抄録
Grain refinement by thermal cycling in Cu-2Be and Cu-2Be-0.15Co alloys has been investigated. The thermal cycling means the alternative heat treatments for the discontinuous precipitation and the dissolution of nodules. The solution treatment was performed at 1023 K and the aging temperatures for Cu-2Be and Cu-2Be-0.15Co alloys were 673 and 773 K, respectively. Microstructures of heat treated specimens were observed by means of an O. M., SEM and TEM. Crystallographic orientation was determined by analyzing SEM-ECP (Electron Channeling Pattern) and SADP.
The fronts of the nodules without contacting each other moved back to the initial position during dissolution. Consequently the grain size did not change. The dissolution of the precipitates occurred both at the moving front and within the nodules. Meanwhile, the dissolution proceeded mainly within the nodules in the Co-added alloy an account of a low mobility of grain boundaries. Therefore, the mechanism of the dissolution is a combination of the grain boundary diffusion and the volume diffusion in the nodules.
Grain refinement was achieved for the fully precipitated specimen. The refinement was explained as a special case of the back motion of the front of nodules, that is, the case of a very low mobility of the front. Grain size reduced from 100 to 19.3 μm by the first thermal cycling in the Cu-2Be alloy. However, only a small change in grain size occurred after the 2nd thermal cycling. On the other hand, such a small change in grain size was also observed by the 1st thermal cycling from the initial grain size of 12 μm. Thus the refinement ratio depends on the initial grain size.
