ABSTRACT
In the early 1970s as Massachusetts Institute of Technology graduate student David Spenser was conducting experiments on hot tearing of metallic alloys, he observed that vigorously stirred Sn. l5wt%Pb had lower viscosity in the partially solidified (i.e., semi-solid) state than did the same material when it was allowed to cool without stirring (1]. Subsequent work showed that the shear stress in the semi-solid state can be orders of magnitude lower when continuous stirring is applied during solidification. Furthermore, Spenser noted that spherical grains developed in those specimens that were stirred during solidification, while an interlocking network of dendritic grains developed in the unstirred specimens. The spherical grains were found to be enriched alpha particles and were surrounded by a eutectic matrix. This structure is to be expected since the solid particles first formed during solidification would naturally solidify with an alpha-rich composition leaving behind a solute-rich liquid that upon further solidification would eventually reach the eutectic composition. Uniform distribution of the solid alpha particles results in the aforementioned billet microstructure. Subsequent work by a wide variety of researchers has shown similar behavior with other alloy systems [2-11}. Figure I shows typical microstructures for billets solidified in stirred and unstirred conditions. The term "semi-solid metal" (SSM*) has
been used to describe those partially molten metallic systems that have been intentionally produced with this globular microstructure.
