Superplastic behavior of a TiAl-based alloy

Superplastic behavior under the conditions of a temperature range from 850 to 1075°C and strain rates varying from 8×10⁻⁵ to 1×10⁻³ s⁻¹ was investigated for Ti–33Al–3Cr–0.5Mo (wt%) alloy with a very fine grain size obtained by the multi-step thermal mechanical treatment. The results show that the TiAl-based alloy with a hot-deformed fine grain size possesses good superplasticity. It exhibits a strain rate sensitivity coefficient of 0.9 at a strain rate of 3×10⁻⁵ s⁻¹ and temperature from 1000 to 1075°C. Moreover, the strain rate sensitivity coefficient is stable during the hot deformation, and a tensile elongation of 517% was obtained at 1075°C and a strain rate of 8×10⁻⁵ s⁻¹. The superplastic behavior of the present fine-grained TiAl-based alloy can be explained by the local strain hardening and high m value during the tensile deformation. Microstructure evolution in the superplastic deformation was also discussed.     

Heat-transfer corrosion behaviour of cast Al alloy

Heat-transfer corrosion behaviour of an ISO 2379 cast Al alloy was studied in antifreeze radiator coolant under heat-rejecting condition. Extensive analyses of microstructures and corroded surfaces were carried out under the optical microscope, scanning electron microscope equipped with energy dispersive spectroscopy and X-ray diffractometer. Heat-rejecting condition led to a cavitation process and cavities were observed within the α-Al matrix. Crevice corrosion was predominant at oxygen depleted regions in heat-transfer corrosion cell. Al₂Cu, Al₁₅(Fe,Mn)₃Si₂ dendrites, Al₄Cu₂Mg₈Si₇ and Si phases served as the effective cathodes resulting microgalvanic corrosion at the anodic site of α-Al matrix.     

Formation behaviour of blister in cast aluminium alloy

Abstract

The formation behaviour of a blister in a die cast aluminium alloy was observed by employing a combined methodology of in situ three-dimensional observation using X-ray microtomography and image based simulation. It has been revealed, via a reverse approach based on the simulation, that nitrogen and carbon dioxide gases fill the blister nucleus. Spontaneous growth of the blister nucleus occurs through creep deformation of the surrounding aluminium due to the blister nucleus’ high internal gas pressure. This internal gas pressure also induces hydrogen precipitation in the form of micropores, which undergo steady growth in a spherical shell region around the blister nucleus. The selective growth of the micropores is attributable to the elevation of hydrostatic stress in directions parallel to the casting surface, thereby promoting the expansion of the blister, also parallel to the casting surface, through the absorption of surrounding micropores into the blister nucleus.     

Creep of cast Fe–36Al–2Ti alloy

Creep of an alloy based on the intermetallic compound FeAl with Ti addition was studied by compressive tests at constant stress in the temperature range from 873 to 973 K. The stress exponent n and the activation energy of creep Q were determined for the minimum creep rate. The stress exponent is slightly decreasing with increasing temperature. Its value is in agreement with stress exponents reported for Fe–Al alloys with similar additions of titanium. The values of n together with the observed shapes of the creep curves are consistent with the expected behaviour for solid solution hardened alloys where the dislocation motion is controlled by viscous glide of dislocations. Annealing of the alloy for 2 h at 1423 K with subsequent oil-quenching had no influence on the minimum creep rate in comparison with the as-cast material. On the other hand, this annealing accelerated “the inverse primary behaviour”.     

Mg-5.5Zn-2Gd-0.6Zr铸造镁合金的蠕变机制

在ZM-1(Mg-5Zn-0.6Zr)合金的基础上,适量增加Zn的含量并加入重稀土元素Gd,设计了Mg-5.5Zn-2Gd-0.6Zr实验合金。采用砂型铸造工艺制备实验合金试样,在不同温度和应力条件下对该实验合金和ZM-1合金的蠕变曲线进行了测试。结果表明:在相同条件下,Mg-5.5Zn-2Gd-0.6Zr实验合金的稳态蠕变速率较ZM-1合金的降低了一个数量级;当施加应力为40 MPa时,实验合金的蠕变激活能Q200-250℃=142.0 kJ/mol,接近镁的自扩散激活能,蠕变受位错攀移控制,而ZM-1合金在相同应力下蠕变激活能Q200-250℃=88.5 kJ/mol,接近镁的晶界扩散激活能,蠕变受晶界滑移控制。合金在200℃条件下的应力指数n=4.21,而ZM-1合金的应力指数n=2.21。因此,认为加入重稀土元素Gd后实验合金的蠕变机制发生改变,200℃时的蠕变机制为位错攀移机制。     

Creep behaviour of long range ordered Cu50Zn alloy

Creep behaviour of long range ordered β′-brass was investigated with the aim of obtaining more information exploitable in an explanation of the physical basis of creep taking place in the ordered structure of metallic materials. In addition to conventional analysis of creep based on an analysis of the applied stress and temperature dependence of the steady-state creep, extensive constant structure experiments in the steady-state creep were performed and analysed. Comprehensive investigations yielded a new view, primarily in the internal stress and mechanisms acting in the creep of the investigated alloy.     

TiAl基合金钎焊技术研究现状及展望

TiAl基合金具有质量轻、耐高温等优点,在航空航天、军用战车和民用汽车的高温部件中具有广阔的应用前景。TiAl基合金实用化要求解决其连接问题,而其本质脆性使得钎焊技术成为实现TiAl基合金连接,特别是与其他材料连接的重要方法,综述了TiAl基合金钎焊技术研究现状,指出了目前存在的问题,进而对钎焊研究方向进行了展望。     

TiAl基合金及其连接技术的研究进展

综述了TiAl基合金熔焊和固态连接的研究状况,主要包括弧焊、激光焊、电子束焊、钎焊、扩散焊、自蔓延高温合成和摩擦焊等连接方法,分析了各种方法用于TiAl基合金连接时的优缺点.由于TiAl基合金室温塑性差,采用熔焊方法连接时焊后冷却速度块,接头组织淬硬倾向大,易形成固态裂纹.固态连接方法大多可控制焊接热循环,焊接过程中加热峰值温度相对较低,对母材组织影响小,可避免裂纹等缺陷,因而采用固态连接方法具有优势.如果能进一步降低冷却速度,则将熔焊方法用于TiAl基合金的连接有很好的应用前景.     

Oxidation behaviour of Ag-containing TiAl-based intermetallics

The effect of alloy composition on the oxidation behaviour of γ-TiAl alloys with silver additions was studied during exposure at 800 °C in air. For this purpose, a number of Ti–Al–Ag alloys with systematic variation of the chemical composition was prepared by levitation induction melting. Subsequently, the alloys were isothermally and/or discontinuously oxidised at 800 °C in air. The results showed that suitable silver additions can promote and stabilise formation of an alumina scale on γ-TiAl up to the maximum test times of 12,000 h. An oxidation map was derived which allows defining which type of surface oxide is formed on various TiAl–Ag alloy compositions. This result can be applied in further work for development of oxidation resistant coatings for γ-TiAl or/and Ti-based alloys.     

Corrosion behaviour of twin belt cast EN AW 7075 alloy

The corrosion behaviour of the twin belt cast EN AW 7075 alloy is governed by intermetallic phases, namely Al₁₂(Fe,Cr,Mn)₃Si, Mg₂Si and CuAl₂, and by Mg(Zn,Cu,Al)₂ precipitates. The former are responsible for pitting activities while the Mg(Zn,Cu,Al)₂ precipitates play a key role in intergranular corrosion. The very fine dispersion of Mg(Zn,Cu,Al)₂ precipitates in samples aged to peak hardness undergo coarsening, particularly along the grain boundaries, when the hot band samples are overaged. Overageing improves the resistance to intergranular corrosion while the samples in T6 temper suffer heavy attack along grain boundaries. While ageing treatments hardly produce any change in the features of the intermetallic particles, they nevertheless seem to impact the pitting response. This may be accounted for also by the precipitation activities which in turn, change the chemistry of the solid solution matrix. Overageing to the T73 temper implies a higher purity matrix and thus changes the microgalvanic effects when exposed to neutral chloride solutions.     

Surface damage of a TiAl-based alloy during high temperature annealing

The near surface stability of a Ti–45Al–2Mn–2Nb alloy has been investigated. Samples of the alloy were annealed at two different temperatures, 1200 and 1000°C for various periods of time. It was found by detailed SEM and TEM investigation that a significant loss of aluminum occurred from the surface of the specimens during annealing, resulting in the formation of Al-depleted damaged layer near the surface of the specimens. The microstructure of the damaged layer was found to be considerably different from that of the bulk. The damaged layer was found to consist of α₂ phase near the interface between the damaged layer and the undamaged bulk, and α₂ phase and B2 phase with ω-type phase precipitates near the outer edge of the specimen. It was also observed that the thickness of the damaged layer increased with the annealing time and also with the annealing temperature. The thickness of the damaged layer exhibited a parabolic relationship with annealing time for both the temperatures.     

Microstructure and properties of a beta-solidifying TiAl-based alloy with different refiners

This study systematically compared the influences of yttrium(Y),boron(B),and carbon(C) on the microstructural refinement and properties of a Ti-43Al-5Nb alloy.The microstructural refinement effect in the TiAl alloy closely depends on the refiner used.The refinement effects of the three elements on colony size and lamellar thickness can be arranged as B Y C and Y C B,respectively.Moreover,a microstructure with a small grain size and ultra-fine lamellar spacing can be obtained by adding B and Y or B and C.The mechanical properties of TiAl alloy are also influenced by the refiners.TiAl alloys with proper B and Y contents exhibit favorable hot workability,tensile properties,and fracture toughness,whereas the C-containing alloy displays poor tensile properties and low fracture toughness.These results indicate that Y and B are more suitable microstructure refiners than C.This study may serve as a reference for practical alloying design.     

TiAl基合金动态再结晶临界模型建立

依据粉末冶金Ti-47Al-2Nb-2Cr合金热模拟压缩实验结果,研究了变形温度为950～1150 ℃、应变速率为0.001～0.1 s(-1)条件下材料的流变力学行为。采用Poliak和Jonas所提出的临界条件动力学理论,确定了该合金的动态再结晶临界应变(ε_c)和临界应力(σ_c),揭示了变形温度与应变速率对ε_c和σ_c的影响规律。结果表明,温度补偿应变速率因子Z与ε_c、σ_c、ε_p(峰值应变)和σ_p(峰值应力)间的关系可以采用指数函数形式表征。建立了该合金动态再结晶临界发生模型:ε_c=1.2×10~(-3)Z~(0.147),动态再结晶临界应变与流变应力曲线峰值应变的比值约为 0.73。根据对模型的分析表明,临界应变与 Z 参数之间呈现正相关性,即随着 Z 参数的减小(变形温度升高或应变速率降低),材料发生动态再结晶的临界应变减小,说明变形温度的升高与应变速率的下降能够促进动态再结晶行为的发生。通过对热变形后微观组织的观察,验证了所建立动态再结晶临界模型的可靠性。     

Electrochemical corrosion behaviour of Bi-11Ag alloy for electronic packaging applications

This study investigated the electrochemical corrosion properties of the solders for die-attach applications in 3.5% NaCl solution. Compared with Pb-5Sn and Zn-40Sn, Bi-11Ag exhibited higher corrosion potential and relatively low corrosion current density. The ductile Ag-rich phase which dispersed in the Bi matrix was able to accommodate the stress arising from the formation of a passive layer and contributed to the two-stage passivation. X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) results confirm that the corrosion products comprised BiOCl, Bi₂O₃ and AgCl.     

粉末冶金TiAl合金排气门的研制

利用元素粉末冶金方法制备了TiAl合金排气阀。为提高排气门杆部的致密度、均匀度 ,设计了特有的径向热压工艺 ,并从理论上分析了与该工艺相关的压坯密度、致密化和应力变化规律。制备出高径比为 10 .7,密度为 3.79g/cm3 的粉末冶金TiAl合金汽车发动机排气门