Microstructures, Mechanical Properties and Transformation Behavior in Ni49.6Ti35.4Hf15 Alloy Produced with High-Pressure Torsion

High-pressure torsion (HPT) was applied for the Ni49.6Ti35.4Hf15 (at.%) alloy up to 1/4, 2 and 16 turns under a pressure of 4.0 GPa. The samples were examined using X-ray diffraction (XRD), transmission electron microscope (TEM) and microhardness measurements. The results indicate that the mixture of an amorphous and nanocrystalline microstructure developed in the investigated NiTiHf alloy as the number of HPT turns was increased to two. The average hardness of the samples increased from 330 Hv to 500 Hv after 16 turns of HPT. Very fine martensite developed when the HPT-processed samples were annealed at 550 C-circle and the finer microstructures were attained with higher HPT turns. Differential scanning calorimetry (DSC) tests were performed in the post-HPT annealing samples to clarify the transformation behavior after severe plastic deformation in HPT and subsequent annealing, so as to provide an experimental basis for the application of the shape memory alloy. The transformation temperature of the alloy decreased remarkably when the number of turns of HPT reached 16. It is suggested that the deformation extent and annealing temperatures should be considered in order to maintain a high transformation temperature while utilizing the strengthening effect of HPT in the NiTiHf alloy.

Automated summary

In this study, high-pressure torsion (HPT) was used to process Ni49.6Ti35.4Hf15 alloy with 1/4, 2 and 16 turns under a pressure of 4.0 GPa. The results showed that a mixture of amorphous and nanocrystalline microstructure developed in the alloy with increased HPT turns. The hardness of the samples increased significantly after 16 turns of HPT. Annealing of the HPT-processed samples at 550°C resulted in the formation of very fine martensite, with finer microstructures obtained with higher HPT turns. Differential scanning calorimetry (DSC) tests confirmed the transformation behavior after severe plastic deformation and annealing, providing experimental basis for the application of shape memory alloy.