Properties of (Ti,Nb)Al-(Ti,Nb)_5Si_3 eutectic composite

摘要

Ti-40Al-5Si and Ti-39Al-5Si-2Nb (in at.%) alloys were studied as prospective high-temperature structural composites consisting of γ-(Ti,Nb)Al + α_2-(Ti,Nb)_3Al matrix and Ti_5Si_3 reinforcement. The alloys were prepared by arc melting under helium. Oxidation resistance was studied at 900 ℃ in air. Thermal stability of alloys was investigated by measuring room temperature hardness and compressive strength after long-term annealing at 900 ℃. To prepare oriented composites, directional crystallization at rates of 5-115 mm/h was carried out by the floating zone technique. It was observed that the addition of 2% Nb to the Ti-40Al-5Si alloy does not modify eutectic structure. Niobium is almost uniformly distributed in all present phases. Both alloys show excellent oxidation resistance at 900 ℃ in air. The Nb-addition causes significant improvement of oxidation resistance due to the doping effect and increase of Al activity in the scales. Room temperature hardness and compressive strength of both as-cast alloys are similar - about 500 HV and 1600 MPa, respectively. Room temperature mechanical properties do not reduce significantly after 300 h annealing at 900 ℃, due to a high morphological stability of eutectic silicides. Directionally solidified alloys consist of columnar Ti-Al grains elongated in crystallization direction and silicides. Niobium refines both Ti-Al grains and Ti_5Si_3 silicides. As a consequence, orientation and elongation of silicides in the Nb-containing alloy are reduced. In the Ti-Al-Si alloy directionally crystallized at 5-115 mm/h, the silicide interparticle spacing λ (in mm) is related to the crystallization rate R (in mm/h) by a following expression: λ~(1.33)?R = 0.32. In the Nb-containing alloy, silicide interparticle spacing does not depend on the crystallization rate.