Thermally induced martensitic transformation and structural properties in Ni-Ta high-temperature shape memory alloys

Abstract.

The morphological, structural and martensitic phase transformation properties of the binary Ni-Ta high-temperature shape memory alloys depending on the composition via SEM-EDX, XRD and DSC observations have been investigated. It was found that the different Ta contents did not change the high-temperature shape memory behaviors of alloy samples with completed martensitic transformation above 220^°C, but had a dramatic effect on the reverse transformation temperatures ($A_{\rm s}$) and transformation hysteresis ($A_{\rm f}-M_{\rm f}$) of alloy samples. As the Ta content of the alloy samples decreased, the $A_{\rm s}$ temperatures shifted to high-temperatures ($> 340$^°C, and the $A_{\rm f}-M_{\rm f}$ values arose to temperatures above 100^°C. However, different Ta contents did not cause considerable changes on the morphological and structural properties of the alloy samples. While a very rare dispersed NiTa₂ precipitate phase was observed in the microstructure of the alloy samples with Ta contents above 25 at.%, Ni-rich areas were rarely detected in the microstructure of the alloy sample with Ta content below 25 at.%. In addition, no crack formation was observed in the microstructures of the alloy samples.