The Effects of Cold Work and Heat Treatment on the Properties of Nitinol Wire

The successful medical application of Nitinol requires precise control of its transformational and mechanical properties. In this study the effects of heat treatments at 300-550 degrees C for 2-180 minutes on Ti-50.8at%Ni wire with 30% and 50% initial cold work were investigated. Transformational and mechanical properties were characterized through the BFR technique and tensile testing. Thermally activated precipitation and annealing processes were observed. Annealing processes tended to increase the maximum slope of the BFR curves. The R-phase was observed with greater frequency and prominence in the 50% cold-worked wire after heat treatment. The general trends in A(f) are summarized in two TTT diagrams; both illustrate a maximum precipitation rate of Ni4Ti3 at 400-450 degrees C. The trends in tensile properties are outlined for all heat treatment conditions. Recovery processes occurred at all temperatures. The onset of recrystallization occurred at approximately 450 degrees C for both wires.