PHASE HARDENING INFLUENCE ON THE DIFFUSION CHARACTERISTICS OF CARBON AND COBALT IN FERROMANGANESE ALLOYS

Abstract

Martensitic transformation (MT) is the main type of phase transformations in solids and is characterized by a number of features that distinguish it from other phase transformations. It is characteristic of many alloys (based on Fe, Cu, Au, Ag, Co, Ti). The main features that unite all martensitic transformations into one group are diffusionlessness, regular movement of atoms, cooperativity and directionality of such movements in the process of restructuring the crystal lattice of the initial phase. MT is a powerful tool for creating materials with a wide range of new properties, primarily diffusional.

As a result of cyclic martensitic transformations (CMT) in metastable alloys, linear and planar defects of the crystal structure (dislocations, packing defects, subgrain and nanograin boundaries) are generated. In the region of such defects, the diffusion mobility of insertion atoms at low temperatures increases by orders of magnitude. Thus, in metastable iron-based alloys, the idea of increasing the intensification of low-temperature diffusion mobility in order to control diffusion processes that form useful properties can be implemented due to the internal factor of the MT without using any high-energy actions. Acceleration of the diffusion of insertion atoms due to γ–α–γ (fcc-bcc-fcc) MT has been shown experimentally. The influence of γ–ε–γ (fcc-gcc-fcc) MT on diffusion processes in alloys with low packing defect energy still remains practically unstudied.

A new stage in the development of methods for processing metallic materials using diffusion processes may be the practical implementation of the possibility of increasing the diffusion mobility of atoms, in particular at low, technologically convenient temperatures. In view of this, the problem of a significant, by orders of magnitude, increase in the diffusion coefficient in metallic systems is of a pronounced fundamental nature. It can be solved by using the strong dependence of diffusion characteristics on the sizes of structural and substructural elements and the main types of defects in the crystal structure.

The diffusion mobility of insertion atoms (for example, carbon) in metastable iron-based alloys increased as internal stresses accumulated and crystal defects accumulated during cyclic γ–ε–γ martensitic transformations. The main increase in the diffusion coefficient and the decrease in the diffusion activation energy were observed at the thermal cycling stage, when the maximum increase in the density of crystal defects was recorded.

It is shown that cyclic γ–ε–γ martensitic transformations significantly increase the diffusion mobility of cobalt and carbon atoms. The difference in the increase in the depth of carbon penetration increased with increasing the number of transformation cycles, but to a certain extent. Optimal conditions for carrying out this type of processing were found based on available experimental data, which also gave impetus to conducting additional research in the search for the least energy-consuming machining modes.

Acceleration of the CMT diffusion of the cobalt and carbon atoms at low processing temperatures opens up the possibility of significantly optimizing processing modes and reducing the cementation temperature of metastable ferromanganese and ferronickel alloys to save resources and energy costs.

The article is a review of previously obtained data, general conclusions are made regarding the diffusion characteristics of substitutional and insertion atoms for the case of iron-manganese alloys.