Search

The effect of overheating on the morphological evolution of primary silicon in undercooled Al–70%Si-0.5%RE alloys was investigated by observing and recording their growth processes and then analyzing the as-solidified microstructure. Investigations revealed that an increase in the overheating and undercooling temperatures, transforms the primary silicon's growth morphology from dendrite with several long and thin branches to a blocky shape and then to equiaxed grains as the silicon atom clusters disappear. In addition, we observed that, in contrast to previously reported and theoretically predicted critical undercooling promoting morphology transition, the critical undercooling actually deceased the transformation from dendrite to blocky shape and then to equiaxed grains. • Highly undercooled Al–70%Si-0.5%RE alloys were studied under various overheating and undercooling temperatures. • Study the morphological evolution of primary silicon. • Study the morphological evolution of solid/liquid and recalescence interfaces. • Study the relationship of overheating and undercooling. • Study the critical undercooling for morphology transition. [ABSTRACT FROM AUTHOR]

The solidification of undercooled Ni–3.3 wt% B alloy was studied by high-speed video analysis and microstructural analysis. For moderate initial undercooling (ΔTp = 75 K), the solidification interface for primary phase transformation manifests a shape of a planar dendrite, and possesses an constant growth velocity, for eutectic transformation whereas the interface presents multi-dendrite shape and spasmodic growth, so that a constant velocity cannot describe the interface exactly. These differences suggest that primary phase solidification is controlled by far-distance diffusion while eutectic solidification by short-distance diffusion. For large initial undercooling (ΔTp = 262 K), a kinds of large “white dendrites”, which is in fact composed of multiple phases, were found in the microstructure, from inside to outside of which, the eutectic phase changes from dot phases (anomalous structure) to irregular eutectic and then to regular eutectic, indicating that the center of “white dendrites” may be the nucleation zone of eutectic reaction. [ABSTRACT FROM AUTHOR]

Abstract: By examining the morphologies of the recalescence interface and the growing crystal, direct evidence for silicon to grow in intermediary mode has been found, and the critical undercooling for silicon from lateral growth to intermediary growth ΔT ∗ and that from intermediary growth to continuous growth ΔT ∗∗ have been determined. A method that predicts the solid–liquid interface energy on the basis of the critical growth transition undercooling has been developed. The solid–liquid interface energy predicted from ΔT ∗ for silicon is in very good agreement with that from ΔT ∗∗. The percentage error between the solid and liquid interface energies predicted from ΔT ∗ and ΔT ∗∗ is smaller than 0.59% when the temperature is in the range 783–1683K. The results obtained for the solid–liquid interface energy predicted from the critical growth transition undercoolings for silicon are also consistent with the reported results from the nucleation method and the grain boundary method. Their percentage errors are in the range 0.24–4.55%. [Copyright &y& Elsevier]

The effects of crystal–liquid interface energy and undercooling on a crystal–liquid interface structure and the growth mode of undercooled semiconductors have been investigated. A factor to predict the structure of the solid–liquid interface has been identified. Criterions expressed as the critical nucleation undercoolings (ΔT*1 and ΔT*2) and kinetic undercoolings (ΔTK1* and ΔTK2*) to judge the transition of growth mode in undercooled semiconductor have been developed. The growth mode of a semiconductor is a lateral one when nucleation undercooling is lower than ΔT*1 or kinetic undercooling is lower than ΔTK1*, a continuous one when nucleation undercooling is higher than ΔT*2 or kinetic undercooling is higher than ΔTK2*, and an intermediary one in the nucleation undercooling region from ΔT*1 to ΔT*2 or kinetic undercooling region from ΔTK1* to ΔTK2*. The critical undercoolings for the growth transition predicted from the present criterions for silicon and germanium are in very good agreement with the experimentally measured results. [Copyright &y& Elsevier]

It is well known that cooling rate ( R c ), undercooling (Δ T ) and microstructure in solidification are not independent, but the simple relationships are rarely reported. We aim to present the relation of R c , Δ T and microstructure through solidification of iron melt by molecular dynamics simulation and classical nucleation theory (CNT). The results show that if R c ⩽ 10 11.97 K/s, the iron melt will solidify as bcc crystal structure; if 10 11.98 K/s ⩽ R c ⩽ 10 12.3 K/s, the mixed structure of crystalline and glass will be obtained; if R c > 10 12.3 K/s, the iron melt will solidify as amorphous structure. From the CNT, a relationship between R c and Δ T can be deduced, by which, the critical cooling rate for forming glass is predicted as 10 11.43±1 K/s that is near the simulation result ( R c > 10 11.97 K/s). [ABSTRACT FROM AUTHOR]