This invention relates to medical devices comprising shape memory alloys which have been subjected to a thermal and mechanical treatment to increase the austenite start temperature As to As′ such that the shape memory alloy is martensitic at body temperature and when subsequently subjected to a controlled deformation, the shape memory alloy preferentially reverts to the parent phase. The shape memory alloy comprises nickel, titanium and a ternary element, preferably 3 at. % to about 20 at. %. The ternary element is insoluble in a Ni—Ti matrix. In a preferred embodiment, the element is selected from the group consisting of niobium, tantalum and zirconium.