The effect of microstructure on tensile properties, deformation mechanisms and fracture models of TG6 high temperature titanium alloy

The tensile properties at room temperature and 600 °C of TG6 titanium alloy with different microstructures {bi-modal microstructures with thick α lamella (BTL) and fine α lamella (BFL), and a mixed microstructure with different morphologies of α phase} were obtained. It was found that the BFL microstructure possessed the highest tensile strength, and the elongations of the BTL and BFL microstructures were almost the same of about 13% at room temperature and 17% at 600 °C, respectively. In addition, the mixed microstructure had the lowest plasticity. The tensile deformation mechanisms of α lamella (α_L), primary α phase (α_p), equiaxed α phase (α_e) and α colonies were researched by the analysis of respective dislocation morphologies. Notably, the accommodative deformations through grain/phase boundaries sliding determined the deformation models of α_L, α_p, and α_e. Compared to the thick α_L and α colony, the fine α_L and α colony activated more slip systems due to their excellent accommodative deformation capability. Furthermore the deformation mechanisms at room temperature and 600 °C were different from each other. Scanning electron microscope (SEM), energy-dispersive spectrometer (EDS) and transmission electron microscopy (TEM) were used to research the crack propagation paths and fracture models. Crack propagation path crossing α colonies and α_p were discussed, respectively. The colonies boundaries, α_p/colonies boundaries, α_e/α_e boundaries and silicide were found to be the stress concentration locations. The micro-plasticity of tensile specimens determined the fracture morphologies and fracture models.     

Simulation of transmission electron microscopy images during tensile fracture of metal foils

Based on computer simulations, we examined a new mechanism of plastic deformation that has been proposed to operate in tensile fracture of metal foils. We constructed a Au crystal containing high concentration of vacancies and/or one subjected to large elastic tensile strain using embedded atom method (EAM) potential, and then calculated transmission electron microscopy (TEM) images of the crystal based on multislice method. Randomly distributed vacancies did not cause a large distortion in the crystal lattice, and did not affect the TEM image intensity appreciably unless the vacancy concentration exceeded several percent. Under a large elastic tensile strain of about 10% along 100, a periodic displacement of atoms whose amplitude was 10% of the atomic distance was induced, reducing the intensity of equal thickness fringe by about half. At around 15% tensile strain along 110, the crystal transformed from fcc to bcc structure, releasing the distortion of crystal lattice.     

Deformation and fracture mechanisms of an annealing-tailored "bimodal" grain-structured Mg alloy

Through investigating and comparing the mechanical behavior of an as-rolled Mg-3%Al-1 %Zn(wt%) alloy before and after annealing treatments,it was revealed that the formation of annealing-tailored bimodal grain structure ensured the 330 ℃/4 h samples having a good combination of tensile strength and plasticity.Failure analysis demonstrated that for the as-rolled and 330 ℃/1 h samples with fine grain structure,their plastic deformation was mainly attributed to basal slips,whereas the deformation mechanism in the bimodal grain-structured samples was dominated by basal slips in fine grains and twinning in coarse grains.For the 330 ℃/8 h samples with coarse grain structure,high densities of twins were activated.Meanwhile,basal slips occurred in the twinned and un-twinned areas of coarse grains and could pass through twin boundaries.For differently treated samples,cracking preferentially occurred along slip bands,resulting in their transgranular fractures.     

Effects of Weaving Laser on Weld Microstructure and Crack for Al 6k21-T4 Alloy

For Al 6k21-T4 overlap weld joint,the shear-tensile strength by using the weaving laser was improved as compared to the case of linear laser.For the specimen of low strength,the porosity was distributed continuously along the intersection between the plates and fusion line.However,for the optimized welding condition,large oval-shaped porosities were located only in the advancing track of the concave part.Therefore,the continuity of cracks and porosities played a key role to determine the strength.And,the weaving width was also the important parameter to control the strength.Furthermore,the concave part had more significant hot and cold cracking in the weld and heat-affected zone(HAZ),respectively,than the convex part.     

Prediction of fracture strength in Al2O3/SiCp ceramic matrix nanocomposites

Based primarily on a recent publication [S.M. Choi, H. Awaji, Sci. Tech. Adv. Mater. 6 (2005) 2–10.], where the dislocations around the nano-sized particles in the intra-granular type of ceramic matrix nanocomposites (CMNCs) were modeled, dislocation activities in Al₂O₃/SiC_p CMNCs were discussed in relation to the processing conditions. The dislocations around the nano-sized particles, caused by the thermal mismatch between the ceramic matrix and nano-sized particles, were assumed to hold out the effect of Orowan-like strengthening, although the conventional Owowan loops induced by the movement of dislocations were unlikely in the ceramic matrix at room temperature. A model involving the yield strength of metal matrix nanocomposites (MMNCs), where the Owowan strengthening effect was taken into consideration, was thus modified and extended to predict the fracture strength of the intra-granular type of CMNCs without and with annealing. On the basis of the characteristics of dislocations in the CMNCs, the load-bearing effect and Orowan-like strengthening were considered before annealing, while the load-bearing effect and enhanced dislocation density strengthening were taken into account after annealing. The model prediction was found to be in agreement with the experimental data of Al₂O₃/SiC_p nanocomposites reported in the literature.     

球形孔泡沫铝合金三明治梁的三点弯曲变形

研究了球形孔泡沫铝合金的单轴压缩性能,得到了抗压强度与相对密度的关系;与多边形孔泡沫铝合金和泡沫纯铝作了对比,发现球形孔使力学性能有了较大的提高.根据球形孔泡沫铝合金三明治梁三点弯曲的载荷(P)位移(δ)曲线研究了四种常见破坏模式并建立了破坏模式图.用极限载荷公式得到的计算值与极限载荷的实验值吻合良好.球形孔泡沫铝合金力学性能高于多边形孔泡沫铝合金及泡沫纯铝,因而其三明治梁的力学性能最好.     

Determination of fracture energy and tensile cohesive strength in Mode I delamination of angle-ply laminated composites

The energy release rate in delamination of angle-ply laminated double cantilever composite beam specimens was calculated using the compliance equation, and interlaminar cohesive strengths were obtained. Instead of the traditional approach of a beam on an elastic foundation, a second-order shear-thickness deformation beam theory (SSTDBT) was considered. The equilibrium equations were obtained using the principle of minimum total potential energy and the system of ordinary differential equations were solved analytically. The problem was solved for [0°]₆ , [±30°]₅, and [±45°]₅ laminates with mid-plane delaminations and the results were verified using experimental evidence available in the literature.     

The fracture process in a fine-grained superplastic 7475 Al alloy

In order to contribute towards alloy design and therefore an improvement in fracture toughness of engineering materials in general, the effect of temperature, strain rate and strain level on the superplastic deformation, cavity nucleation and growth, and fracture behaviour are studied in an important rate-sensitive structural engineering material, 7475 Al, in the light of current models and thinking. The efficacy of hydrostatic pressure in reducing cavitation during superplastic deformation is considered.     

First-principles study of the tensile and fracture of the Al/TiN interface

Employing the density functional theory, we investigate the tensile and fracture processes of the Al/TiN(0 0 1) interface. The simulation presents directly the strain–stress relationship, the ideal tensile strength and the process of bond breaking of the system. Through the analysis of deformation, we find that the softer Al layers deform larger than the harder TiN layers during the tensile process. And fracture occurs between the interface and the sub-interface Al layers. In addition, the results show that during the tensile process, the ripple of the interfacial TiN layer decreases gradually with the increment of the strain. Charge transfer was detected from the Al to TiN layers near the interface area during the tensile process by means of charge density and density of states analyses. The charge transfer affects the fracture process. Compared to our previous study of the Al/TiN(1 1 1) interface, the Al/TiN(0 0 1) interface has smaller work of adhesion and larger tensile strength than the Al/TiN(1 1 1) interface. Our investigation shows that the fractures of the Al/TiN(0 0 1) and (1 1 1) interface systems both happen in the Al layers near the interface.     

原位反应喷射沉积7075铝合金的塑性变形机理

采用原位反应喷射沉积法制备TiCP/7075铝基复合材料,进行热模拟变形实验.研究了TiCP/7075铝基复合材料塑性变形后的微观组织与应力-应变曲线的变化规律,探索其塑性变形机理.结果表明:应力随变形温度的增加而下降,当变形温度达到400℃后,发生动态再结晶现象;变形温度达到450℃后,变形过程中的强化与软化效应保持平衡.     

The effect of precipitates on the mechanical properties of ZK60-Y alloy

This paper investigated the influence of the microstructure on the mechanical properties of the forged ZK60-Y alloy under different heat treatments (T₄, T₅ and T₆). The results show that the alloy at the direct artificial ageing treatment has superior strength and plasticity. X-ray analysis, SEM fracture observation and EDAX spectrum analysis reveal that bulk w-Mg₃Y₂Zn₃ and coarsened rod-like MgZn′(β^′₁) phases have great influence on the tensile properties of the alloy. Further investigations of the alloy aged at 180 °C, 200 °C and 225 °C for 0.5–100 h, respectively, suggest that the mechanical properties and the fracture characteristics are closely related with the variation of the rod-like β^′₁ precipitation. The fracture observation of the samples aged at 200 °C indicates that with the increase of the density and the size of the rod-like β^′₁ precipitation, more semi-cleavage planes will appear on the fracture surfaces.     

7B04铝合金超塑性变形行为

针对7B04铝合金开展了变形温度为470~530℃,应变速率为0.0003~0.01s~(-1)的高温超塑性拉伸实验,研究了材料的超塑性变形行为和变形机制。结果表明,7B04铝合金的流动应力随着变形温度的升高和应变速率的降低而逐渐减小,伸长率随之增加;在变形温度为530℃,应变速率为0.0003s~(-1)时,7B04铝合金的伸长率达到最大1105%,超塑性能最佳;应变速率敏感性指数m值均大于0.3,且随变形温度的升高而增加;在500~530℃的变形温度范围内,m值大于0.5,表明7B04铝合金超塑性变形以晶界滑动为主要变形机制;变形激活能Q为190kJ/mol,表明7B04铝合金的超塑性变形主要受晶内扩散控制;7B04铝合金超塑性变形中在晶界附近有液相产生,且适量的液相有利于提高材料的超塑性能。     

激冷非晶/纳米晶Al65Si35合金的显微组织

用单辊熔体激冷法制得Al65Si35（原子分数/％，下同）的铝硅合金薄带，厚度为0.02-0.03mm，宽度2mm。经过透射电子显微分析、能谱分析和X射线衍射分析。结果表明该合金的组织结构由纳米Si相、acc-Al纳米颗粒主非晶相组成。形成了纳米/非晶复合材料。合金的显微硬度达到HV230。     

Rapidly modifying microstructure and mechanical properties of AA7150 Al alloy processed with electropulsing treatment

Studies show that the proper solid solution treatment (SST) is a key step in the precipitation strength-ening of AA7150 Al alloy.Despite the superior characteristics of the fully dissolved phase,it has major drawbacks,including high consumption of energy and low efficiency.Recently,electropulsing treatment(EPT) has been proposed to study dissolved precipitations and modify microstructures of AA7150 Al alloy faster than conventional SST.Experiments have been conducted in the present article,and the obtained results show that EPT can promote the rapid dissolution of the rl'phase at relatively low temperatures in only 20 s.Meanwhile,the strength and ductility of electropulsed samples decrease drastically.Compared with conventional SST,EPT accelerates recrystallization and obtains relatively fine grains after 20 and 50 s electric pulses.Moreover,as the EPT time increases,the corresponding non-uniform local heating and the electron force promote dislocation generation and annihilation.     

Effect of solute atom concentration and precipitates on serrated flow in Mg-3Nd-Zn alloy

Influence of solute atom concentration and precipitates on serrated flow, i.e., Portevin-Le Chatelier effect,was studied in Mg~(-3) Nd-Zn alloy by tensile test at 250?C with a strain rate of 1 × 10~(-3) s~(-1). Microstructure and tensile property of the Mg~(-3) Nd-Zn alloy in solution and aging conditions were also investigated.Results indicate that the serrated flow was weakened with aging time, and geometry of the serrations changed from sharp to rounded corner. Through analyzing the mechanism of the interactions between dislocations and solute atoms, it was identified that the precipitates did not only weaken the serrated flow due to the decrease in the concentration of solute atom, but also regulate the serration type by restraining the movement of dislocations during high temperature deformation.     

Al—Si—Cu—Mg系合金W（AlxSi4Mg5Cu4）相与Al2Cu相的显示与鉴别

研究了Ａｌ－Ｓｉ－Ｃｕ－Ｍｇ系中富铜的Ｗ（ＡｌｘＳｉ４Ｍｇ５Ｃｕ４）相与Ａｌ２Ｃｕ相的金相显示鉴别法，并用化学染色试剂彩色显示区分铸造铝硅铜镁系合金（α＋Ｓｉ＋Ａｌ２Ｃｕ＋Ｗ）四元共晶中的Ｗ相及Ａｌ２Ｃｕ相。     

A procedure for superposing linear cohesive laws to represent multiple damage mechanisms in the fracture of composites

The relationships between a resistance curve (R-curve), the corresponding fracture process zone length, the shape of the traction-displacement softening law, and the propagation of fracture are examined in the context of the through-the-thickness fracture of composite laminates. A procedure for superposing linear cohesive laws to approximate an experimentally-determined R-curve is proposed. Simple equations are developed for determining the separation of the critical energy release rates and the strengths that define the independent contributions of each linear softening law in the superposition. The proposed procedure is demonstrated for the longitudinal fracture of a fiber-reinforced polymer-matrix composite. It is shown that the R-curve measured with a Compact Tension Specimen test cannot be predicted using a linear softening law, but can be reproduced by superposing two linear softening laws.     

Ti6A14V合金的低温超塑性拉伸变形行为

在700℃-850℃的温度范围内对Ti-6%Al-4%V(质量分数)合金板材进行超塑性拉伸试验,研究了应变速率为3×10-4-5×10-38-1条件下的拉伸变形行为.结果表明:Ti6A14V合金在空气中表现出良好的低温超塑性变形能力.在800℃初始应变速率ε=5×10-4s-1条件下,延伸率达到536%.在较低的700℃下变形(ε=5×10-4s-1),延伸率仍然超过了300%.在整个变形温度区间内,应变速率敏感性指数m均为0.3左右,最大值为0.63.在850℃变形激活能与晶界自扩散激活能十分相近,表明晶界扩散控制的晶界滑动是超塑性变形的主要机制.在700-750℃,变形激活能远大于晶界自扩散激活能,位错运动是激活能升高的原因.在800℃变形的激活能介于两者之间,表明随着温度的降低变形机制逐渐发生改变.     

Fatigue and fracture properties of thin metallic foils

Metallic thin foils are essential structural parts in microsystems,which may be subjected to fatigue loading caused by thermal fluctuations and mechanical vibrations influencing their reliability in numerous engineering applications. It is well known that the fatigue properties of bulk material cannot be adopted for small scaled structures. For a better understanding of the `size-effect' in the present investigation fatigue crack growth near threshold in the high cycle fatigue regime and associated fracture processes were studied. Free- standing rolled and electrodeposited Cu-, Mo- and Al foils of thickness from 20 m to 250 m in different conditions have been tested in a special experimental set up operating at R=–1 and a testing frequency of 20 kHz. At a given constant strain value the fatigue crack growth behaviour has been recorded accompanied by intermittent observation of the change of the dislocation structure in the vicinity of the growing crack by use of the electron channeling contrast imaging (ECCI)-technique in a scanning electron microscope (SEM). In a load shedding technique fatigue threshold stress intensity factor values have been derived and compared with data of bulk material. Typical crack growth features were detected depending on thickness and grain sizes of the foils. Various criteria (compliance, extent of plastic zones and plastic strain gradients) were selected for the explanation of this anomalous behaviour. Additionally fractomicrographs of uniaxial strained and fatigued foils have been studied to obtain further insight of the effect of dimensional constraint.