Al-Li-Zr和Al-Li-Cu-Mg-Zr合金的显微组织和力学性能

研究了两种Al-Li基合金的时效硬化、拉伸和冲击等力学行为,用透射电镜和扫描电镜对合金的显微组织结构,变形和断裂的特征进行了分析。结果指出:同一成分、不同热处理态的合金变形和断裂的机制截然不同。分析了时效态合金韧性下降和产生冲击各向异性的原因。讨论了改善Al-Li合金塑、韧性的可能途径。     

含有不同尺度量级第二相的高强铝合金断裂韧性模型

基于合金中不同尺度第二相对其塑性的影响以及塑性与韧性指标之间的人在联系，建立了高强铝合金中粗大第二相颗粒，中间尺寸第二相颗粒以及时效强化第二相颗粒形态与其断裂韧性间的多元非线性关系解析模型。借助于该模型的理论分析，可以得出不同尺度第二相颗粒尺寸，体积分数改变对高强铝合金断裂韧性的定量影响，并且可以解释铝合金板材断裂韧性的取向效应以及加工变形对断裂韧性的弱化作用，更进一步地能够给出在确保铝合金高强度的前提下改善其断裂韧性的优化方案。     

AN INVESTIGATION OF HIGH-TEMPERATURE DEFORMATION STRENGTHENING AND TOUGHENING MECHANISM OF TITANIUM ALLOY

1.IntroductionItiswellknownthatalpha-betatitaniumalloysareusuallyprocessedbyconventionalforging,i.e.,materialsareheatedandprocessedat30--50"Cbelowbetatransustemper-ature.Theconventionallyforgedtitaniumalloyshaveanequiaxedmicrostructure.Ingeneral,equiaxedmicrostructuresshowahigherductilityandhigherstrength,buthavelowerhigh--temperatureproperties,lowerresistancetofatiguecrackpropagationandlowerfracturetoughness[1].Inthelate50s,anewforgingmethod,calledbetaforging,waspro-posedbyCroan[2].Itbroket…     

Fracture mechanism of Ni3Al alloys and their composites with ceramic particles at elevated temperatures

Ni₃Al alloys and their matrix composites reinforced with fine ceramic particles have been successfully fabricated by reactive hot-pressing. This paper investigates the embrittlement mechanism of these materials at intermediate temperatures using a mechanical fracturing technique, i.e. a single edge chevron-notched beam method with variation of loading rate. In the case of monolithic alloys, extrinsic embrittlement originating from diffusion of atomic oxygen into plastic deformation zone coincides with the inherent brittleness connected with deterioration of grain boundary cohesion and unique dislocation motion at 673–1073 K. Oxygen embrittlement predominates over other mechanisms at 673 K, because significant loading rate dependence of fracture toughness is observed in air. The fracture toughness of the alloys intrinsically decreases at 873–1073 K. However, the mechanical behavior of their matrix composites is quite different, depending on the kind of reinforcement particles. Although the composites with TiN particles have high strength and ductility, their fracture toughness decreases at intermediate temperatures, in a similar manner to the monolithic alloys. The fracture toughness of TiC particle reinforced composites is exceptionally constant between 300 and 900 K.     

Understanding fracture toughness in gamma TiAl

The ambient-temperature ductility and fracture toughness of TiAl-base intermetallic alloys have been improved in recent years by both alloy additions and microstructural control. Two-phase TiAl alloys have emerged as a new class of lightweight, high-temperature materials with potential importance for aerospace applications. This overview summarizes recent advances in the basic understanding of the fracture processes and toughening mechanisms in TiAl-base alloys and the relationships between microstructures and mechanical properties.     

微量Ag对自然时效态Al—Li—Cu—Mg—Zr合金低温拉伸行为的影响

本文研究了两种不同量的Ag的加入对自然时效状态Al-Li-Cu—Mg—Zr合金在290—77K温度范围内的拉伸性能及断裂行为的影响。结果表明,随着实验温度的降低,合金的拉伸性能在153—77K之间明显改善,不含Ag合金的延伸率则基本保持不变;所有合金在室温时的变形与断裂模式基本相同,主要为平面滑移引起的穿晶剪切断裂;但低温下的拉伸行为随Ag的加入而表现出不同的特点,即含Ag合金的变形在低温下趋于均匀,其低温延伸率的改善可归因于此;低温下合金分层及沿晶开裂的倾向增加     

含两种尺度微裂纹的高强铝合金的断裂韧性模型

基于高强铝合金在断裂过程中萌生不同尺度微裂纹的机制,用断裂力学建立两种尺度微裂纹影响应力应变场的规律,导出高强铝合金拉伸延性与两种尺度微裂纹的关系,由断裂韧性与拉伸延性的关系建立了高强铝合金断裂韧性与双级微裂纹的非线性关系模型。通过模型解析,分析两种尺度微裂纹体积分数对高强铝合金断裂韧性的影响规律。结果表明:随着一级微裂纹体积分数的增加,材料的断裂韧性开始迅速下降,然后缓慢降低;在较大尺度微裂纹之间萌生小尺度微裂纹,将显著降低合金的断裂韧性。将高强铝合金的结晶相作为一级微裂纹,将弥散相和粗大析出相作为二级微裂纹,预测高强铝合金断裂韧性随两种尺度相(微裂纹)的变化,其规律与实验结果较为吻合。利用模型解析与实验验证结果,提出了改善高强铝合金断裂韧性的组织控制方向。     

A newly developed Tribaloy alloy with increased ductility

An outstanding combination of mechanical, wear and corrosion performance has been achieved in Tribaloy intermetallic materials. However, conventional Tribaloy alloys usually have lower ductility and fracture toughness compared with ductile materials due to the large volume fraction of Laves phase. This research was aimed at developing an advanced Tribaloy alloy with increased ductility.     

Role of Mn-dispersoid in the fracture toughness enhancement of Al-Zn-Mg-(Mn) alloys

The effects of Mn dispersoid and fabrication methods on the fracture toughness in peak aged Al-Zn-Mg-(Mn) alloys have been studied. Sphere- or rod-shaped Mn dispersoids of the size in range from 0.05 μm to 0.5 μrn are formed by the addition of Mn in Al-Zn-Mg alloy. The extruded alloys containing Mn have higher fracture toughness with higher strength and show transgranular ductile fracture surface. as compared with those of Al-Zn-Mg alloy. These phenomena are found to be obtained from the improved load bearing capacity and the effective accommodation of the applied stress due to the dispersion hardening effect and homogeneous deformation by the Mn-dispersoids. Comparing the mechanical properties between the extruded and the rolled alloys containing 0.8 wt% Mn. the extruded alloy is shown to have higher strength and better fracture toughness than those of the rolled one. This result can be explained by the dispersion of stress concentration and the improved homogeneous deformation attributed to the fine grain structure and the existence of deformation texture for the extruded alloy.     

High temperature fracture experiments on tungsten–rhenium alloys

A big problem when using tungsten as plasma facing components in a future fusion reactor is the very low fracture toughness at low temperatures. Tungsten–rhenium alloys outclasses other tungsten-based materials in terms of increased ductility. We study the reason for this positive effect by investigating the influence of rhenium on the fracture process of tungsten–rhenium alloys at different temperatures (between room temperature and 900 °C). The experiments are performed in a furnace-equipped tensile testing machine with a vacuum chamber, which allows us to perform fracture experiments at elevated temperature without oxidizing the material. Antecedent and subsequent electron backscattered diffraction scans are used to analyse the extent of plastic deformation and the interaction of plastic deformation and the fracture process. Furthermore, the consequences of recrystallization on the fracture process of tungsten–rhenium alloys will be analysed.     

7050-T7452高强铝合金的开坯与锻造工艺研究

为了研究7050高强铝合金开坯工艺的变形均匀性及锻件的断裂韧性,采用数值模拟、微观组织分析及紧凑拉伸试样的KIC试验等,分析研究7050高强铝合金铸坯改锻和挤压坯锻造的变形均匀性、显微组织和断裂韧性,得到了两种开坯工艺条件下坯料的平均等效应变、变形均匀系数、晶粒尺寸大小及分布和断裂韧度值。结果表明:铸坯改锻工艺的变形均匀性较好,其平均等效应变约为7.2,变形均匀系数约为0.475;挤压坯锻造工艺的变形均匀性较差,其平均等效应变约为1.9,变形均匀系数约为0.954;铸坯改锻工艺的平均晶粒尺寸与挤压坯锻造工艺基本相当,约为7.5μm,但铸坯改锻工艺的晶粒尺寸比挤压坯锻造更均匀;铸坯改锻工艺下锻件的KIC满足AMS4108F标准要求,而挤压坯锻造工艺不满足,其原因是挤压坯锻造后脆性较大,并残留有挤压织构。     

改善颗粒增强金属基复合材料塑性和韧性的途径与机制

评述了影响颗粒增强金属基复合材料塑性和韧性的各种因素,在此基础上深入研究了颗粒形状对ＳｉＣｐ／ＬＤ２复合材料塑性和断裂韧性的影响规律。采用有限单元法分析不同形状的ＳｉＣ颗粒增强的ＬＤ２复合材料的微区力学环境和整体力学行为,结果表明颗粒的尖锐化导致基体内应变集中和颗粒尖端断裂的可能性加剧,因而降低材料的塑性;而在外加载荷的作用下,由于复合材料基体整体均处于较高的加工硬化状态,因此颗粒形状对材料断裂韧     

Ti3Al基合金的拉伸形变和断裂

测试了三种显微组织Ｔｉ３Ａｌ基合金的室、高温拉伸性能；并用ＳＥＭ和ＴＥＭ详细观察样品的形变和断裂特征，结果发现，材料的力学性能与断口和位错组态的变化密切相关，随实验温度升高，强度降低，延性增加；随固溶温度提高，强度增加，延性降低，三种组织室温拉伸均为解理断裂，温度呈现解理与沿晶混合断裂。     

Quantitative evaluation of fracture toughness-microstructural relationships in alpha-beta titanium alloys

The fracture toughness of two alpha-beta titanium alloys containing an alpha platelet in a transformed beta matrix has been examined in terms of the microstructural parameters controlling the fracture initiation and propagation in the alloys. Equations have been formulated that show that the highest toughness values of both alloys were associated with the finest platelet spacings and the thickest alpha platelets. It is proposed that the fracture initiation process in both alloys is controlled by the distance between the platelets, the fracture toughness of the alloys being dependent on the distance between active centers of void nucleation, i.e., as a function of the alpha platelet thickness and spacing between the platelets. Seven models of ductile fracture relating fracture toughness to mechanical property and microstructural parameters have been compared in their ability to predict the toughness of the alloys after solution treatments, which produce varying platelet thickness and inter-platelet spacings. The principle has been adopted following Rice and Rosengren and Hutchinson (HRR)^[1,2] that there must be a 1/x energy singularity at the crack tip, which also prescribes the stress and strain distribution ahead of a crack tip. Any model not incorporating these requirements should be rejected.     

表面机械研磨处理对热轧态Mg-Gd-Y合金微观组织和力学性能的影响

镁合金由于塑性变形能力较差,采用传统热轧工艺得到的晶粒组织往往较为粗大,导致力学性能不佳。本文采用表面机械研磨处理(SMAT)在热轧态Mg-Gd-Y合金中获得梯度组织,以提高合金的强韧性并揭示相关机理。热轧态合金在峰时效之后,强度显著上升但塑性急剧下降,这是因为沉淀相容易在粗晶内引起应力集中,诱发解理开裂、造成脆性断裂。SMAT试样在峰时效之后,强度相当、而塑性增加,显示了较好的强韧性。这是因为表面细晶层由于均匀变形能力较好导致延性断裂,阻碍了试样内部粗晶层解理开裂对试样截面的贯穿,从而抑制了早期开裂,使塑性得以提升。     

Environmental effect on mechanical properties of aluminide matrix composites

This paper investigates the validity of the toughness measurement with a variation of the loading rate for distinguishing the fracture mechanism of aluminide intermetallics and their composites. The ductility and fracture toughness of Ni₃Al alloys and their composites are governed by inherent grain boundary brittleness and moisture-induced embrittlement at ambient temperatures. Although B doping is effective in suppressing both factors, remarkable improvement of toughness mainly depends on grain boundary strengthening. The toughness of the alloys is influenced by the dislocation locking mechanism and the extrinsic embrittlement promoted by diffusion of oxygen at intermediate temperatures. Extrinsic embrittlement is the predominant mechanism in determining the toughness at 673 K. Restriction of the dislocation motion is the predominant factor in determining toughness at 873 and 1073 K. The composites reinforced with TiC particles exhibit exceptionally constant toughness at 300 to 900 K.     

Strength and ductility in TiAl alloys

Tensile behavior of two-phase TiAl alloys at room temperature (RT) is analyzed for duplex and lamellar microstructural forms. The Hall-Petch relationship with high constants in fully-lamellar material is explained as a combined function of grain-size and deformation-anisotropy. The low ductility and its inverse relationship with grain size are explained using the anisotropic tensile properties of lamellar structures and assuming that the fracture is controlled by the crack nucleation process involving the pile-ups of dislocations under shear stress. The crack initiation toughness and associated strains near the crack tip are used to explain the inverse relationship between ductility and toughness.     

粗大第二相及时效析出相对Al-Mg-Si合金延性断裂的耦合影响

针对Al-Mg-Si合金采用不同固溶处理和时效处理获得粗大第二相颗粒与析出相颗粒之间体积分数的相对变化,研究粗大第二相颗粒与析出相颗粒对合金断裂应变的耦合影响.结果表明:两种颗粒间含量的相对变化对Al-Mg-Si合金断裂应变的影响呈非单调性,粗大第二相颗粒含量较高的合金经较高温度时效时,其断裂应变值高于粗大第二相含量较低但经较低温度时效的合金的断裂应变值.根据强韧化效果相对变化对此试验现象进行分析讨论,采用多尺度断裂模型很好地模拟了该两相颗粒对断裂应变的耦合影响.     

High strength ductile Cu-base metallic glass

Usually, bulk metallic glasses exhibit strength values superior to conventional crystalline alloys, often combined with a large elastic limit and rather low Young's modulus. This combination of properties renders such alloys quite unique when compared to commercial materials. However, the major drawback for engineering applications is their limited room temperature ductility and toughness due to the localized deformation processes linked to shear banding, where high plastic deformation is accumulated in a very narrow region without contributing to macroscopic deformation, work hardening or yielding. In this work we report on a new class of metallic glass in a simple Cu-base alloy. Addition of 5 at.% Al increases the glass-forming ability of binary Cu₅₀Zr₅₀. The resulting Cu_47.5Zr_47.5Al₅ glass exhibits high strength (2265 MPa) together with large room temperature ductility up to 18%. After yielding a strong increase in the flow stress is observed during deformation. The structure of the metallic glass exhibits atomic-scale heterogeneities that enable easy nucleation and continuous multiplication of shear bands. The interaction and intersection of shear bands increases the flow stress of the material with further deformation, leading to a ‘work hardening’-like behavior and yields a continuous rotation of the shear angle up to fracture resulting in a high compressive ductility.     

Heat treatment-modulated coupling effect of multi-scale second-phase particles on the ductile fracture of aged aluminum alloys

Experiments and multi-scale modeling were carried out in order to study the heat treatment-modulated coupling effect of multi-scale second-phase particles on the ductile fracture of two typical kinds of heat-treatable aluminum alloys, i.e. an Al–Cu–Mg alloy and an Al–Mg–Si alloy. It was revealed experimentally and theoretically that an appropriate combination of the multi-scale second-phase particles, which could be achieved by appropriate cooperation of the heat treatment steps, i.e. the solution, quenching and aging treatments, is necessary and sufficient for obtaining an excellent fracture toughness for the heat-treatable aluminum alloys. The experimental phenomenon, that the alloys containing more detrimental constituents but aged at a somewhat higher temperature exhibit ductility and fracture toughness superior to those of the alloys containing less detrimental constituents but aged at lower temperatures, could be reasonably explained by the cooperative effect of the heat treatment steps. Contours of the fracture toughness with respect to the technological parameters of the heat treatment, e.g. the aging temperature and quench factor, were developed to show the cooperative effect of the heat treatment steps on the fracture toughness of the aged aluminum alloys quantitatively. The good agreement between the calculations and the experimental results indicated that the present modeling is applicable for describing the heat treatment-modulated coupling effect of the multi-scale second-phase particles in aged aluminum alloys.