Nb-ti-al (si), High-temperature Oxidation Behavior Of Niobium Alloys

Nb-based multi-phase alloys show promise for high-temperature applications due to their high melting point, low density and high strengh at elevated temperatures. However, one major impediment to the development of the Nb-based alloys is their poor oxidation resistance. Till now, very little experimental research is available on the oxidation behavior of the Nb-based multi-phase alloys and the theoretical model of the oxidation behavior of the Nb-based multi-phase alloys has not been established yet. In this work, Nb-Ti-Al single/two-phase alloys are prepared by arc melting and Nb-Ti-Si-Al two phase alloy is produced by mechanical alloying followed by hot-press process. The effection of composition and microstructure on the oxidation behavior of these alloys are investigated for the first time, by means of OM, XRD, SEM and EDS. On the base of experimental research, the theoretical analysis is made on the oxidation behavior of niobium alloys at elevated temperature, and the critical condition required for selective oxidation of active elements proposed by Wagner is improved. The following results are obtained:1. The Ti element decreases the solubility and diffusion of Oxygen in Nb-Ti-Al alloys. Two-phase β+δ alloys Nb-30Ti-15Al and Nb-15Ti-11Al show good oxidation properties because evenly distributed and shaped 8 phase decreases the solubility of oxygen in the matrix. The segregation of Ti in single-phase β alloy Nb-35Ti-6Al leads to the internal oxidation which speeds up the oxidation of Nb-35Ti-6Al.2. The powder metallurgy alloy Nb-38Ti-l lSi-3Al exhibits the best oxidation resistance among all the Nb-based alloys in this work owing to the oxygen solubility decreasing and external oxidation of titanium in α-Ti lamellar structure, and to the much lower oxidizing speed as well as oxygen solubility decreasing in (Ti,Nb)₅Si₃ phase.But the existence of a-Ti lamellar structure results in high Nb content in [3 phase which easily attacked by oxygen.3. The weight gain of Nb-based alloys both in experiment and in literatures at different temperatures is predicted via theoretical calculations. The independent oxidation model of two-phase Nb-based alloys is established and evaluated by experimental and literature datas. The critical condition of the external oxidatin of active elements in the two-phase Nb-based alloys is discussed. The calculating results show that only when u (u = 1 / 2^kf ID ) is between 0.1 ⁰.124 can the external oxidation of element Al happen, and 0.185⁰.192 of element Si.