Thesis for Doctor of Philosophy Variant selection of Allotriomorphic Ferrite in Steels

The orientation of crystals of allotriomorphic ferrite in austenite has been investigated in order to understand the mechanism by which particular relationships are favoured over others via a process known as “variant selection”. This kind of selection in the context of diffusional transformations contrasts with displacive products where the interaction of transformation plasticity with an external field is the dominant selection criterion. With diffusional reactions the local environment of the growing crystal controls its crystallography. Variables such as the location of the nucleation site, the number and orientation of parent grains with which the site is in contact, interfacial velocity and impingement effects have been identified here to play a systematic role in variant selection. It is conceivable that an allotriomorph may seek to adopt an orientation which matches well with all the adjacent austenite grains. Such simultaneous lattice matching has been investigated theoretically in order to predict the probability of such events as a function of polycrystalline aggregates of austenite grains which themselves may be textured. It is found that both using the theory, and through experimental determinations, that the probability of simultaneous matching depends not only on the intensity of texture in the austenite, but also the presence of