Macroscopic tensile properties of AlxCoCrCuFeNi (x = 0.3 and 1) before and after heat treatment

Abstract

The relationship between morphology (grain-size order) and mechanical properties of AlxCoCrCuFeNi (x = 0.3 and 1) was evaluated. Both alloys were prepared using the arc-melting method. Crystal structures with x = 0.3, face-centered cubic (FCC), and body-centered cubic (BCC) structures were observed for x = 1, the typical crystal structures of AlxCoCrCuFeNi. The alloys prepared via arc melting exhibited two different microstructures: a Cu-rich region (area I) and an equiatomic region (area II). Microstructures of the as-fabricated specimen were homogenized through heat treatment at 600 and 1000 °C for 5 h. Although the homogenization of the microstructure proceeded, areas I and II remained after the heat treatment. In particular, the area I composition was inconsistent with the predicted phase diagram. Tensile tests for these alloys revealed that the tensile strength of x = 1 (∼310–1100 MPa) is higher than that of x = 0.3 (∼320–660 MPa), whereas the fracture strain of x = 1 (∼0.03–0.09) is lower than that of x = 0.3 (∼0.06–0.26). These results indicate that the increase in Al content caused an increase in strength and brittle fracture because it also caused an increase in the formation of the BCC and B2 phases, which required higher stresses for the movement of dislocations than the FCC phase. Because the tensile properties of AlxCoCrCuFeNi are comparable to those of conventional alloys, such as Ti alloys and steels, a design with a moderate composition for stronger and tougher AlxCoCrCuFeNi is required to apply high entropy alloys to structural materials.