Effect of Small Amount Addition of Co on Dynamic Recrystallization of Cu–Be Alloys

The effect of small addition of Co on dynamic recrystallization (DRX) of Cu–Be alloys was studied by means of tensile testing and microstructural observation at 1123 K and at strain rates from 10−5 to 10−1 s−1. The high temperature flow stresses were scarcely affected by the addition of Co and, in contrast, the room temperature hardnesses of the hot deformed matrix as well as the DRX structures were clearly dependent on it. The latter can result from cuboidal inclusions existing only in the Cu–Be–Co alloys, which were identified as the (CoCu)Be intermetallic compound of CsCl-type cubic lattice. DRX of the Cu–Be alloy was almost the same as that of pure copper in the whole range of deformation condition, and also that of the Cu–Be–Co alloy only in the region of flow stresses higher than 50 MPa. In the lower stress region of the Cu–Be–Co alloy, however, DRX and the growth of (CoCu)Be particles took place simultaneously during deformation, leading to the evolution of finer DRX grain structures. It is concluded that the critical condition in which the Cu–Be–Co alloy changes the DRX characteristics is Ds=Dlim. (Here Ds and Dlim are the stable DRX grain size evolved under zero second phase particles and the final grain size stabilized by them, respectively.) When Ds<Dlim in the higher stress region, dynamic grains can be controlled by deformation conditions (T and ε), leading to Ds. Conversely, when Ds>Dlim in the lower stress region, they can be controlled by second phase particle, leading to Dlim.