Macrosegregation and sedimentation in liquid-phase sintering

Macrosegregation has been observed in a series of Pb-Sn alloys isothermally treated in an alpha + L (or beta + L) two-phase region. Depending on the alloy composition, the Pb-rich alpha (or the Sn-rich beta) solid phase quickly settles (or floats) to the bottom (or top) of the crucible as the alloy is heated into the solid-plus-liquid two-phase region. This settling or floating persists until such time as a skeletal solid structure is formed at the crucible bottom or top. The skeleton (the "mushy" zone) resembles liquid-phase-sintered structures; that is, it consists of interconnected solid and liquid phases. The initial settling can give rise to macrosegregation. Elimination of macro segregation requires substantial time and is accompanied by a reduction in length of the mushy zone, giving rise to apparent sedimentation of this zone. We argue that this sedimentation results from the compositional readjustments accompanying macrosegregation elimination. On this basis, it appears that many observations of sedimentation in liquid-phasc-sintered structures are due to these readjustments.