ON SHEAR NUCLEATION SITES AT PHASE TRANSFORMATIONS IN STEEL

The article considers the question of nucleation sites during phase transitions, which, even for thoroughly studied martensitic transformations, remains an essential one. Generality of shear nucleation and nucleation site for diffusion (normal) and martensitic transformations is discussed. Specific aspects of nucleation have been studied through surface relief observation of 30KhGSA and U12 steel and technical iron by means of high-temperature metallography, and also by microstructural study of the initial stages of phase transformations of porous sintered steels with carbon content of 0.40 and 1.57  %. Pictures of austenite origin from low-angle and large-angle boundaries are presented, which testify to shear mechanism of its formation. Numerous acts of ferrite nucleation at γ-phase grain boundary are shown for both slow and fast cooling, which forms the morphology of the acicolar (Widemanstatten) ferrite. In 30KhGSA and U12 steels martensite is formed along the grain boundaries, although other nucleation areas, for example, such as packing defects, inclusions, special dislocation configurations, are not excluded, but they are not preferential. In samples­ of porous steel, predominant growth of twin crystals of martensite from pores takes place. The obtained experimental data on embryos of austenite and ferrite indicate a shear mechanism of nucleation at the initial stage of diffusion transformations, which subsequently is replaced by normal growth mechanism with formation of equiaxed grains. It is concluded that, despite the differences in phases nucleation nature and conditions, acts of nucleation in all cases occur in the same way, and the differences­ begin at the stage of growth. It is shown that shear nucleation can start from grain, subgrains boundaries and also from free surfaces (for example,­ pores in sintered steel). Nucleation in these places is facilitated by relaxation of transformation stresses and liberated part of the grain-bound-ary energy. Revealed rhomboidal morphology of martensitic­ crystals formed on pores can be explained by the fact that nucleation from free surface occurring under conditions of minimal influence of elastic fields of the surrounding matrix encourages implementation of general regularities of martensitic crystals growth.

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