Compression of semi-solid dendritic Sn-Pb alloys at low strain rates

Abstract

The behavior of semi-solid dendritic Sn-Pb alloys was studied at one degree above the eutectic. Small cylindrical samples were deformed at an initial strain rate of 1.3 x 10⁻² s⁻¹ in a parallel-plate apparatus. The friction between the sample and the plates was found to affect strongly both the strength of the material in compression and the resultant liquid-solid segregation. For low friction a maximum stress occurred at strains of about 0.3. Above this strain large cracks were observed. High friction resulted in a much higher degree of segregation than observed for low friction. No maximum stress and no cracking was observed, even for strains as large as 1.2 for high friction. Cylindrical samples were extruded through cylindrical dies at constant piston velocities ranging from 8.5 x 10≓5 ms~’ to 8.5 X 10≓4 ms≓1, and with reduction of area ranging from 2:1 to 8:1. The deformation occurred in two distinct modes. First, a “compaction≓ mode, during which liquid was expelled and the solid compacted but did not flow through the die, under increasing stress. Second, a “flow≓ mode, during which compacted solid flowed through the die, under a constant stress, σ_rextr, which was found to be proportional to the natural logarithm of the area reduction. Experiments involving compression over a filter and compression between parallel plates of alloys of different compositions were performed to examine the effects of the fraction liquid on the rheology of semi-solid dendritic alloys. The stress required for deformation was seen to exhibit the same pseudo-plastic strain-rate dependence for parallel plate, piston-filter and in forward extrusion experiments. This strain-rate dependence can be summarized by the power-law expression: σ ∝ ɛ^0.23.