Effect of interfacial bonding strength on tensile ductility of multilayered steel composites

The effect of the bonding strength of the laminate interface on ductility in the tensile deformation of multilayered steel composites was investigated. Multilayered steel composites consisting of alternating layers of as-quenched martensitic and austenitic steels were prepared with various bonding strengths, ranging from weak bonding obtained by alpha-cyanoacrylate adhesive to strong bonding obtained by cold rolling with a subsequent heat treatment. Tensile tests and peel tests were conducted to investigate the relationship between tensile behavior and bonding strength at the interface. It was demonstrated that tensile ductility could be markedly enhanced as the bonding strength increased, and that three types of tensile fracture behavior were identified depending on the bonding strength of the interface.     

Effect of bonding variables on bonding mechanisms in press bonding superplastic 8090 aluminium alloy

Abstract

Press bonding was carried out to study the bonding ability of superplastic 8090 aluminium alloy sheet in air. Surface roughness, deformation, time, and microstructure were the bonding variables chosen, to investigate the effects of these parameters on the bonding mechanisms. The mechanisms involved in press bonding to eliminate voids along the bond interface were instantaneous deformation on loading, plastic flow during heating, and a diffusion dominant mechanism. The contributions of the bonding mechanisms to the bonding effect were highly dependent on the original status of the specimens and the bonding variables. Plastic flow during heating was the major effect during bonding. The bonding effect response to time was related to the surface roughness and evolution of the microstructure during bonding. The effect of surface roughness on bonding depended on the bonding mechanisms.     

高压扭转对SiC_P/Al复合材料微观组织和力学性能的影响

为研究大塑性变形对金属基复合材料微观组织和力学性能的影响,利用高压扭转工艺(HPT)在200℃下将纯Al粉末和经氧化处理的SiC粉末混合固结成10wt%SiC_P/Al复合材料。采用TEM观察HPT变形后不同圈数试样的SiC-Al界面及Al基体微观组织,采用EDS能谱仪分析界面处原子扩散现象,采用万能拉伸试验机测试研究不同扭转圈数试样的力学性能。结果表明:不同圈数试样Al基体内出现大量位错、非平衡晶界等晶格缺陷;组织内存在两种SiC-Al界面,含SiO_2层的原始界面和因颗粒破碎而新生成的界面。两种界面结合良好,界面处元素相互扩散;随着扭转圈数的增加,10wt%SiC_P/Al复合材料抗拉强度增加,延伸率得到较大提高。分析发现高压扭转后不同圈数组织内产生的大量晶格缺陷和细小晶粒,促进界面处元素的相互扩散,使界面结合良好,同时大量晶格缺陷和细小晶粒的产生以及结合良好的SiC-Al界面是SiC_P/Al复合材料力学性能大幅提升的主要原因。     

An investigation of grain boundary sliding in superplasticity at high elongations

Experiments were performed on the superplastic Zn-22% Al eutectoid alloy to determine the contribution of grain boundary sliding at both low (35%) and high (235%) elongations. The tests were conducted at two different strain rates in the superplastic Region II, and the results show that, within the accuracy of the measurements, there is a large sliding contribution at both elongations. By taking detailed measurements of both the magnitude of the sliding offset and the type of interface, it is shown that the average offsets are generally a maximum at the Zn-Zn boundaries, there is less sliding at the Zn-Al interfaces, and the offsets are a minimum at the Al-Al boundaries. In addition, the distributions of the magnitudes of the sliding offsets are similar at both the low and high elongations. It is concluded that grain boundary sliding is an important deformation process in the superplastic Region II and that it remains important even when the elongation is very high. The nature of the results indicates also that experimental observations of the deformation behaviour in superplastic materials at low elongations (up to 50%) provide meaningful information on the behaviour at much higher (superplastic) elongations.On leave from Mechanical Engineering Department, Nanjing Aeronautical Institute, Nanjing, Jiang-su 210002, People's Republic of China.     

Semi-analytical solution for orthotropic piezoelectric laminates in cylindrical bending with interfacial imperfections

The static and dynamic problems of an imperfectly bonded, orthotropic, piezoelectric laminate in cylindrical bending are investigated based on the equations of piezoelasticity. A general spring layer is adopted to model the bonding imperfection at the interfaces of the laminate. A recently proposed semi-analytical approach, which makes a hybrid use of the state-space method (SSM) and the differential quadrature method (DQM), is employed. This approach allows us to efficiently analyze the laminate with a large number of plies and with arbitrary boundary conditions at the two ends. Numerical examples are considered and discussed to show the efficiency of the present semi-analytical solution and the effects of relevant parameters on the behavior of the laminate.     

Investigation of the deformation behavior of samples with superplastic layers

The tensile deformation behavior of samples made of magnesium and titanium alloys with superplastic layer(s) was investigated. It was observed that deformation took place by means of layer-by-layer shear in the superplastic region. Traces of such shear were visible to the unaided eye on the surface of the deformed specimens. The spacing of these shear surfaces was about 6–8 grains. It is suggested that superplastic deformation proceeds by cooperative grain shear along two intersecting grain boundary systems oriented at approximately 45°–60° to the tensile axis. Using optical microscopy and back-scattered electron imaging on a scanning electron microscope, traces of shear systems were also observed on the prepolished surfaces of deformed magnesium alloy samples and the etched surfaces of deformed titanium alloy samples.     

A finite element analysis of fracture initiation in ductile/brittle periodically layered composites

A near-tip plane strain finite element analysis of a crack terminating at and normal to the interface in a laminate consisting of alternate brittle and ductile layers is conducted under mode-I loading. The studies are carried out for a system representing steel/alumina composite laminate. The Gurson constitutive model, which accounts for the ductile failure mechanisms of microvoid nucleation, growth and coalescence, is employed within the framework of small deformation plasticity theory. Evolution of plastic zone and damage in the ductile layer is monitored with increasing load. High plastic strain localization and microvoid damage accumulation are found to occur along the brittle/ductile interface at the crack-tip. Fracture initiation in the ductile phase is predicted and the conditions for crack renucleation in the brittle layer ahead of the crack are established for the system under consideration. Ductile fracture initiation has been found to occur before plasticity spreads in multiple ductile layers. Effects of material mismatch and yield strength on the plastic zone evolution are briefly discussed. This revised version was published online in August 2006 with corrections to the Cover Date.     

Interfacial microstructure and shear strength of Ti–6Al–4V/TiAl laminate composite sheet fabricated by hot packed rolling

A two layer Ti–6Al–4V(wt.%)/Ti–43Al–9V–Y(at.%) laminate composite sheet with a uniform interfacial microstructure and no discernible defects at the interfaces has been prepared by hot-pack rolling, and its interfacial microstructure and shear strength were characterized. Characterization of the interfacial microstructure shows that there was an interfacial region of uniform thickness of about 250 μm which consisted of two layers: Layer I on the TiAl side which was 80 μm thick and Layer II on the Ti–6Al–4V side which was 170 μm thick. The microstructure of Layer I consisted of massive γ phases, needlelike γ phases and B2 phase matrix, while the microstructure of Layer II consisted of α₂ phase. The microstructure of the interfacial region is the result of the interdiffusion of Ti element from Ti–6Al–4V alloy layer into the TiAl alloy layer and Al element from the TiAl alloy layer into the Ti–6Al–4V alloy layer. The shear strength measurement demonstrated that the bonding strength between the TiAl alloy and Ti–6Al–4V alloy layers in the laminate composite sheet was very high. This means that the quality of the interfacial bonding between the two layers achieved by the multi-path rolling is high, and the interface between the layers is very effective in transferring loading, causing significantly improved toughness and plasticity of the TiAl/Ti–6Al–4V laminate composite sheet.     

Particle characterization and cavity nucleation during superplastic deformation in Al-Li-based alloys

The role of intermetallic particles on the cavity formation during superplastic deformation has been studied in two aluminium-lithium-based alloys of identical chemical composition. They were, however, manufactured by two different routes — one by ingot metallurgy (IM) and the other by rapid solidification powder metallurgy (PM). A large number of particles of different shapes and sizes were found in both the alloys. In the IM alloy, particles were aligned in stringers in the direction of rolling. Iron and silicon, which were present as impurity elements, formed intermetallic phases in the IM alloy whereas only silicon-rich particles were found in the PM alloy. The particles of the PM alloy were of smaller size and were rather uniformly distributed in comparison to the IM alloy. During superplastic deformation, cavities first nucleated at the interface of large particles. The cavities of the IM alloy formed around the aligned stringers of large particles, whereas in the PM alloy they were uniformly distributed. It is shown that because both alloys contained particles of varying sizes, cavity nucleation occurred continuously during superplastic deformation.     

Generalized theoretical analysis method for free-edge delaminations in composite laminates

A simplified method for determining the individual mode components of the strain energy release rate of free-edge delaminations in composite laminates is proposed. Interlaminar stresses are evaluated as an interface moment and interface shear forces obtained from equilibrium equations of stress resultants at the interface between the adjacent layers. The deformation of edge-delaminated laminate is calculated by using a generalized quasi-three dimensional classical laminated plate theory developed by the authors. The analysis provides closed-form expressions for the Mode-I, Mode-II and Mode-III component of the strain energy release rate by combining the deformation of the edge-delaminated laminate with the interface moment and the interface shear forces. The presented method is compared with existing method suggested by Li for the asymmetry laminate. Comparison of the results with a finite element analysis using the virtual crack closure technique shows good agreement.     

SiC晶须增强铝基复合材料超塑性

采用高温拉伸、透射电镜、X射线衍射仪、差示扫描量热计和超塑性经典理论,对低压浸渗、小挤压和热轧制备的SiC晶须增强2024Al基复合材料超塑性的力学行为和变形机制进行了研究。研究表明:复合材料的晶粒细小,尺寸约为1 μm;在温度为788 K、初始应变速率为3.3×10-3s-1的拉伸条件下,超塑伸长率为370％;DSC曲线上有一小的初期熔化吸热峰,其温度相应于偏晶反应:Al+Al₂Cu+Cu₄Mg₅Si₄Al_<em>x→液相+Mg₂Si,785 K;超塑性变形的主导机制为传统的晶界扩散机制和适量液相共同控制的晶界(界面)滑动。     

Effect of interface bonding on the phase-transformation-aided magnetoelectric effect in ferromagnetic/ferroelectric composites

Based on modified constitutive equations and finite element method, calculations have been performed to study the effect of interface bonding on the phase-transition-aided magnetoelectric (ME) response in a new kind of NiMnGa/lead–zirconate–titanate (PZT) multiferroic laminate composites. The results quantitatively show that the ME effect is remarkably dependent on both the interface layer characteristics and the interface layer thickness. Stiffer and thinner interface layers are apt to produce higher ME effect. Calculations are in good agreement with available experimental results. Furthermore, the theoretical approach was improved to consider the enhancement in the magnetostriction of martensites induced by pre-applied opposing stress. Predictions reveal that the usage of single crystal Fe₇Pd₃ as ferromagnetic phase to form magnetoelectric composite with PZT can produce a high ME up to ~1 V/cm Oe.     

Role of particles on microstructure and mechanical properties of the severely processed 7075 aluminium alloy

The 7075-T6 Al alloy was processed by accumulative roll bonding (ARB) up to five passes by a 3:1 thickness reduction per pass at 300, 350 and 400 °C. Microscopical examinations revealed that the particle distribution varies with the processing parameters. As a consequence, the particle pinning effect decreases with increasing processing temperature and the number of ARB passes. For this reason, the evolution of grain structure with the number of passes is different for each processing temperature, and the superplastic properties are determined not only by the microstructural features before deformation, but also by the alloy thermal stability. The best superplastic properties are attained by the sample processed up to three passes at 300 °C, obtaining elongations to failure in excess of 200 %. Finally, ARB processing at high temperature leads to a decrease of hardness with respect to the peak-aged as-received alloy, especially with increasing number of passes and with decreasing processing temperature, due to the variation of the interparticle mean spacing and the amount of solute atoms.     

对称型陶瓷层状复合材料中的残余应力分析

针对由层间约束引起的层内残余应力，提出了用于描述层合材料应力应变状态的非均匀应变模型。利用非均匀应变模型推导出对称型层合材料由层间约束引起的层内残余应力的解析表达式，得到层内应力和界面应力沿长度方向分布的变化规律，指出轴向残余应力是层间界面剪应力造成的，是位置的函数；论证了由于表层材料受力的非对称性，界面必定存在正应力，且界面正应力须自平衡，界面正应力亦为长度方向上位置的函数，针对Si3N4-Si3N4/TiN-Si3N4三层及多层（2N＋1）对称型陶瓷基层状复合材料，研究了残余应力对强界面结合的层合材料宏观力学性能和裂纹扩展行为的影响和作用。结果表明，材料的宏观性能随着残余应力的变化而变化，其变化规律与理论计算的结果吻合。     

层状复合材料界面结合强度非传统评价方法

简要总结了层状复合材料界面结合强度的非传统评价方法。非传统界面结合强度评价方法包括:波振法(激光层裂法、应力波法、超声波法、电磁波法)、划痕法(激光划痕法)、辅助分析法(X射线衍射法、有限元法、解析法)和其他方法(电阻法)。波振法是将载荷以冲击波的形式施加在界面位置处,实现复层和基体分离。划痕法是将高能激光作用于复层,分离界面,实现界面结合强度的测量。电阻法通过建立界面电阻和界面强度之间的关系,评价界面结合强度。与传统界面评价方法对比,非传统评价方法有特殊优点:对材料的破坏程度小,有效抑制材料的弹塑性变形,测得界面强度接近界面本征强度。但非传统法也仍存在一些亟待解决的问题,随着新材料的不断出现,需要不断改进现有测试方法,使界面结合强度评价方法向着简单易行、无损、自动化的方向发展。     

Micro-mechanical finite element analysis of Z-pins under mixed-mode loading

This paper presents a three-dimensional micro-mechanical finite element (FE) modelling strategy for predicting the mixed-mode response of single Z-pins inserted in a composite laminate. The modelling approach is based upon a versatile ply-level mesh, which takes into account the significant micro-mechanical features of Z-pinned laminates. The effect of post-cure cool down is also considered in the approach. The Z-pin/laminate interface is modelled by cohesive elements and frictional contact. The progressive failure of the Z-pin is simulated considering shear-driven internal splitting, accounted for using cohesive elements, and tensile fibre failure, modelled using the Weibull’s criterion. The simulation strategy is calibrated and validated via experimental tests performed on single carbon/BMI Z-pins inserted in quasi-isotropic laminate. The effects of the bonding and friction at the Z-pin/laminate interface and the internal Z-pin splitting are discussed. The primary aim is to develop a robust numerical tool and guidelines for designing Z-pins with optimal bridging behaviour.     

Principles of superplastic diffusion bonding

Abstract

Superplastic diffusion bonding is now established as an important technique for solid state bonding. There are two types of superplastic diffusion bonding using either transformation or isothermal superplasticity. In this overview, the principles of these two types are summarised and the applications are discussed, including superplastic forming with concurrent diffusion bonding (the SPF-DB process), pressure welding, and the use of superplastic diffusion bonding in sintering.

MST/835     

The dynamic behavior of a surface-bonded piezoelectric actuator with a bonding layer

The performance of smart structures depends on the dynamic electromechanical behavior of piezoelectric actuators and the bonding condition along the interface, which connects the actuators and the host structures. This paper provides a theoretical study of the influence of material parameters of the bonding layer on the coupled electromechanical characteristics of piezoelectric actuators, which are subjected to high frequency electric loads. A one-dimensional actuator model with a bonding layer, which undergoes a shear deformation, is proposed. Analytical solutions based on the integral equation method are provided. Detailed numerical simulation is conducted to evaluate the effect of the bonding layer under different loading frequencies. The results indicate that the properties of the bonding layer, the loading frequency, the material combination and the geometry of the actuator have a significant effect on the load transfer between the actuator and the host medium.     

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.     

湿热环境对碳纤维增强聚苯硫醚层合板感应焊接接头性能的影响

以碳纤维增强聚苯硫醚(Carbon fiber reinforced polyphenylene sulfide, CF/PPS)复合材料层合板为研究对象,采用感应焊接方法对CF/PPS层板进行了焊接,重点研究了湿热环境对CF/PPS层板焊接接头性能的影响,实验结果表明:吸湿前后PPS树脂未发生化学变化;室温环境下,随着吸湿时间的增加,焊接接头剪切强度逐渐下降,与干态焊接接头相比分别降低了15%、18%、23%、32%和38%,不锈钢网-树脂基体-碳纤维界面处的湿应力不断增大,削弱了焊接接头界面的结合性能,影响了焊接接头的失效形式;120℃环境下,不同吸湿时间焊接接头剪切强度的下降率分别为12%、15%、22%、37%和44%,高温高湿使不锈钢网-树脂基体-碳纤维界面处热应力和湿应力增大,加剧了界面结合的损伤,界面脱粘成为焊接接头主要的失效形式。