SOLID-STATE CRYSTAL-GROWTH OF REFRACTORY-METALS BY ARC-ZONE MELTING

Crystal growth by the arc-zone melting technique has been used over the past 25 years to prepare single crystals of the refractory metals and their alloys. During arc-zone melting only the upper surface of the metal is melted yet the entire cross section of the rod becomes single crystalline. The mechanism for growth of single crystals by this technique is not understood and a process parameter study was conducted to determine the operating mechanisms of crystal growth. It was found that crystal growth occurs in the solid state and is a result of large scale grain boundary migration. A minimum power input to the arc was required to prepare crystals and this was related to minimum temperature necessary for boundary migration. A series of quenching experiments indicates that boundary migration is driven by abnormal grain growth processes and is independent of the initial microstructure of the metal. The length and size of crystals prepared are determined by the number and frequency of boundary intersections with the advancing molten pool. There was no preferred orientation observed for the crystals prepared.