Solidification of undercooled dilute tin-lead alloys

The rate of solidification of dilute tin-lead alloys has been measured as a function of the initial undercooling (up to 45°C) and the solute content (up to 2 wt pct lead). Solidified specimens were examined by metallography and X-ray diffraction to obtain information on the solidification process and the resulting grain structure. Over an intermediate range of undercoolings, it was found that dendrites grow in the tin-lead alloys as much as four times faster than in pure tin at the same undercooling. This result is inconsistent with any present theories for dendrite growth kinetics in binary alloys. At both lower and higher undercoolings there is no evidence for growth by simple extension of dendrites along the specimen, and solidification rate measurements made under these conditions are probably not indicative of normal dendrite growth kinetics. A. W. Urquhart and G. L. F. Powell were formerly at the Thayer School of Engineering.