The Structure Evolution of the Interstitial High Entropy Alloy Fe50Mn30-xCr10Ni10Cx Produced by Mechanical Alloying

Three non-equiatomic Fe50Mn30-xCr10Ni10Cx (x = 0.2, 0.5, and 0.8 atomic%) high entropy alloys were prepared by mechanical alloying with milling times up to 25 h. Green compacting and sintering at 1200 °C for 6 h were conducted to the milled samples. The phase diagram of each of the studied high entropy alloy system was predicted under equilibrium conditions. Microstructure characterization and chemical composition analysis were carried out using x-ray diffraction, scanning electron microscope, and transmission electron microscope. A dual-phase structure is established from the austenitic FCC, with Fe and Ni as the main elements, and the carbide precipitates that have a cuboidal shape and are rich in both Cr and Mn. There is an obvious grain size heterogeneity with some localized grain boundaries decohesion; besides, sigma phase appears at the grain boundaries for all studied alloys. By increasing the C content, both the yield strength and microhardness are enhanced and ductility is decreased with closer values in the higher C content alloys. Compressed alloys show fragmentation and fracturing in cuboidal second phase precipitates, while slip bands are observed in the more ductile FCC grains. In addition, nano-twins are observed in the transmission electron microscope images.