Faceted Dendrites Research Articles

Interface configurations during the directional growth of Salol—I. Morphology

The facet crystallography and morphological transitions in directionally grown transparent salol is studied. The facet cell morphologies are observed in situ in experiments where the velocity of growth is imposed and also during recalascence in the presence of a strong temperature gradient. It is found that the growth interface of salol is bounded by various combinations of (111) planes. The choice of a particular (111) plane is influenced by the root tip undercooling of the interface. At low undercooling values, facet cracking is observed. At a higher undercooling of the interface, the facet tip angle is noted to decrease while retaining the (111) planes which bound the interface. A model is presented for explaining the growth over a wide range of interface undercooling and the associated cracking phenomena. Stress assisted faceted dendrites are also noted during growth within glass cells which are separated by less than 12 microns. The origin of the dendrites and the emergence of the related spacings are discussed.

Growth Process of Primary Silicon in Al-Si Alloys / Wachstum von primären Siliziumkristallen in Al-Si-Legierungen

Samples of Al-20 Si alloy were solidified using different cooling rates and investigated by optical and scanning electron microscopy. The growth of the silicon crystals depends primarily on the silicon content of the liquid melt surrounding it. Faceted dendrites are formed. In the case of sufficient silicon atoms in the liquid, the dendrite branches coalesce to polyhedral crystals. If the growth is impeded due to a smaller quantity of silicon in the liquid, the crystals assume a spider cloth-like shape

Microstructures induced in germanium-rich GeAl films by laser-induced melting and rapid solidification

We report metastable phase formation in germanium-rich Ge 1−xAl x amorphous films ( x = 0.35 and 0.24) induced by laser melting. The results show that the nucleation phase for solidification is germanium and that germanium faceted crystals and dendrites are formed. The four known metastable crystalline phases of GeAl are observed and compared with those obtained by other rapid solidification techniques. For the range of compositions in the present study, the hexagonal phase is found to nucleate the rhombohedral phase and it is the first time that the orthorhombic phase is found as a consequence of a melting-solidification process.

Manufacture of amorphous GeSn alloys

R.f. sputter deposition has been used to manufacture amorphous Ge 1−xSn x fims with x upto 0.21 and 0.78 for substrates held at 300K and 77K respectively. Futher increases in Sn concentration cause two phase formation with an increasing size and quantity of β-Sn islands embedded in a fine scale polycrystalline or amorphous Ge matrix. A similar microstructure is formed in thermally evaporated films with x = 0.5. At high Sn concentrations, r.f. sputter deposited films consist of a fine scale polycrystalline extended solid solution of Ge in β-Sn. Melt spun ribbons of Ge 1−xSn x with x between 0.19–0.8 consist of faceted dendrites with β-Sn in the interdendritic regions.

Disorder in Al-Li-Cu and Al-Mn-Si Icosahedral Alloys

Faceted dendrites of icosahedral AL(6)Li(3)Cu have been studied by high-resolution x-ray scattering. The samples display long-range icosahedral symmetry both in their diffraction patterns and in their macroscopic morphology. Despite the appearance of well-defined facets, the samples have a high degree of atomic disorder. The Bragg peaks have symmetry-dependent shapes and widths that scale linearly with G perpendicular (phason momentum). The peak widths are surprisingly similar to those found in icosahedral Al-Mn-Si alloys in both their absolute magnitude and their dependence on G perpendicular. The origin of these features in models for the icosahedral phase is discussed.