Marforming and tempering of binary Ni–Ti alloys including precipitation effects

The influence of plastic deformation in the martensitic state followed by tempering was studied. The amounts of true plastic deformation up to ϕ=1,1 were obtained by cold rolling. To achieve complete transformation into the martensitic state, Ni rich alloys with low transformation temperatures were cooled in a solution of liquid nitrogen and alcohol below Mf. The effects of workhardening on strength and transformation behaviour were investigated by differential scanning calorimetry, tensile tests and micro-hardness measurements, as well as by light and electron microscopy. Increasing deformation leads to higher reverse transformation temperatures (As, Af). Finally the alloys lose their martensitic retransformability (dead martensite), i.e. As and Af reach the temperature range in which diffusion effects become considerable. By tempering at 200<TMT<800°C the austenitic state, consequently reversible martensitic transformability and shape memory, is regained by passing through several stages. These are: recovery in martensite, nucleation and the coarsening growth of austenite. In addition to these processes, the precipitation in martensite or austenite was examined by transmission electron microscopy. The tensile tests provided evidence for the improvement of strength properties and fatigue resistance by some thermomechanical treatments.