Crystal-Growth Transition and Homogenous Nucleation Undercooling of Bismuth
- PDF / 3,088,259 Bytes
- 12 Pages / 593.972 x 792 pts Page_size
- 53 Downloads / 215 Views
TRODUCTION
TURNBULL and Cech carried out a notable experiment in 1950. In this experiment, 14 kinds of metals (Au, Ag, Cu, Fe, Pt, Co, Ni, Ge, Bi, etc.) were allowed to reach an undercooling of approximately 0.18 Tm (where Tm is the melting point).[1,2] On the basis of the experimental results, Turnbull[2] put forward a classical theory of nucleation, which points out that the homogenous nucleation undercooling for a metal is around 0.18 Tm. Over the past several decades, the classical nucleation theory has been the foundation of the solidification theory. To date, in many monographs and books that discuss solidification, the homogenous nucleation undercooling for a metal is taken as 0.18 Tm.[3,4] However, given improvements in undercooling techniques, the undercoolings achieved in metal melts have far exceeded 0.18 Tm. For example, the undercoolings of copper, germanium, tin, bismuth, and gallium have reached 0.26,[5] 0.34,[6] 0.37,[7] 0.41,[7] and 0.58 Tm,[7] respectively. These findings, as well as the principle that the homogenous nucleation undercooling should be greater than the heterogeneous nucleation undercooling in a given [1,2]
ZENGYUN JIAN and FANGE CHANG, Professors, and JI CHEN, Research Fellow, are with the School of Materials and Chemical Engineering, Xi’an Technological University, Xi’an 710032, People’s Republic of China. Contact e-mail: [email protected] WANQI JIE, Professor, is with the State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an 710072, People’s Republic of China. Manuscript submitted September 28, 2010. Article published online August 12, 2011 METALLURGICAL AND MATERIALS TRANSACTIONS A
sample, have cast doubt on the belief that the homogenous nucleation undercooling for a metal is 0.18 Tm.[7,8] On the other hand, several researchers have suspected that bismuth and gallium could be undercooled to 0.41 and 0.58 Tm, respectively. Therefore, the question as to what is the homogenous nucleation undercooling for a metal remains unanswered. Theoretically, the homogenous nucleation undercooling could be calculated according to the formula of the homogenous nucleation rate.[2–11] Except for the solidliquid interface energy, the parameters in the formula of the homogenous nucleation rate are available in handbooks. Thus, if the solid-liquid interface energy is known, the homogenous nucleation undercooling can be predicted. Over the past several decades, more and more attention has been paid to research on solid-liquid interface energies.[2,8–30] The common methods for measuring the solid-liquid interface energy include the nucleation-undercooling[2–4,9–11] and the grain-boundary methods.[12–20] The accuracy of the nucleation-undercooling method depends on the measured nucleation undercooling, that is, if homogenous nucleation occurs. However, no resolution has been reached as to how to determine the nucleation form. For the grain-boundary method, the solid-liquid interface energy is determined in terms of the curvature and the temperature measured in
Data Loading...
