Columnar grain growth of superelastic CuAlMn alloy during directional recrystallization

Directional recrystallization (DR) was utilized to obtain columnar grains in a superelastic CuAlMn alloy. The effects of hot-zone temperature and drawing velocity on the microstructural evolution of the alloy during DR, and the abnormal grain growth mechanism for the formation of columnar grains were systematically investigated by OM and EBSD. It was found that columnar grains could be obtained when the hot-zone temperature was above the alpha-phase solvus temperature of-730 degrees C. The optimum drawing velocities for obtaining columnar grains with the largest aspect ratio increased with increasing hot-zone temperatures, i.e. at 800 degrees C, 850 degrees C and 900 degrees C, the velocities were 2 mu m/s, 5 mu m/s and 15 mu m/s, respectively. The apparent activation energy of the columnar grain growth was estimated to be-177 kJ/mol. The equiaxed grains that were produced by primary recrystallization all had a similar (111)[110] orientation. Thus, there was little driving force for grain growth. In contrast, grains that do not have this orientation are surrounded by high-angle grain boundaries, which have high mobility, and form columnar grains.