Advanced Materials Research Vols. 15-17

Abstract: Copper powders with very dense, uniform silver coating layer were prepared by electroless plating method in aqueous system at room temperature. Silver-copper composite powders were obtained by strictly controlling the activation and complexing process variables such as NH4OH/(NH4)2SO4 molar ratio, tartrate concentration, and feeding rate of silver ion solution. The mechanism of composite powders formation and their characteristics were discussed. It is noted that completely cleansing the oxide layers and protecting the copper particles surface from hydrolysis are key factors to obtain high quality Ag-Cu composite powders.

Abstract: Tensile tests of epoxy molding compounds were carried out using specimens composed of epoxy molding compounds which are transfer molded and post cured. The mechanical characteristics of the epoxy molding compounds change significantly due to changes in temperature and strain rate. In addition, the effect of nonlinear viscosity is large in both elastic and plastic regions. The characteristics of the visco-elastic-plastic behaviors of the epoxy molding compounds were examined. The behavior characteristics of the epoxy molding compounds during loading and unloading were shown in detail.

Abstract: Gradient-dependent plasticity where a characteristic length is involved to consider the microstructural effect (interactions and interplaying among microstructures due to the heterogeneous texture) is introduced into Johnson-Cook model considering the effects of strain-hardening, thermal softening and strain rate sensitivity. Effects of initial static yield stress, strain-hardening coefficient and exponent, strain-rate and thermal-softening parameters on the occurrence of phase transformation and the thickness of phase transformed adiabatic shear band (ASB) in deformed ASB are numerically investigated. Higher initial static yield stress, strain-hardening coefficient, strain-rate parameter and lower strain-hardening exponent lead to earlier occurrence of phase transformation (lower plastic shear strain). Effect of thermal-softening parameter on plastic shear strain corresponding to the onset of phase transformation is not monotonous. Transformed ASB is located at the center of deformed ASB since the position has higher temperature exceeding the temperature of phase transformation. The thickness of transformed ASB increases with decreasing flow shear stress and the increasing tendency becomes slow. For the same flow shear stress, the thickness of transformed ASB is wider for higher initial static yield stress, strain-hardening coefficient and exponent, strain-rate and thermal-softening parameters. Compared with classical elastoplastic theory applicable to completely homogenous material, gradient-dependent plasticity considering the microstructural effect predicts that phase transformation occurs earlier and that the thickness of transformed ASB changes with flow shear stress.