The Use of Model Systems Based on Fe-30 wt%Ni for Investigating the Precipitation and Transformation Behaviour of Microalloyed Austenite

The development of physically-based models for the microstructural evolution during thermomechanical processing of metallic materials requires knowledge of the internal state variable data, such as microstructure, texture and dislocation substructure characteristics, over a range of processing conditions. This is a particular problem for steels, where the transformation of austenite to a variety of transformation products eradicates the hot deformed microstructure. This paper reports on a model Fe-30wt%Ni based alloy, which retains a stable austenitic structure at room temperature, and has therefore been used to model the development of austenite microstructure during hot deformation of conventional low carbon-manganese steels, thus providing for an understanding of the role of austenite microstructure on the subsequent transformation behaviour. It also provides an excellent model alloy system for microalloy additions, providing insight relating to the precipitation location and kinetics in the deformed austenite. This research will discuss the results from a microalloyed Fe-30% Ni-Nb alloy in which the strain induced precipitation mechanism was studied directly. The work has shown that precipitation can occur at a much finer scale and higher number density than hitherto considered, but that pipe diffusion leads to rapid coarsening. The implications of this for model development are discussed. Additional results will be shown for the effect of strain path reversal on the microstructural development of a model Fe-30wt%Ni alloy, compared with a C-Mn steel processed in a similar manner in order to establish a clearer understanding of the role of the austenite microstructure on subsequent transformation behaviour.