GROWTH OF MOLYBDENUM-BASED ALLOY SINGLE-CRYSTALS USING ELECTRON-BEAM ZONE-MELTING

The effects of scan rate on both the solute segregation and the crysta l growth characteristics in Mo-Nb, Mo-Hf and Mo-Hf-C alloys were exami ned in this study. Little difficulty was encountered when growing the Mo-Nb alloys. The ease of crystal growth in the Mo-Nb binary system wa s attributed to the small temperature difference between the solidus a nd the liquidus lines and the complete solid solution characteristics of the Mo-Nb system. A scan rate of 55 mm/h was found to be suitable f or producing satisfactory single crystals of Mo-Nb with up to 11%Nb. H owever, for the Mo-Hf alloys, the highest scan rates for single crysta l formation were found to depend on Hf content in an inverse manner. E xperimental results show that the critical maximum scan rates for the Mo-3.75%Hf, Mo-2.75%Hf and Mo-1%Hf alloys to be 11 and 22 and 55 mm/h, respectively. Optimum scan rates were determined based on the comprom ise between the ease of formation of single crystals and the extent of solute segregation.