Processing of single crystals for high temperature applications

Refractory metals and intermetallic alloys are important in high temperature engineering systems, where the deformation must be maintained at a low level. This paper first reviews the work at Auburn University in the processing of these materials in a single crystalline form and will focus on two approaches. The first is based on a traditional float zone melting method with a focused electron beam as the heat source. This method requires the melting and solidification of the material in a controlled manner and is therefore power intensive. This method was used to successfully process both single phase Ni3Al intermetallic compound and molybdenum based refractory metals in the single crystalline form. Due to the complex Ni3Al phase diagram, slight 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 presented and discussed. The second method used is a chemical vapor deposition (CVD) approach based on the reduction of selected chlorides and fluorides of the refractory metals at significantly lower temperatures. In the latter approach, refractory metal depositions were made using a flowing system. The results presented in this paper concentrate on the production of tungsten single crystal layers using this technique. A comprehensive analysis for the effects of operating parameters on single crystal layer growth has been conducted and optimized growth conditions have been achieved. Characterization of the processed materials was conducted using a variety of techniques including X-ray diffraction (Laue back reflection) and electron microscopy with energy dispersive analysis.