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

Abstract

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 ) B19 0 ) 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 * 13 up to * 55% and gradually decreased with temperature increasing above 100°C.

B2 ⇒ B19′ ⇒ B2T Martensitic Transformation as a Mechanism of Plastic Deformation of NiTi

“…Commercial superelastic NiTi wire produced by Fort Wayne Metals in cold work state (FWM #1, Ti-50.9 at.% Ni, 42.1% CW, diameter 0.1 mm) was heat treated by electropulse method to prepare NiTi wire samples with desired microstructure and properties [20]. 30 mm long wire segments were crimped by two steel capillaries, prestressed to * 300 MPa, constrained at current length and subjected to the short pulse of controlled electric power (power density 160 W/mm 3 , pulse time 16 ms).…”

“…Hence, the wire used in this work is called ''16 ms NiTi wire''. It has fully recrystallized microstructure with grain size * 500 nm and transformation temperatures M s = -40°C and A f = -10°C in [20].…”

“…The appearance of unusually long upper stress plateaus in tensile tests at elevated temperatures is not, however, specific property of the 16 ms NiTi wire. Long upper stress plateaus were observed in tensile tests on NiTi wires possessing wide range of virgin microstructures [6,20]. The temperature range, at which the long plateaus were observed vary significantly with the starting microstructure of the NiTi wire.…”

“…First reason is that the B2 ) B19 0 ) B2 T transformation becomes activated in all NiTi wires, only the temperature and stress at which it happens vary significantly, since they heavily depend on the virgin microstructure of the wire [6,20]. While the long plateaus and deformation bands were observed at 60°C/800 MPa on 20 ms NiTi wire, they appeared above 150°C/1400 MPa in tensile tests on 12 ms NiTi wire, the properties of which are similar to commercial superelastic NiTi wires [20].…”

“…The PTG scheme allows for prediction of deformation twinning systems in the B19 0 martensite [17] bringing about unrecoverable strains claimed by Gao et al to be associated with functional fatigue [18] and cousing grain refinement [19] of NiTi transforming cyclically under high stress. Chen et al [6,20,21] carried out a systematic investigation of tensile deformation of NiTi wires having a range of microstructures in wide temperature range. Based on the results of the observations of {114} austenite twins in microstructures of deformed wires and synchrotron x-ray experiments informing the evolution of phase volume fractions upon loading, they claimed that the deformation twinning in oriented martensite plays an instrumental role in superelastic deformation at elevated temperatures [6] and plastic deformation of NiTi wires from -100 to 200°C.…”