Transmission electron microscopy investigation of dislocation slip during superelastic cycling of Ni–Ti wires

Thermal cycling and isothermal deformation response of polycrystalline NiTi: Simulations vs. experiment

“…This NiTi alloy does not exhibit a strong, "flag-like" pseudoelastic response, as typically observed above h AF in Ni-rich NiTi alloys [12][13][14][15]28,29,[38][39][40][41][42] or cold-worked and annealed Ti-rich NiTi alloys [43]. The more idealized pseudoelastic response, with clear loading and unloading plateaus and large recoverable strains, is obtained in microstructures that suppress plastic deformation [44] yet permit stress-induced transformation.…”

“…On this smaller scale, retained martensite can affect texture evolution, and the stress redistribution in the vicinity of the plates may promote dislocation substructure [27][28][29][30] and affect the critical driving force for martensite formation (f c , Eq. (9)).…”

“…The more idealized pseudoelastic response, with clear loading and unloading plateaus and large recoverable strains, is obtained in microstructures that suppress plastic deformation [44] yet permit stress-induced transformation. In practice, this is achieved through grain refinement [28,42], cold work with partial recovery [43] or precipitation strengthening of Ni-rich NiTi alloys through suitable heat treatment [38][39][40][41]. Such strategies were not pursued here since the goal was to study the simultaneous effects of plasticity and transformation.…”

“…Some recent efforts have included martensite plasticity, either in J2-based [18,26] or crystal-based [17] forms. However, austenite rather than martensite plasticity has been observed in recent transmission electron microscope studies of pseudoelastically deformed, solutionized 50.7 at.% Ni-Ti single crystal pillars [27] and cold-worked 50.8 at.% Ni-Ti wires [28], as well as thermally cycled, singlecrystal 50.4 at.% Ni-Ti [29]. In particular, a detailed analysis reveals that transformation-induced dislocations have Burgers vectors and slip planes with rationale directions in the austenite basis rather than the martensite basis [27].…”

Martensite stabilization during superelastic cycling of laser welded NiTi plates

“…In particular, the existing martensite in the as-welded material should have undergone detwinning during solicitation up to 10%, contributing to the irrecoverable strain of the welded joint. As for the austenite, it is expected that, aside from the introduction of dislocations which are known to occur during cyclic solicitation of NiTi [11,12], some retained martensite is found to occur due the blockage of the reverse martensitic transformation upon unloading [13,14]. In either case, the same phenomenon occurs: martensite stabilization.…”