Effect of withdrawal rates on microstructures and room temperature fracture toughness in a directionally solidified Nb–Ti–Cr–Si based alloy

Integrally directional solidification of an Nb–Ti–Cr–Si based ultrahigh temperature alloy is conducted with the use of special ceramic crucibles. The effect of withdrawal rates on microstructure and room temperature fracture toughness is investigated. The microstructure of arc-melted alloy is composed of primary niobium solid solution (Nbss), Nbss/(Nb,X)5Si3 eutectic and fine Nbss/Cr2Nb eutectic colonies, and the directionally solidified microstructure is composed of primary Nbss, Nbss/(Nb,X)5Si3 eutectic cells and fine Nbss/Nb3Si/Cr2Nb colonies (here X represents Ti and Hf elements). The room temperature fracture toughness decreased with increase in the withdrawing rates. The room temperature fracture toughness of the directional solidification specimen with withdrawing rates 10 and 20 μm/s are higher than the arc-melted counterparts. The effects of withdrawing rates on the microstructure and room temperature fracture toughness are discussed based on the microstructural and compositional characters.