Promoting the sintering densification and mechanical properties of gas-atomized high-entropy alloy powder by adding boron

Powder metallurgy (PM) with pressure-assisted sintering is a versatile process for producing high-entropy alloys (HEAs) with a fine grain size and high strength. Unfortunately, HEA powders are extremely difficult to sinter with a general press-and-sinter PM process due to the sluggish diffusion effect in HEAs. One economical means to effectively densify the PM materials is liquid phase sintering (LPS). According to the phase diagrams, boron (B) is a feasible element for facilitating LPS of an AlCoCrFeMoNi powder because B can react with Co, Cr, Fe, Mo, and Ni to generate a low-temperature liquid phase. The objective of this study was thus to conduct the first investigation of the influences of 0.8 wt% B on the sinterability, microstructure, and mechanical properties of the AlCoCrFeMoNi HEA powder. The results showed that after 1200 degrees C sintering, the densification behaviors of both the AlCoCrFeMoNi and AlCoCrFeMoNi+0.8B alloys were undesirable. However, adding 0.8 wt% B to AlCoCrFeMoNi and sintering at 1220 degrees C significantly increased the densification and reduced the porosity from 11 vol% to 1 vol%. The onset temperature for liquid generation was identified as 1205 degrees C by differential scanning calorimetry curve. The microstructure of the AlCoCrFeMoNi alloy sintered at 1220 degrees C was face-centered cubic and sigma phases. The 0.8 wt% B additive led to the generation of Mo2FeB2 boride at the expense of sigma phase. Furthermore, adding 0.8 wt% B to the AlCoCrFeMoNi alloy sufficiently improved the yield strength by 42%, ultimate compressive strength by 66%, and fracture strain by 92%, due to the LPS effect. The AlCoCrFeMoNi powder was successfully sintered by adding B to facilitate LPS. In the future, this alloy design can be also applied to the other HEA powders that contain principal elements that react with B to form a low-temperature liquid.

Automated summary

By adding 0.8wt% B, the sinterability and mechanical properties of AlCoCrFeMoNi HEA powder can be significantly improved, reducing porosity and increasing density, leading to better microstructure. Boron facilitates the liquid phase sintering (LPS) effect in AlCoCrFeMoNi alloy, enhancing its strength and toughness.