Tailoring hardness gradient in Ni50·3Ti29.7Hf20 high temperature shape memory alloy through thermal treatment

Abstract

NiTiHf alloys are functional materials with the ability to recover shape changes at temperatures above 100 °C because of their high phase transformation temperatures. Herein, the tailoring of the hardness gradient of Ni50·3Ti29.7Hf20 (at.%) alloy through thermal treatment is reported. Upon tailoring the hardness gradient, the compressive ductility and shape memory properties remain unaffected through a careful selection of thermal treatment. After oxidation treatment at 1000 °C for 2.5 h, the hardness of the sample increases remarkably from 508 HV in the central region to 745 HV in the peripheral region. Moreover, almost no losses in compressive ductility and shape memory properties (with 6% pre-strain) are observed. Detailed microstructural analysis indicates that a composition gradient is formed from the center to the edge of the sample due to the oxidation of Ti and Hf at elevated temperatures. The composition gradient results in different precipitation behavior, because of which the hardness gradient is observed from the center to the sample edge. The results obtained in this study can allow additional potential applications of NiTiHf high temperature shape memory alloys.