EFFECT OF THERMAL CYCLING ON THE MARTENSITE FORMATION IN EQUIATOMIC TiNi

Abstract
The structural effects of thermal cycling on the martensite formation in equiatomic TiNi-alloys were investigated by electron microscopy. On quenching from high temperatures (650°C<T) orientation variants and {1[MATH]1}-twins of the monoclinic martensite are formed with a structure of relatively coarse alternating platelets. Low temperature annealing (below 200°C) or temperature cycling decreases the number of variants, and produces intensive twinning both on the {111}- and (010)-martensite planes. During the reverse transformation the interface between the martensite and high temperature (B2) phase is close to {112} B2. The following characteristics are observed during one complete temperature cycle in the electron microscope. Starting from the high temperature phase a premartensitic rhombohedral phase is formed on cooling above M_s. followed by dislocation rearrangements just before the martensite transformation. The martensite forms in a discontinuous manner, and every growth step is preceded by dislocation rearrangements. The reverse transformation results in a tetragonal phase behind the interface and new dislocations are generated. The density of dislocations is increasing with each cycle. These dislocations are thought to play an important role in the formation of the thermo elastic martensite. Finally the best reversibility of the martensite is obtained after several cycles when a high density of dislocations is obtained having Burgers vectors, which can be incorporated in both the martensite and the high temperature B2 phase.