Enhanced mechanical properties of Nb-18.7Si alloy by addition of ceramic nano particles for microstructural control

The phase composition, microstructure and mechanical properties of arc-melted eutectic Nb-18.7Si (at.-%) alloys with different nano-ceramic particle addition (Al₂O₃, TiC, SiC, 5 mol.-%) were investigated. The results showed that ceramic Al₂O₃ and TiC nanoparticle are thermally and chemically stable and can be used to tailor phase composition and refine the microstructure, while SiC dissolves completely in the melt. Al₂O₃ and TiC nanoparticle were found mainly in two different areas: (1) at grain boundaries of eutectic structures and (2) on the phase boundaries of silicides inside the eutectics. The presence of the particles refined the microstructure down to nano-scale lamellae by functioning as heterogeneous nuclei for the silicide phase. Without nano particle addition, the Nb-18.7Si alloy was mainly composed of Nb solid solution (Nb_ss) and Nb₃Si. The addition of 5 mol.-% Al₂O₃ promoted the decomposition of the Nb₃Si phase and an ultrafine nano-scale lamellar eutectic structure (Nb_ss + α-Nb₅Si₃) formed. With the addition of 5 mol.-% TiC, primary Nb₃Si and coarse Nb_ss were observed, as well as fine eutectic Nb_ss + γ-Nb₅Si₃ structures. The complete dissolution of SiC led to a hypereutectic alloy with primary Nb₃Si and γ-Nb₅Si₃ phase, coarse Nb_ss and eutectic Nb_ss + γ-Nb₅Si₃ structures. The compressive strength was increased from 3068 MPa to 3446 MPa by adding 5 mol.-% Al₂O₃ due to the formation of the high strength α-Nb₅Si₃ phase, the ultrafine nano-scale lamellar structures of Nb_ss + α-Nb₅Si₃ and the strong interface of Nb_ss/α-Nb₅Si₃. However, the small grain size of the Nb_ss phase was not effective in inhibiting crack propagation. Crack bridging and branching seem to be important mechanisms at the Nb_ss phase to inhibit crack propagation. Therefore, the size and distribution of Nb_ss play a key role. The results indicate that a continuous Nb_ss phase with embedded silicide phase and coarse Nb_ss phases can inhibit crack propagation.