G. A. Colligan

The growth rate of dendrites in undercooled tin

AbstractThe dendrite growth velocity has been determined for tin in melts undercooled as much as 40°C (approximately twice the maximum undercooling reported previously). The results can be represented approximately asV = 0.8 (ΔT)2 WhereV is the growth velocity in mm s−1 and ΔT is the undercooling in degrees centrigrade.

Halo formation in eutectic alloy systems

Halo formation was studied in several faceted-nonfaceted and faceted-faceted eutectic systems. Ascribing halo formation to the amount of thermal, under cooling of the secondary phase in the presence of a primary phase is shown incorrect. Rather it is proposed that halo formation is due to growth rate phenomena of the phases involved.

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.

Nucleation in undercooled Bi-Te alloys

Thermal analysis of bulk samples of Bi-Te alloys as a function of compositional change from hypo-eutectic to hyper-eutectic composition has proved that the intermediate phase Bi7Te3 with a rhombohedral crystal structure is a very efficient nucleant for bismuth. This technique is a modification of an earlier one applied by the authors as a critical experiment to test Jackson’s objection to Sundquist and Mondolfo’s droplet technique for determination of nucleation catalysis. The modified method described in this paper is unique and eliminates another possible source of error in the determination of nucleation capacity in eutectic systems.

Solidification of Undercooled Sn-Bi and Pb-Sb Alloys

In hypoeutectic and eutectic alloys of Sn-Bi and Pb-Sb, the first phase to solidify from the melt is the nonfaceted phase, tin and load, respectively. Since primary tin and lead are poor nucleants for the second phase of the eutectic, the faceted phase, substantial undercooling below the equilibrium eutectic temperature occurs before the eutectic is nucleated. Once nucleated the faceted phase does not grow preferentially to return the melt composition to the equilibrium eutectic composition. Instead growth of a complex regular structure takes place from a melt whose composition is displaced from the equilibrium eutectic composition towards the faceted component. Reheating experiments have confirmed that the complex regular structure is rich in the faceted component. This structure grows at a temperature depressed several degrees below the equilibrium eutectic temperature. An hypothesis is presented to explain the growth of a structure rich in the faceted component at a depressed temperature.