Abstract
The changes that accompany the plastic deformation of pewter have been studied by microscopical, thermal and analytical techniques in addition to the measurement of mechanical properties. It has been found that deformation facilitates the decomposition of the supersaturated tin based solid solution formed on casting, with the consequent precipitation of the beta phase (SbSn). Recrystallisation occurs in pewter alloys at room temperature, and the fine recrystallised grain size is stabilised by the fine dispersion of SbSn. At room temperature and above it is shown that rolled pewter with a sufficiently small grain size can deform at least in part by a superplastic mechanism, which involves grain boundary sliding, at a lower stress than would otherwise be necessary for slip on a macroscopic scale. The softening of rolled pewter as measured in a conventional, hardness test is found to be a direct consequence of the increased creep rate due to grain boundary sliding during the indentation period. This has been confirmed by tensile testing at low strain rates where rolled pewter is shown to have a lower strength than cast pewter of the same composition. At high rates of testing this loss of strength is not shown as the grain boundary sliding mechanism is unable to make any significant contribution to deformation. Alternative alloy compositions have been developed that do not show such severe softening as conventional pewters at large rolling reductions.