Methods of producing nanostructured materials such as powder metallurgy, inert gas condensation, mechanical milling, melt quenching, or crystallization of an amorphous material have received much attention[1, 2]. Another approach for the preparation of highly dispersive materials is cyclic plastic deformation, which is viable for particular classes of metallic materials.
The crystallographic orientation of initial austenite in Fe-based alloys is nonideally restored after reverse martensite transformation. This is associated with the incomplete reversibility of direct and reverse martensite transformations and an accumulation of dislocations in the reverted austenite[4, 5]. Formation of structure defects on martensite transformations results in azimuthal tailing of diffraction reflections of single-crystalline samples. From the magnitude of tailing and from the increase of the misorientation angle of crystal lattice regions after multiple martensitic transformations, one can deduce the capability of fragmentation and grain refinement of an austenite phase[4, 6].
In Fe-based alloys, three types of martensitic transformations are realized: γ-α-γ in Fe-Ni-based alloys with face-centered cubic (f.c.c.)-body-centered cubic (b.c.c.)-f.c.c. structure rebuilding, γ-ϵ-γ in Fe-Mn-based alloys with f.c.c.-hexagonal close-packed (h.c.p.)-f.c.c. transformation, and γ-ϵ′-γ in Fe-Mn-based alloys with f.c.c.-18-layer rhombic (18R)-f.c.c. transformation[8, 9]. It is shown experimentally that the restoration of the initial austenite structure after cyclic γ-ϵ-γ and γ-ϵ′-γ transformations turned out to be superior against that of alloys with γ-α-γ transformations. This regularity is based on the fact that the density of dislocations increases by more than 103 after cyclic γ-α-γ transformations connected with a high volume change - up to 3% to 4%, while it increases only by 10 after cyclic γ-ϵ-γ transformations (with a smaller volume change - up to approximately 0.75%) and practically does not change after γ-ϵ′-γ transformations (volume change - up to approximately 0.5%)[4, 7]. In the austenitic phase, additional subgrain boundaries can form under conditions of dislocation generation by direct and reverse martensite transformations, for example, by means of wall formation by one-sign dislocations. On account of these processes, the fragmented structure of reverted austenite is received. The process of structure fragmentation can be essentially different for alloys with different types of martensitic transformations.
In the present article, the effect of multiple martensitic transformations of different types is studied in Fe-Ni- and Fe-Mn-type alloys. The development of austenitic structure fragmentation and the capability of particular alloys to form highly dispersive structures due to the accumulation of structure defects are elucidated.