Creep Behavior Of Multilayer Structures And Creep Test On Tin-Lead Solder Under Mechanical Stress And Electric Current

Multilayer structures have been used extensively in many engineering areas such as mi-croelectronics, micromachinery, aeronautics and astronautics. Most current studies about the multilayer systems only focus on the linear elastic region and lack of studies on the mechani-cal behaviors of multilayer systems under high-temperature creep condition. This has been an obstacle for further development of multilayer systems. Aiming at the creep behaviors of mul-tilayer structure and materials, studies on the interfacial behaviors of multilayer structures, effects of creep deformation on the stress distributions in multilayer systems, interaction be-tween scale growth and creep deformation, tensile creep behavior of Sn63Pb37 alloy under the coupling effect of electric current and mechanical stress, have been systemically investi-gated in this contribution. The main contents and conclusions of this thesis are as follows:Based on the balance conditions of force and moment, a general theoretical solution for prediction stresses in multilayer systems with consideration of creep effect under residual stress and external bending has been developed. Results derived from the current model agree well with that derived from finite element analysis (FEA). Based on the current model, effects of creep on stresses distribution and redistribution in the multilayer structures have been sys-temically analyzed and effects of creep on the curvature evolution has been obtained. These provide a basis for creep strength design and analysis of multilayer structures.A model to simulate the scale growth process under an applied bending load has been presented. This model explores the interaction between creep deformation and scale growth process, and is able to predict the stress-driven scale growth rate, the creep response of the multilayer and the stress accumulation in the thin scale. Thus it is helpful for better under-standing the high-temperature oxidation behaviors of multilayer structures. Due to the shifting of bending axis (or neutral axis) can be avoided in the proposed model, it demonstrates a sig-nificant improvement compared with Limarga's model.Experiments on the creep behavior of Sn63Pb37 alloy under the coupling effect of elec-tric current and mechanical stress has been carried out. Results indicate that the electric cur-rent can accelerate the creep deformation of Sn63Pb37 alloy and the steady state creep strain rate and the creep activation energy of Sn63Pb37 alloy increases with increasing the electric current while the creep exponent decreases with increasing the electric current. Creep mecha- nism of Sn63Pb37 alloy is mainly affected by electromigration. Heat treatment will lead to the Pb phase coarsening while the effect of mechanical stress on the Pb phase coarsening is not very obvious. Under the coupling effect of mechanical stress and electric current, the migra-tion of Sn phase causes the concentration of Pb phase. The concentrated and coarsened Pb phase was drawn to form a threadlike dendritic structure.Finite element analysis on the interfacial stresses of bonded structures with consideration of creep-damage coupled effect has been given for the first time. Meanwhile, edge effect, ef-fect of thickness and effect of creep parameters on interfacial stresses distribution have also been discussed. Results show that the existed elastic closed-form solutions could not be used for predicting interfacial stresses distribution when creep occurs. Improved solution with con-sideration of creep effect should be given. The maximum damage generates at the edge of the interface where failure often occurs and the damage will increase with increasing the value of modulus ratio of the substrate to the film. The damage distribution along the interface is very similar to that of the peeling stress. The interfacial shear and peeling stresses derived from creep-damage analysis is the lowest than that derived from linear elastic and creep analysis.