Strengthening mechanisms of femtosecond laser fabricated multi-phase ZrNbMoTaW body-centered cubic refractory high entropy alloy with enhanced hardness

The present work reports a new fabrication method for refractory high-entropy alloy (RHEA) based on femtosecond laser. The strengthening mechanisms in ZrNbMoTaW body centered cubic RHEA prepared via femtosecond laser fabricating are discussed comprehensively. By analyzing the mechanical properties of femtosecond laser fabricated ZrNbMoTaW RHEAs with different processing parameters, the optimal femtosecond laser fabricating window of ZrNbMoTaW RHEAs with enhanced microhardness and low porosity was obtained. The grain morphology, phase structure, elemental distribution, local misorientation and lattice parameters of ZrNbMoTaW RHEA at optimized processing parameters were investigated. The instantaneous fabricating temperature was measured to analyze the effect of femtosecond laser on powder melting. The results show that the microstructure of ZrNbMoTaW RHEA consists of homogeneous BCC1 and BCC2 phase distribution with refined grains (sizeave = 2.1 mu m). Based on the numerical deduction of RHEA with enhanced hardness, contributions to the multiple strengthening mechanisms are given to establish the relationship between the microstructure and hardness of femtosecond laser fabricating ZrNbMoTaW RHEAs, including frictional stress (16.4%), dislocation hardening (17.9%), grain boundary strengthening (9.9%) and solid solution strengthening (55.8%), respectively. Adequate solid solution effect, high dislocation density and low grain size endow ZrNbMoTaW RHEA with high hardness (Vickersvalue = 565.7 HV). These suggest that femtosecond laser is influential in governing the mutual solubility of multiple elements and introduction of strengthening factors. The revealed strengthening mechanisms can serve as a basis for design and the improvement of mechanical properties in NbMoTaW-based RHEAs.