Papers by Author: G.P. Zhang

Abstract: Two different laminated composites with submicron-scale grain size and strong interface bonding toughness, Cu/Al and Cu/Cu, were fabricated by cold-roll bonding at ambient temperature, and then annealing of the laminated composites was conducted to get different interface bonding toughness. It was found that a better strength-plasticity combination for the laminated composites could be obtained through stronger interface bonding toughness, which effectively delayed the onset of plastic instability and premature local necking of the material. Uniform elongation of both Cu/Al and Cu/Cu laminated composites was enhanced compared with that of the cold-rolled Cu. At the same strength level, plasticity of the Cu/Cu laminated composite is better than that of the Cu/Al one and that of the cold-rolled Cu. Mechanisms of plasticity instability and fracture of the laminated composites were evaluated.

Abstract: A sort of biological shells (Saxidomus purpuratus), which belongs to Bivalve, was selected as the target material, and hardness and dynamic three point bending fatigue tests were conducted to examine its mechanical properties. Microhardness measurements showed that the inner layer is the hardest. The indentation on the specimen with a lower bending strength was damaged more seriously by the same load. Three point bending fatigue tests demonstrated that this kind of the shells with a special structure comprising mineral and organic matrix can experience the repeated loads instead of immediate breaking. The fatigue results on a single shell investigated here indicated that the fatigue strength is usually less than the static bending strength. Most of the fatigue lives of the specimens are less than 2105 cycles. In addition, fatigue fracture surfaces are observed by scanning electron microscopy.

Abstract: Deformation and fracture behaviors of Cu/Au and Cu/Cr multilayered composites with different length scales were investigated by using instrumented-indentation and three-point-bending methods. It is found that with decreasing the length scale (layer thickness and grain size), both multilayers tend to produce plastic instability via localized shear banding under indentation load in spite of high hardness they have, while quasi-brittle fracture under relatively low fracture stress prevails at three-point-bending test. Especially, the compressive flow stress and the tensile fracture stress exhibit inverse trend of variation with the length scales, which implies different mechanisms. Such length scale dependent deformation and plasticity were analyzed concerning size and interface effects under different stress state.

Abstract: Nano-scale Au/Cu multilayers were investigated by nano/microindentation. It was found that the hardness of the multilayers increases with decreasing individual layer thickness (λ), and shear band deformation can occur more easily in the multilayer with small λ. For comparison, the same experiments were also performed on Cr/Cu multilayers with the same layer structure. The results show that the Cr/Cu multilayer can be more effective in resisting shear band deformation than the Au/Cu multilayer. Finally, the λ dependence of shear band deformation and the difference between plastic deformation behaviors of the two multilayers were analyzed based on dislocation plasticity.

Abstract: An in-situ testing system for thermal-mechanical fatigue of thin metal lines was setup inside a dual-beam focused ion beam (FIB)/scanning electron microscope (SEM) system. Alternating currents (AC) were applied to narrow Au lines 200-nm-thick through nanomanipulator needles. Preliminary results show that severe thermal-mechanical fatigue damage can be generated by the action of the applied AC. The in-situ recording of the evolution of the damage has been carried out and the possible mechanism of the thermal-mechanical fatigue damage in the Au lines resulted from the joule heating was discussed.

Abstract: Microstructures of long-term serviced F12 steel exposed at 545 °C have been investigated by electron microscopes. The hardness of the material was measured to be correlated with the variation of the microstructures. Fatigue properties of the material with different running time were evaluated and analyzed. The experimental results show that the coarsening of the precipitated carbides along boundaries and the formation of subgrains accelerate the degradation of the long-term creep properties of the steel. Fatigue crack initiation threshold from a notch linearly deceases with increasing the running time due to the variation of the distribution and the shape of the precipitated carbides. The degradation mechanisms of the F12 steel during their long-term service at high temperature are discussed.

Abstract: Fatigue tests of nanometer-thick Cu films as deposited and annealed in vacuum were conducted under constant load ranges at room temperature. Fatigue strengths of the Cu films, which is defined as the critical load range being able to cause crack initiation within 106 cycles, are determined. The experimental results show that fatigue strength increases with decreasing film thickness. Fatigue cracking behaviors were characterized by electron microscope. It is also found that fatigue cracking resistance is dependent on film thickness and increases with decreasing film thickness. Size effects on fatigue properties of the nanometer-thick Cu films are discussed.

Abstract: Fatigue damage behaviour in micron and sub-micron thick Cu films has been investigated using focused ion beam (FIB) microscopy and transmission electron microscopy (TEM). The observations show that cyclic strain localization in the fatigued thin films is affected by the physical dimensions of the material, as evidenced by changes in the extrusion dimensions and by changes in the dislocation structures. The significant decrease in extrusion dimensions and the suppression of the development of bulk-like dislocation structures with decreasing film thickness and grain size is attributed to the strong inhibition of dislocation mobility and activity at small length scales.