碳纳米管含量对Ti-48Al-2Cr-2Nb合金组织及力学性能的影响

采用真空电弧熔炼炉制备了碳纳米管(CNTs)增强Ti-48Al-2Cr-2Nb合金,研究不同CNTs添加量对Ti-48Al-2Cr-2Nb合金微观组织及力学性能的影响.结果 表明:向Ti-48Al-2Cr-2Nb合金中添加CNTs后有Ti2AlC相析出,Ti2AlC相的含量随着CNTs含量的增加而增加.添加0.3 at...     

钛合金在汽车上的应用和展望

综述了近三十年来国外汽车用钛合金的研究开发现状汽车用钛合金包括Ti-5Al-2Cr-1Fe,Ti-6Al-2Sn-4Zr-2Mo,Ti-6Al-4V,Ti-3Al-2.5V,Ti-6Al-4V/TiB,Ti-Al-Sn-Zr-Nb-Mo-Si/TiB,Ti-3Al-8V-6Cr-4Mo4Zr,Ti-6V-2Sn-4Zr-2Mo,Ti-6Al-2Sn-4Zr-2Mo-0.1Si,Ti-6Al-2.7Sn-4Zr-0.4Mo-0.45Si,Ti-3Al-2V-0.2S-0.47Ce-0.27La,Ti-6Al-1.7Fe-0.1Si/10TiB,Ti-4.5Fe-6.8Mo-1.5Al/10TiB,Ti-6Al-2Fe-0.1Si等.估计到2001年,全世界汽车用钛合金将达到6000t.     

冷却速率对Ti-48Al-2Cr-2Nb合金组织及性能的影响

TiAl基合金铸造成形过程多采用陶瓷型、金属型,少量涉及石墨型,文中对比石墨型、金属型和陶瓷型成形Ti-48Al-2Cr-2Nb合金的凝固组织及力学性能,研究冷却速率对凝固组织及力学性能的影响规律。研究结果表明:采用冷却能力较强的石墨型成形Ti-48Al-2Cr-2Nb合金的组织层片间距最小,金属型的次之,而采用陶瓷型成形的组织层片间距最大。通过对Ti-48Al-2Cr-2Nb合金宏观组织特征的分析,总结一种评价铸型冷却能力的方法。通过对显微组织层片间距的定量统计分析,测量出三种铸型条件下,石墨型和金属型的试样层片间距相近,无明显差别,而陶瓷型的试样层片间距明显高于其他两种。铸型的蓄热系数和热导率是影响Ti Al合金组织和力学性能的主要因素。     

Ti-Al合金抗氧化性能的影响因素初探

研究了氧化条件、显微组织和成分对Ti-Al合金抗氧化性能的影响。结果发现,Ti-48Al-2Cr-2Nb合金在700℃和800℃下具有较好的抗氧化性能,氧化形成的氧化膜为4层结构。在800℃下恒温氧化100h后,Ti-48Al-2Cr-2Nb合金的氧化增重从低到高依次为双态组织、近片层组织和片层组织。在800℃下恒温氧化100h后,3种成分的Ti-Al合金的氧化增重从高到低依次为Ti-48Al-2Cr-2Nb、Ti-46.5Al-2.5V-1C和Ti-45Al-8Nb-0.2W-0.1B-0.1Y。具有双态组织的Ti-45Al-8Nb-0.2W-0.1B-0.1Y合金在800℃下恒温氧化100h后增重4.8g/m~2。     

Ti-46Al-2Cr-4Nb-Y合金的高温变形及加工图

采用Gleeble-1500 热压缩模拟试验机进行压缩实验,在变形温度为1 100～1 250 ℃、应变速率为10-2～ 1 s-1的范围内,研究Ti-46Al-2Cr-4Nb-Y合金的高温变形行为,并基于动态材料模型,建立Ti-46Al-2Cr-4Nb-Y合金的加工图.结果表明:Ti-46Al-2Cr-4Nb-Y合金的高温变形流变应力对温度及应变速率敏感;流变应力随应变速率的增大而增大,随温度的升高而减小;动态再结晶是导致流变软化及稳态流变的主要原因;Ti-46Al-2Cr-4Nb-Y合金的安全热加工区域为温度1 200～1 230 ℃,应变速率10-2～10-1 s-1.     

Ti-48Al-2Cr-2Nb-( TiB2, Ni)合金凝固组织演变规律及力学性能研究

TiAl合金已成为航空航天工程升级换代的关键材料,然而其铸态晶粒尺寸粗大,室温塑性和强度低,限制其进一步工程应用。本文采用真空感应凝壳熔炼工艺系制备铸锭,系统研究TiB2和Ni元素共同添加对Ti-48Al-2Cr-2Nb合金凝固组织和力学性能的影响。结果表明,TiB2及Ni合金化后,合金的凝固路径和初生相并未改变,晶粒尺寸从700μm细化至100μm,生成片状TiB2和富镍τ3相。T4822-( Ni, TiB2)合金的室温拉伸强度与基体合金相近,断裂伸长率提高30%。700-900℃时,T4822-(Ni, TiB2)合金的抗拉强度始终高于基体合金,在900℃时抗拉强度达到365MPa,较基体合金提高9%。800℃和900℃时T4822-(Ni, TiB2)合金的断裂伸长率分别达到25.3%和36.1%,远高于基体合金。晶粒尺寸的细化和晶界处的块状γ相是T4822-(Ni, TiB2)合金塑性提升的主要原因,其良好的高温强度则可以归因于片层团内部和界面处的硬质硼化物和富镍τ3相。     

电子束选区熔化增材制造TiAl合金的高温硬度及氧化行为研究

钛铝合金是航空航天领域具有广阔应用前景的轻质高温合金,电子束选区熔化（EBM）增材制造技术是制备复杂结构TiAl合金有效途径,但目前对其高温性能研究较少。本文重点研究了电子束选区熔化增材制造Ti-48Al-2Cr-2Nb合金的显微组织、高温硬度及其高温氧化行为。结果表明,EBM成形Ti-48Al-2Cr-2Nb合金呈现出由等轴γ晶粒和双相区组成的独特层状组织;在800 ℃下恒温氧化100 h,表现出较低的氧化速率常数,形成的氧化膜主要由TiO2、Al2O3及TiO2 / Al2O3混合交替层组成,抗氧化性能优于传统方法制备的Ti-48Al-2Cr-2Nb合金和其他TiAl合金。此外,900℃以下,该合金具有良好的高温硬度,显微硬度随温度的升高未发生明显的下降趋势。     

电子束增材制造用TiAl预合金粉末表征

本文采用无坩埚电极感应气雾化法(EIGA)制备了Ti-48Al-2Cr-2Nb与Ti-47.5Al-6.8Nb-0.2W预合金粉末,并对粉末的特性进行了对比研究。结果表明,两种Ti Al预合金粉末具有良好的球形度,且粒度分布符合正态分布;粉末的气体含量较低,其中氧元素的含量保持一致,Ti-47.5Al-6.8Nb-0.2W预合金粉末中氮元素、氢元素含量较高;粉末的显微组织形貌表现为树枝晶状,XRD分析结果表明Ti-48Al-2Cr-2Nb预合金粉末的主要相为γ相,而Ti-47.5Al-6.8Nb-0.2W预合金粉末的主要相为α2相。     

冷却速度对快速凝固Ti-48Al-4Cr合金组织转变规律及力学性能研究

采用单辊旋淬快速凝固设备制备了不同辊速条件下的Ti-48Al-4Cr(at.%)薄带,研究冷却速度对快速凝固Ti-48Al-4Cr合金的组织及力学性能变化规律。结果表明,快速凝固Ti-48Al-4Cr合金凝固在辊速为10m/s和20m/s时,基体为等轴的γ相,基体中含有少量的B2相、α₂相颗粒和片层组织;辊速进一步增加至30m/s时,基体转变为α₂相,片层组织消失。快速凝固Ti-48Al-4Cr纳米硬度随着冷却速度的增加而增加,纳米硬度由常规凝固时的5.04±0.09GPa增加至辊速为30m/s时的10.48±0.13GPa。该结果为研究TiAl合金组织转变,减小TiAl合金偏析,提高其力学性能提供了基础。     

冷却速度对快速凝固Ti-48Al-4Cr合金组织转变规律及力学性能

采用单辊旋淬快速凝固设备制备了不同辊速条件下的Ti-48Al-4Cr(原子分数,%)薄带,研究冷却速度对快速凝固Ti-48Al-4Cr合金的组织及力学性能变化规律。结果表明,快速凝固Ti-48Al-4Cr合金凝固在辊速为10和20 m/s时,基体为等轴的γ相,基体中含有少量的B2相、α_2相颗粒和片层组织;辊速进一步增加至30 m/s时,基体转变为α_2相,片层组织消失。快速凝固Ti-48Al-4Cr纳米硬度随着冷却速度的增加而增加,纳米硬度由常规凝固时的5.04±0.09 GPa增加至辊速为30 m/s时的10.48±0.13 GPa。该结果为研究Ti Al合金组织转变,减少Ti Al合金偏析,提高其力学性能提供了基础。     

The influence of thermal processing route on the microstructure of some TiAl-based alloys

The influence of a range of different continuous cooling rates from the alpha phase field has been investigated in large-grain-sized (∼1500 μm) Ti48Al2Cr2Nb and in grain-refined Ti48Al2Cr2Nb1B alloys. In addition the microstructure of direct laser-fabricated samples of Ti48Al2Nb2Mn has been examined in order to assess the influence of the complex heating cycle and high cooling rate on this alloy. It has been found that the critical cooling rates to obtain a particular structure are higher in grain-refined Ti48Al2Cr2Nb1B than in the alloy without boron (B). The increase in the amount of the lamellar structure during cooling takes place in different manners in the two alloys. The formation of the lamellar structure in laser-treated samples has been found to be very different with the formation of Al-rich gamma regions separating regions with a lamellar structure. The possible factors affecting phase transformations are discussed in terms of the role of grain boundaries and B in solution in the solid state transformations and, in the case of the laser-treated samples, of melting and remelting.     

The solidification path related columnar-to-equiaxed transition in Ti–Al alloys

Columnar-to-Equiaxed Transition (CET) of binary Ti–Al alloys and multi-component Ti–48Al–2Cr–2Nb alloys is studied using Bridgman solidification technique. The effect of aluminum concentration and growth rate on CET is determined. It is found in Ti–46Al and Ti–50Al alloy ingots equiaxed grains develop ahead of the moving solid–liquid interface with a growth rate of 500 μm/s; microstructures in Ti–49Al alloy stay columnar dendrites with the same growth rate. CET in Ti–Al alloys are not only influenced by growth rate, but also by the solidification path that is related to alloying composition. CET in Ti–Al alloys is predicted using the dendritic growth model based on the criterion of growth at marginal stability. According to the calculated results and directionally solidified microstructures, values of the nucleation undercooling for α and β phases are given. The growth rates to avoid CET in Ti–48Al–2Cr–2Nb alloy are suggested.     

Stability of lamellar microstructures in a Ti–48Al–2Nb–2Cr alloy during heat treatment and its application to lamellae alignment as a quasi-seed

The stability of lamellar microstructures in the Ti–48Al–2Nb–2Cr alloy during heat treatment depends more on heating rate and less on holding time and cooling rate. When the heating rate is higher than 61 °C/min, the original lamellae orientation at room temperature can still maintain unchanged even though the lamellae are transformed to single α grains at a temperature close to melting point. Therefore, the high-temperature α grains, which are transformed from the α₂/γ lamellae within a quasi-seed, are successfully employed to align the lamellar microstructure by directional solidification.     

Effects of heat shock at 800 °C on mechanical properties of γ-TiAl alloys

Effect of heat shock on the mechanical properties and the microstructure of TiAl alloys (Ti–47Al–2Cr–2Nb and Ti–45.3Al–2Cr–2Nb–0.1W–0.15B) with near fully lamellar structure were investigated. After heat shock process from room temperature to 800 °C for 500 cycles, the microstructure demonstrated that lamellar microstructure has been destructed by the presentation of some γ and α₂ block phases in lamellar structure, resulting in the ductility of as-polished Ti–47Al–2Cr–2Nb alloy decreased by about 70% and the ultimate tensile strength (UTS) reduced by about 25%, and the ductility of as-unpolished Ti–47Al–2Cr–2Nb alloy decreased by more than 70% and the ultimate tensile strength (UTS) was reduced by about 35%. The ductility of Ti–45.3Al–2Cr–2Nb–0.1W–0.15B alloy decreased by about 60% and the ultimate tensile strength (UTS) reduced by about 18% after heat shock, which was resulted from the appearance of small α₂ block phase at interfaces of lamellar colonies and microcracks at the interfaces of ribbon boride and lamellar structure.     

The isothermal and cyclic high temperature oxidation behaviour of Ti–48Al–2Mn–2Nb compared with Ti–48Al–2Cr–2Nb and Ti–48Al–2Cr

The isothermal and cyclic oxidation behaviour of Ti–48Al–2Mn–2Nb (at%) were studied at high temperatures in air in comparison with the intermetallic alloys Ti–48Al–2Cr–2Nb and Ti–48Al–2Cr. Tests were performed in air between 800 and 900°C. At 800°C Ti–48Al–2Mn–2Nb showed an excellent oxidation resistance under isothermal and cyclic conditions, comparable with Ti–48Al–2Cr–2Nb, and superior to Ti–48Al–2Cr. At 900°C the isothermal oxidation rate of Ti–48Al–2Mn–2Nb was similar as found for Ti–48Al–2Cr–2Nb, but much lower as that of Ti–48Al–2Cr. Upon cooling the oxide scale formed on Ti–48Al–2Mn–2Nb was prone to spallation. During the cyclic oxidation at 900°C, a steady state condition is reached for both niobium bearing materials, with a net linear mass loss rate, due to spallation and (re-)growth of the oxide scale. The linear mass loss rate for the Ti–48Al–2Mn–2Nb was higher than that of Ti–48Al–2Cr–2Nb, indicative of a higher susceptibility for spallation. During the initial stage of oxidation of all tested materials a complex multi-phased and multi-layered scale was formed consisting of α-Al₂O₃, TiO₂ (rutile), TiN and Ti₂AlN. After longer exposure times the outer scale was dominated by TiO₂. In case of the niobium containing materials no loss of protectivity of the oxide scale was found during the growth of the outer TiO₂ layer (under isothermal conditions). Two-stage oxidation experiments with isotope tracers were performed to study the oxidation mechanism in more detail.     

Isothermal oxidation behavior of powder metallurgy beta gamma TiAl–2Nb–2Mo alloy

A new TiAl–2Nb–2Mo beta gamma alloy was synthesized by powder metallurgy process. HIP’ed and vacuum heat treated specimens were isothermally oxidized at 800 °C and 900 °C in air up to 500 h. The TiAl–2Nb–2Mo alloy oxidized parabolically up to 500 h at both 800 °C and 900 °C. The oxides consisted of outer TiO₂ layer, intermediate Al₂O₃ layer, and inner TiO₂ rich mixed layer and the oxidation mechanisms of the alloy were identical at both temperatures. During oxidation, the degradation of lamellar colonies formed a diffusion zone just below the oxide/substrate interface consisting of γ-TiAl matrix and dispersed beta phases which contained high concentration of Nb and Mo. The oxidation rate of the TiAl–2Nb–2Mo alloy is more sensitive to temperature than those of the Ti–48Al–2Nb–2Cr and Ti–48Al–2Nb–2Cr–W alloys.     

Crack propagation behavior in TiAl–Nb single and Bi-PST crystals

The fracture toughness of directional solidified Ti–(45,47)Al–3Nb, Ti–(45,47)Al–3Nb–0.2Si–0.1C, Ti–(45,47)Al–3Nb–0.3Si–0.2C type I alloys and their contribution to crack growth resistance of TiAl–Nb alloys were studied using PST (polysynthetically twinned) crystals produced by directional solidification in FZ (floating zone) furnace. Lamellar orientations in the individual colonies are described using two angles defined with respect to the notch orientation: an in-plane kink angle and a through-thickness twist angle. Therefore, lamellar misorientation across an individual colony boundary is quantified as differences in these angles across the boundary. Crack growth resistance in colony boundary was identified by three-point bend test and crack advance was monitored by interrupted in situ test. From three-point bend test, it was found that the colony boundary could offer significant resistance to crack growth under large twist angle difference. Fracture toughness of type I specimens (in which crack propagates against lamellae boundaries) of the alloys decreased slightly with increasing Si and C contents and increased rapidly with decreasing Al content. The toughness for type I specimens was controlled by α₂–α₂ spacing in which the delamination-type separation occurred. Compared to 47Al alloys, α₂–α₂ spacing in 45Al alloys increased by decreasing Al content, therefore, fracture toughness increased rapidly. These results are discussed and the ability to improve toughness by changing Al content, Si and C addition in TiAl–Nb alloys produced by directional solidification is suggested.     

Improvement in mechanical and oxidation properties of TiAl alloys with Sb additions

The objective of this study is to improve the mechanical properties and oxidation resistance by control of both alloy addition and microstructure in two-phase TiAl alloys based on Ti–48Al (at%). It is found that with the addition of up to 0.15Sb, the room temperature bending deflection and fracture strength (σ_b) can be enhanced significantly. For example, σ_b reaches as high as 1345 MPa in the alloy of Ti–48Al–0.15Sb. These improved properties are attributed mainly to the refinements of the colony grain size and the interlamellar spacing with the addition of Sb. Additionally, the high-temperature oxidation resistance can also be substantially increased in the presence of 0.15Sb, being higher than those for Ti–48Al and Ti–48Al–2Cr–2Nb. The cause for this increase is suggested to be the formation of a protective layer of Al₂O₃ stabilized by Sb.     

Effects of Nb and Al on the microstructures and mechanical properties of high Nb containing TiAl base alloys

The influence of Nb and Al contents on the microstructure and yield strength of high Nb containing TiAl base alloys was investigated. The experimental results show that the yield strength at 900 °C of the alloys with the same type of microstructure, such as fully lamellar (FL), nearly lamellar (NL) and degraded fully lamellar (DFL), increases with increasing Nb content and decreasing Al content in the composition range of 0–10 at.% Nb and 44–49 at.% Al. DFL is the degraded form of FL microstructure after exposure at 1050 °C for 30 h. It is shown that the Nb addition in the alloys increases the value of the σ₀ term in the Hall–Petch relation of yield stress vs. lamellar spacing. This result has been related to TEM observations of dislocation structure in deformed specimens. The observations indicated that high level of Nb solute in the γ-TiAl matrix leads to a high critical resolved shear stress (CRSS) of dislocation loops. High Nb addition also reduces the degradation rate of FL microstructure after exposure at 1050 °C for 30 h. Both effects of high Nb addition are related to the change of the directionality of Ti–Ti (Nb) and Nb–Al bonds in the lattice. The decrease in Al content results in an increase in the volume fraction of α₂ phase, which leads to a decrease in the lamellar spacing of the lamellar structure. The high temperature strength of the alloys is determined by the lamellar spacing λ through the Hall–Petch equation k_λλ^−1/2.     

Real-time observation of grain nucleation and growth during solidification of aluminium alloys

The crystallisation kinetics of liquid aluminium–titanium alloys with microscopic TiB₂ particles added to refine the grain size in the solidified material was studied by X-ray diffraction measurements at a synchrotron source. Real-time observation of the formation and growth of individual grains reveals the central role played by the added TiB₂ particles during solidification. Prior to the main transformation, weak reflections of a metastable TiAl₃ phase were detected. This observation finally pinpoints the highly debated mechanism responsible for enhanced grain nucleation in Al–Ti–B alloys.