Phase stability and mechanical properties of a single-phase and compositionally complex quinary σ phase
A brittle intermetallic known as σ phase is often found to form in several engineering alloys [Hsieh_2012, Reed_1999] as well as high-entropy alloys (HEAs) [Laplanche_2018] when exposed to elevated temperatures. Its precipitation causes a deterioration of the alloys ductility and corrosion resistance [Hsieh_2012, Pohl_2007]. Although the σ phase precipitates in several important compositionally complex alloy systems, comprehensive data regarding its phase stability and intrinsic mechanical properties, especially in HEAs, is still lacking. Therefore, the aim of this work is to produce a single-phase, monolithic σ phase with a nominal composition of Cr46.0Mn15.2Fe16.3Co17.0Ni5.5 in at.% [Laplanche_2018] and subsequently to study its stability and properties.
To investigate the phase stability of our monolithic σ phase, several heat treatments followed by water quenching were performed. The results show that the σ phase transforms into a body-centered cubic (BCC) phase when annealed above 1170°C. Based on these results, a cast ingot was annealed for 72 h at 1100°C to stabilize the σ phase. Subsequent microstructural analyses including SEM, TEM, and XRD that our alloy mainly consists of σ phase (99%) with minor secondary phases (~1 %) such as a face-centered cubic phase and M7C3 carbides. The precipitation of these secondary phases is believed to be due to the presence of small amounts of carbon, which are either present in the raw materials used to cast the alloy or result from contamination during the arc-melting process. As the alloy solidifies as a BCC phase, which has a low carbon solubility, solidification results in a carbon enrichment of the melt next to the solidification front. At the late stage of solidification, the rest of the melt has been so enriched in carbon that it may stabilize the FCC phase and the M7C3 carbides. As a result of the brittle nature of the σ phase, neither tensile nor compression specimens could be prepared out of the cast and annealed ingot. Therefore, micro- and nanoindentation were conducted to determine its hardness and Young’s modulus. Additionally, an attempt was made to evaluate its fracture toughness using a model based on the length of the cracks that formed at the corners of the microindents during microhardness testing [Niihara_1982, Schiffmann_2011].