HIGH-PERFORMANCE DISPERSION-STRENGTHENED CU-8 CR-4 NB ALLOY

A new high-temperature-strength, high-conductivity Cu-Cr-Nb alloy with a Cr:Nb ratio of 2:1 was developed to achieve improved performance and durability. The Cu-X Cr-4 No alloy studied has demonstrated remarkable thermal and microstructural stability after long exposures at temperatures up to 0.98 T-m. This stability was mainly attributed to the slow coarsening kinetics of the Cr2Nb precipitates present in the alloy. At all temperatures, the microstructure consists of a bimodal and sometimes trimodal distribution of strengthening Cr,Nb precipitates, depending on precipitation condition, i.e., from liquid or solid solution, and cooling rates. These precipitates remain in the same size range, i.e., large precipitates of approximately 1 mu m and small precipitates less than 300 nm, and effectively pin the grain boundaries, thus retaining a fine grain size of 2.7 mu m after 100 hours at 1323 K. This grain-boundary pinning and sluggish coarsening of Cr2Nb particles explain the retention of good mechanical properties after prolonged holding at very high temperatures, e.g., two-thirds of the original yield strength after aging for 100 hours at 1273 K, The main sources of strengthening are the Hall-Fetch and Orowan mechanisms due mostly to small particles. The coarsening kinetics of the large precipitates are most likely governed by grain-boundary diffusion and, to a lesser extent, volume diffusion mechanisms.