Tensile Deformation of Superelastic NiTi Wires in Wide Temperature and Microstructure Ranges

Superelastic NiTi wires were prepared from a single cold worked wire by various electropulse heat treatments. The wires having a wide range of virgin microstructures were subjected to tensile tests until rupture and cyclic superelastic tensile testing in a wide temperature range. The results were complemented by TEM observation of lattice defects created by superelastic cycling. It appeared that the yield stress depends significantly on the wire microstructure and less on the test temperature. The upper plateau stress varies with the microstructure through its effect on the Ms temperature and increases with increasing temperature in accord with the Clausius-Clapeyron equation up to a maximum temperature characteristic for each microstructure. The upper plateau strains exhibit pronounced maxima (12-18%) at test temperatures and microstructures (pulse times), at which the upper plateau stress approaches the yield stress. The instability of cyclic superelastic deformation was found to be inversely related to the difference between the yield stress and upper plateau stress. Cyclic deformation introduces dislocation slip in the microstructure of the cycled wire from the 3rd cycle and promotes formation of {114} austenite twins upon later cycling. These observations were explained by the activation of deformation twinning in oriented martensite and the stress induced B2 double right arrow B19 double right arrow B2(T) martensitic transformation in specific range of microstructures and temperatures. The ductility of the tested wires was observed to vary stepwise with microstructure from similar to 13 up to similar to 55% and gradually decreased with temperature increasing above 100 degrees C.