Bainite self accomodating shear strains
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.
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.
The transformation can have a pronounced interaction with an externally applied stress only if it is accompanied by a shape change (a strain). Alberry) Ferritic Steels for Fast Reactor Steam Generators, British Nuclear Engineering Society, London 1977 1-4. This shape change is an invariant-plane strain with a large shear component. The much smaller dilatational component of strain is directed normal to the habit plane (invariant-plane). For example, the freezing of water to ice is a reconstructive transformation where there is mass transport but no change in composition. Similarly, austenite in pure iron can transform into ferrite, either by a reconstructive mechanism involving diffusion, or by a martensitic mechanism. Edmonds) Materials Science and Technology 3 1987 441-449.