Low-Hysteresis Shape Memory Alloy Scale-Up: DSC, XRD and Microstructure Analysis on Heat-Treated Vacuum Induction Melted Ni-Ti-Cu-Pd Alloys

Quaternary Ni-Ti-Cu-Pd formulations were cast by vacuum induction melting (VIM) with the aim of preparing low-hysteresis shape memory alloys and verifying the applicability of the Co-Factor theory in conventional industrial manufacturing processes. The cast alloys showed lower transformation hysteresis width in DSC measurements than binary Ni-Ti, but struggled to achieve a near zero hysteresis, as predicted by the theoretical framework, despite being close to satisfy the first Co-Factor condition (CC I) that foresees minimum hysteresis for formulations in which the middle eigenvalue of the martensitic transformation matrix λ2 approaches one. The microstructure of the annealed Ni-Ti-Cu-Pd alloys exhibited a considerable amount of mostly sub-micron-sized secondary phases, which distort the matrix composition and prevent it from reaching the optimum stoichiometry for satisfying the CC I. In addition, this class of materials is prone to aging effects, leading to the formation of semi-coherent tetragonal precipitates, which tend to also form at the grain boundaries after low-temperature annealing, further affecting the transformation hysteresis in DSC experiments depending on the thermal history. This work reveals the importance of considering typical casting effects that alter the theoretical λ2 of ideal materials in the compositional design for the development of high-performance low-hysteresis alloys.

Influence of Stress on Kinetics and Transformation Plasticity of Ferrite Transformation Based on Hysteresis Effects

Transformation plasticity and kinetics play an essential role in the prediction of residual stresses resulting from transformation. This paper is devoted to the investigation of the influence of stress on the kinetics and transformation plasticity of ferrite for H420LA steel. It has been shown that under small external stresses, lower than the yield stress of the weaker phase, the ferrite transformation is inhibited at the beginning of the transformation in the continuous cooling process and the mechanical stabilization of austenite is observed, due to transformation hysteresis effects. This phenomenon affects the metallurgical and mechanical behaviors of the transformation progress. However, most existing models ignore these effects, leading to deviations in the description of transformation plasticity during the transformation progress. Considering the hysteresis effects, the micromechanical model for kinetics and transformation plasticity is reexamined. A general formulation of austenite decomposition kinetics accounting for these effects is developed to better describe the phase transformation under a continuous cooling process. In addition, the influence of hysteresis effects on the evolution of transformation plasticity is analyzed. Consideration of the hysteresis effects decreases the discrepancy between the calculated and experimental values. This will allow better prediction of residual stresses in the thermomechanically controlled processes.

Influence of Pre-Deformation Strain on Recovery Performance of Ni47Ti44Nb9 Alloy Φ8mm Pipe Joint

Using the method of expanding diameter in low temperature and water bath heating, this paper studied the effect of pre-deformation strain on shape memory recovery characteristics of Ni47Ti44Nb9 alloy Φ8 mm pipe joint. The results showed that the transformation hysteresis was relative stable along with the pre-deformation strain more than 12.1%, which was 130°C. And the radial recoverable strain got maximum value 8.4% when the pre-deformation strain was in 10.8%~12.5%. But the radial recovery rate of inside diameter declined smoothly with the pre-deformation strain. Systems considering the three parameters of transformation hysteresis, recoverable strain and strain recovery rate, it seemed the excellent predeformed strain for Ni47Ti44Nb9 alloy Φ8 mm pipe joint was just in 12%~17%.

Elucidating the origins of phase transformation hysteresis during electrochemical cycling of Li–Sb electrodes

In this combined computational and experimental study, we identify the origins of phase transformation hysteresis during cycling of Sb electrodes, which undergoes alloying reactions upon lithiation.