EFFECT OF LOW-TEMPERATURE AGING ON THE TRANSFORMATION BEHAVIOR OF NEAR-EQUIATOMIC NITI

A unique three-stage transformation behaviour on cooling has been obse rved in a Ti-50.2 at % Ni alloy after a low temperature ageing treatme nt. The cooling transformation in the aged alloy occurred as a three-s tage process of austenite-to-R phase transition followed by two separa te martensitic transformations. The R phase transition developed durin g ageing with a clear second order nature initially and gradually evol ved into a predominantly first order process. The occurrence of the R transition is not associated with a decrease in the martensitic transf ormation temperature during ageing. The reverse transformation to aust enite occurred in one step, regardless of the nature of the forward tr ansformation on cooling. An all-round shape memory effect was observed in aged samples. These experimental observations suggest that precipi tation induced by the ageing treatment is responsible for the unusual transformation behaviour.