Beryllium–copper reactivity in an ITER joining environment

AbstractBeryllium–copper reactivity was studied using test parameters being considered for use in the ITER reactor. In this application, beryllium–copper tiles are produced using a low-temperature copper–copper diffusion bonding technique. Beryllium is joined to copper by first plating the beryllium with copper followed by diffusion bonding the electrodeposited (ED) copper to a wrought copper alloy (CuNiBe) at 450°C, 1–3 h using a hot isostatic press (HIP). In this bonded assembly, beryllium is the armor material and the CuNiBe alloy is the heat sink material. Interface temperatures in service are not expected to exceed 350°C. For this study, an ED copper–beryllium interface was subjected to diffusion bonding temperatures and times to study the reaction products. Beryllium–copper assemblies were subjected to 350, 450 and 550°C for times up to 200 h. Both BeCu and Be2Cu intermetallic phases were detected using scanning electron microscopy and quantitative microprobe analysis. Growth rates were determined experimentally for each phase and activation energies for formation were calculated. The activation energies were 66 mol and 62 kJ mol−1 for the BeCu and Be2Cu, respectively. Tensile bars were produced from assemblies consisting of coated beryllium (both sides) sandwiched between two blocks of Hycon-3. Tensile tests were conducted to evaluate the influence of these intermetallics on the bond strength. Failure occurred at the beryllium–copper interface at fracture strengths greater than 300 MPa for the room-temperature tests. At 300°C, the fracture strength was decreased significantly and, in contrast to the room-temperature tests, the fracture initiated in the copper–copper bond. The change in fracture initiation is attributed to a decrease in the residual stresses at the beryllium–copper interface at the higher temperatures and a decrease in the intrinsic fracture strength of the ED copper.