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 Ni50.8Ti49.2 is aff ected by thermal cycling. With increasing the number n of thermal cycles, t he IF peak P-AM (P-MA) occurring at the austenite/martenlite transition tem perature decreases to disappear almost completely. Meanwhile the associated E(T) minimum at the beginning (n < 250) deepens and then becomes progressi vely shallower and wider (3 x 10(3) less than or equal to n < 18 x 10(3)). The strong sensitivity of P-AM (P-MA) to thermal cycling and to impurity (h ydrogen) contents suggests that this peak is predominantly associated with stress-assisted corrective motions of twin boundaries located inside the ma rtensite platelets embedded within the austenite phase, rather than with th e martensitic transition itself. However, as the high temperature tail of P -AM starts at temperatures appreciably higher than the martensite start-tem perature M-s, a premartensitic contribution to damping is also present. The widening of the E(T) minimum indicates that, for n greater than or equal t o 3 x 103, the direct transformation is to some extent hindered by the disl ocation 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 rr. (C) 2000 Elsevier Science S.A. All rights reserved.