High entropy alloys(HEAs) alloyed with more than five elements at equivalent atomic ratios have a high entropy effect, a lattice distortion effect, a sluggish diffusion effect, and a cocktail effect due to the complex interaction of the elements. As a result, it has better mechanical, electrical and thermal properties than commercial alloys. A lot of research has been published as a new concept material that can replace various conventional structural materials.
Recently, HEAs are generally produced by arc melting casting, which may cause shrinkage defects, dendrite, and segregation in the casting process. Therefore, it are required multi-layer melting and solidification process. In order to solve the problems, research on newly HEAs fabricated by powder metallurgy process is needed. So, we investigate the phase transformation and separation of HEAs using mechanical alloying, and develop a new HEAs with high mechanical properties.
In order to establish optimal alloying process conditions, AlCoCrFeNiTi powders mixed in equivalent element ratio and then alloyed with high energy milling and cryomilling respectively. Alloyed powders were densified by spark plasma sintering(SPS). Also, phase separation and transformation behavior were analyzed.
Also, to investigate the phase transformation behavior and mechanical properties of HEAs according to the additive elements, HEAs was alloyed with Cu, Mn, Mg, Ti or Zn to AlCoCrFeNi by high energy milling process.
After densification, microstructural changes according to process steps and additive elements were compared and analyzed. The phase transformation behavior on HEAs was studied. As the valance electron concentration(VEC) of the HEAs increases, the fraction of the face centered cubic(FCC) phase growth. Also, AlCoCrFeNi and AlCoCrFeNiMn with body centered cubic(BCC) and FCC formed a fine sigma phase Cr and Fe rich phase, due to lower the energy level in matrix.
The change of the mechanical properties according to the additive element is researched, Cu forms a nano scale liquid phase during the densification process. Ti has a higher hardness and a lower lattice constant than other elements so that alloying does not occur well during high energy milling and densification processes, by the reason, the nano scale Ti phase is distributed in the grain boundaries. Mg and Zn form pores due to vaporization. Pore and rich phase induced deterioration of mechanical properties on HEAs.
On the other hand, Mn is a simple cubic structure, and its diffusion rate is speedy due to the large lattice constant. So, alloying, phase stabilization and separation quickly were occurred during mechanical alloying and densification processes. For a reason, fine ordered BCC and sigma phase are formed. Finally, HEAs with excellent mechanical properties were fabricated in this study.
In this study, various HEAs which have not been studied were fabricated with the mechanical alloying process. Also, the optimal process conditions for successfully producing HEAs using high energy milling and cryomilling were established. And, the phase transformation, separation behavior and microstructural changes of HEAs were investigated according to the added elements. The relationship between microstructure and mechanical properties was investigated.
It is expected to be applied to various research including high-strength light-weight HEAs based on Al, Ti and Mg and low cost HEAs based on Al, Mg, and Zn. Also, the optimal mechanical alloying condition which is established in this study can be applied to various powder metallurgy processes.
