Effects of heat treatments on the microstructure and microhardness of directionally solidified Nb-Ti-Si based ultrahigh temperature alloy

Directionally solidified specimens of Nb-Ti-Si based ultrahigh temperature alloy with the withdrawing rates of 2.5μm/s and 50μm/s were homogenized at 1250, 1350°C and 1400°C for 50h, respectively. The constituent phases were Nbss, β(Nb, X)₅Si₃ and γ(Nb, X)₅Si₃. The needle-like precipitates of Cr₂(Nb, Ti) appeared in the specimens directionally solidified at the withdrawing rate of 2.5μm/s after homogenizing treatments, and its amount increased with the homogenizing temperatures. After homogenizing treatments, the straight boundaries and sharp edges of the primary hexagonal silicides became smooth, the fine silicides in the center of the eutectic cells became spheroidzed, and some silicide blocks in the eutectic cells broke into small ones. The transformation from β(Nb, X)₅Si₃ to α(Nb, X)₅Si₃ did not occur during the homogenizing treatments. After homogenizing treatments, the microhardness of the alloy increased remarkably, and its value first increased and then decreased with the increasing of the homogenizing temperatures. The maximum value of microhardness of the constitute phases occurred in the specimens homogenized at 1350°C for 50h.