A Modified Model Of The Interface Between Martensite-austenite

Martensitic transformation is a very important phase transformation, through which the properties of materials can be improved, i.e. the strengthening of steel and the research and development of shape memory alloys. As invariant plane transformation, martensitic transformation has habit plane, i.e. the interface between mother phase and product phase, implying that there should be no transitional layer on the interface. The recent HREM experiments, however, observed the intermediary layer. To explain this phenomenon, we attempt to set shear strain as the order parameter to build modified model to describe the transitional layer and to calculate the effects of materials'parameters on the layer.The modified model successfully characterizes the transitional layer. On the interface profile, there is a platform, on which the order parameter changes slowly, belonging to neither martensite nor austenite. This platform is defined as the interface preferred state and used to explain the atomic arrangement on the transitional layer, demonstrating that the modified model matches better with experiments than Falk's model.According to the calculated results, interface width and interface energy depend on the material's elastic modulus, the energy density of interface preferred state as well as the driving force of martensitic transformation: lower energy density of interface preferred state is the prerequisite for the existence of transitional layer. Moreover, for the materials with soft mode phenomenon during martensitic transformation, the fall of modulus will bring down the interface energy, generating the tweed structure. The driving force of phase transformation, however, will only influence the transfer from preferred state to martensite. The interface width will relate negatively with the driving force. Thus the thermo elastic martensitic transformation with lower driving force will have wider interface, keeping the area between two phases coherent. While for the non thermo elastic transformations, the larger driving force will force the transitional layer to be sharp, on which there are misfit dislocations relaxing part of strain energy.The results of modified interface model firstly illustrate the inherent reasons for the variations of interface width in different materials and explain clearly the important role the interface preferred state played during the process of phase transformations. The modified model will also help discover the fine structure of martensite-austenite interface.