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

Eutectic Nb-18.7Si alloys with different ceramic nanoparticle addition (Al 2 O 3 , 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-Nb 5 Si 3 and small Nb 3 Si phases from the as-cast alloy decomposed into α-Nb 5 Si 3 and Nb ss /α-Nb 5 Si 3 -eutectoids. Larger Nb 3 Si 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 Al 2 O 3 nanoparticles, due to the ultrafine nano-scale lamellar structures of Nb ss + α-Nb 5 Si 3 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. • Heat treatment at 1500 °C for 24 h changes the Nb-18.7Si alloy microstructure from lamellar to coarse eutectic • Al 2 O 3 nanoparticles enhance anomalous eutectic transformation due to ultrafine Nb ss + α‑Nb 5 Si 3 lamellae in the as-cast state. • The Al₂O₃ and TiC nanoparticles do not pin grain boundaries or affect interface migration during heat treatment. • Microstructure coarsening and a continuous Nb ss matrix with spherical silicides enhance crack bridging, deflection, and inhibition.