Shock-induced transformation behavior in NiTi shape memory alloy

Abstract

The high-strain dynamic behavior of NiTi shape memory alloy has significant applications in several fields such as military af- fairs, aerospace. In order to investigate the transformation behavior in NiTi alloy, induced by dynamic mechanics, the shock-loading experiments are performed using a single stage gas gun at different temperatures and different shock velocities. Differential scanning calorimeter (DSC) and comprehensive physical property measurement system are employed to analyze the phase transformation in- duced by residual effects of shock waves in NiTi alloy. Three endotherms are observed in the first heating cycle, showing the presence of three-step reverse phase transformation; whereas during the second heating only one endotherm is seen, because the other two en- dotherms attributed to stress-induced martensite have disappeared. The exothermic and endothermic peak, owing to the transformation of shock-treated specimens, become small and their transformation temperature regions are broadened. This tendency indicates that the internal defects in the specimens, introduced by shock-treated, increase the resistance of phase transformation. The exothermic peaks of specimens, shock-treated at low velocity and high velocity, all shift to the low-temperature-zone, because the dislocations increase the hindrance to martensitic transformation. However, the endothermic peaks of specimens with low velocity shock-treated shift to high-temperature-zone, illustrating that the reverse martensitic transformation is also opposed by dislocations; while the endothermic peaks shift to low-temperature-zone for high velocity shock-treated, due to the decrease of transformation energy, caused by the re-duction of recoverable martensite. A small shoulder is detected in exothermic peak, whose shape becomes sharper with shock rate increasing. This result reveals that the intermediate phase (R-phase) results in two-stage phase transformation. The electrical resistivity measurement result further confirms that the two types of phase transformations associated with austenite to rhombohedral (A→R) and rhombohedral to martensite (R→M) can occur at the same time in a certain temperature range.

Dynamic Behavior in Ti-5553 Alloy with β Phase under Compression Loading

“…The overall mobility of the dislocations was starting to be impeded by the ω phase whose hardness is very high (HB≈500), which significantly improves the tensile strength, hardness, modulus of elasticity of this alloy [18]. At low shock wave pressure this alloy is subjected to quenching from β to αʺ phase [19] The substructures within the martensite αʺ phase is a set of dislocations, whose burgers vector are substantially to balance the frequency of the positive and negative. In low pressure the rearrangement of dislocations is caused by shock wave, within whose process there are no new dislocations generated.…”