The effects of heat treatment on the recovery stresses of shape memory alloys

The influence of annealing temperatures on the thermo-mechanical behavior of NiTi alloy in terms of transformation temperatures, mechanical properties at ambient temperature and the recovery stress under constrained end conditions were investigated experimentally. An attempt is made to correlate results obtained from the DSC test and tensile test at room temperature with results from recovery stresses at elevated temperatures. It is found that annealing the alloy above the recrystallization temperature (600 °C) reduces the maximum recovery stress significantly, even though the alloy still exhibits thermal transformations and a stress-induced martensite plateau at an annealing temperature above 600 °C. A pre-strained amount above approximately 2.4% strain is sufficient to achieve the maximum recovery stress at constrained end conditions. It is recommended that the alloy used in this study be annealed at temperatures below 450 °C in order to produce the desired thermo-mechanical properties in the alloy for applications that exploit the shape memory effect.