Transformation In NiTi Alloy Research Articles

The Relationship between Chemical Composition of the Alloy and the Parameters of the Martensitic Transformation in NiTi Alloys

The Relationship between Chemical Composition of the Alloy and the Parameters of the Martensitic Transformation in NiTi Alloys

Repetitive Nanosecond Laser-Induced Oxidation and Phase Transformation in NiTi Alloy

NiTi shape memory alloys, known as Nitinol, are highly valuable in medical fields for their unique attributes, including superelasticity, wear resistance, and biocompatibility. Laser treatment provides precise control over surface characteristics, enhancing biocompatibility. This study focuses on the effects of laser irradiation on NiTi alloy surfaces, particularly considering the number of laser scans and their impact on surface features. Even at low laser power, multiple high-frequency scans significantly alter surface roughness and induce phase transformation. After 16 repeated laser irradiations, amorphous Ti oxide transforms into crystalline anatase. Remarkably, anatase can further transform into rutile due to the influence of Ni nearby and TiO, due to insufficient oxygen content. The most notable outcome is the formation of a thick Ti oxide layer, causing unbound Ni to emerge on the surface, resulting in a Ni oxide layer. These findings highlight the importance of precisely adjusting laser parameters to achieve tailored surface properties for medical applications, addressing challenges and enhancing biocompatibility.Graphical abstract

Neutron diffraction study on strain rate dependent mechanical response in superelastic NiTi alloy: Bulk structural evolution and constitutive model

As expanded service environments, it is necessary to investigate the deformation behavior of NiTi shape memory alloy under dynamic and impact loadings. In this paper, we study bulk structural responses of a superelastic NiTi alloy in strain rates from 10−5 s−1 to 3 × 103 s−1 by neutron diffraction method. The objective is to quantify structural parameters of NiTi alloy at various strain rates, and compare the differences in deformation mechanisms. Further, we derive a phenomenological model based on experimental cognition to describe the strain rate dependent mechanical responses. The results demonstrate that impact loading obviously prevents stress-induced martensitic transformation in NiTi alloy. The austenite parent phase undergoes more significant slip deformation during high-speed deformation, which is hardly observed in quasi-static compression. Meanwhile, the austenitic crystal orientation during impact loading is not essentially different from that of quasi-static condition. Through an estimation of temperature effect, the strain rate effect is caused by adiabatic temperature rise. In validation of the phenomenological model, it is proved that the model well reproduces not only stress-strain curves but also structural parameters obtained from the neutron diffraction experiments.

An atomistic study of self-accommodation martensite morphologies and microstructure evolution during forward and reverse martensitic transformations in single crystal and bicrystal NiTi alloys

The forward and reverse martensitic transformations in NiTi alloys with different crystallography conditions, in terms of [100]-oriented and [110]-oriented single crystal models as well as a bicrystal model possessing twist grain boundary, are studied deeply using molecular dynamics simulations. An atomic tracing method is proposed to identify the specific numbers of B19′ martensite variants, hence the self-accommodation martensite morphologies and microstructure evolution during cooling and heating are clearly demonstrated at the atomic scale. The triangular self-accommodation morphology consisted of three correspondence variants (CVs) possessing {1¯1¯1}M type I twin relationships is formed in the NiTi single crystals at the beginning of the martensitic transformation, and stays settled till the end of the transformation. Whereas three types of self-accommodation morphologies including triangular, “herring-bone” and mixed morphologies are formed in the bicrystal, due to the geometrical constraint induced by the grain boundary. The mixed self-accommodation morphology is unstable, which occurs briefly during martensitic transformation and thus evolves into “herring-bone” morphology ultimately. During reverse transformation, B2 austenite phase nucleates preferentially at the triple-junctions of the triangular self-accommodation morphology in NiTi single crystals, and propagates along the junction plane rather than the (001)M compound twin boundaries in the bicrystal alloy.

Influence of heat treatment on the structure and martensitic transformation in NiTi alloy produced by wire arc additive manufacturing

The influence of heat treatment on the structure and martensitic transformation in the 5-layered NiTi sample deposited by wire arc additive manufacturing on Ti substrate was studied. The Ti2Ni phase was found on the boundary of the Ni-rich NiTi grains after annealing that was not typical for NiTi alloys produced by conventional techniques. This was due to the as-built sample contained Ti-rich NiTi phase in the interdendritic space. During annealing, this phase decayed to the NiTi and Ti2Ni phases and led to the formation of the Ti2Ni precipitates on grain boundaries. Annealing led to homogeneous composition of the NiTi phase within the layer but kept it to be different in various layers. This was caused by the Ti2Ni and Ni4Ti3 (Ni3Ti2) precipitates formed during annealing, but the diffusion of the Ti and Ni elements between layers hardly occurred. Annealing decreased the difference in Ni concentration in the NiTi phase of the 3rd to 5th layers that decreased the temperature range of the martensitic transformation. It was shown that the properties of the NiTi sample deposited to Ti substrate after post-production annealing are close to the properteis of the NiTi sample deposited on the NiTi substrate.

Dual beam-shear differential interference microscopy for full-field surface deformation gradient characterization

We propose a new experimental mechanics method: dual beam-shear differential interference microscopy (DInM) for full-field surface deformation measurement. The method integrates the principles of differential interference contrast, photoelasticity, digital image correlation and the theories of continuum mechanics for a 4D quantification of the surface topography. Our first DInM prototype provides the lateral resolution of 787nm for up to 12% measurable out-of-plane strains. The resolution can be improved by using the shorter spectrum light and higher magnification objective lens. We use our system to characterize the buckling profile of Si microribbon on an elastomer substrate, which was verified by atomic force microscopy. We also demonstrate the stress-induced phase transformation in NiTi alloy by our system. The evolution of the surface topographies and the full-field deformation gradients are successfully captured and quantified. The dynamic measurement provides the information to calculate the relative transformation strains between phases of different symmetries. The establishment of dual beam-shear DInM opens a new avenue in the field of experimental meso/micromechanics.

Ductile–Brittle Variation Phenomenon and a Special Transformation-Induced Plasticity Effect in NbTi-NiTi Composite

In order to study the transformation-induced plasticity (TRIP) effect in a NiTi shape-memory alloy composite, an in situ NbTi-NiTi composite was prepared by vacuum arc melting, hot forging and wire drawing. An unusual ductile–brittle variation phenomenon was observed by means of a series of tensile tests. The composite was brittle in the absence of transformation at 200 °C, and the fracture strain was about 2%. With lowering the tensile temperature, the composite became ductile when stress-induced martensitic transformation occurred, and the elongation increased to about 18% (a ninefold increase) during tensile test at room temperature. This ductile–brittle variation phenomenon indicates that the stress-induced martensitic transformation of NiTi alloy helps to improve the ductility of the composite, which is just the exhibition of TRIP effect in the NbTi-NiTi composite. Different from conventional TRIP steels, the TRIP behavior in NiTi induces only an increase in the elongation, but not any increase in fracture strength.

DSC and microstructure analysis of high temperature Ni-Ti-Hf, low hysteresis Ni-Ti-Cu and conventional super-elastic and shape memory Ni-Ti alloy ingots and wires

The progression of martensitic transformations in calorimetric experiments on NiTi-based shape memory alloys (SMA) depends significantly on the material composition, processing history and the thermo-mechanical treatment which is performed to prepare the material for use in application. The phase transformations in Ni-Ti alloys are first order transitions, i.e. the microstructure symmetry changes occur spontaneously and superheating and supercooling effects emerge. Thus, the thermal scanning rate in differential scanning calorimetry (DSC) affects the observation of important material parameters for the characterization of shape memory alloys like onset temperatures, hysteresis width and change in enthalpy of phase transformations. This work depicts how the transformation characteristics as observed by DSC measurements vary significantly by altering the alloy composition and processing conditions of the material. An in-depth calorimetric study on a wide range of NiTi-based alloys which were prepared as heat treated ingot and processed wire was performed. In order to evaluate also the rate effect on the transformation of distinct alloys, DSC rates from 1 to 30 °C/min were tested, highlighting the interplay between thermal scanning velocity and material parameters. Manifold results are presented on how chemistry, microstructure, DSC sample geometry and processing stresses in the materials contribute to award the alloys diverging sensitivity to the thermal scanning rate. Deviations in binary Ni-Ti composition alter the sensitivity to heating and cooling rate, e.g. martensitic transformation in annealed super-elastic (SE) wire is stronger affected by the rate than shape memory wire due to differences in the precipitation behavior. Ni-Ti-Hf high temperature shape memory alloys show a rather small enlargement of hysteresis, but a huge shift in transformation temperatures, while Cu addition to Ni-Ti causes very sensitive hysteresis changes up on modifying thermal rates which is assigned to B19 orthorhombic transformation. Transformation parameters of wires are less affected by DSC rates than ingots due to their high surface to volume ratio, while sample weight and surface finish is observed to have a minor impact. Small grains and residual strains owing to processing history of drawn wire tend to stabilize the transformation. In materials which exhibit R-phase transformation, the low heat capacity of this phase could be responsible for its minor sensitivity to variations in DSC rate.

An improved phase field method by using statistical learning theory-based optimization algorithm for simulation of martensitic transformation in NiTi alloy

An improved phase field method by using statistical learning theory based optimization algorithm is developed for solving the phase field equations through building simple relationships between the key phase field variables and the phase evolution driving force, and using statistical analysis of mass computed data during phase field simulation. Phase field simulation results of growth of R phase and the B2–R phase transformation in a Ni-rich Ni50.5Ti49.5 alloy by using the proposed statistical strategy algorithm are compared with that using the conventional numerical algorithm, which demonstrates that with coupling the statistical learning theory, i.e., by means of the optimization algorithm, the credible simulated microstructure is obtained while maintaining high accuracy, and meanwhile the computational time has been significantly reduced.

Martensitic transformation in TiNi alloy after surface modification done by hydroxyapatite layer deposition

ABSTRACTA hydroxyapatite (HAp) layer was deposited on the surface of an NiTi alloy using a spark plasma technique. The effects of spraying conditions on the martensitic transformation were investigated in three heat-affected zones: the zone under the handle, the transition zone and in a representative zone with HAp coverage. In the representative area, a reduction in the amount of non-transformable phases was observed. A part of the sublayer showed a nanocrystalline or completely amorphous structure. This sublayer was approximately 30 µm thick. Structural changes contributed to the reduction of the temperature of the reverse martensitic transition down to 25°C. Moreover, modification of the surface revealed a positive influence on the transformation course by narrowing its temperature range and increasing its enthalpy.This paper is part of a thematic issue on Titanium.

Effect of hydrostatic pressure on thermally induced phase transformation in NiTi alloy: A molecular dynamics study

Molecular dynamics simulations are performed to study the influence of hydrostatic pressure P on the thermally induced phase transformations in NiTi alloy during thermal cycling between 500 K and 5 K. It is found that the martensitic transformation path shows strong dependence on the P: During cooling process, the NiTi alloy undergoes single transformation of B2 → B19′ at 0 ≤ P ≤ 3 GPa, two-step transformation of B2 → B2 + R → B19′ at 3 < P ≤ 5 GPa, and B2 → R at 5 < P ≤ 8 GPa. The martensite start temperature (Ms) decreases from 293 to 236 K with increasing P from 0 to 5 GPa, and the start temperature of R phase (Rs) decreases from 333 to 233 K with increasing P from 6 to 8 GPa. The shift of Ms or Rs can be explained by the Clausius-Clapeyron equation. Our work provides novel insights into the martensitic phase transformations in NiTi alloy.

Dissipation of cavitation-induced shock waves energy through phase transformation in NiTi alloys

Energy dissipation in aluminum plates coated with a thin layer of Nickel–Titanium (NiTi) alloy subjected to a shock loading is studied. An experimental framework is designed and implemented to apply a shock load on the plates, using a liquid jet generated by collapsing of spark-generated bubbles under water. In this setup, a high-speed camera and a hydrophone are used to characterize the generated bubbles. Moreover, a laser Doppler vibrometer (LDV) and multiple strain gauges are implemented to investigate the vibration and mechanical responses of plates subjected to the water jet induced by cavitation bubbles. Comparing the mechanical response of an Al–NiTi plate with the same system in which the NiTi coating layer is replaced with an Al layer of the same thickness, shows that the propagation of shock waves is activating the forward and reverse phase transformations in NiTi that consequently results in dissipation of the shock wave energy in the material. The results approve the excellent cavitation erosion resistance of NiTi alloys which originates from their superior capability in attenuating stress waves and energy dissipation due to phase transformation, when the shock wave propagates in the material. Finite element simulations are also performed to investigate the energy dissipation capability of aluminum plates coated with a NiTi layer, when the structure vibrates by exerting and removing a concentrated transverse force at the center. When the energy of incident shock waves caused by cavitation bubbles is strong enough, the energy dissipation due to global structural vibrations can complement the energy dissipation due to the phase transformation. The computational results show that the observed energy dissipation in the experiments is exclusively caused by the shock wave propagation, not the transverse vibrations, and also investigate the details of energy dissipation for both NiTi and Al–NiTi composite plates subjected to a cyclic transverse load at the center.

Symmetrical tilt grain boundary engineering of NiTi shape memory alloy: An atomistic insight

The role of austenitic symmetric tilt grain boundaries (STGBs) on the martensitic transformation in NiTi alloy during thermal cycling between 500K and 10K are studied from molecular dynamics simulations. Four austenitic STGBs including Σ3[11¯0](111), Σ3[11¯0](112), Σ9[11¯0](114) and Σ9[11¯0](221) have been considered. Through martensitic transformation, the austenitic STGBs transform to martensitic grain boundaries (GBs) and martensitic twin boundaries form in the interior of the simulation models. After austenitic transformation, the martensitic GBs recover to the austenitic STGBs and the twin boundaries disappear. We find that the roles of STGBs on the martensitic transformation can be divided into three groups based on the martensite start temperature (Ms). (1) The Σ3[11¯0](112) and Σ9[11¯0](221) STGBs retard the martensitic transformation, i.e., decreases Ms, by raising the potential energy of the grain interior. (2) The Σ9[11¯0](114) STGB promotes the martensitic transformation by lowering the potential energy of the grain interior. (3) The effects of the Σ3[11¯0](111) STGB depend on whether twins form during cooling. The evolutions of the microstructures during thermal loadings are analyzed using the von Mise shear strain; insight obtained in this work paves the way for engineering NiTi shape memory alloy through grain boundaries.

Thermokinetic EMF under direct phase transformation

This paper presents an analysis of dependence of thermokinetic EMF on temperature in the cooling zone in the course of direct phase transformation in TiNi alloy. The distribution of temperature along TiNi wire sample is modelled for the case of movement of the cooling zone with a constant rate. Increase of thermokinetic EMF occurring when cooling zone passes through locally deformed area.

Расчет термокинетической ЭДС при обратном фазовом переходе в никелиде титана

It is known that a thermokinetic electromotive force (EMF) in TiNi is induced due to thermoelastic phase transformations in a local zone. The initiation of these transformations is possible during the movement of both the local heating and cooling zones along a TiNi alloy sample. Local heating of the conductor results in a reverse phase transfomation (T≥Аs), while cooling results in a direct phase transfomation (T≤Мs). Since the amount of the martensite phase in TiNi based materials is determined by the temperature of a sample, it has been assumed that the value of thermokinetic EMF depends on the temperature in the heating zone too. Therefore, the aim of the present work was to determine the dependence of the thermokinetic EMF on heating temperature. In this paper, we propose a physical model, in which the thermokinetic EMF is initiated as a result of the potential difference in areas with direct and reverse phase transformations. These areas occur during the motion of a heating zone (T>As) along the TiNi alloy. Mathematical relationships which allow for a calculation of thermokinetic EMF initiated by reverse phase transformation in TiNi alloys and its dependence on temperature in the heating zone have been obtained. It has been found that thermokinetic EMF is induced when alloy temperature in the heating zone reaches As and then grows to 0.23 mV with increasing of alloy temperature up to Af. A further increase of the heating zone temperature does not affect the value of thermokinetic EMF. The results of calculations of thermokinetic EMF in TiNi alloys during reverse phase transformation obtained are in good agreement with experimental data.

Anti-precursor effect of Fe on martensitic transformation in TiNi alloys

The lack of information about point defects presents a big hurdle to a complete understanding of the premartensitic phenomena in shape memory alloys. Using first-principles density functional theory calculations, we have studied the behavior of substitutional Fe in both B2 and R phases of Ti50Ni50−xFex, at different concentrations. We show that with no need of aggregation, an Fe atom can incur a drastic atomic scale local lattice distortion in the R phase and makes its nearest neighbor environment like an intermediate structure between B2 and R. In addition, its solution energy is lower in B2 than in R phase, indicating that Fe tends to stabilize the austenite against the martensite. This means that Fe exerts an anti-precursor effect in the B2 to R transformation, in contrast to the assumption that Fe fosters the growth of R-like nanodomains. The Fe–Fe interaction is very weakly attractive in both phases, suggesting that only short-range variation in concentration is possible and the formation of precipitates is unlikely. The difference in cohesive energy of B2 and R phases, and the martensitic transformation temperature as well, decreases with the increasing Fe concentration, until the Fe content exceeds a critical level at which point the R phase turns into B2 phase automatically and the martensitic transformation is suppressed altogether. Such a complete suppression is an excellent analogue to the concept of frozen strain glass. Since Fe atoms at different Ni sites in R phase have divergent solution energy, the anti-precursor effect of Fe may vary at different spot in the material, and so will be the local transformation temperature.

Investigation on Crack Tip Transformation in NiTi Alloys: Effect of the Temperature

The effect of the temperature on crack tip transformation in Nickel–Titanium (NiTi) shape memory alloys was analyzed in this work by means of experimental and analytical approaches. In particular, single edge crack specimens were analyzed for two different values of the testing temperature in the pseudoelastic regime of the alloy, i.e., T = 298 K and T = 338 K. The thermal-dependent phase transition mechanisms occurring at the crack tip region were studied by analyzing data obtained from digital image correlation as well as by nanoindentation experiments performed near the crack tip. Finally, experimental results were compared with predictions of a recent analytical model. Results revealed that an increase in temperature causes a decrease of the phase transformation zone and that both the techniques are well suitable in capturing the thermal effect on the phase transformation mechanisms near the crack tip.

Martensitic Transformation in NiTi Alloy Covered by Protective Multi-layers

Commercial NiTi shape memory alloy was covered with multi-layers. First, titanium nitride and titanium oxide was deposited on polished surface of the NiTi alloy. Layers were deposited in one process - a glow discharge technique. Second, on the top of the TiN/TiO2 layer the hydroxyapatite (HAp) was deposited using electrophoresis. The HAp layer was sintered at temperature from 5000C up to 9000C for 5minutes. Influence of the thermal treatments, realized during processing stages, on a course of the martensitic transformation was studied using differential scanning calorymetry (DSC).

Study on Measurement of Group Velocity of Lamb Waves in Shape Memory Alloy Sheets

This paper describes the nondestructive evaluation of microstructure using laser-excited Lamb waves to detect the phase transformation in NiTi shape memory alloy sheets. Lamb waves were applied in the NiTi sheet using a pulse laser beam. Piezoelectricity transducers were used to receive the Lamb waves, the group velocities of which were measured using a time-frequency analysis method at different temperatures. Results show that a marked variation in the group velocity occurs during the phase transformation in the NiTi alloy. The dependence of group velocity on temperature provides a effective means of inspecting microstructure transformation in NiTi alloys.

R-phase transformation in NiTi alloys

In near equiatomic NiTi alloys, the thermoelastic transformation between austenite and the R-phase shows unique properties, which make the R-phase transformation very promising for applications. In the present paper, the fundamental issues related to the R-phase transformation, especially the effects of different thermomechanical treatments, are reviewed. Inspired by the literature review, recent work on controlling the R-phase transformation temperature by low temperature aging treatments is summarised.