Microstructural investigation of phase transformations and properties during thermal processing of a 6 w/oAl-modified Ni–Cr–Fe ternary alloy

A differential scanning calorimetric study was carried out on a P/M processed Inconel 600 (Ni–Cr–Fe)-based superalloy modified by the addition of 6 w/o Al to study sequence of phase transformations, their reaction modes and products on heating a green compact from room temperature to the sintering temperature of 1300 °C. Two different heating rates were employed for the DSC study, 2.5 °C/min and 10 °C/min. Transformation reactions were also studied by heating the samples in a DSC to the points of exo/endothermicity and quenching in argon followed by phase identification by X-ray diffraction, and microstructural analysis by SEM equipped with EDS capability. A series of AlxNiy- and AlxFey-type intermetallics were observed to form by phase transformation at temperatures from 540 °C to 1120 °C. The sequence of the formation of intermetallics by those phase transformation closely replicated the intermetallics that were predicted by the binary Ni–Al and the Fe–Al equilibrium phase diagrams to form, with the final microstructure being Ni3Al (γ′), AlNi, AlFe in a Ni–Cr–Fe (γ) matrix. Most of these phase transformations were diffusion-controlled, however at 640 °C AlNi formed by combustion synthesis. Selected mechanical property of the alloy at 1120 °C which corresponds to the completion of transformation was estimated in terms of micro-hardness and compared to the as-sintered ternary.