Structural evolution during the cycling of NiTi shape memory alloys
The structural evolution in NiTi shape memory alloys subjected to pseudoelastic cycling is examined in the present study. Single crystals with  and  orientations were subjected to repeated compressive cycles and then studied by transmission electron microscopy (TEM). TEM observations were made at cycle numbers 1, 2, 5, 10, and 20 since the majority of degradation occurs during these initial cycle numbers. Under compression, single crystals with  orientations degraded much faster than crystals with  orientations. Under tension, single crystals with  orientations fractured in the elastic region, and crystals with  orientations showed considerable degradation as a function of cycling. Intermittent TEM observations on single crystals oriented along the  direction showed an increase in dislocation density on multiple active slip systems as a function of cycling. Single crystals oriented along the  orientation show a less dramatic increase in dislocation density as a function of cycling. TEM observations have revealed that dislocation structures formed near martensite plates have a similar periodicity as internal twin modes within the martensite. This observation implies that, although the interface between the martensite and parent phase is a low-energy boundary, the local disruptions due to internal twins create preferential nucleation sites for the formation of lattice defects.
Hurley, J; Ortega, AM; Lechniak, J; Gall, K; Maier, HJ
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