DUCTILE JOINING OF BERYLLIUM TO COPPER

BERYLLIUM APPEARS TO BE THE CHOICE FOR PLASMA FACING COMPONENTS OF THE HIGH ENERGY FUSION REACTOR INTERNATIONAL EXPERIMENTAL THERMNUCLEAR REACTOR (ITER) DESIGN. CURRENT DESIGN HEAT FLUX CONDITIONS FOR THE ITER ARE FORMIDABLE AND MUST BE DEALT WITH SAFELY AND RELIABLY. COMPONENTS SUCH AS THE DIVERTOR, AS WELL AS OTHERS, MUST BE ACTIVELY COOLED. THIS REQUIREMENT MAKES THE DEVELOPMENT OF SUCCESSFUL METHODS FOR JOINING OF BERYLLIUM TO COPPER A CRITICAL ISSUE IN THE FUSION REACTOR DEVELOPMENT PROGRAMS. EVEN THOUGH VARIOUS APPROACHES HAVE BEEN ADVANCED FOR THE JOINING OF BERYLLIUM TO ITSELF AND TO OTHER METALS, THE JOINING TECHNOLOGY CURRENTLY ADVAILABLE CANNOT BE USED IN VIEW OF THE SPECIAL REQUIREMENTS IMPOSED BY THE FUSION REACTOR ENVIRONMENT. BERYLLIUM IS AN EXTREMELY REACTVE METAL AND FORMS BRITTLE INTERMETALLIC COMPOUNDS WHEN IN METALLURGICAL CONTACT WITH MOST METALS. THERE IS ALSO THE HIGH REACTIVITY WITH OXYGEN RESULTING IN THE DEVELOPMENT OF A TENACIOUS OXIDE SKIN THAT MAKES WETTING OR DIFFUSION BONDING VERY DIFFICULT WITHOUT EXTREMELY CAREFUL SURFACE PREPARATION TECHNIQUES. THE NEED FOR A DUCTILE JOINT IN THE CASE OF BRAZING BERYLLIUM CANNOT BE OVER EMPHASIZED. AN INNOVATIVE SURFACE TREATMENT PROCEDURE FOR BERYLLIUM IS PLANNED THAT WILL FACILITATE ITS BRAZING TO COPPER USING BRAZE ALLOYS WITH EITHER HIGH OR LOW MELTING TEMPERATURES. THE TECHNIQUE INVOLVES CONSIDERATIONS OF METALLURGICAL COMPATIBILITY AND THIN FILM DIFFUSION BARRIER CONCEPTS.