Effect of Si and Ti on microstructures and mechanical properties of Nb-6Hf-4Zr-2B alloy at room temperature

To strike a balance between strength and plasticity of niobium alloys, Nb-xSi-6Hf-4Zr-2B-yTi (x=4,8;y=10,30;at%) alloys with an Nbss-dominant microstructure are developed, and the effect of Si and Ti additions on microstructures and mechanical properties of this kind of alloy at room temperature is evaluated. It is found that the alloy contains three phases: Nbss, Nb₃Si and Nb₅Si₃. The microstructures of the as-cast 4Si-(10, 30)Ti alloys are composed of the dendrite Nbss and silicides (Nb₃Si and Nb₅Si₃); as for the as-cast 8Si-(10, 30)Ti alloys, their microstructures are composed of the dendrite Nbss, eutectic (Nbss + Nb₅Si₃) and silicide Nb₃Si. The fraction of silicides goes up with Si and Ti contents increasing. Upon annealed at 1 600°C for 50 h, part of the Nb₃Si decomposes into eutectoid and the microstructural morphology and volume fraction of each phase are changed. The hardness and strength enhance with Si and Ti contents increasing, whereas the plasticity and fracture toughness decrease. The fracture mode of the dendrite Nbss is cleavage, while the silicides fracture in a brittle manner.