Stress-induced Martensite Reorientation Research Articles

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.

Loading influence on the corrosion assessment during stress-induced martensite reorientation in nickel-titanium SMA

The effect of stress-induced martensite reorientation on the corrosion behavior of a nickel-titanium (NiTi) shape memory alloy (SMA) was characterized by mechanical-electrochemical techniques. This martensite reorientation influenced the passive state of NiTi SMA. These passive to active state transitions could be sensed by the global interfacial electrochemical measurements, such as electrochemical impedance spectroscopy (EIS), open circuit potential (OCP) and linear polarization resistance (LPR) methods. The increase in the loading when exposed to a corrosive environment resulted in the breakdown of the passive layer. A later repassivation stage was reached due to the formation rate becoming more dominant than the breakdown rate during loading conditions. The integration of the mechanical loading by tensile testing, strain distribution by digital image correlation (DIC) and electrochemical methods by OCP and EIS, characterized and detected the influence of the mechanical effect in the corrosion assessment and interfacial mechanisms.

Phase field simulation to one-way shape memory effect of NiTi shape memory alloy single crystal

Based on the Ginzburg-Landau theory and in the framework of thermodynamics, a three-dimensional phase field model was developed to simulate the one-way shape memory effect (OWSME) of NiTi shape memory alloy (SMA) single crystal originated from the martensite transformation and reorientation. A new reorientation strain was simultaneously introduced when a transformation strain was considered in the proposed model, and the phase field equations corresponding to the martensite transformation and reorientation were derived, respectively. From the simulations to the OWSME, it is found that: the martensite variants are grown in a self-coordinating manner and combined by twinned interfaces to reduce the local elastic energy during the temperature-induced transformation from B2 phase to B19′ one; then, the stress-induced martensite reorientation is completed through the migration of twinned interfaces and the variants with optimal orientations are obtained during the subsequent loading, and a residual strain is produced after the unloading; during the further heating after the unloading, a uniform temperature-induced reverse transformation from the single-oriented martensite variant to B2 austenite phase occurs, and the residual strain produced by the stress-induced martensite reorientation is progressively recovered.

Phase field study of the microstructure evolution and thermomechanical properties of polycrystalline shape memory alloys: Grain size effect and rate effect

Shape memory alloys (SMAs) are a class of metallic smart materials which possess pseudoelasticity (PE) and shape memory effect (SME), depending on the ambient temperature. The unique thermomechanical properties of SMAs originate from the temperature-induced as well as stress-induced martensitic transformations (MTs). Former research work has shown that the thermomechanical properties of polycrystalline SMAs are grain size and rate dependent. In order to explore the physical mechanisms behind this phenomenon, a phase field (PF) model is developed in this paper to study the microstructure evolution and the thermomechanical responses of polycrystalline SMAs under dynamic loadings. The inertial effect, the latent heat release and conduction during the phase transformation are simultaneously considered. The grain boundary energy change during the phase transformation is also accounted for in the PF model. Numerical simulations are then conducted to study the temperature-induced MT as well as the stress-induced martensite reorientation of nanocrystalline SMAs with different grain sizes. The influences of the grain size, the latent heat effect and the loading rate on the microstructure evolution process and the stress–strain curves are systematically discussed.

Influence of Strain Rate on Stress–Strain Response of Ni–Mn–Ga Ferromagnetic Shape Memory Alloy Single Crystals

Stress–strain behaviors of Ni–Mn–Ga single crystals were characterized by measurement of stress-induced martensite reorientation under constant magnetic fields. For this purpose, a new setup was designed and manufactured. This setup was supplemented to Santam STM-50 compression testing machine to carry out compression tests, while the specimen was subjected to a constant magnetic field. All the experiments were performed at room temperature in which the sample was at martensite phase. The results indicate that strain rate has no significant effect on stress–strain response of the specimen for strain rates less than 2 × 10−2 s−1 at complete and incomplete loading–unloading cycles.

Temperature rise of high-frequency martensite reorientation via Type II twin boundary motion in NiMnGa Ferromagnetic Shape Memory Alloy

We measured the temperature rise of the high-frequency magnetic-field-induced martensite reorientation in Ni–Mn–Ga Ferromagnetic Shape Memory Alloy and observed the morphology of the associated twin boundaries. It was found that this temperature rise is mainly caused by low-friction Type II twin boundary motion, even though high-friction Type I twin boundary motion is dominant in quasi-static stress-induced martensite reorientation. A simple formula considering the frictional twin boundary motion and heat transfer is derived to describe this temperature rise.

Magnetic field and stress induced martensite reorientation in NiMnGa ferromagnetic shape memory alloy single crystals

Magnetic shape memory properties of a single crystal Ni 2MnGa alloy were characterized through monitoring magnetic field induced strain (MFIS) as a function of compressive stress, and applied stress induced strain as a function of magnetic field. Compressive stress and magnetic field were applied perpendicular to each other along the [1 0 0] and [0 1 1] axes, respectively. The critical magnetic fields for variant reorientation, first cycle effect and cyclic evolution of MFIS are reported as a function of stress level. It was revealed that increasing constant magnetic field level significantly increases the stress required for the reorientation, i.e., magnetostress and leads to superelasticity in martensite. Possible microstructural mechanisms, considering the interplay between stress and magnetic field favored martensite variants, magnetic domains and magnetization rotation, are proposed. Moreover, it was observed that the MFIS evolution is field rate dependent as was evidenced by a rate dependent two-stage reorientation where the maximum MFIS magnitude increases as the field rate increases. This effect was attributed to the difference between the nucleation and propagation barrier strength for twin boundary motion in NiMnGa alloys. The magnetostress (5.7 MPa), blocking stress (5 MPa) and maximum MFIS (5.8%) combination observed in this study is the highest reported to date in NiMnGa alloys. The high blocking and magnetostresses are a consequence of the low test temperature (−95 °C) where the magnetocrystalline anisotropy energy is high and detwinning stress is low. Thus, for magnetic shape memory alloys, the selection of the operating temperature with respect to martensite start and Curie temperatures is critical in optimizing actuator performance since both magnetocrystalline anisotropy energy and detwinning stress are a strong function of temperature below the characteristic temperatures.

Acoustic emission of Ni–Mn–Ga magnetic shape memory alloy in different straining modes

Abstract We investigated a Ni 49.7 Mn 29.1 Ga 21.2 single crystalline sample exhibiting 5.7% magnetic-field-induced strain (magnetic shape memory effect) by means of acoustic emission (AE) experiments. The AE was monitored during thermally- and stress-induced martensitic transformations and during compressive stress-induced martensite reorientation. Partial cycles were measured for the stress-induced transformation. All processes are acoustically active. The highest AE activity was detected during stress-induced transformation from martensite to austenite upon unloading. Observed difference between AE activity during stress-induced forward and reverse transformations might be attributed to different dynamics of transformation processes. Austenite remains acoustically inactive during loading and unloading as well as during thermal cycling. Martensite exhibits acoustic activity during stress-induced reorientation, during loading and unloading, and lower activity during thermal cycling. This acoustic activity can be tentatively ascribed to twinning processes.