Phase formation and strengthening mechanisms in a dual-phase nanocrystalline CrMnFeVTi high-entropy alloy with ultrahigh hardness

Abstract

A novel equimolar CrMnFeVTi high-entropy alloy (HEA) was synthesized by mechanical alloying (MA) and spark plasma sintering (SPS). The phase formation and microstructure in the milled powders and in the sintered bulk HEA were characterized using a combination of X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). It was found that a body-centered cubic (BCC) phase formed gradually in the milled powders during MA. After SPS, a very small fraction of the BCC phase transformed into a face-centered cubic (FCC) phase. Thus, the sintered bulk CrMnFeVTi HEA exhibits a dual-phase structure of BCC and FCC phases, with an average nano-scale grain size of ∼97 nm. Thermodynamic analysis demonstrates that it is the high concentration of titanium in the bulk CrMnFeVTi HEA that stabilizes this dual-phase microstructure over a single-phase microstructure. Titanium has the largest atomic radius and the highest enthalpy of mixing with the other elements, leading to a phase transformation from the BCC phase to FCC phase during SPS. The bulk CrMnFeVTi HEA exhibits extremely high values of compressive strength (2279.53 MPa) and hardness (835 HV), which are significantly higher than those reported for most single-phase BCC structured HEAs. Calculations performed for the contributions of different strengthening mechanisms in this HEA indicate that dislocation and grain boundary strengthening play the most significant roles, whereas the effect of solid solution strengthening effect is very limited because of the release of lattice distortion energy during the BCC to FCC phase transformation in the SPS process.