|dc.description.abstract||A new paradigm of alloy design concept, named as multi-principal element alloys or high entropy alloys (HEAs), has been proposed to overcome properties limit of conventional alloys. The HEAs consist of more than five metallic elements with equi- or near equi-atomic concentration. The HEAs exhibit exceptional properties, including high strength at various temperature, moderate ductility, and considerable resistances to wear, oxidation and corrosion. Several HEA systems have been intensively investigated, including Cantor alloy (CoCrFeMnNi), combined systems of Al and 3d transition metals, and refractory HEAs (RHEAs). Particularly, the RHEAs, which are predominately composed of refractory elements such as W, Mo, Nb and Ta, have been developed recently for applying to high temperature aerospace industries. Some of the RHEAs were reported as having superior compressive yield strength at room temperature and elevated temperatures compared to commercial Ni-base superalloys. However, there are several issues for RHEAs, heavyweight, poor ductility and oxidation resistance, and limited understanding on fundamentals of strengthening mechanisms at room and elevated temperature.
In this research, lightweight CrNbVMo (Al0), Al0.1CrNbVMo (Al0.1), Al0.5CrNbVMo (Al0.5), and Al1.0CrNbVMo (Al1.0) RHEAs, were newly designed and developed, and their microstructure and mechanical properties at room and elevated temperature were systemically characterized. Al was added to CrNbVMo system, which is a benefit for reducing density, and decreasing shear modulus and valence electron concentration (VEC) to increase ductility. When it comes to processing method, powder metallurgy (PM), which has several advantages including homogeneous microstructure without chemical segregation, energy efficiency and easiness to fabricate precise and small components compared to conventional casting process, is utilized to synthesis the AlxCrNbVMo RHEAs. By using the PM method, homogeneous microstructure without chemical segregation and micropores was achieved. The concept of the optimization of the milling time are proposed to minimize the C and O contamination, which is resulting to the formation of carbide and oxide. The AlxCrNbVMo RHEAs showed outstanding mechanical properties compared to those of other RHEAs processed by conventional casting. Especially, the specific yield strength of the Al0 alloy was increased by 27% and 87% at 25 and 1000℃, respectively, compared to the AlMo0.5NbTa0.5TiZr RHEA, which exhibited so far the highest specific yield strength among the cast RHEAs.
Comprehensive studies on the strengthening mechanisms of the AlxCrNbVMo RHEAs were conducted to understand fundamentals on the exceptional mechanical properties. Among various strengthening mechanisms, solid solution strengthening was the predominant contributor, accounting for over 50% of strengthening in the alloys. Advantages were obtained by high-energy ball milling and SPS, i.e., a relatively large dislocation density and refined grain structures on the sub-micron scale, which made large contributions to strength due to dislocation and grain boundary strengthening effects in the PM-processed AlxCrNbVMo RHEAs. The high temperature deformation behavior of AlxCrNbVMo RHEAs was identified through flow stress analysis with Al1.0 as the representative alloy. The hot-deformation mechanism of Al1.0 alloy was dislocation glide creep. The activation energy of the hot-deformation of Al1.0 alloy was higher than that of the self-diffusion owing to the large strain field and the presence of oxide inclusions.
Oxidation resistance of the AlxCrNbVMo RHEAs, which was evaluated by thermogravimetric analysis, was not attractive for practical high-temperature applications. In order to improve the oxidation resistance of the AlxCrNbVMo RHEAs while maintaining the excellent mechanical properties, Ta was added to Al0.5 RHEA to form protective CrTaO4. The effect of the addition of Ta to microstructure, mechanical properties and oxidation resistance of Al0.5 RHEA was analyzed.||-