Recoverable and plastic strains generated by forward and reverse martensitic transformations under external stress in NiTi SMA wires

Abstract: Superelastic NiTi 1 shape memory alloy (SMA) wire was subjected to thermomechanical loading tests in tension to evaluate stress and temperature conditions under which the wire deforms plastically. Although the alloy demonstrated a very high resistance to plastic deformation in both the austenite and martensite phases, incremental plastic strains were recorded whenever the B2 cubic to B19' monoclinic martensitic transformation (MT) took place under external stress. To shed light on the origin of the functional fatigue presumably originating from the accumulated plastic strains, special closed-loop thermomechanical loading tests were performed to evaluate recoverable and plastic strains generated by the forward and reverse MTs under external stress. These experiments revealed that both forward and reverse MTs generate well-defined incremental plastic strains only if they occur above certain stress thresholds. Specifically, the forward MT upon cooling under stresses up to 500 MPa does not produce plastic strain or lattice defects, whereas the reverse MT upon heating generates plastic strains even under stress as low as 100 MPa. It is proposed that plastic deformation generated by martensitic transformations under low stresses takes place by 100 dislocation slip in martensite and while under high stresses it occurs via kwinking deformation in martensite. The characteristic thresholds and magnitudes of plastic strains generated by the forward and reverse MTs define the functional fatigue limits for NiTi. Specifically, the stress-strain-temperature responses recorded in closed-loop thermomechanical tests remain stable if both forward and reverse MTs occur below these thresholds. Conversely, they become unstable and exhibit predictable ratcheting if the MTs exceed these thresholds.