Faceting behaviour of primary phase in Ag-Bi alloys

Abstract

In the past the faceting behaviour of crystals, during their growth from the melt, has been the subject of extensive research. It is generally considered that factors which influence the structure of the solidliquid interface predominantly control the growth morphology of crystals. One of the first and simplest analyses of the interface structure of pure component was provided by Jackson [1]. The faceting behaviour of the interface was correlated using the concept of an a-factor or roughness parameter, which is a function of the entropy of fusion (AS f) of the component. It was shown that phases with a 16.8 Jmo1-1 K -1 exhibit faceted morphology during crystallization from their melt. According to this analysis most of the semiconductors or metalloids grow with faceted morphology, whereas common metals have non-faceted or dendritic growth features. The Jackson criteria for pure materials has been further extended by Kerr and Winegard [2] for the solid solution phase. These investigators used the entropy of solution (AS ~L) of the phase in the expression of the a-factor. Later, more general analysis by Jackson [3] suggested that the entropy of solution of the phase is indeed an important parameter, not only to control the faceting behaviour of crystals but also to determine the rate of crystal growth and its anisotropy. Although the a-factor criterion is adequate to explain most of the experimental observations [4], a few notable exceptions to this rule was observed in the cases of CuA12 [5] and phosphorus [6]. Even though these materials have a-factor <2, they still exhibit faceted growth morphology. In a recent publication Saroch et al. [7] proposed a thermodynamic model to emphasize the importance of composition and temperature dependence of entropy of solution (dASaL/dT) of the primary phase in the A1-Sn system to explain the faceting behaviour of the primary Al-rich phase. The origin of large values of this parameter was traced to the occurrence of retrograde solid solubility and nearly fiat liquidus lines in this system. Similar features are also observed in the phase diagram of the Ag-Bi system [8].