ON THE NUCLEATION AND PROPAGATION OF PHASE-TRANSFORMATION FRONTS IN ANITI ALLOY

In this paper we present an experimental methodology for simultaneous full field monitoring of the deformation and thermal changes in NiTi d uring mechanically unstable regimes associated with the pseudoelastic material response. The deformation history is established by photograp hically recording surface changes of a brittle coating as austenite-ma rtensite phase transition fronts traverse the specimen. Temperature ch anges are monitored by infrared thermal imaging. Synchronized sets of optical and thermal images and the corresponding stress histories are presented for uniaxial experiments conducted at two relatively slow en d-displacement rates. The results help clarify the interactions among the prevailing heat transfer conditions, the loading rate and the fund amental sensitivity of transformation stress to temperature. It is sho wn that, during loading, nucleation of martensite in an austenitic reg ion is a distinct event requiring a higher stress than the stress requ ired subsequently to continue the transformation. By contrast, the nuc leation stress of austenite in a martensitic region during unloading i s lower than the stress required to continue the transformation. This distinction between the nucleation and propagation stresses, coupled w ith the local temperature change caused by the latent heats of the two transformations, govern the number of nucleations of a new phase. It is also shown that coexisting transition fronts tend to propagate at t he same speed which is inversely proportional to the number of fronts in the specimen. Copyright (C) 1997 Acta Metallurgica Inc.