Fabrication and characterization of WNbTaVMo and WNbTiVMo high entropy alloys with powder metallurgical process

Abstract

Unlike conventional alloys synthesized with one main element with addition of minor elements, High entropy alloys (HEAs) consist of more than five metallic elements with equi- or near equi-atomic ratio. Due to the characteristics of the high entropy alloys

High configurational entropy, lattice distortion, atomic size difference and cocktail effect, the HEAs exhibit excellent mechanical properties and be noted as novel materials. Since the concept of the HEAs was established, a variety of the researches to find new HEAs system have been conducted. Especially, HEAs system consisted with refractory elements of high melting point (W, Ta, etc.) are widely studied due to the high potential of HEAs to replace the refractory Ni-Superalloys. However, the fabricate process of refractory HEAs is only focused to the arc-melting and casting process which has following problems: inhomogeneous microstructure come from segregation of alloy elements, grain coarsening during the cooling. Therefore, in this study, to enhance the mechanical properties and homogenize the microstructure, powder metallurgical process was applied to fabricate the refractory HEAs. In case of this process, precise composition control is possible compared with the melting process. Mechanical alloying and spark plasma sintering process were carried out to obtain homogeneous microstructure with enhanced mechanical properties of fabricated HEAs. Here, two refractory HEA system of WNbTaVMo and WNbTiVMo was produced and characterized. WNbTaVMo system is known for the best mechanical property at elevated temperature, but it has high density, low elongation at room temperature with expensive cost of raw materials. To resolve these weakness, in terms of density and cost of raw materials, WNbTiVMo system was newly designed and developed. Mechanical alloying behavior was analyzed by the X-ray diffraction patterns and the cross sectional microstructure of the HEA powders. To reduce the contaminations during the mechanical alloying, minimum time of mechanical alloying was conducted. The mechanical properties of fabricated bulk specimen was evaluated by Vickers hardness test and compressive test and as a result, it was obvious that the HEAs fabricated by powder metallurgical process exhibit enhanced mechanical properties than those of arc-melting and casting process. Based on this study of fabricate refractory HEAs with powder metallurgical process, it is expected to be applied for further researches of finding new refractory high entropy alloy and maximizing the mechanical properties by introducing reinforcements.