Determining the Effects of Transformation Induced Plasticity and Creep Plasticity on the Functional Fatigue Life of NiTiHf High Temperature Shape Memory Alloys

Abstract

With their high work output during thermal cycling under load, NiTiHf high temperature shape memory alloys became promising candidates for smart actuator design for aerospace industry. However, high temperature applications under loading are susceptible to creep deformation. It has been already shown that functional properties of high temperature shape memory alloys degrade faster since they work at high temperatures. For instance, their actuation abilities decrease and transformation temperatures shift due to the dislocation formation via plastic deformation with martensite-austenite phase transformation. In this work creep deformation behavior of 50at%Ni – 25 at% Ti – 25 at% Hf high temperature shape memory alloy was studied at 450°C, 500°C, 550°C and 600°C temperatures under 200 MPa, 300 MPa, 400 MPa and 500 MPa loading conditions. Creep model parameters were found and creep behavior was discussed in comparison with creep results in the literature on NiTi alloys. Creep deformation rates were also compared with the deformation rates that were acquired from functional fatigue experimental results. Functional fatigue experiments yielded 40 times higher deformation rate than that was obtained from creep experiments for the same loading condition. It is important to mention that upper cycle temperature which the samples were heated during functional fatigue experiment and creep test temperature was same. Additionally, functional fatigue cycles with subsequent creep experiments were run consecutively in order to determine the effect of the deformation of one on the other. Creep deformation was found to have little to no effect on functional fatigue cycles, on the other hand functional fatigue cycles led to an increase in dislocation density that changes creep behavior of the material.