Fabrication of step functionally graded NiTi by laser heating

Abstract

This paper investigates the application of localized laser heating to produce a step pseudoelastic behavior of a cold worked Ni-rich Ti-50.9 at%Ni alloy. The localized heating was done using a custom-made laser heating apparatus. The aim is to create step graded martensitic phase transformation plateaus with large stress window and good recoverability. A cold-worked NiTi wire was locally heated using laser at 400 °C for 10 min, 475 °C for 30 min and 525 °C for 20 min along its length. These temperature and duration settings were carefully chosen based on isothermal heat treatment of the same Ti-50.9 at%Ni wire. Under tensile load, the wire exhibited deformation in a sequence of stress plateaus (step deformation), starting from the segment heated at the highest temperature and ending at the segment heated at the lowest temperature. The step gradient stress plateau produced 310 MPa and 270 MPa stress windows on the forward and reverse transformations, respectively. The residual strain on the recovery was about 1%. This novel heat treatment technique provides the possibility to produce a step pseudoelastic behavior of Ni-rich NiTi alloys to improve the mechanical responses of this smart material in actuator applications.