Determination of phase equilibria in the Fe 3Al–Cr–Mo–C semi-quaternary system using a new diffusion-multiple technique

Abstract

A new diffusion-multiple (DM) technique was proposed to allow us mapping of quaternary phase diagrams. This technique utilizes a combination of DM technique to introduce two-dimensional composition gradients in a matrix phase and a conventional annealing heat treatment to have precipitation reactions in the matrix. Phase equilibria at 800 °C in the semi-quaternary Fe 3Al–Cr–Mo–C system were determined with an assist from this technique. The following five carbide phases were precipitated in the Fe 3Al matrix phase (B2) with composition gradients of Cr, Mo and C: κ-Fe 3AlC, M 6C H, M 6C L, Cr 7C 3 and M 2C (M: Mo, Cr and Fe, the two M 6C phases are distinguished by their C and Cr contents.). It was found from our DM technique that the four-phase equilibrium of M 6C H + M 6C L + Cr 7C 3 + B2 exists. Microstructure analysis of several bulk alloys as well as DM samples revealed that the three-phase equilibrium of M 6C L + κ + B2 in the Fe 3Al–Mo–C system changes to that of M 6C L + Cr 7C 3 + B2 with increasing Cr content through the following three four-phase equilibria: (1) M 6C H + M 6C L + κ + B2, (2) M 6C H + Cr 7C 3 + κ + B2, (3) M 6C H + M 6C L + Cr 7C 3 + B2. The M 2C phase is thought to be not in equilibrium with the Fe 3Al matrix phase at 800 °C.