Effect of thermal cycling through the martensitic transition on the internal friction and Young’s modulus of a Ni50.8Ti49.2 alloy

The internal friction (IF) and Young’s modulus (E) of a Ni_50.8Ti_49.2 is affected by thermal cycling. With increasing the number n of thermal cycles, the IF peak P_AM (P_MA) occurring at the austenite/martensite transition temperature decreases to disappear almost completely. Meanwhile the associated E(T) minimum at the beginning (n<250) deepens and then becomes progressively shallower and wider (3×10³≤n<18×10³). The strong sensitivity of P_AM (P_MA) to thermal cycling and to impurity (hydrogen) contents suggests that this peak is predominantly associated with stress-assisted collective motions of twin boundaries located inside the martensite platelets embedded within the austenite phase, rather than with the martensitic transition itself. However, as the high temperature tail of P_AM starts at temperatures appreciably higher than the martensite start-temperature M_s, a premartensitic contribution to damping is also present. The widening of the E(T) minimum indicates that, for n≥3×10³, the direct transformation is to some extent hindered by the dislocation network introduced by thermal cycling. A not thermally activated IF peak P_TWM, which is believed to be due to stress-assisted motions of (001) compound twin boundaries in the homogeneous martensite state, grows with increasing n.