Thermal analysis of mechanically activated Cr–B mixture

Abstract

A chromium-boron mixture with the atomic composition corresponding to CrB₂ was subjected to mechanical activation in a high-energy planetary mill for 6–40 min at a balls-to-material ratio of 20:1. The milled powders were analyzed by X‑ray diffraction and other methods. Then the mixture was heated at an initial rate of ~ 6 °C/s to a temperature of 1000 °C. Exothermic peaks associated with the formation of borides were observed on the heating curves. The onset temperature of this transformation decreased with the milling time to 15 min and then reached a plateau. This is due to the formation of a thin layered structure as the milling time increases and the limit of structure thinning is reached. The optimal milling time for preparing the mixture for self-propagating high-temperature synthesis should be 15–20 min, since the thinning of the roll layers after this practically stops or becomes very slow. At such milling duration, boride formation does not occur in the grinder, that is, the energy charge of the mixture is not consumed. At the same time, some literature data suggest that during milling for 40 h, up to 50% of boron is converted into CrB₂.