Formation of slip traces during grain growth of thin metal sheets

During a study of the mechanical properties of polycrystalline Ni3Al produced by recrystallizing B-free Ni3Al single crystals, it was discovered that {111} slip traces formed during grain growth of cold-rolled single crystal thin sheets annealed at 1200 degrees C for 10 hours. Grain growth stops when the slip starts. The slip traces are revealed by the formation of thermal grooves at 1200 degrees C. The formation of slip traces seems to be a general phenomenon during the recrystallization of thin sheets of metals, as slip traces also formed on cold-rolled polycrystalline Cu thin sheets annealed at 922 degrees C for 10 hours. It is proposed that the residual strains/stresses in the thin recrystallized sheets are responsible for the slip processes and also provide a primary driving force for grain growth. Slip begins when the grain size has increased to the point where the yield stress, as determined by the Hall-Fetch relation, equals the residual stress. As the residual stresses and strains are relieved by the slip, grain growth stops, and the traces seen in this study are produced. The resulting grain size can be controlled by changing the level of the yield stress or the residual stresses thereby changing the critical grain size at which the two stresses are equal. Thus thinner specimens will have a smaller grain size because the yield stress decreases with decreasing sample thickness in sheet specimens. Other factors, such as annealing temperature or amount of cold-work also directly influence the final grain size.