Microstructure and viscosity of semi-solid mixtures

Abstract

The semi-solid state of metallic alloys is characterized by the coexistence of solid and liquid phases at thermal equilibrium. The rheology of such diphasic materials not only depends on the solid fraction, but also is highly correlated with the degree of aggregation of the solid particles. The viscosity of a diphasic (30% and 40% solid fractions) Ag-Cu alloy was measured using a contactless method: at 923 °C, we get = (2 ± 1) ×103 Pa s for a solid fraction of 30% and = (2 ± 1) ×104 Pa s for a solid fraction of 40%. The solid, copper-rich phase, is dispersed into globules of about 50 µm in size. The large value of suggests that these particles form a random interconnected network. This type of microstructure is likely to give rise to the thixotropic rheological properties of many semi-solid materials. The structure and therefore the viscosity are sensitive to the applied shear rate . Two distinct regimes of aggregation, depending on the actual value of the shear rate, are discussed: `compact clusters' versus `random network'. We calculate the characteristic size of the compact clusters and discuss the parameters governing the state of aggregation.