Chapter 2 - Friction Stir Microstructure for Superplasticity

Abstract

The microstructural prerequisites for a superplastic material are well established in metallic alloys. The first prerequisite is a fine-grain size, typically less than 15μm. The optimum strain rate for superplasticity increases with decreasing grain size when grain boundary sliding (GBS) is the dominant process. For a given strain rate, the finer grain sizes lead to lower flow stresses, which is beneficial for practical forming operation. The second prerequisite is thermal stability of the fine-grained microstructure at high temperatures. Single-phase materials generally do not show superplasticity because grain growth occurs rapidly at elevated temperatures. Hence, presence of second phases at grain boundaries is required to resist excessive grain growth. An appropriate amount of fine, uniformly distributed, and thermally stable second-phase particles is necessary to keep stable microstructure during superplastic deformation. The third prerequisite is high grain boundary misorientation. High-angle grain boundaries (HAGBs, angle ≥15°), particularly random ones promote GBS, whereas low-angle grain boundaries are generally believed to be not suitable for GBS. The fourth prerequisite is equiaxed grain shape. With equiaxed grains, grain boundaries can experience shear stress easily promoting GBS. The fifth prerequisite is mobility of grain boundaries. During GBS, stress concentration could be produced at various grain boundary discontinuities such as triple points. The migration of grain boundaries could lead to reduction in stress concentration. Thus, GBS can continue as a major deformation mechanism.