Microstructure and viscosity of semi-solid mixtures

The semi-solid state of metallic alloys is characterized by the coexistence of solid and liquid phases at thermal equilibrium. The rheology of such di phasic materials not only depends on the solid fraction, hut also is highly correlated with the degree of aggregation of the solid particles. The visc osity eta of a diphasic (30% and 40% solid fractions) Ag-Cu alloy was measu red using a contactless method: at 923 degrees C, we get eta = (2 +/- 1) x 10(3) Pa s for a solid fraction of 30% and eta = (2 +/- 1) x 10(4) Pa s for a solid fraction of 40%. The solid, copper-rich phase, is dispersed into g lobules of about 50 mu m in size. The large value of eta suggests that thes e particles form a random interconnected network. This type of microstructu re is Likely to give rise to the thixotropic rheological properties of many semi-solid materials. The structure and therefore the viscosity are sensit ive to the applied shear rate (gamma) over dot. Two distinct regimes of agg regation, 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.