Simultaneously improved the strength and ductility of laser powder bed fused Al-Cr-Fe-Ni-V high-entropy alloy by hot isostatic pressing: Microcrack closure and precipitation strengthening

Abstract

Hot isostatic pressing (HIP) is usually applied to reduce the defects including cracks and pores in the materials prepared by laser powder bed fusion (LPBF). In the present research, in order to improve the relative density and mechanical property, HIP was employed on the LPBF-processed Al-Cr-Fe-Ni-V high-entropy alloy (HEA) with microcracks and pores. The microstructure evolution and property improvement induced by HIP were investigated. In the LPBF-processed HEA, the microcracks were caused by residual stress and element segregation, and these microcracks as well as the pores reduced significantly after HIP treatments. Remarkably, HIP temperature has a more critical effect on the microcrack closure than the holding time, thus, microcracks and pores still existed after HIP-1 treatment (1273 K, 8 h), while HIP-2 treatment (1473 K, 4 h) could close the microcracks significantly. The crack closure was attributed to the interfacial diffusion of the alloying element under high temperature accompanied by high pressure, and the degree of element diffusion at both interfaces of the cracks determined the bonding strength after crack closure. Higher temperatures at high pressure induced more adequate element diffusion and higher bonding strength. The above high temperature and high pressure also induced the growth of the L12 phase and the precipitation of the B2 phase in HEA. Consequently, the tensile strength and elongation of the LPBF-processed HEA after HIP-2 treatment were simultaneously enhanced (80.7% and 222.5% higher than that of LPBF-processed HEA, respectively). This could be attributed to the combined effect of microcrack/pore closure and precipitation strengthening. The strengthening effect of the B2 phase and L12 phase accounted for 53% (dislocation by-pass mechanism) and 47% (dislocation shearing mechanism) of the total precipitation strengthening, respectively.