| 摘要 | 纯TiAl金属间化合物在氧化的过程中形成TiO2＋Al2O3混合型氧化膜而不能生成具有保护性的Al2O3膜。而添加Nb、Mo、W等元素则可提高TiAl合金的抗氧化性，其中Nb不仅能显著提高其抗氧化性而且改善蠕变抗力及室温强度。前人对系列不同Nb含量的TiAl基合金氧化增重研究结果表明，Nb原子百分含量在20 at%以内都能对抗氧化性起到促进作用，且随Nb含量的增加氧化速率呈下降趋势。
本文选择了具有代表性的系列不同Nb添加量的TiAl基合金Ti-47Al-2Nb- 2Cr-0.15B (第二代TiAl合金)、Ti-46.5Al-5Nb (第三代TiAl合金)、Ti-45Al-8Nb-0.2W-0.2B-0.02Y (第四代TiAl合金)及Ti-45Al-15Nb (at %) 在900°C静态空气气氛下进行了不同阶段的等温氧化实验。综合运用EFTEM、EDS、EELS、SAED、HREM等手段，对TiAl基合金的初期氧化行为、Nb元素的分布特征及对TiAl抗氧化性的影响、氮化层中相结构的演变及氧化膜中亚稳态Al2O3相的相转变过程进行了系统的研究。
对TiAl基合金的初期氧化行为进行了研究，结果表明：Cr的加入会导致提前在过渡层中形成亚稳态X相及NbCr2相。当Nb的原子百分比加入在8 at%左右时可以促使Al2O3与富TiO2混合层的分层，在外层形成Al2O3膜，而Nb的加入量太低或太高时，在初期均未发生氧化膜的分层。当Nb的含量在15 at%时，初期氧化时没有形成氮化层。由于氮化物的PBR接近于1，所以形成的氮化层与基体结合性良好。而TiO2及Al2O3的PBR都大于1，所以在该层存在应力的集中，导致该层与下层之间形成明显的分界。随着氧化膜厚度的增加应力集中会不断升高，形成了强氧化膜弱界面结构，随着氧化时间的延长在这两层之间存在破裂的趋势。初期氧化时基体合金吸氧并发生晶粒细化，使晶界的体积分数增加，同时基体内部存在大量位错及晶格畸变。
添加合金元素的TiAl的氧化在不同的氧化时间总是存在氧化膜及过渡层。氧化膜中的各层随合金成分及氧化时间的变化而演变；过渡层的相组成较为复杂，随添加元素及氧化时间的改变也发生变化。研究了添加元素Nb对TiAl基合金抗氧化性的作用，结果表明：TiAl合金中添加Nb元素对抗氧化性起作用原因之一是由于 +5价的Nb离子在富TiO2层的掺杂，对减缓阴阳离子在富TiO2层的传输、促进富Al2O3层的形成起到作用。而 +3价的Cr所起作用与Nb相反。添加元素Nb会在过渡层重新反应形成Nb的化合物，所形成的化合物对阻止X相在过渡层中的连续形成起到作用，从而提高TiAl合金的抗氧化性。高铌TiAl合金在氧化过程中动力学曲线发生转折，这是由于转折点前后 所存在的扩散通道发生变化引起，这是该类合金内在特征的表现。
研究了氮化层中相的形成及转变机理，结果表明：TiN相与Ti2AlN相具有取向关系：
(111) TiN // (0001) Ti2AlN, [0-11] TiN // [11-20] Ti2AlN
这种取向关系使得两相间的低指数方向及密排面保持严格平行，倒空间的重叠体积最大。TiN相在氧化膜与基体合金的界面处首先形成，而基体中Al元素的向外扩散降低了TiN相的堆跺层错能，从而在该相中形成大量的层错；Al的富集诱导TiN相向Ti2AlN相转变，使得在靠近基体方向形成Ti2AlN相。电子束的辐照能够提供能量使Ti2AlN相中的Ti-Al键断开，Ti-N间的重新组合形成非化学计量比的TiN相，Al在该相中富集。
对TiAl基合金氧化过程中Al2O3相的转变进行了研究，结果表明：TiAl基合金氧化过程中有亚稳态gamma-Al2O3相、theta-Al2O3相及kappa-Al2O3相形成，其中gamma-Al2O3相易于以孪晶形式存在而kappa-Al2O3相则具有120°三重旋转孪晶共存，不同的存在形式与其形成机理相关。TiAl基合金氧化与NiA和FeAl氧化过程中Al2O3相的转变过程相同，主要遵循以下两个路径：
gamma-Al2O3->delta-Al2O3->theta-Al2O3->alpha-Al2O3
kappa-Al2O3->alpha-Al2O3; A TiO2＋Al2O3 mixed layer but not a protective Al2O3 layer formed on TiAl based alloy during high temperature oxidation. However, it was shown that the addition of Nb, Mo, and W alloying can effectively improve the oxidation resistance of TiAl based alloy. Among these elements, Nb not only significantly enhances the oxidation resistance of the materials, but also improves their creep resistance and room temperature toughness. According to literature reports, Nb with the content smaller than 20 at% can improve the oxidation resistance of TiAl based alloys and the oxidation rates of the alloys decrease with the increase of Nb content.
The oxidation behavior of different stages of four kinds of Nb additional TiAl based alloy, Ti-47Al-2Nb-2Cr-0.15B, Ti-46.5Al-5Nb, Ti-45Al-8Nb-0.2W-0.2B-0.02Y and Ti-45Al-15Nb (at %) has been investigated at 900 °C in static air. The initial stage oxidation behavior, the distribution of Nb and corresponding effect on TiAl oxidation, phase transformation in the nitride layer and the evolution of metastable Al2O3 phase during TiAl oxidation were investigated by using EFTEM (energy filter transmission electron microscopy), EDS (energy dispersive X-ray spectrometer), EELS (electron energy loss spectrum), SAED (select area electron diffraction) and HREM (high resolution electron microscopy).
The initial stage of oxidation was studied. The result indicated that the addition of Cr element will lead to the formation of X phase and NbCr2 phase in advance. 8 at% Nb can promote Al2O3 separating from TiO2 enriched layer. However, this phenomena does not exist when Nb content is lower or higher than 8 at% at the initial stage. The nitride layer does not form when Nb content is 15 at%. The adherence between nitride layer and base alloy is good because the PBR of nitride is near 1. However, the PBR of TiO2 or Al2O3 is higher than 1, stress will be concentrated in this layer which leads to an obvious interface between the mixed layer and the under layer. The inner stress increases with the increase of oxide scale thickness, which leads to the formation of strong oxide scale and weak interface. This kind of interface will crack during the oxidation process. At the initial oxidation stage, oxygen absorption and refining of the base alloy lead to the increase of the volume fraction of grain boundary. Many dislocations and distortions are formed in the base alloy.
Oxide scale and transition layer are always formed during element additional TiAl alloys oxidize. They are variety following the variation of additional alloy and oxidation time. The effect of Nb addition on the oxidation behavior of TiAl based alloy was investigated. The first reason why Nb addition improves the oxidation resistance of TiAl based alloy is that the doping of Nb5+ in the TiO2 enriched layer can not only reduces the diffusion path of Ti cation and O anion but also promote the formation of Al2O3 enriched layer. However, the function is contrary when Cr3+ doped in the TiO2 enriched layer. Nb based compound was formed at the subsurface layer, which can prevent a continuous formation of X phase. This is the second reason why Nb addition improves the oxidation resistance of TiAl based alloy. The oxidation process of high Nb content TiAl alloy can be divided into three stages during 100 hr at 900C. The change of ion diffusion path leads to the discontinuity of the oxidation curve.
Phase transformation in nitride layer was investigated during the oxidation of TiAl alloys. The OR (orientation relationship) between TiN and Ti2AlN phases is,
(111) TiN // (0001) Ti2AlN, [0-11] TiN // [11-20] Ti2AlN
In this OR between TiN and Ti2AlN, low-index direction and close-packed planes are exactly parallel. The formation of the OR between TiN and Ti2AlN can be attributed to the largest overlapping volume of TiN and Ti2AlN in reciprocal space. TiN phase was formed firstly at the interface of oxide scale and base alloy. Aluminum element reduces TiN phase twin boundary energy. Many stacking faults form in TiN phase following Aluminum diffusion from base alloy to outer. The enrichment of Aluminum in TiN phase induced phase transformation from TiN to Ti2AlN, which lead to the formation of Ti2AlN phase at the bottom of the nitride layer. Electron beam irradiation can provide energy to break the Ti-Al bonds in hexagonal Ti2AlN phase. TiN phase with non-stoich- iometry would form following Ti2AlN phase decompose. Aluminum element is enrichment in TiN phase.
Metastable Al2O3 phase transformation was studied during the oxidation of TiAl alloys. Metastable gamma-Al2O3 phase, theta-Al2O3 phase and kappa-Al2O3 phase form during the oxidation process. gamma-Al2O3 platelet forms with high number density {111} twins. Meanwhile, three twin-related kappa-Al2O3 phases rotating by 120 with respect to one another are coexistence. The different states are correlation with different formation mechanism. The phase transformation of Metastable Al2O3 during the oxidation of TiAl alloys is consistent with NiAl and FeAl alloys. Alumina transformation is mainly obey two routes, that is
gamma-Al2O3->delta-Al2O3->theta-Al2O3->alpha-Al2O3
kappa-Al2O3->alpha-Al2O3 |
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