Surface hardening of Al_0.1CoCrFeNi and Al_0.5CoCrFeNi high-entropy alloys by low-temperature gaseous carburization

The effect of low-temperature gaseous carburization on Al_xCoCrFeNi high-entropy alloys (HEAs) (x = 0.1 and 0.5) was investigated. The results show that low-temperature gaseous carburization can be successfully applied to surface hardening of Al_xCoCrFeNi HEAs. After carburization, a hardened case is formed in the surface region of HEAs, which shows that the maximum nano-hardness varies from 12.9 to 11.9 GPa and is accompanied by a case depth ranging from 16 to 10 μm when the Al content is changed from x = 0.1 (2.4 at.%) to x = 0.5 (11.1 at.%). The surface hardness originates from a hard carbide-free carbon-enriched case. For carburized Al_0.1CoCrFeNi the case is a single-phase supersaturated interstitial solid solution of C atoms in FCC (expanded FCC). For carburized Al_0.5CoCrFeNi the case is a dual-phase and consists of expanded FCC phase and a supersaturated interstitial solid solution of C atoms in BCC (expanded BCC). Increasing the Al content reduces the potential for surface hardening by low-temperature gaseous carburization, because the Al-stabilized BCC phase has a relatively low capacity to dissolve C atoms, despite a high Cr content.