Crystal-Growth Transition and Homogenous Nucleation Undercooling of Bismuth

Abstract

The cross-sectional and surface morphologies of highly undercooled bismuth samples are investigated by optical microscopy and scanning electron microscopy. It is found that the grain morphology can be classified into three types. When the undercooling is less than 49 K (49°C), flaky grains with pronounced edges and faces are arranged parallel to each other, showing the feature of lateral growth. When the undercooling is over 95 K (95°C), refined equiaxial grains with several smooth bulges on the surface of each grain are randomly arranged, showing the feature of continuous growth. In the undercooling region from 49 K to 95 K (49°C to 95°C), the features of both lateral and continuous growth are observed. The microstructures within the sample grains obtained at different undercooling regions are dissimilar, but they all show features of anisotropic growth. Based on the critical growth-transition undercoolings, direct expressions that express the relationship between the solid-liquid interface energy and temperature are determined. Homogenous nucleation undercooling is also predicted according to the solid-liquid interface energy obtained from the critical growth-transition undercooling. The predicted results of homogenous nucleation undercooling for bismuth are in good agreement with the experimental results.

Morphological Heredity of Intermetallic Nb5Si3 Dendrites in Hypereutectic Nb-Si Based Alloys via Non-Equilibrium Solidification

“…This can explain the direct formation of Nb 5 Si 3 dendrites in EBSM alloy, which is an indication of continuous growth. Such microstructural transition from faceted to non-faceted growth enabled by a larger kinetic undercooling has been observed in the solidification of some other materials with high entropy of fusion like primary Mg 2 Si in Al-Mg 2 Si alloys [15], silicon [34,35], bismuth [17], germanium [36] and Al 2 O 3 [37], etc. It is interesting to find that the rapidly solidified β-Nb 5 Si 3 phases can transform to the α-Nb 5 Si 3 phases via annealing due to its better thermal stability at room temperatures, indicated by the Nb-Si binary phase diagram [38].…”

“…For rapidly solidified alloys, the growth mode of thermodynamics-controlled faceted phases, may be altered concomitantly, e.g., the transition from faceted to nonfaceted growth of intermetallic phases with high entropy of fusion [14,15]. The mechanism behind this morphology transition is mainly due to the change in the solid/liquid interface structures and roughness [16,17]. Previous literature [6,12,18] has reported that beneficial microstructure modification of Nb-Si based alloys is enabled by rapid solidification, resulting in improvements of oxidation resistance and fracture toughness.…”