Characterization of Microstructure and Tensile Properties of a New Alumina-Forming Austenitic Alloy at High Temperatures

High temperature structural materials are essential for the proper function of ultra-supercritical power generation devices. The natural reductions in strength and elongation of the material at high temperatures of operation (from 600 to 750 °C) reduce the service life of the generator. In this research, a novel alumina-forming austenitic (AFA) stainless alloy was produced and studied as a promising option for such devices. After the design and production of the novel AFA alloy, detailed mechanical tests were conducted at elevated temperatures. The evolution of the fracture morphology, microstructure, and phase transformation was evaluated at different temperatures. According to an analysis of the microstructure, the strengthening mechanism of the alloy was a combination of solution strengthening and dislocation strengthening. Through dynamic recovery, the density of the dislocations decreased and the degree of work-hardening reduced, leading to a decrease in the strength with an increase in the temperature. The segregation of Fe and Cr at the grain boundary made the grain boundary brittle and caused ductility reduction. The electron backscatter diffraction results also showed reductions of the ratio of low-angle grain boundaries and the mean number of geometrically necessary dislocations. However, this novel AFA kept a high tensile strength and a large elongation at 750 °C, which confirmed it as a good option for ultra-supercritical power generation devices.