Effect of heat treatment temperature on microstructure and electrochemical behavior of additive-manufactured AlCoCrFeNi high entropy alloy

Abstract

In this paper, a high-entropy AlCoCrFeNi alloy were prepared using laser melting deposition (LMD) and the effect of heat treatment on their microstructure and corrosion resistance was studied. XRD, OM, SEM, EDS, EBSD and electrochemical tests were used to investigate the structure, microstructure, and corrosion resistance. The LMD and LMD-500 °C samples showed a single BCC phase structure. After the temperature was increased to 800 °C, a small amount of a Cr-rich and Fe-rich FCC phase and σ phase precipitated at the grain boundaries, Al and Ni were enriched within the crystals. At 1100 °C, the σ phase disappeared, and the FCC phase content increased from 1.4% to 5.2%. The average grain size was the largest at 500 °C, reaching 31.29 μm. In 3.5% NaCl solution, the LMD-500 °C sample showed a high self-corrosion potential and the lowest corrosion current density; the polarization resistance (Rp) and charge transfer resistance (Rct) were 5.266 × 105 Ω∙cm2 and 1.248 × 108 Ω∙cm2, respectively, with the shallowest surface corrosion and the best corrosion resistance. In 0.5 mol/L NaOH solution, there were only a few corrosion holes in the samples before and after the heat treatment, and all the samples showed a good corrosion resistance. In 0.5 mol/L H2SO4 solution, with the increase of temperature, the Cr, Al and other elements underwent intensified segregation. The corrosion resistance gradually improved, the Rp and the Rct was 2.785 × 102 Ω∙cm2 and 7.569 × 103 Ω∙cm2, respectively, at 1100 °C, providing the best corrosion resistance.