The role of internal stresses on the phase transformation of iron alloys

Abstract

It is well established that in multiphase materials, where two or more components have different thermal expansion coefficients (TEC), internal stresses are developed during cooling from a high temperature (e.g., processing or heat treatment). In iron alloys second phase inclusions (e.g., Al{sub 2}O{sub 3}, spinels, CaO{center_dot}2Al{sub 2}O{sub 3}, TiC, TiN) derived from the processing route are frequently present. These particles have a significantly lower TEC than the iron alloy matrix. Therefore, internal stresses are developed during heat treatments, which affect the mechanical properties. Although the effect of the internal stresses on the mechanical properties of iron alloys has received great attention, their influence on the microstructure has not been investigated. Iron is a polymorphous metal and, therefore, the application of internal stresses may cause solid state phase transformations in steels. Practically, it is not easy to determine the effect of these thermally generated stresses on the microstructure of iron alloys, because a reference sample (i.e., inclusion free) with exactly the same chemical composition as the matrix in the iron alloy is needed. In the present study, theoretical and experimental evidence indicates that extensive martensitic transformation may be induced in iron alloys containing second phases, because of the application of thermallymore » generated internal stresses. The proposed transformation mechanisms are undoubtedly an oversimplification of a very complex problem. However, this observation could initiate a more detailed investigation of the effect that second phases (e.g., carbides, nitrides, oxides), with a significantly lower TEC, have on the microstructure of the polymorphous steel material. Furthermore, similar phenomena and conclusions may take place also in other polymorphous materials such as titanium, cobalt, TiAl, shape memory alloys, etc.« less