Femtosecond laser sintering of irregular NbMoTaWZr powders to fabricate fine grain, single-phase body-centered cubic solid solution refractory high-entropy alloys

The properties of NbMoTaWZr RHEAs depend on the phase and microstructure, which are difficult to control, as they contain refractory elements with high melting points. In this work, a single-phase body-centered cubic (BCC) high-entropy solid solution NbMoTaWZr RHEA with a compositionally homogeneous refined microstructure was achieved using femtosecond laser sintering. The average grain size was significantly refined to 0.52 µm with a power density of 1.88 × 106 W/cm2. The formation of a single BCC phase high-entropy solid solution was achieved by the complete α to β Zr phase transition during recalescence. The formation of core-shell structured grains induced by liquid phase separation confirmed the mass transport in liquid phase during sintering, resulting in homogeneous mixing of the multi-component RHEA solid solution. The maximum lattice temperature calculated by the two-temperature model (1794 K) was insufficient to support the sintering of the NbMoTaWZr RHEA, implying the existence of other unrevealed heating factors. We showed that localized plasmon resonance (LPR) induced electric field enhancement triggered femtosecond laser sintering of the irregular-shaped NbMoTaWZr refractory powders, providing new insights into the sintering mechanism. The spikes of the irregular-shaped powder particles constituted highly concentrated areas of the electric field due to LPR, which led to a dramatic temperature increase. Excitation of LPR was amplified upon continuous femtosecond laser pulse input, manipulating successive heat generation within all of the powder particles in the entire system to realize sintering. The sintered NbMoTaWZr RHEA presented an ultrahigh microhardness of 8.68 GPa which was mainly due to solid solution strengthening. The combined effects of LPR enhancement, no selectivity for powder shape, and low energy input made femtosecond laser sintering an effective, energy-saving solution towards fabricating of RHEAs.