| Abstract Scope |
Annealing twins are known since long to be very common in Face Centered Cubic (FCC) metals and alloys with low to medium stacking fault energy, but, quite surprisingly, neither the exact mechanisms by which they appear and evolve during thermomechanical processing nor their precise role in property control are yet fully understood. If twin boundaries, and more generally of low- CSL boundaries, can be beneficial in situations involving interfacial diffusion of chemical species and chemical reactions (e.g with regards to corrosion resistance), their role is much more ambiguous when dealing with mechanical properties. The most striking example is that of fatigue resistance where, on the one hand, wide coherent twin boundaries have clearly been identified as preferential crack nucleation sites, but on the other hand high fractions of special boundaries have been reported to decrease the crack growth rate. Controlling these defects is therefore a possible way of tailoring properties, but this implies understanding the mechanisms by which they form as microstructure is evolving during thermomechanical processing.
Those mechanisms were addressed some ten years ago in a joint project with Pr. A.D. Rollett, co-funded by the French Research National Agency (ANR) and the US National Science Foundation (NSF). This project provided evidence that twins mostly form during recrystallization, while grain growth leads to a decrease in their density. Besides that, this work has given rise to some very interesting discussions concerning 3D topology and the stereological biases that are inherent to microstructure observation and quantitative analysis in 2D sections. Another outcome if this work is a better definition of the roadmap for the development of models to take account of the twin formation of twins in full-field numerical simulations. This project will be at the heart of the presentation, but a number of other joint projects and a few anecdotes drawn from two decades of cooperation with Prof. A.D. Rollett will also be mentioned. |