Bainite self accomodating shear strains
The latter case represents displacive transformations, in which the pattern of atomic arrangement is altered by deformation. There is no diffusion of iron or substitutional solutes during displacive transformations in iron alloys. Cementite and -carbides are not included in the above classification, but it is believed that they grow by a displacive transformation mechanism at low temperatures. Both kinds of reactions lead to changes in shape (Fig. The simplest of these occurs during reconstructive transformations in which the densities of the parent and product phases are different. The ability of an external stress to influence the development of microstructure during solid-state transformation is well established in theory. Harada) Acta Metallurgica et Materialia 42 1994 1081-1090.
For very small plastic strains, the first two of these deformation modes are conservative - they preserve an atomic correspondence between the deformed and undeformed parts of the crystal so that the crystal contains a memory of its original shape. Alberry) Residual Stresses in Welded Constructions, Welding Institute, Cambridge 1977 Paper 2.
There is a need for this subject to develop further before serious quantitative applications become viable, although very useful general principles already exist which can help in the assessment of weld phenomena.
We therefore consider first the basic principles which are applicable to both the heat-affected zone and fusion zone of a weld, and which are useful in phase transformation theory per se.
Hence, when austenite transforms into grain boundary allotriomorphic ferrite, there is a uniform expansion, and vice-versa.
Much more interesting deformations are associated with displacive transformations, where the shape change is in general described as an invariant-pane strain with a large shear component.
Classification of the transformation products of austenite .