EFFECT OF CYCLIC MARTENSITIC γ-ε TRANSFORMATIONS ON DIFFUSION CHARACTERISTICS OF COBALT IN AN IRON-MANGANESE ALLOY

This review article analyses the results of experimental investigations of the influence of the cyclic direct gamma-epsilon (f.c.c.-h.c.p.) and reverse epsilon-gamma (h.c.p.-f.c.c.) martensitic transformations (MT) on the diffusion characteristics of Co atoms in Gamma 18C2 (Fe-18.3 wt.% Mn-2.1 wt.% Si) alloy with low stacking-fault energy. With using of the radioactive isotopes via autoradiography and the layer analysis methods, it is shown the significant intensification of mobility of Co atoms by gamma <-> epsilon transformations is determined by two different independent mechanisms: due to the MT (athermal mechanism) and by means of the mechanism of thermal activation in the area of structural defects formed during the gamma-epsilon and epsilon-gamma transformations. The possibility of Co atom transport by means of the athermal mechanism in the process of cyclic martensitic transformations (CMT) by moving of interstitial atoms and their complexes along the close-packed (111)(gamma) and (001)(epsilon) planes in the crystal lattices of f.c.c. austenite and h.c.p. martensite, respectively (crowdion mechanism) is analysed. The ability of the crowdion complexes to move with a velocity exceeding the velocity of sound in a crystal in the field of high internal stresses arising during high-rate deformation of austenite in a MT process is taken into account. The regularities of accumulation of such structural defects in the CMT process as disorientation of the crystal lattice and chaotic stacking faults (CSF) are investigated by the x-ray methods for the single-crystalline and polycrystalline samples. The intensification of diffusion processes in a phase-hardened alloy by the thermal-activation mechanism is attributed to the increase in the Co-atoms' mobility in the region of accumulation of defects in the crystal structure. An analysis of the regularities of accumulation of different types of defects with an increase in the phase-hardening degree made it possible to establish a certain sequence of their influence on the diffusion mobility of Co atoms. The results of the investigation develop the physical notions about the diffusion of substitutional atoms in alloys with a developed system of the structural defects of different types (dislocations, small-angle sub-boundaries of fragments, large-angle grain boundaries, deformation twinning boundaries, CSF). The obtained new experimental data can be used to develop a model of diffusion in the region of the linear and planar structural defects at low temperatures (below the half of the melting point). The perspective of practical application of the diffusion-intensification regularities for optimization of chemical-and-thermal treatment regimes and dispersion hardening processes for metastable alloys is determined. A significant acceleration (by means of the CMT) of the diffusion of substitutional atoms at low temperatures opens up new additional possibilities of the dispersion solidification technology.