Primary carbide transformation in a high performance micro-alloy at 1000 °C

The transformation from M₇C₃ → M₂₃C₆ in a high performance micro alloy (HP-MA) was studied using different techniques. Following the cooling during the centrifugal casting, the as-cast HP-MA alloy consists of an austenitic matrix with the primary carbide network consisting of a combination of M₇C₃ (M being mainly chromium) and MC (M being mainly niobium). During the heat treatment at 1000 °C, the primary chromium carbides transform from M₇C₃ to M₂₃C₆. The incompletely transformed primary carbide consists of an outer shell of M₂₃C₆ carbide (coherent with the austenitic matrix) and a core of M₇C₃ type carbide. The experimentally determined M23C6 shell thickness agrees reasonably well with the estimated diffusion distance of carbon in M₂₃C₆ based on Zener's relation implying that the M₇C₃ → M₂₃C₆ transformation is mainly controlled by the diffusion of the carbon from M₇C₃ to the matrix through the M₂₃C₆ shell. This mechanism is discussed vis-à-vis the literature reported mechanism on the transformation.