Plane front solidification of two phase peritectic tin–cadmium alloys: Part 1

In earlier studies, plane front solidification of binary peritectic alloys has typically produced a banded microstructure, resulting from the deposition of alternate layers of the primary α phase and peritectic β phase. The mechanism by which this structure is formed has not been satisfactorily clarified. In the present study, thin specimens of binary Sn–Cd peritectic alloys were horizontally solidified, unidirectionally, under very stable growth conditions at speeds of freezing R as low as 10 nm S⁻¹, with a temperature gradient G of about 2 K mm⁻¹, giving the high G/R necessary for plane front freezing. Steady plane front growth of Sn–2·58 wt-%Cd samples gave only α phase. Metastable growth of α phase was observed at temperatures as much as 7 K below the peritectic temperature, producing α phase having a composition well beyond its solubility limit at the peritectic temperature. Undercooling experiments showed that the β phase is nucleated at an undercooling of 1–2 K below the peritectic temperature in the presence of the a phase in this system. Extension of the α liquidus indicated that the observed metastable growth occurred at an undercooling of about 0·7 K below the stable β liquidus. Disruption of the steady growth conditions by, for example, a sudden acceleration in growth rate, caused the β phase to appear. It was concluded that metastable growth of the α phase can occur in this system for values of undercooling at the a phase/liquid interface of less than about 1 K. Introduction of the peritectic β phase is caused by irregularities in growth conditions or by the onset of cellular freezing, which is accompanied by local segregation, leading to local freezing at lower temperature and higher cadmium content.

MST/3174