Experimental study on rate dependence of macroscopic domain and stress hysteresis in NiTi shape memory alloy strips

NiTi polycrystalline shape memory alloys, when stretched, can deform through the formation and growth of localized macroscopic martensite domains. In this paper, we study the effects of stretching rate on the stress-induced domains and stress hysteresis in NiTi strips. Synchronized measurements of the nominal stressstrain curve, macroscopic domain pattern and the associated temperature field were conducted in the strain rate range of 10 ^-410^-1/s. It was found that the nominal stressstrain curve changed from the near-isothermal plateau-type with distinct stress drops at the very low strain rate to the near-adiabatic smooth hardening-type in the high strain-rate region. The corresponding deformation mode changed from the nucleation propagation mode with a few parallelepiped martensite domains to the near-homogeneous multiple-nucleation mode with many fine alternating austenitemartensite stripes. The number of the domains (domain spacing) increased (decreased) monotonically with the strain rate and followed a power law scaling, while the stress hysteresis (or material damping capacity) changed non-monotonically with the strain rate, reaching a peak at strain rate of 2.0×10^-3/s. We show that, though the rate dependence of both pattern and hysteresis originates from the transfer of the released/absorbed heat and the thermo-mechanical coupling, the domain spacing in the test of static air is mainly controlled by heat conduction and the hysteresis change is mainly controlled by the heat convection with the ambient.