Effects of heat treatment cooling methods on precipitated phase and mechanical properties of CoCrFeMnNi–Mo5C0.5 high entropy alloy

Abstract

C and Mo in the form of solid solution atom or compound phase is distributed in CoCrFeMnNi high entropy alloy (HEA), which have the effects of solution strengthening or second phase strengthening. CoCrFeMnNi–Mo5C0.5 HEA were prepared by adding C and Mo elements to CoCrFeMnNi HEA, and then which heat treatment were carried out by furnace-cooling, air-cooling and water-cooling respectively. the microstructure and phase composition of as-cast or heat treated CoCrFeMnNi–Mo5C0.5 HEA were examined by scanning electron microscopy (SEM), energy dispersive spectrometry (EDS) and X-ray diffraction (XRD), the mechanical properties of as-cast or heat treated CoCrFeMnNi–Mo5C0.5 HEA were tested by the tensile test and hardness test, and the effects of heat treatment cooling methods on precipitated second phase and mechanical properties of CoCrFeMnNi–Mo5C0.5 HEA were investigated. The results show that precipitated second phase in CoCrFeMnNi–Mo5C0.5 HEA is MC phase, which is rich in Mo, Cr and C elements. With change of heat treatment cooling methods from furnace-cooling, air-cooling to water-cooling respectively, heat treatment cooling rate is increased. With increase of heat treatment cooling rate, the amount and size of precipitated second phase are all decreases, and the position of second phase precipitation changes from grain boundaries to intragranular. The tensile strength and elongation of heat treatment water-cooled CoCrFeMnNi–Mo5C0.5 HEA are 739 MPa and 41% respectively, which are realizes the simultaneous increased of strength and plasticity. With increase of heat treatment cooling rate, the hardness of CoCrFeMnNi–Mo5C0.5 HEA decreases, the uniform plastic deformation and the elongation are all increases, and the plasticity increase. The fractures of as-cast, heat treatment furnace-cooled, air-cooled and water-cooled CoCrFeMnNi–Mo5C0.5 HEA are all brittle fracture with cleavage characteristics, and the plastic deformation occurs mainly in the uniform plastic deformation stage. The fracture mode of as-cast CoCrFeMnNi–Mo5C0.5 HEA is intergranular fracture, the fracture mode of heat treatment furnace-cooled, air-cooled and water-cooled CoCrFeMnNi–Mo5C0.5 HEA are all transgranular fracture.

Keywords

• CoCrFeMnNi high entropy alloy (HEA)
• Adding C and Mo
• Heat treatment cooling methods
• Precipitating second phase
• Mechanical properties
• Fracture mechanism