鈦鋁介金屬合金(TiAl-Nb)顯微結構與高溫性質研究

Abstract

This study aims to investigate the microstructure, creep behavior and high temperature oxidation behavior of novel high niobium-containing Ti-40Al-xNb (x=10,12,15,16) intermetallic alloy. The microstructure of the as-cast Ti-40Al-10Nb alloy consists of dense Widamanstatten α2 laths in the B2 matrix. The microstructure of the as-cast Ti-40Al-xNb (x=12,15,16) alloy is composed of the primary β dendrites and dense γ phases with various morphologies, such as lathy, feathered and irregular shapes. Following heat treatment, the microstructure of the heat-treated Ti-40Al-10Nb alloy resembles that of the as-cast Ti-40Al-10Nb alloy. The homogenized Ti-40Al-12Nb alloy and Ti-40Al-15Nb alloy have a two-phase microstructure of B2+γ, while the homogenized Ti-40Al-16Nb alloy has a four-phase microstructure of B2+γ+α+σ.The microstructure of the as-cast Ti-40Al-16Nb-0.4wt% X (X=Sc or Mm) alloy contains many Sc-rich oxides with cubic or cauliflower-shapes and La2O3 oxides having strip-like or spherical shape in the inter-dendrite region. The formations of these precipitates are caused by the internal oxidation during solidification. After homogenization, numerous fine particles with sub-micrometer scale are present in the Ti-40Al-16Nb-0.4wt% X (X=Sc or Mm) alloy. This is due to the fact that during long-term heat treatment at high temperature, Sc or Mm elements, initially dissolving in the as-cast alloy, may react with oxygen atoms by internal oxidation and reproduce fine-scale particles. The creep responses of the Ti-40Al-xNb (x=15,16) alloy are strongly correlated with tertiary creep behavior. The deformation of creep converges mainly at the B2 phase. A stress exponent of 4.5 estimated indicates that the mechanism of controlling creep behavior is dislocation climb. The creep curve of the Ti-40Al-xNb (x=15,16) alloy does not exhibit a steady-state region, resulting from the absence of the subgrain structures of dislocations in the alloys during secondary creep. The creep activation energy of the Ti-40Al-xNb (x=15,16) alloy is about 365 KJ/mole. The calculated values of activation energy for the alloys are quite close to the activation energy of Ti self-diffusion in the β phase (~353KJ/mole). The creep fracture of the alloys is dominated by cleavage fracture over the entire fracture surface. The brittleness of the σ phase causes most of the cracks to run through it immediately, indicating no resistance to their propagation. Therefore, the creep life of the Ti-40Al-16Nb alloy is shorter than that of the Ti-40Al-15Nb alloy. The strengthening effects of minor elements added (Sc or Mm) are apparent on the properties of tertiary creep rate and rupture life of the alloys. The fine particle formed after homogenization is an effective obstacle to the motion of dislocations, further increasing the creep fracture life of the alloys. The fracture of the Ti-40Al-xNb (x=15,16) and Ti-40Al-16Nb-0.4wt% X (X=Sc or Mm) alloy during tertiary creep is caused by microstructural instabilities. The results of the isothermal oxidation tested at the temperature of 800℃ for the Ti-40Al-xNb (x=10,12,15,16) alloys reveal that the difference among the oxidation resistances of these four alloys arise from their various microstructures. The oxidation resistance of α2 is inferior to that of γ. At a higher temperature of 900℃, the effect of Nb content on the oxidation resistance of the Ti-40Al-xNb (x=10,12,15,16) alloy becomes more pronounced. For the Ti-40Al-xNb (x=10,12,15,16) alloys, the increased Nb content promotes the formation of Al2O3 oxides. Therefore, the Ti-40Al-15Nb alloy has the strongest oxidation resistance among these four tested alloys. But at 1000℃, the Ti-40Al-xNb (x=10,12,15,16) alloys show a severe scale spallation, indicating that these alloys could no more resist the oxidation and lost their surface protection at the temperature.