Refractory metal joining for first wall applications

The potential use of high temperature coolant (e.g. 900 degreesC He) in fir st wall structures would preclude the applicability of copper alloy heat si nk materials and refractory metals would be potential replacements. Brazing trials were conducted in order to examine techniques to join tungsten armo r to high tungsten (90-95 wt%) or molybdenum TZM heat sink materials. Palla dium-, nickel- and zirconium-based filler metals were investigated using br azing temperatures ranging from 1000 degreesC to 1275 degreesC. Palladium-n ickel and palladium-cobalt braze alloys were successful in producing genera lly sound metallurgical joints in tungsten alloy/tungsten couples, although there was an observed tendency for the pure tungsten armor material to exh ibit grain boundary cracking after bonding. The zirconium- and nickel-based filler metals produced defect-containing joints, specifically cracking and porosity, respectively. The palladium-nickel braze alloy produced sound jo ints in the Mo TZM/tungsten couple. :Substitution of a lanthanum oxide-cont aining, fine-grained tungsten material (for the pure tungsten) eliminated t he observed tungsten grain boundary cracking. (C) 2000 Elsevier Science B.V . All rights reserved.