Effect of Ti Addition on the Microstructure and High-Temperature Oxidation Property of AlCoCrFeNi High-Entropy Alloy

This study investigated the effect of Ti addition on microstructure and high-temperature oxidation property of AlCoCrFeNi high-entropy alloy. Ti content was controlled at 0 at% and 1 at%. The two alloys were found to have BCC single phase, and the average grain sizes of Ti0.0 and Ti1.0 were 47.3 mu m and 49.7 mu m, respectively, showing similarity. The EDS mapping of the inside of grains found that both alloys were characterized to be divided into Al-Ni element rich region and Cr-Fe element rich region. As a result of high-temperature oxidation test at 1100 degrees C, oxidation weight gains were measured at Ti0.0: 0.75 mg/cm(2) and Ti1.0: 0.17 mg/cm(2), respectively. The results indicate that Ti addition largely improved high temperature oxidation resistance of AlCoCrFeNi HEA. Ti1.0 alloy, in particular, showed remarkably more excellent 1100 degrees C high-temperature oxidation resistance than other previously reported major ones such as NiCrAl and FeCrAl. In the surface and cross-section observations after oxidation tests, both alloys were found to have Al2O3 oxides mostly. While the Ti0.0 material was observed to have Al2O3 spallation macroscopically, the Ti1.0 alloy showed Al2O3 spallation only in some local areas. In addition, a unique result was found in AlCoCrFeNiTix(x=0,1) alloy that BCC -> FCC phase transformation was accelerated, and FCC phase layer was formed in the surficial area where Al element had been depleted due to high-temperature oxidation. Moreover, as Ti was added, the thickness of FCC layer induced by high-temperature oxidation decreased. Based on the results, it was also discussed on how to improve the high-temperature oxidation resistance of AlCoCrFeNiTix HEA.