Effect of heat treatment on nanoparticle reinforced Nb-18.7Si alloy

Eutectic Nb-18.7Si alloys with different ceramic nanoparticle addition (Al₂O₃, TiC, SiC, 5 mol.-%), prepared by arc-melting, were heat-treated for 24 h at 1500 °C. Phase composition, microstructure transition and mechanical properties were investigated. The results show that the γ-Nb₅Si₃ and small Nb₃Si phases from the as-cast alloy decomposed into α-Nb₅Si₃ and Nb_ss/α-Nb₅Si₃-eutectoids. Larger Nb₃Si phases still existed after heat treatment. Additionally, after heat treatment the Nb_ss phases were interconnected to a continuous matrix, while silicides appeared coarsened and spherical while being uniformly distributed in the Nb_ss phase. This effect was most pronounced in the alloy with addition of Al₂O₃ nanoparticles, due to the ultrafine nano-scale lamellar structures of Nb_ss + α-Nb₅Si₃ in its as-cast condition. While the added nanoparticles had a strong effect on the initial microstructure of the as-cast state, they showed no noticeable effects like pinning of grain boundaries during heat treatment. A decrease in hardness was observed due to the coarsening of the microstructure and the reduced area fraction of silicide phases. However, the transformation to the coarser microstructure with a continuous Nb_ss matrix and spherical silicide phases seems to be effective in inhibiting crack propagation by promoting crack deflection and bridging over the Nb_ss phase.