Refractory TaTiNb, TaTiNbZr, and TaTiNbZrX (X = Mo, W) high entropy alloys by combined use of high energy ball milling and spark plasma sintering: Structural characterization, mechanical properties, electrical resistivity, and thermal conductivity

Refractory TaTiNb, TaTiNbZr, and TaTiNbZrX (X = Mo, W) high entropy alloys were synthesized by combined use of high energy ball milling (HEBM) and spark plasma sintering (SPS). Powders of predominantly bcc TaTiNbZrX (X = Mo, W) refractory high entropy alloys (RHEAs) were successfully prepared by short-term HEBM (60 min) and then SPS-consolidated at 1373 K for 10 min. TEM analysis of the TaTiNbZrW HEA powder obtained after 60 min of HEBM revealed its nanocrystalline structure with an average grain size of up to 50 nm and predominantly uniform distribution of the elements on an atomic scale. The SPS consolidation at 1300⁰C led to an increase in grain sizes up to 100–300 nm. Thus prepared bulk RHEA materials showed ultra-high Vickers hardness of 8.5 GPa and 13 GPa for TaTiNbZrMo and TaTiNbZrW alloys, respectively. The room-temperature compressive strength of TaTiNbZrW RHEA alloy sintered from HEBM powders attained a value of 2665 МPа, which is 30% higher than that for the same alloy produced from non-milled powders. Bulk samples of synthesized RHEA show a higher electrical resistivity (r) compared to the samples prepared from non-milled powder blends. Within the temperature range 298–573 K, the maximum r value for TaTiNbZrW RHEA alloy was found to vary between 132 and 143.6 Ω cm. A decrease in thermal conductivity was observed: (a) upon introduction of Zr, Mo, and W atoms to TaTiNb-based alloys due to additional phonon scattering on lattice distortions caused by different radius and mass of Zr, Mo, and W atoms and (b) for the HEBM-prepared bulk alloys because of additional phonon scattering on the surface of mechanocomposites.