SUPERELASTICITY OF Ni-Ti ALLOY AND IN SITU OBSERVATION OF ITS PHASE TRANSITION BY SEM

An in situ observation by SEM was made of the phase transition induced by stain in the specially heat treated 51.6 at.-%Ni-Ti ahoy of which the superelasticity might be reached to such extent of its maximum elastic strain up to 17%. The phase transition is shown as two stages, namely, pre-martensitic and martensitic. The alloy reveals a behaviour for superelasticity, i. e., if the strain exceeds no more than its elastic limit, the whole transition process may be reversible, and the macro-or micro-feature may be back to beginning after unloading at any point of the curve. However, the intergranular cracks and the internal slips may be appeared, as well as the unreversible residual strain may be then remained if it exceeded the limit.