Microstructure and room temperature mechanical properties of hypereutectic Nb-Si based alloy processed by directional solidification

In this investigation, Nb-16Si-24Ti-4Cr-2Al-2Hf alloy was directionally solidified and then heat treated, and the microstructure, fracture toughness KQ and tensile strength σ_b of the alloy were studied. The directionally solidified microstructure consisted of primary Nb ₅Si₃ blocks and Nb_ss+Nb₅Si ₃ eutectic cells. The cross section of eutectic cells changed from the square morphology to the nearly round morphology with the increasing withdrawal rate. During heat treatment, the boundaries of the round eutectic cells were eliminated as the Nb_ss connected to be a net-like structure. The values of KQ and σ_b increased with the increase of withdrawal rate. The toughening from the crack deflection of continuous Nb_ss network was significant and refinement of the microstructure improved the tensile strength in heat treated samples.