Sigma phase evolution in Co-Re-Cr-based alloys at 1100 °C

Co-Re-based alloys have been introduced as a novel metallic system that possesses a wide exploitable composition range and high melting temperatures. The poor oxidation resistance of the binary system can be improved by alloying chromium. However, adding chromium also leads to the occurrence of the sigma phase of type Cr2Re3. In the present study, we investigate the evolution of the sigma phase during creep and aging at 1100 °C for three selected alloys based on the ternary composition Co-17Re-23Cr (at.%). In all alloys, sigma phase populates the grain boundaries of the hexagonally close-packed (HCP) matrix phase in a blocky morphology. Additionally, a fine dispersion of lamellar sigma phase in the grain interiors has formed during the initial processing or forms during thermal exposure. This precipitation takes place by a cellular reaction that transforms a supersaturated HCP phase into alternating lamellae of a near-equilibrium HCP phase and the sigma phase. The process therefore has the character of a discontinuous precipitation. Using orientation imaging microscopy, we observe an orientation relationship between the lamellae, which describes the basal plane of the HCP phase to form a coherent interface with the base layer of atoms of the tetragonal sigma phase. After long-term thermal exposure to 1100 °C, overaging of the lamellar structure results in spheroidization of the sigma lamellae and subsequent Ostwald ripening.