Effect of Cr on high temperature oxidation of TiAl

The oxidation behavior of TiAl-Cr（mole fraction of Cr：0-20%） was investigated at 1 173 K in air. The microstructure and composition of the oxide scale were studied by X-ray diffractometry（XRD）, scanning electron microscopy（SEM） and electro-probe micro-analyses（EPMA）. The results show that with the addition of Cr content from 0 to 8%, the oxidation resistance decreases, especially at 3%, which is mainly attributed to the doping effect of Cr^3＋. TiAI-Cr（mole fraction of Cr： 15%-20%） has good oxidation resistance, and the protective alumina layer is preferentially formed on the surface of TiAI alloy, which is due to an increase of mole ratio of Al to Ti in TiAl-Cr alloys.     

Effect of chromium on the oxidation resistance of TiAl intermetallics

The effect of 10 at.%Cr on the oxidation resistance of TiAl intermetallic compound at 800–1100°C in air was investigated. The results indicated that 10 at.%Cr equally substituting for Ti and Al in TiAl alloy had duplex effects on the isothermal kinetics of DAL At lower temperatures (800–900°C), Cr increased the oxidation rates as a result of the doping effect of Cr in the scale and at higher temperatures (1000–1100°C), especially at 1100°C, Cr significantly reduced the oxidation rates as a result of the formation of a continuous Al₂O₃ film on the surface. 10 at.%Cr only substituting for Ti in TiAl alloy remarkably reduced the oxidation rates at all temperatures by about two orders of magnitude. Moreover, 10 at%Cr significantly improved the cyclic-oxidation rsistance of TiAl alloy.     

Effect of Cr, Nb, Mn, V, W and Si on high temperature oxidation of TiAl alloys

Alloys of Ti−(47,51)Al, Ti47Al−4Cr, Ti48Al−2Cr2Nb, Ti47Al−1Mn, Ti39.4Al−10V, Ti48.4Al−1.9W, and Ti43Al−2W0.1Si were oxidized at temperatures between 800 and 1000 °C in air to determine the effect of each alloying element on oxidation behavior. Among the alloys tested, the Ti48.4Al-1.9W alloy displayed the best oxidation resistance, due to the beneficial effects of W, whereas the Ti39.4Al-10V alloy displayed the worst oxidation resistance, due to the formation of volatile V-oxides. Cr was harmful while Nb was beneficial. The oxidation rate of each alloy was not strictly proportional to temperature, because each alloying element had different temperature sensitivity with respect to oxidation rate.     

Effects of Nb and Si on high temperature oxidation of TiAl

The isothermal oxidation behavior of TiAl based alloys at 900℃ in air with a combination of Nb (5%-10%, mole fraction) and Si (1%-5%, mole fraction) was investigated. The microstructure and the composition of the oxidation scale were studied by using XRD, TEM and EPMA. The results show that the combination of Nb and Si can improve the oxidation resistance of the alloys significantly. The element Si can change the typical microstructure of oxidation scale on TiAl based alloys. In alloys with Si addition, the compact Al2O3 forms in the interior side of oxidation scale. When x (Si)3%, the Ti5Si3 phase forms and the coarse crystal TiO2 forms on Ti5Si3 phase after oxidation. The increase of Nb content in the TiAl based alloys impedes the growth of Ti5Si3 phase, and the formation of TiO2 on surface and on Ti5Si3 phase is also impeded.     

Mechanism of isothermal oxidation of the intel-metallic TiAl and of TiAl alloys

The oxidation behavior of Ti36Al, Ti35Al-0.1C, Ti35Al-1.4V-0.1C, and Ti35 Al-5Nb-0.1C (mass-%) in air and oxygen has been studied between 700 and 1000°C with the major emphasis at 900°C. Generally an oxide scale consisting of two layers, an outward- and an inward-growing layer, formed. The outward-growing part of the scale consisted mainly of TiO₂ (rutile), while the inward-growing part is composed of a mixture of TiO₂ and -Al₂O₃. A barrier layer of Al₂O₃ on TiAl between the inner and the outer part of the scale was visible for up to 300 hr. Under certain conditions, the Al₂O₃ barrier dissolved and re-precipitated in the outer TiO₂ layer. This shift leads to an effect similar to breakaway oxidation. Only the alloy containing Nb formed a longlasting, protective Al₂O₃ layer, which was established at the metal/scale interface after an incubation period of 80–100 hr. During this time, Nb was enriched in the subsurface zone up to approximately 20 w/o. The growth of the oxide scale on TiAl-V obeyed a parabolic law, because no Al₂O₃ barrier layer formed; large Al₂O₃ particles were part of the outward-growing layer. A brittle 2-Ti₃Al-layer rich in O formed beneath the oxide scale as a result of preferential Al oxidation particularly when oxidized in oxygen. Oxidation in air can lead also to formation of nitrides beneath the oxide scale. The nitridation can vary between the formation of isolated nitride particles and of a metal/Ti₂AlN/ TiN/oxide, scale-layer system. Under certain conditions, nitride-layer formation seemed to favor protective Al₂O₂₃ formation at the metal/scale interface, however, in general nitridation was detrimental with the consequence that oxidation was generally more rapid in air than in oxygen.     

EFFECT OF NANOCRYSTALLIZATION ON OXIDATION RESISTANCE OF TiAl INTERMETALLIC COMPOUND

ＥＦＦＥＣＴＯＦＮＡＮＯＣＲＹＳＴＡＬＬＩＺＡＴＩＯＮＯＮＯＸＩＤＡＴＩＯＮＲＥＳＩＳＴＡＮＣＥＯＦＴｉＡｌＩＮＴＥＲＭＥＴＡＬＬＩＣＣＯＭＰＯＵＮＤ①ＴａｎｇＺｈａｏｌｉｎ，ＷａｎｇＦｕｈｕｉ，ＷｕＷｅｉｔａｏＮａｔｉｏｎａｌＫｅｙＬａｂｏｒａｔｏ...     

Cyclic-oxidation behavior of TiAl and of TiAl alloys

The cyclic-oxidation behavior of (in w/o) Ti-36Al, Ti-35Al-0.1C, Ti-35Al-1.4V-0.1C and Ti-35Al-5Nb-0.1C was studied between 800 and 1000° C in air. A few experiments were also performed in oxygen. Scale spallation after oxidation in air occurs during cooling on TiAl, TiAl-C, and TiAl-V at or close to the metal/scale interface when a critical scale thickness has been achieved. This process repeats and can lead to a stratified scale. These three materials form scales composed of an inward-growing fine-grain mixture of TiO₂-Al₂O₃ and an outward-growing coarse-grain TiO₂ layer or TiO₂+Al₂O₃ mixture. The TiAl-Nb alloy had a significantly different behavior. The scale on this material grew very slowly because a protective Al₂O₃ layer formed at the metal/scale interface. This behavior resulted in much better resistance to spallation because the critical scale thickness was reached only after a much longer time, and is different from the behavior of the other three alloys. Oxidation in air leads to slight nitridation of the subsurface zone beneath the scale. In comparison to oxidation in air, oxidation in oxygen improves the cyclicoxidation behavior. Whereas the scale formed in air was uniformly thick over the entire surface, the scale grown in oxygen varied locally in structure and thickness. A large fraction of the surface was covered with a thin Al₂O₃ layer, while the remaining part formed a two-layer scale similar to that formed in air. The results are discussed briefly in the light of a recently published model for scale spallation under compressive stress, however, quantitative estimations are not possible due to a lack of relevant data.     

扩散处理对离子镀铝TiAl合金高温氧化性能的影响

利用多弧离子镀技术在TiAl合金表面制备纯铝层,分别进行720℃×2 h、780℃×2 h和840℃×2 h高温扩散处理,采用SEM、EDS和XRD分析了扩散铝层的微观组织及其相组成,并测试了不同扩散热处理工艺对合金高温氧化性能的影响.结果表明:随着扩散温度升高,镀铝扩散层厚度不断增加,扩散层由表及里形成Al2O3/TiAl3/TiAl2的结构特征,且内部Al元素浓度呈梯度分布;镀铝经扩散处理后的TiAl合金在850℃空气中的氧化速度主要受扩散层元素与基体元素之间的热扩散过程控制.氧化动力学曲线呈对数变化规律.当扩散工艺为840℃×2 h时,镀铝扩散层具有优良的抗高温氧化性能.     

Ti-42Al-8Nb合金氧化膜的XPS研究

以Ti-42Al-8Nb(at%)合金为研究对象,将其置于900℃空气气氛中氧化20 h,重点采用XPS手段对合金表面氧化膜结构、相组成及成分分布进行了分析。结果表明:合金经20 h氧化后,XRD结果显示氧化膜主要由Al_2O_3和Ti O_2混合物组成,而经XPS结果分析可知氧化膜最外层只有Ti O_2一种物质,次外层由Al2O3、3种不同价态的Ti氧化物及少量Ti N组成,且沿基体到外表层的方向,Ti的高价氧化物含量逐渐增多,最外表面只有Ti O_2,另外2种低价Ti氧化物含量为零,说明整个系统通过O原子的溶解、扩散完成氧化过程,导致氧化反应随时间和样品深度发生阶段性变化,从而使氧化膜沿深度方向表现出物质种类及其含量的不同。     

TiAl基合金离子渗碳与高温抗氧化性研究

研究了辉光离子渗碳处理对TiAl基合金的渗层组织、表面硬度和抗高温氧化性的影响。实验显示,渗碳处理可在TiAl基合金的表面形成由碳化物和过渡层组成的复合相结构,经不同渗碳处理的试样,其表面硬度和抗高温氧化性均得到明显提高。     

Wear and isothermal oxidation kinetics of nitrided TiAl based alloys

Gas nitridation of TiAl based alloys in an ammonia atmosphere was carried out.The evaluation of the surface wear resistance was performed to compare with those of the non-nitridd alloys.It is concluded that high temperature nitridation raised wear resistance of TiAl based alloys markedly.The tribological behaviors of the nitrided alloys were also discussed.The oxidation kinetics of the nitrided TiAl based alloys were investigated at 800-1000℃ in hot air.It is concluded that nitridation is detrimental to the oxidation resistance of TiAl based alloys under the present conditions.The nitrided alloys exhibit increased oxidizing rate with the prolongation of nitridation time at 800℃.However,alloys nitrided at 940℃ for 50h display a sign of better oxidation resistance than the other nitrided alloys at more severe oxidizing conditions.The parabolic rate law is considered as the basis of the data processing and interpretation of the mass gain vs time data.As a comparison with it ,attempts were made to fit the data with the power law.The oxidation kinetic parameter kn,kp and nwere measured and the trends were discussed.     

Comparisons of high temperature oxidation behavior between reactive-sintered and melted Ti-45at.%Al-1.6at.%Mn intermetallics

In order to investigate high temperature oxidation behavior, isothermal oxidation experiments for Ti-45at.%Al-1.6at.%Mn intermetallics prepared by both reactive sintering and melting were carried out in both oxygen and air environments at temperatures of 900, 1000 and 1100°C. In the oxygen environment, reactive sintered Ti-45at.%Al-1.6at.%Mn was found to have excellent oxidation resistance due to the formation of a continuous A1₂O₃ layer at temperatures up to 1100°C; however, when melted it showed very poor oxidation resistance at and above 1000°C. Weight gains of the melted specimen after 12 hours, especially at the temperature of 1000°C and 1100°C, were nearly 12 times that of the reactive sintered one. In air, the reactive-sintered intermetallics showed a poorer oxidation resistance man did that in the oxygen environment. Especially, the weight gains in air were 6 times larger than the weight gains in oxygen when oxidized at 1100°C for 12 hours.     

静止空气中TiAl断续氧化动力学

用热重法研究了５种铸态ＴｉＡｌ在静止空气中的高温（７００℃～１０００℃）断续氧化行为。ＴｉＡｌ在７００℃,１００ｈ戏微。８００℃,１００ｈ～１０００℃,１００ｈ氧化动力学曲线可用幂函数△Ｍ＾ｎ＝ｋｎｔ描述,通常１〈ｎ〈２,这表明。ＴｉＡｌ的断续氧化规律一般介于线性规律与抛物线规律之间。中、高温（≤９００℃）氧化时,合金成份对ＴｉＡｌ的氧化行为影响显著,但随着氧化温度进一步增主,不同ＴｉＡｌ基合金间     

Fe和Mo元素对TiAl金属间化合物显微组织和性能的影响(英文)

β相可以提高TiAl金属间化合物的塑性。通过显微组织分析和变形行为的评估研究β稳定性元素Fe和Mo对Ti-45Al-xFe-yMo(x,y=1,2,3,4)合金的影响。结果表明:合金中的B2(β)相随着Fe和Mo元素含量的增加而增多,Mo表现出强的β稳定性。加入3%Fe和2%Mo后,合金的晶粒得到细化,其尺寸达到12-m。由于具有一定量的β相,细化后的Ti-45Al-3Fe-2Mo合金在790℃具有良好的塑性。     

显微组织对TiAl合金热腐蚀行为的影响

研究了均匀化和铸态TiAl合金在(Na,K)2SO4混合熔盐中的热腐蚀行为,讨论了α2相在均匀化和铸态TiAl合金热腐蚀中的作用机理.结果表明:含少量α2相的均匀化TiAl合金呈现良好的抗热腐蚀行为,腐蚀产物中的富Al2O3相成层状分布,在腐蚀产物/基体界面形成连续的富Ti硫化物; 而含有大量层状组织的铸态TiAl合金则遭受严重的热腐蚀,在腐蚀初期,α2 γ构成的层状组织优先腐蚀,铸态TiAl合金的腐蚀层主要由富Ti的氧化物和硫化物组成,形成的混合膜黏附性差,较易剥落.     

Cyclic oxidation behavior of TiAl composites incorporated with TiB2 dispersoids

TiAl alloys incorporated in (0,3,5,10) wt.% TiB₂ dispersoids were manufactured via mechanical alloyingspark plasma sintering (MA-SPS), and their cyclic oxidation characteristics were studied at 800, 900 and 1000°C in air. The cyclic oxidation resistance of the prepared TiAl-TiB₂ composites effectively increased with increases in TiB₂ content. The oxide scale formed consisted of an outer TiO₂ layer, an intermediate Al₂O₃ layer, and an inner (Al₂O₃+TiO₂) mixed layer. The scale adherence was relatively good, and much thinner oxide scales, when compared to TiB₂-free TiAl alloys, were formed on the prepared composites. The incorporated TiB₂ dispersoids oxidized to TiO₂ and B₂O₃ which evaporated during oxidation.     

Effect of SiC, Si3N4 and TiB2 additions on the oxidation resistance of TiAl alloys

The oxidation behavior of TiAl alloys containing dispersed particles of (5, 10, 15 wt.%) SiC, (3,5 wt.%) Si₃N₄ or (3, 5, 10 wt.%) TiB₂ was studied between 800 and 1200°C in atmospheric air. The TiAl−(SiC, Si₃N₄) alloys oxidized to TiO₂, Al₂O₃, and SiO₂. The TiAl−TiB₂ alloys oxidized to TiO₂, Al₂O₃, and B₂O₃ which evaporated during oxidation. Improvement in oxidation resistance accompanied by thin, dense scale formation due to the addition of dispersoids originated primarily from the enhanced alumina-forming tendency, improved scale adhesion by oxide grain refinement owing to the beneficial effect of dispersoids, and the incorporation of SiO₂ within the oxide scale in the case of TiAl−(SiC, Si₃N₄) alloys.     

Effect of Hf addition (0.24 w/o) on the oxidation behavior of TiAl at high temperatures

The isothermal oxidation behavior of TiAl coupons containing 0.24 wt.% Hf has been studied in the temperature range 1100 to 1400 K in a flow of purified oxygen at atmospheric pressure. The addition of Hf is very effective in decreasing the overall oxidation rate at temperatures up to 1300 K, although the oxidation rate was slightly increased during the initial period of about 10 ks, after which a very low oxidation rate was maintained. However, at 1350 K the effect becomes small, and at 1400 K it is inverted. The excellent oxidation resistance obtained is attributable to the formation of two alumina-rich layers in the scale; one is beneath the outer rutile layer and the other at the scale/substrate interface. In addition, no internal oxides were formed in the substrate up to 1300 K.     

TiAlCr涂层对TiAl金属间化合物抗高温氧化性能的影响

研究了溅射Ｔｉ－５０Ａｌ－１０Ｃｒ涂层在８００－１０００℃对ＴｉＡｌ金属间化合物抗高温氧化性能的影响。结果表明,ＴｉＡｌ合金由于形成Ａｌ２Ｏ３和ＴｉＯ２的混合氧化物而表现出较差的抗高温氧化性能,而ＴｉＡｌＣｒ层由于形成粘附性了的Ａｌ２Ｏ３膜而大大提高了ＴｉＡｌ的恒温及循环氧化性能,经９００℃１０００ｈ长时间氧化。ＴｉＡｌＣｒ涂层对ＴｉＡｌ合金仍能提供很好的防护,涂层和基格结合我孔洞及裂纹出现,表现