Strain arrest during isothermal uniaxial deformations of nickel–titanium shape memory alloys and rate independent phase diagrams

Abstract

Strain arrest during uniaxial tests on nickel–titanium polycrystalline wires indicates that beyond a certain stress level at which the microstructure is stable at a given temperature, there is an additional stress needed to sustain pseudoelasticity or reorientation. This additional stress converges asymptotically to a non-zero value with a decrease in deformation rate in case of reorientation as well as pseudoelasticity (in the latter case, the rates are low enough such that isothermal conditions are expected to prevail). The additional stress is sensitive to the value of the overall strain only when there is a change in crystallinity (during pseudoelasticity), but not when the crystallinity does not change (during reorientation). The effect is studied as a function of temperature, and a rate independent stress–temperature phase diagram is proposed. Noting that thermomechanical coupling is a significant feature of the phase transformation in shape memory alloys, a connection is made between the strain arrest results presented in the present paper and the body of work on thermal arrests reported in the literature.