铝硅合金超塑性研究

本文对 Al—13Si 共晶合金在超塑性变形时产生早期断裂的原因进行了研究。对影响Al—Si 共晶合金断裂的因素——第二相 Si 粒子形状、Si 粒子长大以及 Si 相与α相高温硬度差进行了详细分析。文章揭示出 Al—13Si 共晶合金的超塑性拉伸断裂是外部无颈缩的空洞型断裂。Si 粒子周围产生空洞是由于晶内位错堆积在 Si 粒子周围,造成应力集中,以及 Si 相与α相高温硬度相差悬殊,不能协调变形引起的。第二相 Si 粒子为带有尖角的短棒状,使得空洞沿尖角指向不均匀扩展,导致该合金发生早期断裂。     

Pb-Sn共晶合金超塑变形时晶粒三维重排物理模型的建立

利用动态定点观察和对界面易动性的测试,发现 Pb—62Sn 双相(α+β)合金的超塑性变形是借助晶粒三维重排和转动而实现晶粒沿拉伸轴方向重新排列的过程。由于α相和β相晶粒易动性的程度不同,导致各自的重排方式有很大差别。文中提出了一个双相合金超塑性变形晶粒三维重排模型。作者认为:在超塑性变形初期,晶界滑动主要是以晶界位错运动实现的,当晶界产生微空洞时,则由晶界位错运动和晶界空洞运动共同实现的;不同界面对晶界滑动的协调方式不同:β/β界面是以晶界迁移的方式协调,而α/β界面则以β相一侧晶界附近区域的塑性变形方式进行协调。     

QBe2铍青铜棒热处理断裂原因分析

通过宏观检验、化学成分分析,金相检验和超声波探伤等方法,对某批QBe2铍青铜棒在固溶处理出炉时个别发生断裂的原因进行了分析。结果表明:该QBe2铍青铜棒断裂主要是因为固溶处理炉局部温度过高,造成材料局部过烧、晶粒粗大、力学性能下降所致。     

30CrMnSiA钢超塑流变中的力学行为和显微组织的研究

经两次调质热处理细化的30CrMnSiA 钢在770℃以■=2.78×10~(-4)·S~(-1)的应变速率拉伸下呈现了良好的超塑性:δ=867%,σ=34.3MN/m~2,m=0.41。在拉伸时,经预处理的非平衡组织会经碎化而变成微细等轴的晶粒组织,并具有两相体积分数近似相等的α+v 双相组织。这种组织在超塑性流变过程中具有很高的稳定性。此钢对空洞的敏感性比较低,但在变形后期,由于空洞的形成、长大和连接而导致试样呈空洞型沿晶断裂。     

冷轧变形量对5 A02铝合金管材组织和性能的影响

在室温下,对热挤压态5A02铝合金管材进行四种不同工艺参数(A～D)的两道次冷轧,四种工艺均将尺寸为Φ65. 5 mm×5. 5 mm(外径×壁厚,下同)的坯料冷轧成尺寸为Φ58 mm×2. 5 mm的管材,总冷轧变形量为57. 6%。第一道次与第二道次冷轧变形量(%)分别是:A工艺(20. 2%、47. 3%),B工艺(34. 4%、35. 9%),C工艺(43. 6%、25. 4%),D工艺(52. 7%、11. 1%)。两道次冷轧后的管材均采用410℃/1 h完全退火处理。利用金相显微镜、扫描电镜(SEM)、能谱仪、电子万能试验机等对冷轧管材的显微组织、力学性能、拉伸断口形貌等进行分析,研究冷轧变形量对5A02铝合金管材组织与性能的影响。A～D工艺第一道次冷轧后晶粒与第二相逐渐变小,抗拉强度与断裂延伸率逐渐变大; D工艺第二道次冷轧后管材的晶粒最细小,晶粒尺寸在15～55μm,抗拉强度为186 MPa,断裂延伸率为29%; A～D工艺第二道次冷轧后第二相大小及分布均匀。结果表明:随着冷轧变形量逐渐变大,管材晶粒逐渐变小,力学性能变好;在总冷轧变形量相同的情况下,单道次变形量大的冷轧工艺退火后获得的晶粒更细小均匀,力学性能更好;第二相粒子的尺寸随着冷轧变形量的逐渐增大而变小,在总冷轧变形量相同的情况下,第二相粒子的尺寸也相同。     

Al-6Cu-0.5Zr合金超塑变形后微观结构的研究

相分析表明该合金由Al基固溶体和CuAl_2、ZrAl_3两种粒子所组成。合金超塑性变形后,利用标记线法测定了晶界滑移量对总变形量的贡献为60％左右。在δ≈30％的试样上观察到晶界变宽,在晶界上呈现折皱区,并在遇到第二相时改变方向。透射电镜分析表明,晶界滑移时出现晶界位错,在三晶交界处或晶界坎处向晶内激发位错,晶界是位错源与壑,激活的晶内位错通过滑移和攀移会形成位错亚晶界,晶内位错的激活与运动是晶界滑移的重要协调机制,晶界滑移与晶界位错运动有关。合金超塑性变形时,在晶界和CuAl_2相界处有空洞形成,研究了空洞面积分数与面缩率的关系。靠近断口处,空洞数和面积分数急剧增加,说明空洞的增殖、扩展和连接导致断裂。     

预变形对2050铝锂合金晶粒细化及超塑性的影响

通过形变热处理工艺制备2050铝锂合金细晶板材,采用光学显微镜、扫描电镜等研究预变形对第二相分布、晶粒组织及板材超塑性的影响。结果表明:采用预变形后,高温过时效过程中板材晶内形成大量亚晶,大量的亚晶界促进了T_B相的析出同时提高了粗化速率,显著增加了晶内T_B相的尺寸,使得有效激发再结晶形核第二相粒子体积分数由0.92%提高至3.28%。同时与未预变形板材相比,板材中心层平均晶粒尺寸由12.59μm降低至9.59μm,表层平均晶粒尺寸由10.79μm降低至8.60μm,晶粒细化效果得到明显改善,超塑性变形能力显著提升,在490℃,2×10~(-4)s~(-1)的变形条件下,伸长率由230%提高至470%。     

辐照后A508-3钢冲击功异常试样的显微组织研究

采用带屏蔽的光学显微镜(OM)、扫描电子显微镜(SEM)和能谱仪(EDS)等手段对中子辐照后国产A508-3钢冲击功异常试样的冲击断口形貌、显微组织、晶粒尺寸、孔洞和第二相夹杂物进行了观察和分析,并探讨了产生异常的可能原因。结果表明,在本次辐照实验条件下(中子注量2.97×1019 n/cm2,辐照温度(290±15)℃,国产A508-3钢试样辐照前后的显微组织均为贝氏体组织,且晶粒尺寸未见明显变化,不是导致冲击功产生异常的主要原因,其直接原因可能是基体组织中体缺陷(孔洞)数量的差异;组织中的孔洞分为两种类型,一种为填充了Al2O3、MnS、Al-Mg-O三元化合物等层片状复合第二相粒子的孔洞,另一种为未填充任何第二相的空洞,且孔洞中第二相与基体结合较差,易导致材料韧性降低。     

超塑性变形微观结构变化的研究

本文研究了A1-10Si-1Mg合金在超塑性拉伸过程中微观组织的变化。揭示出该合金在超塑性变形中随应变量增加,晶内位错密度增加,是由两方面原因造成的:一是晶粒长大和晶界上第二相粒子给晶界滑移造成困难,使晶内位错调节作用增强;二是晶粒内的第二相粒子阻碍了晶内位错运动使位错堆积在第二相粒子周围。     

30CrMnSiA钢超塑性变形中的机理问题

利用扫描电镜和透射电镜研究了30CrMnSiA钢超塑性变形中组织结构变化。结果表明,变形中合金元素的扩散导致横向晶界的宽化,并且富集了Si、Cr、Mn元素。三角晶界上呈现的显微空洞宏观调节了晶粒的三维重排过程。未溶碳化物与晶格位错、晶界以及晶界位错之间有相互作用关系。扩散和位错运动微观调节了晶界滑动,并导致它的发展。     

Fracture behaviour of Cu-Zn-Co and Cu-Zn-Ni alloys during superplastic deformation

Cavitation and fracture behaviour in two commercial /gb brasses, one modified with 2wt% Co (Cu-Zn-Co) and the other with 2wt% Cr (Cu-Zn-Cr), have been investigated in Region II of superplastic flow. These alloying elements form cobalt-rich (0.3 m average diameter) and chromium-rich (5 m average diameter) precipitate particles which are distributed uniformly in the matrix and which play an important role in cavitation and inhibiting grain growth during deformation. Void size distributions, volume fraction of voids and the number of voids per unit area have been measured as a function of strain in Region II and the results show a very marked difference in the degree of cavitation in Cu-Zn-Co and Cu-Zn-Cr alloys. Experiments show that the deformation is quasi-uniform with little or no necking in the specimens of Cu-Zn-Co alloy in Region II, and the final fracture occurred by the growth and interlinkage of internal voids. On the other hand, in the specimens of Cu-Zn-Cr alloy a sharp or localized neck developed early in the deformation in Region II and the specimen pulled down to a fine point leading to failure by necking. The importance of diffusion or slip accommodation of grain boundary sliding in void formation during superplastic flow is discussed and a criterion for failure is suggested.     

Ductile rupture in thin sheets of two grades of 2024 aluminum alloy

The damage and rupture mechanisms of thin sheets of 2024 aluminum alloy (Al containing Cu, Mn, and Mg elements) are investigated. Two grades are studied: a standard alloy and a high damage tolerance alloy. The microstructure of each material is characterized to obtain the second phase volume content, the dimensions of particles and the initial void volume fraction. The largest particles consist of intermetallics. Mechanical tests are carried out on flat specimens including U-notched (with various notch radii), sharply V-notched and smooth tensile samples. Stable crack growth was studied using “Kahn samples” and pre-cracked large center-cracked tension panels M(T). The macroscopic fracture surface of the different specimens is observed using scanning electron microscopy. Smooth and moderately notched samples exhibit a slant fracture surface, which has an angle of about 45° with respect to the loading direction. With increasing notch severity, the fracture mode changes significantly. Failure initiates at the notch root in a small triangular region perpendicular to the loading direction. Outside this zone, slant fracture is observed. Microscopic observations show two failure micromechanisms. Primary voids are first initiated at intermetallic particles in both cases. In flat regions, i.e. near the notch root of severely notched samples, void growth is promoted and final rupture is caused by “internal necking” between the large cavities. In slanted regions these voids tend to coalesce rapidly according to a “void sheet mechanism” which leads to the formation of smaller secondary voids in the ligaments between the primary voids. These observations can be interpreted using finite element simulations. In particular, it is shown that crack growth occurs under plane strain conditions along the propagation direction.     

Cavitation in alloy steels during superplastic deformation

A study of cavitation during superplastic tensile straining of two microduplex steels has been made using density measurements and quantitative optical metallography. The steels were of basically similar composition with the exception of a trace addition of boron made to one alloy. During deformation cavities formedα/γ boundaries and matrix-carbide interfaces; the growth and coalescence of these cavities led to failure. Density measurements showed that the extent of cavitation increased with increasing strain and decreasing strain-rate, but the level of cavitation was reduced by the presence of boron. A time dependence of overall void volume of 1.4 to 2.0 was observed. Quantitative metallographic studies of the nucleation and growth contributions to the overall rate of void formation showed that boron inhibited each of these processeS. However, both the nucleation rate and the magnitude of the time exponent of void volume increase suggested that a substantial number of voids grew from pre-existing nuclei which were probably present as non-coherent carbide-matrix interfaces.     

Low-temperature superplasticity of β-stabilized Ti-43Al-9V-Y alloy sheet with bimodal γ-grain-size distribution

The superplasticity of Ti-43Al-9V-0.2Y alloy sheet hot-rolled at 1100 ℃ was systematically investigated in the temperature range of 750-900 ℃ under an initial strain rate of 10-4 s-1.A bimodal γ grain-distribution microstructure of TiA1 alloy sheet,with abundant nano-scale or sub-micron γ laths embed-ded inside β matrix,exhibits an impressive superplastic behaviour.This inhomogeneous microstructure shows low-temperature superplasticity with a strain-rate sensitivity exponent of m =0.27 at 800 ℃,which is the lowest temperature of superplastic deformation for TiAl alloys attained so far.The maximum elongation reaches ～360％ at 900 ℃ with an initial strain rate of 2.0 × 10-4 s-1.To elucidate the softening mechanism of the disordered β phase during superplastic deformation,the changes of phase composi-tion were investigated up to 1000 ℃ using in situ high-temperature X-ray diffraction (XRD) in this study.The results indicate that β phase does not undergo the transformation from an ordered L20 structure to a disordered A2 structure and cannot coordinate superplastic deformation as a lubricant.Based on the microstructural evolution and occurrence of both y and β dynamic recrystallization (DR) after tensile tests as characterized with electron backscatter diffraction (EBSD),the superplastic deformation mecha-nism can be explained by the combination of DR and grain boundary slipping (GBS).In the early stage of superplastic deformation,DR is an important coordination mechanism as associated with the reduced cavitation and dislocation density with increasing tensile temperature.Sufficient DR can relieve stress concentration arising from dislocation piling-up at grain boundaries through the fragmentation from the original coarse structures into the fine equiaxed ones due to recrystallization,which further effectively suppresses apparent grain growth during superplastic deformation.At the late stage of superplastic de-formation,these equiaxed grains make GBS prevalent,which can effectively avoid intergranular cracking and is conducive to the further improvement in elongation.This study advances the understanding of the superplastic deformation mechanism of intermetallic TiAl alloy.     

Einfluss der Al2Cu‐Phase auf die Superplastizität der AlCuMn Legierung

Influence of the Al₂Cu‐phase on the superplasticity of AlCuMn alloy High‐temperature creep‐resistant AlCuMn wrought alloy has been investigated and optimised with respect to their superplastic deformability; a maximal elongation ε of 850 per cent was thus attained at a deformation temperature of 530°C. Prerequisites for superplastic deformation behaviour and for the associated high elongation values of these aluminium alloys are an especially fine‐grained structure as well as a decrease in the amount of Al₂Cu phase and a uniform distribution of this phase in the structure. Superplastic deformation (SPD) results in a pronounced change in the shape of the large particles of the θ‐phase; the particles of this phase thereby form veins along the boundaries of adjacent grains. During deformation, the grains lose their equiaxial shape and elongate in the direction of tension as a result of pronounced intragranular sliding dislocation in the microstructure. Transmission electron micrographs of the deformed structure have revealed a pile‐up of dislocations in the grains of the aluminium alloy. The grain size of commercially available sheets of AlCuMn wrought alloys with a thickness of 1 mm is approximately 30 μm. After optimising, the grain size of the sheets produced by the new method was on 12 μm until 5 μm. The new technique differs only slightly from industrial manufacture.     

Fractal nature and quantitative evaluation of microstructures in metallic materials

The fractal nature of microstructures was investigated using metallic materials containing second-phase particles, grain-boundary reaction (GBR) nodules or creep voids. The area fraction of the precipitates or the creep voids in the specimens was correlated with the scale of the analysis. The microstructures of these specimens exhibited a fractal nature between the lower and the upper critical scales, and could be regarded as the aggregate of the unit pattern with the size of the upper critical scale. The fractal dimension of a given microstructure was generally larger in specimens containing a larger area fraction of the second phase. The lower critical scale was close to the average size of second-phase particles or GBR nodules or the size of a large creep void. The upper critical scale, above which the area fraction of the precipitates or the creep voids did not show a scale dependence, was generally much larger than the average size and the average spacing of the precipitates, but it was almost the same as or a fair degree smaller than the grain size in specimens containing the second-phase particles or the GBR nodules. In the creep-ruptured specimens, the upper critical scale was much larger than the initial grain size and the grain size at rupture. The true area fraction of the second phase or the creep voids corresponding to the upper critical scale was also obtained.     

Dynamic Tensile Fracture Behaviours of Selected Aluminum Alloys under Various Loading Conditions

Experiments investigating dynamic tensile fracture were performed on the extruded rods of 2024‐T4 and 7075‐T6 aluminum alloys under varying loading conditions. The initial yield stress and fracture strain of 7075‐T6 alloy obtained in spilt Hopkinson tension bar tests are higher than that of 2024‐T4 alloy. But the initiation fracture toughness and spall strength of 2024‐T4 alloy are higher than those of 7075‐T6 alloy in three‐point bending and plate impact experiments, which indicates that 2024‐T4 alloy has better crack initiation tolerance and stronger spall failure resistance. Based on metallurgical investigations by using optical and scanning electron microscopes, it is revealed that the microstructure has a profound effect on the dynamic tensile fracture mechanism of each aluminum alloy. The 2024‐T4 alloy is relatively brittle due to voids or cracks nucleated at many coherent CuMgAl₂ precipitate phases in the grain interiors, and the fracture mode is predominantly transgranular. The 7075‐T6 alloy exhibits relatively ductile fracture because voids or cracks growth is partly intergranular along the grain boundaries and partly transgranular by void formation around coarse intermetallic particles. The obvious differences of damage distribution and void coalescence mechanisms for 2024‐T4 and 7075‐T6 alloys under plate impact are also discussed.     

异步轧制AZ31镁合金板材的超塑性工艺及变形机制

经过异步轧制工艺获得AZ31镁合金薄板。在300~450℃范围内,分别通过5×10-3,1×10-3s-1和5×10-4s-1不同应变速率进行高温拉伸实验研究其超塑性变形行为,计算应变速率敏感指数m值、超塑性变形激活能Q及门槛应力σ0值。通过EBSD分析和扫描电镜观察拉伸断裂后的断口形貌,分析AZ31镁合金的超塑性变形机制。结果表明:AZ31镁合金的塑性变形能力随着变形温度的升高及应变速率的降低而增强。当拉伸温度为400℃、m=0.72、应变速率为5×10-4s-1时,AZ31具有良好的超塑性,伸长率最大为206%。温度为400℃时,异步轧制AZ31镁合金的超塑性变形是以晶格扩散控制的晶界滑移和基面滑移共同完成的。