鈦鋁介金屬合金(TiAl-Nb)顯微結構與高溫性質研究

Abstract

本文主要研究高鈮含量鈦鋁介金屬合金Ti-40Al-xNb(x=10,12,15,16)的顯微結構、潛變性質及氧化行為等性質。鑄造狀態的Ti-40Al-10Nb合金，其顯微結構為灰色長針狀Widamanstätten組織密集的束狀交錯分佈於B2基地。而Ti-40Al-xNb(x=12,15,16)合金，則是初析β樹狀晶及樹狀晶間高密度的羽毛狀γ相所組成。均質化熱處理後，Ti-40Al-10Nb合金的顯微結構除了灰色針狀Widamanstätten組織，尚有黑色顆粒狀的γ相析出；Ti-40Al-12Nb合金及Ti-40Al-15Nb合金的顯微結構則有許多不同型態的γ相分佈於B2基地；而Ti-40Al-16Nb合金另有塊狀白色σ相生成及少量深灰色α2相存在Ti-40Al-16Nb-0.4wt% X(X=Sc or Mm)合金的鑄造顯微結構，乃是有矩形或花瓣狀的富Sc氧化析出物及條狀或圓顆粒狀的La2O3氧化物存在於初析β樹狀晶間，而上述的氧化物的析出乃是熔煉時內部氧化導致。而均質化後，Ti-40Al-16Nb-0.4wt% X(X=Sc or Mm)合金基地內有許多次微米尺寸的顆粒狀氧化物產生，此乃固溶於基地中的Sc或Mm於長時間熱處理後再經內氧化反應而析出。Ti-40Al-xNb(x=15,16)合金的潛變變形主要由三期潛變主導，而基地B2相為潛變變形時主要的變形相。應力指數4.5，表示差排爬移為Ti-40Al-xNb(x=15,16)合金的潛變變形機構。Ti-40Al-xNb(x=15,16)合金潛變曲線並未有穩態潛變區域存在，此乃是基地B2相於潛變變形時並無形成穩定差排結構所導致。而Ti-40Al-xNb(x=15,16)合金的潛變活化能約為365 KJ/mole，此活化能應與B2相的晶格活化能有關。Ti-40Al-xNb合金的破裂型態主要以穿晶劈裂破斷為主，而Ti-40Al-16Nb合金則因有脆性σ相存在，所以其抗潛變性比Ti-40Al-15Nb合金差。Ti-40Al-16Nb-0.4wt% X(X=Sc or Mm)合金於抗潛變性上的強化效應，乃來自於微細析出物對差排運動的阻礙，而增加其潛變壽命。Ti-40Al-xNb(x=15,16)合金及Ti-40Al-16Nb-0.4wt% X(X=Sc or Mm)合金的三期潛變破斷，主要是由於顯微結構的不穩定所導致。Ti-40Al-xNb合金的800℃恆溫氧化測試結果顯示，各合金的抗氧化性差異主要來自於顯微結構的不同，α2相的抗氧化性較γ相及基地B2相差。在900℃時，鈮含量的多寡影響抗氧化性的效應較為顯著，而鈮元素對氧化行為的影響，在於促進Al2O3氧化物的生成。經氧化測試後的Ti-40Al-15Nb合金，其表層為一富Al2O3的氧化層。但1000℃的氧化測試則顯示，Ti-40Al-xNb合金的氧化層產生嚴重的剝落，此表示在1000℃測試溫度下，Ti-40Al-xNb合金已失去抗氧化性。

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 Widamanstätten 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.