The Effects of Applied Strain and Heat Treatment on the Properties of NiTi Wire During Shape Setting

摘要

NiTi components are commonly subjected to thermo-mechanical heat treatments during production and fabrication. This study investigates the effects of applied strain of 0-10% and heat treatments of 300-600ºC for times of 2-30 minutes during shape-setting of Ti–50.8 at% Ni wire with a nominal diameter of 0.495 mm and an initial transition temperature, Af, of 12ºC. Strain was applied prior to heat treatments by coiling NiTi wire, essentially producing coiled springs, around different diameter steel mandrels to obtain different strain levels. The samples of NiTi wire under applied strain were heat treated in a salt bath, followed by a rapid quench. Transformation temperatures and mechanical properties were characterized using the differential scanning calorimeter and tensile tests. Changes in the Af, UTS, and elastic modulus due to ageing processes and applied strain were observed. Following theory, precipitation rates of Ni-rich phases generally increased with increased temperature (up to a certain point), time and applied strain levels. Reaction rates to achieve a particular Af might be faster than expected if the sample is under strain during the shape-setting process. Due to precipitation strengthening, an increase in UTS with increased heat treatment time was observed between 300-450°C; annealing processes dominate at higher temperatures, resulting in a decrease in UTS above 500°C. A decrease in UTS with increasing level of applied strain was observed. Trends in elastic modulus were highly inconsistent in this study. As shown by this study, applied strain in NiTi during heat treatment affects the Af and mechanical properties.