Temperature investigations of mechanosynthesized cementite

Methods of differential thermal analysis and Mössbauer spectroscopy ( 57 Fe) 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 Δ l / 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(4 a ) and C(4 b ), 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).