On the nucleation and propagation of phase transformation fronts in a NiTi alloy

In this paper we present an experimental methodology for simultaneous full field monitoring of the deformation and thermal changes in NiTi during mechanically unstable regimes associated with the pseudoelastic material response. The deformation history is established by photographically recording surface changes of a brittle coating as austenite-martensite phase transition fronts traverse the specimen. Temperature changes 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 end-displacement rates. The results help clarify the interactions among the prevailing heat transfer conditions, the loading rate and the fundamental sensitivity of transformation stress to temperature. It is shown that, during loading, nucleation of martensite in an austenitic region is a distinct event requiring a higher stress than the stress required subsequently to continue the transformation. By contrast, the nucleation stress of austenite in a martensitic region during unloading is lower than the stress required to continue the transformation. This distinction between the nucleation and propagation stresses, coupled with 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 the same speed which is inversely proportional to the number of fronts in the specimen. Copyright (C) 1997 Acta Metallurgica Inc.