Discovery of new alloys has always projected civilization forward. Commonly, alloys are synthesized with one or two main elements with addition of minor elements to further improve material properties. High Entropy Alloys (HEAs) however, provide a new approach to the creation of new alloys, which allows many new combinations of elements which has never been explored.
By combining five or more elements with equiatomic distribution, a plethora of new alloys can be created. In general, HEAs are benefited by severe lattice distortion effect and sluggish diffusion effect, which will benefit mechanical properties and thermal stability. Amongst the many combinations of HEA systems possible, CoCrFeMnNi is one of the highly researched systems. It has high hardenability, ductility, and increased mechanical properties at cryogenic temperatures. However, its low yield strength severely limits possible applications of this HEA.
To improve mechanical properties, solid solution strengthening and dispersion strengthening was selected. Solid solution strengthening was achieved by addition of aluminum. Further strengthening is also achieved by addition of $Y_2O_3$ .
In this study, $Al_{0.3}CoCrFeMnNi$ with different $Y_2O_3$ contents are synthesized by Mechanical Alloying and Spark Plasma Sintering. Addition of aluminum added a BCC phase within the originally sole FCC microstructure and also increased solid solution strengthening. While addition of $Y_2O_3$ increased yield strength greatly due to dispersion strengthening.
새로운 물성을 가진 합금의 개발은 매우 중요한 과제이다. 널리 이용되는 기존의 합금은 1~2개의 핵심 원소를 바탕으로 다른 원소의 양을 조절하는 방식을 통해 개발되어 왔다. 기존의 합금과는 달리, 하이엔트로피합금은 5개 이상의 원소가 비슷한 분율로 혼합된 새로운 형태의 합금으로, 격자뒤틀림, 낮은 확산 속도 등으로 인해 우수한 기계적 물성을 보유하고 있다. CoCrFeMnNi계 하이엔트로피 합금의 경우 높은 연신율, 저온안정성 등으로 인해 널리 연구되고 있으나, 낮은 항복강도가 극복해야 할 과제이다.
본 연구에서는 CoCrFeMnNi계 합금의 기계적 물성을 향상시키기 위해 Al원소 첨가를 통한 고용강화와 $Y_2O_3$ 첨가를 통한 분산강화 원리를 적용하였다.기계적 합금화와 방전플라즈마 소결을 이용해Al0.3CoCrFeMnNi하이엔트로피 합금에 $Y_2O_3$ 를 복합화한 신합금을 제조하였다. Al원소 첨가를 통해 BCC상을 형성시켰고, $Y_2O_3$ 첨가로 항복강도를 크게 향상시키는데 성공하였다.
