Invited Paper IE-MoM5
Understanding Dislocation Dynamics and the link to Macroscopic Properties

Monday, October 15, 2007, 9:20 am, Room 618

The behaviour of dislocations under an applied load and their interaction with obstacles, such as grain boundaries, precipitates, voids etc., can be revealed be conducting deformation experiments in real time in situ in the transmission electron microscope. Linking the insight gained form such studies to macroscopic measurements of property changes remains challenging but significant progress has been made. It will be shown that grain and twin boundaries serve as sources and sinks for, and barriers to perfect and partial dislocations. From information learned from these studies, criteria for the transfer of slip across grain boundaries and interfaces have been established. However, the microstructure is not static and evolves with increasing strain. For example, the process of slip transmission can result in the destruction of the grain boundary locally and this influences subsequent deformation activity. These observations provided a basis for developing strategies for incorporating grain boundary effects in large-scale predictive models of mechanical behaviour. Studies of the interaction of dislocations with precipitates with different interfacial character as a function of temperature have revealed a rich variety of complex dislocation-precipitate interactions and by-pass mechanisms. The interaction type depends on the particle coherency and size, the nature and Burgers vector of the dislocation, the geometry of the interaction, the number of interactions, and the test temperature. It is also possible and common for multiple slip systems to interact with the particle consecutively or simultaneously and this changes the nature of the interactions. The number and complexity of the interactions and the richness of the possibilities have significant implications for current models of mechanical properties of precipitate-hardened systems. This paper emphasizes what can be learned from conducting dynamic experiments in the electron microscope and how such insights can and are being used as a basis for formulating physically-based constitutive relationships to predict macroscopic mechanical properties of thin and thick films.