Microstructural development in nanostructured AlCoCrFeNi-ZrO2 high-entropy alloy composite prepared with mechanical alloying and spark plasma sintering methods

In this research, ZrO2-reinforced AlCoCrFeNi high-entropy alloy (HEA) composites were fabricated using mechanical alloying (MA) and spark plasma sintering (SPS) methods. It was found that after 30 h of milling a body-centered cubic (BCC) solid solution structure with an average particle size of 54 m for HEA sample and 46 m for HEA-10 wt% ZrO2 could be obtained. The addition of 10 wt% ZrO2 particles to Al-Co-Cr-Fe-Ni system resulted in finer particle size during the mechanical alloying process. Consolidation of MA-processed HEA powders with SPS method at 1000 °C was led to the formation of a combination of FCC and BCC phases due to the high heating rate of SPS process and limited time for the completion of BCC to FCC phase transformation. With the aid of Rietveld refinement analysis, it was found that the addition of zirconia particles has a restrictive effect on the grain growth of HEA phases during SPS. Interestingly, it was observed that while the weight percentage of FCC phase was almost similar in HEA and HEA-ZrO2 composites, after the addition of zirconia reinforcements the content of BCC phase was decreased. It is well known that the introduction of hard ceramic particles to metal matrices can enhance the interfaces in the composite samples and facilitate the outward diffusion of aluminum from the BCC phase. Thus, the content of BCC phase was decreased in the HEA-based composite samples compared to the pure HEA sample.