Temperature investigations of mechanosynthesized cementite

Methods of differential thermal analysis and Mössbauer spectroscopy (⁵⁷Fe) have been used to study the process of the formation of cementite in α-Fe upon the low-temperature mechanosynthesis (T < 375 K) in the medium of liquid hydrocarbons. It has been established that this process occurs in the absence of austenite and corresponds to a two-stage model suggested previously for describing the mechanism of the decomposition of the quench martensite α″-(Fe)C with the precipitation of the θ phase in the process of aging or tempering. Upon dilatometric studies of the single-phase samples of cementite in the range of 750 ≤ T ≤ 925 K, a significant increase was revealed in the linear elongation Δl/l ₀ and linear thermal expansion coefficient (LTEC). It has been assumed that this increase is not connected with the localization of carbon atoms in the positions C(4a) and C(4b), but rather is determined by the anomalously high concentration of equilibrium carbon vacancies V _C in the unit cell of cementite. The concentration of this type of vacancies can be sufficient for the growth of a graphite component of the carbon layer on the surface of the particles of the mechanosynthesized cementite (θ phase).