The precipitation behaviour of ITER-grade Cu-Cr-Zr alloy after simulating the thermal cycle of hot isostatic pressing

The precipitation behaviour of Cu-0.75Cr-0.105Zr (wt%) alloy for fusion reactor applications has been investigated after thermal treatments similar to those expected during joining procedures by hot isostatic pressing. A bimodal distribution of coarse precipitates with spacings larger 1 mu m and tiny precipitates with spacings between 30 and 40 nm was observed by scanning electron microscopy (TEM) and transmission electron microscopy (TEM), respectively. The coarse precipitates resist solution annealing treatments and do not contribute to mechanical strength. Two types of microscopically small precipitates were observed, which exhibit strain field contrast in TEM and are responsible for the mechanical strength of the aged alloy. A fine dispersion of these precipitates with a mean size of 2.3 nm and a density of 3.3 x 10(22) m(-3) is related with maximum strength. Hardening is most compatible with an Orowan mechanism. These precipitates are only obtainable when the joining procedure is terminated with a minimum cooling rate of about 1 K/s. (C) 2000 Elsevier Science B.V. All rights reserved.