Creep behavior of 50at%Ni 25at%Ti 25at%Hf high temperature shape memory alloy under constant load

Abstract

In the last three decades, aerospace industry has required shape memory alloys with higher phase transformation temperatures. In response to this technical challenge, High Temperature Shape Memory Alloys that can be actuated at higher temperatures were developed. However, high temperature actuation applications also lead to observe much higher dislocation motion and thus plastic deformation due to creep and phase transformation. In this study, 50at%Ni 25at%Ti 25at%Hf high temperature shape memory alloy was used for the investigation of creep deformation under tensile loads corresponding to stress magnitudes between 200 MPa and 500 MPa. Deformation behavior of the alloy in austenitic phase was investigated by means of creep deformation experiments at 450 °C, 550 °C, and 600 °C. 50at%Ni 25at%Ti 25at%Hf alloy has similar creep behavior as NiTi despite higher loading conditions. Almost no noticeable creep strain values were observed under 200 MPa and 300 MPa at 550 °C and 600 °C. Creep parameters such as the stress exponent and the activation energy values were calculated using power law creep model and viscous glide creep was found to be the main mechanism in 50at%Ni 25at%Ti 25at%Hf high temperature shape memory alloy.