Mechanochemical synthesis in the Nb-Al-Si and Nb-Al-Si-C systems

Abstract

Ternary and quaternary alloys of the Nb-Al-Si and Nb-Al-Si-C systems have been produced by mechanochemical synthesis. Our results demonstrate that the milling of a 62Nb + 19Al + 19Si ternary mixture, whose composition corresponds to that of the intermetallic phase Nb₁₀Al₃Si₃, leads to the formation of an amorphous phase, whereas the milling of a 86Nb + 9Al + 5Si mixture, corresponding to a solid solution, results in the formation of a two-phase mixture, consisting of an amorphous phase and a nanostructured (7 nm) bcc solid solution. The addition of graphite to the starting mixtures homogenizes the MS products and reduces their particle size to ≃2 nm. The milling of a mixture with the composition Nb₁₀Al₃Si₃C₁₀ causes no changes in phase composition. The milling of a 78Nb + 8Al + 5Si + 9C mixture leads to the formation of an additional phase: nanoparticulate niobium carbide. Using differential thermal analysis, we have identified the sequence of transformations underlying the transition of the MS alloys to an equilibrium state. After the compaction of the MS powders by hot isostatic pressing, the samples prepared from the graphite-containing mixtures were more homogeneous in the case of both the alloys based on the intermetallic phase and the Nb-based solid solutions. The carbide nanoparticles precipitating on grain boundaries increase the hardness H _V of the alloys by almost 30%: from 12.75 to 18.13 GPa for the samples corresponding to the intermetallic phase and from 4.76 to 6.85 GPa for the samples corresponding to solid solutions.