Processing of single crystals for high temperature applications

Refractory metals and intermetallic alloys are important in high temperatur e engineering systems, where the deformation must be maintained at a low le vel. This paper first reviews the work at Auburn University in the processi ng of these materials in a single crystalline form and will focus on two ap proaches. The first is based on a traditional float zone melting method wit h a focused electron beam as the heat source. This method requires the melt ing and solidification of the material in a controlled manner and is theref ore power intensive. This method was used to successfully process both sing le phase Ni3Al intermetallic compound and molybdenum based refractory metal s in the single crystalline form. Due to the complex Ni3Al phase diagram, s light variations in the compositions, resulted in significant difference in the growth characteristics. In the case of molybdenum alloys, the ease of single crystal formation depends on the selection of the solute. The growth characteristics and parameters for processing these alloys will be present ed and discussed. The second method used is a chemical vapor deposition (CV D) approach based on the reduction of selected chlorides and fluorides of t he refractory metals at significantly lower temperatures. In the latter app roach, refractory metal depositions were made using a flowing system. The r esults presented in this paper concentrate on the production of tungsten si ngle crystal layers using this technique. A comprehensive analysis for the effects of operating parameters on single crystal layer growth has been con ducted and optimized growth conditions have been achieved. Characterization of the processed materials was conducted using a variety of techniques inc luding X-ray diffraction (Laue back reflection) and electron microscopy wit h energy dispersive analysis. (C) 2001 Elsevier Science B.V. All rights res erved.