Microstructural, thermomechanical and tribological behavior of refractory high-entropy AlxCr0.25Nb0.5Ta0.5Ti1.5 (x = 0.5, 1) alloys

Refractory high-entropy alloys (RHEAs) with varying Al composition AlxCr0.25Nb0.5Ta0.5Ti1.5 (x = 0.5, 1) possessing a single body-centered cubic phase were prepared by arc melting followed by casting in a copper mold. The developed RHEAs were subjected to mechanical, thermal and tribological evaluation. The RHEAs demonstrated a homogeneous equiaxed microstructure within which fine dendritic structures grew depending on the Al content in the alloy. The density, hardness, and coefficient of thermal expansion decreased with increased Al content from x=0.5 to x=1. Furthermore, it was found that the coefficient of friction increased from 0.7 to 0.8 while the specific wear rate increased by about 87 % when the Al content increased from x = 0.5 to x = 1. This was attributed to the microstructural changes, crystal size difference, and reduced density resulting in a reduction in the hardness of the alloy with increasing Al content from x=0.5 to x=1. Abrasive wear through ploughing coupled with severe plastic deformation was found to be the dominating wear mechanism as revealed by the field emission scanning electron microscopy and energy-dispersive X−ray spectroscopy analysis.