Influence of processing route on microstructure and properties of Al13.45FeCrNiCo high entropy alloys

The Al13.45FeCrNiCo high entropy alloys (HEAs) were obtained using two different processing routes, namely (i) mechanical alloying (MA) and spark plasma sintering (SPS), and (ii) vacuum induction melting – copper cast molding. A final homogenizing treatment step was applied to the as synthesized materials. Based on thermodynamic and empirical criteria for phase formation, the alloy was designed to develop a mixture of the face-centered-cubic (FCC) and body-centered-cubic (BCC) solid solution. The contribution of aluminum content on the alloy microstructure in terms of hardening of solid solution and precipitation hardening was highlighted. The influence of the processing route on the microstructure evolution and the relationship between the microstructure and properties of HEA materials were established by performing X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), transmission electron microscopy (TEM), and high-resolution (HR) TEM analyses, as well as microindentation and compression tests. The powder metallurgy processes produced a two-phase structure consisting of at least two types of crystalline grains based on FCC and BCC solid solutions. Increasing the sintering temperature from 950 °C to 1050 °C contributed to structural changes in the materials. Lower sintering temperatures (950 °C and 1000 °C) generated a single FCC phase structure, while at 1050 °C two FCC and BCC phases were obtained without any evidence of the liquid phase sintering. The as cast route generated a hypoeutectic type structure with a very fine and preferentially oriented dendritic structure, in which the interdendritic space is occupied by a second phase arranged in a network where the microstructure appears to be the result of a eutectic transformation with precipitation of secondary phases rich in Ni and Al. All the synthesized alloys revealed high hardness and a good mechanical behavior in compression. Sintered HEA materials exhibited higher Vickers hardness than as cast HEA, even after the heat treatment process, while the as cast HEA material has better ultimate strength and deformation behavior than sintered HEAs. The processing route did not affect the nature of the FCC and BCC solid solution phases formed in the Al13.45FeCrNiCo HEA materials but was responsible for the variation of phase proportion in the HEA’s microstructure, which in turn influenced the mechanical properties.