Co–Ta multilayered films also provided some...

Atomistic simulations are utilized to quantify the effects of surface composition on stress-induced B2 to body centred tetragonal (BCT) martensitic phase transformations in intermetallic nickel aluminium (NiAl) nanowires. The simulations show that the phase transformation is observed in all considered cases, regardless of the material composition of the transverse {100} surfaces of the initially B2 wires. The results indicate that, for ⟨100⟩ oriented B2 wires with {100} transverse surfaces, the {100} orientation and not the material composition of the {100} surfaces is the dominant factor in controlling the ability of NiAl alloys to undergo martensitic phase transformations at nanometer scales.

The role of electrons during the martensitic phase transformation in NiTi-based shape memory alloys

Low hysteresis and high cyclic stability in a Ti50Ni45.2Cu1Fe3.8 shape memory alloy

Shape memory alloys (SMAs) exhibit the ability to absorb large dynamic loads and, therefore, are excellent candidates for structural components where impact loading is expected. Compared to the large amount of research on the shape memory effect and/or pseudoelasticity of polycrystalline SMAs under quasi-static loading conditions, studies on dynamic loading are limited. Experimental research shows an apparent difference between the quasi-static and high strain rate deformation of SMAs. Research reveals that the martensitic phase transformation is strain rate sensitive. The mechanism for the martensitic phase transformation in SMAs during high strain rate deformation is still unclear. Many of the existing high strain rate models assume that the latent heat generated during deformation contributes to the change in the stress-strain behavior during dynamic loading, which is insufficient to explain the large stress observed during phase transformation under high strain rate deformation. Meanwhile, the relationship between the phase front velocity and strain rate has been studied. In this dissertation, a new resistance to phase transformation during high strain rate deformation is discussed and the relationship between the driving force for phase transformation and phase front velocity is established. With consideration of the newly defined resistance to phase transformation, a new model for phase transformation of SMAs during high strain rate deformation is presented and validated based on experimental results from an austenitic NiTi SMA. Stress, strain, and martensitic volume fraction distribution during high strain rate deformation are simulated using finite element analysis software ABAQUS/standard. For the first time, this dissertation presents a theoretical study of the microscopic band structure during high strain rate compressive deformation. The microscopic transformation band is generated by the phase front and leads to minor fluctuations in sample deformation. The strain rate effect on phase transformation is studied using the model. Both the starting stress for transformation and the slope of the stress-strain curve during phase transformation increase with increasing strain rate.