Influence of micro-segregation on the microstructure, and microhardness of MoNbTa_xTi(_1-x)W refractory high entropy alloys: Experimental and DFT approach

In recent times, MoNbTaW Refractory High Entropy Alloys (RHEAs) have drawn attention due to their exciting high-temperature applications and efforts are being made to improve their properties further. Alloying addition is an effective and practical approach to enhancing material properties. Ti is well known for its strength, low density, and corrosion resistance and its role in the properties of MoNbTaW-based RHEAs is required to be understood. RHEAs are commonly synthesized by vacuum arc melting (VAR) process and micro-segregation of the elements is reported. However, the effect of micro-segregation on MoNbTaW RHEAs is not well understood. Hence, the present work deals with the addition of Ti and micro-segregation on the structure, microstructure, and indentation behavior of MoNbTaW RHEAs. A novel non-equiatomic MoNbTaxTi((1-x))W (x = 0, 1, and 0.5) RHEAs were prepared by vacuum arc melting (VAR) process. The structure and microstructure of these RHEAs were ascertained through X-ray diffraction (XRD) and transmission electron microscopy (TEM), scanning electron microscopy (SEM) equipped with EDS for ascertaining the nominal chemical composition. The as-cast MoNbTaxTi((1-x))W (x = 0, 1, and 0.5) RHEAs contains two BCC phase having slightly different lattice parameter i.e., BCC1 (a = 0.324 nm) and BCC2 (a = 0.323 nm) close to Ti and W respectively. These as-cast RHEAs having dendritic and inter-dendritic regions were found to be enriched in W and Ta, and Mo, Nb and Ti respectively. The microhardness of these RHEAs was investigated through instrumented indentation techniques. The microhardness of the BCC1 and BCC2 phases was found to be similar to 4.5 GPa and 5.7 GPa. Further, the influence of Ti addition on the structure, elastic constant, and microhardness were estimated from the perspective of the first principle approximation. The special quasi-random structure (SQS) was used for first-principle calculations. The strengthening mechanisms influencing the mechanical properties in these BCC RHEAs were discussed in details. The experimental results were in good agreement with the results pertaining to theoretical modelling, demonstrating the effectiveness of these methods in predicting the structure and mechanical properties of RHEAs.