NATURE OF MARTENSITIC TRANSFORMATIONS

Theoretical concepts are described which explain the structural and kinetic peculiarities of martensitic transformations. A martensitic transformation is considered as a first order phase transition in solids that proceeds under conditions where the initial phase maintains metastability. The coherent interphase boundary of the growing martensitic crystal has a small energy, possesses a very high mobility and serves as a source of internal stresses. The tendency toward stress relaxation leads to the formation of new phase plates with regular orientations and internal substructure (polysynthetic twins or slip packets). The internal substructure of the plate when equilibrated depends on the plate size and the deviation from the phase equilibrium temperature. The plates form regular groups in such a way that the free energy should be at a minimum. Thermodynamic analysis permits calculation of martensitic structure: habit orientations, dimensions of the plates, as well as their mutual arrangement and internal structure. In the limiting case, the thermodynamic approach coincides with the invariant-plane conception, which is the basis of the phenomenological theory of the crystallography of martensitic transformations. Calculation of the nucleation energy of different substructural forms of martensite permits theoretical evaluations of the transformation start temperatures which appear to be in satisfactory agreement with the experimentally observed Ms points. It also provides a natural explanation of the different structural-kinetic types of transformations: athermal, isothermal, intermediate and martensite-like transformations resulting in Widmanstätten structures. Finally, the role of plastic relaxation of the internal stresses during the transformation is considered. © 1979.