Zhen Xu

Precipitation strengthening of body-center cubic (A2) alloys via ordered B2 nanoprecipitates is expected to achieve a desirable combination of strength and ductility. In this work, the A2/B2 configuration is manipulated by adjusting Fe content in medium-entropy AlCrFe_xNi (x = 0, 0.5, 1.0, 1.5, 2.0, 2.5 and 3.0) alloys fabricated via arc-melting for improved mechanical properties and wear resistance. As Fe content increases, the fraction of A2 phase increases, and A2 nanoprecipitates in the B2 matrix change to a weave-like A2/B2 structure. Continuously increasing Fe content leads to a mixture of BMAP (B2 matrix with A2 precipitates) and AMBP (A2 matrix with B2 precipitates), and finally to a complete AMBP structure. The yield strength decreases and fracture strain increases with increasing Fe content except x = 0. The alloy of x = 0 displays slightly higher hardness because of its relatively brittle B2 matrix. Cracks tend to propagate along A2/B2 interfaces. AMBP structure exhibits greater toughness than the BMAP structure. The alloy of x = 0 displays the second-greatest wear volume loss due to its relatively brittle B2 matrix. When Fe is added, the wear volume loss decreases considerably but shows a trend of an upward parabola with respect to the Fe content. After achieving the highest volume loss at x = 1.5 with a mixture of AMBP and BMAP, the volume loss decreases again. A completely uniform AMBP structure at x = 3.0 shows the least volume loss.

Microstructure, mechanical properties, wear resistance, corrosion and corrosive wear resistance of AlCrFeNiW_x (x = 0, 0.1, 0.2, 0.3 and 0.4) medium-entropy alloys (MEAs) prepared by vacuum arc melting process were investigated. Results of the study show that the microstructure of as-cast AlCrFeNiW_x alloys is composed of one disordered Fe-Cr rich BCC phase (A2) and one ordered NiAl-rich (B2) phase when x = 0 and 0.1; W particles appear when x ≥ 0.2. Hardness and strength of AlCrFeNi alloy are markedly increased by the W addition. The strengthening mechanisms include solid-solution strengthening, nano-sized precipitation strengthening and second phase strengthening. The excellent ductility of AlCrFeNi is retained with minor W addition (x ≤ 0.2) but it considerably declines as more W is added, resulting from the precipitation of excessive large-sized W particles. W addition improves the pitting resistance and passivation property of AlCrFeNi HEA in 3.5 wt. % NaCl solution. It is shown that AlCrFeNiW_0.3 possesses the highest corrosion resistance, as reflected by the highest E_corr and the lowest I_corr. Tungsten notably enhances the resistance of this HEA to wear and corrosive wear. AlCrFeNiW_0.4 with the highest strength and hardness exhibits the lowest wear volume loss under both dry and corrosive wear conditions.