Phase Transformation In NiTi Alloys Research Articles

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

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.

Chip formation and phase transformation in orthogonal machining of NiTi shape memory alloy: microstructure-based modelling and experimental validation

Phase transformation and shape memory response of NiTi alloys are sensitive to the variation of temperature and stress. Thus, the phase transformation of NiTi alloys becomes more complex during machining process. This study presents findings from a major study involving modelling of machining-induced phase transformation of NiTi alloys performed by modifying Helmholtz free energy-based microstructure model. Orthogonal cutting tests were performed to validate the predicted outputs from the simulation, such as cutting forces, temperatures and chip morphology. This work provides a strong evidence that the developed new model can accurately predict the experimentally recorded outputs in machining of NiTi alloys.

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.

Phase transformation of NiTi alloys during vacuum sintering

The aim of this study is to ascertain the Phase transformation of NiTi alloys during vacuum sintering. NiTi shape memory alloys (SMA) of atomic ratio 1:1 were prepared through press forming and vacuum sintering with the mixture of Ni and Ti powders. Different samples were prepared by changing the sintering time and the sintering temperature. Phase and porosity of the samples were investigated by X-ray diffraction (XRD) and scanning electron microscope (SEM). The results show that in the process of sintering NiTi2 and Ni3Ti phases are formed firstly and then transform into NiTi phase. The quantity of NiTi2 and Ni3Ti phases gradually decreased but not eliminate completely with increase of sintering time. The porosity of specimen sintering at 900°C decreases slightly with increase of sintering time. With increase of sintering time the porosity of specimen sintering at 1050°C decreased firstly and then increased because of generation rich titanium liquid in the process of sintering.

A model of the kinetics of the temperature-induced phase transformation in NiTi alloys and its experimental verification

A model, describing the kinetics of the temperature-induced austenite phase transformation processes in shape memory alloys (SMAs) is presented. The model is formulated for a general stress and temperature state in the solid body and allows the computation of the austenite phase fraction evolution in it, that is, the dependence of the amount of the austenite phase fraction on the values of the temperature and stress. It is shown that the predicted amount of the austenite phase fraction is not dependent on the control volume discretization. The model does not take into account microstructural changes in the material and the existence of different phases in the material (R-phase, twinned martensite). The experimental verification of the model was conducted using SMA nitinol wire that was prestrained (detwinned martensite state) and subjected to a heat load. It was shown that the austenite phase fraction predicted by the model agrees very well with the tabulated (reference) and experimental data.

The Twinning Modes of B2-B19′ Martensitic Transformation in NiTi Alloys - A Phase Field Study

A three-dimensional phase field model applicable for the B2-B19′ martensitic phase transformation of NiTi alloys was developed to predict the twinning modes in the B2-B19′ transition. The phase field simulation results showed that by taking into account the transformation induced elastic strain, the martensite variants were self-accommodated and sheared along the specific interface in the entire transition stage for reducing the elastic energy, whether the formed interface between the variants belongs to twinning plane could be determined by calculating the minimum value of long range elastic interaction energy, Bpq. Through comparison with the existing analytical solutions, it is demonstrated that the phase field model can be used to predict the type I twinning modes in the B2-B19′ phase transition with good precision.

612 モンテカルロ法を用いたNi-Ti合金に関する相変態現象の解析(GS-1 材料科学)

612 モンテカルロ法を用いたNi-Ti合金に関する相変態現象の解析(GS-1 材料科学)

Predicting in vivo failure of pseudoelastic NiTi devices under low cycle, high amplitude fatigue.

Due to the large reversible strains achievable through the stress-induced austenite-martensite phase transformation in NiTi alloys, NiTi has replaced stainless steel in the majority of large-strain biomedical applications such as root canal enlargement. However, the pseudoelasticity of NiTi is currently overshadowed by the short fatigue life of NiTi wires used in this low cycle (200-2000 rpm), high amplitude (epsilon(a) > 2.5%) application, resulting in in vivo fracture or premature retirement of otherwise reusable NiTi-based wire devices. In this study, the failure of pseudoelastic 55.8 wt % Ni-Ti wire is investigated experimentally, as a function of experimental parameters that include the clinically relevant regime. The effects of radius of curvature, angle of curvature, wire diameter, strain amplitude, cyclic frequency, volume under strain, and specific heat of the surrounding environmental fluid are considered systematically. These data indicate that the lifetime or cycles to failure N(f) of a rotating NiTi wire can be predicted via a modified Coffin-Manson relation that is a strong function of both strain amplitude and volume under strain, and a weaker function of frequency and fluid specific heat. The resulting quantitative relation can be used to predict useful device lifetime under clinically relevant conditions and thereby reduce incidences of in vivo failure.