Microstructure evolution during severe plastic deformation

Abstract

Radiotracer diffusion studies of severely deformed, ultra-fine grained materials have revealed the presence of ultra-fast transport paths, which include “non-equilibrium” grain boundaries and free volume. Under some experimental conditions, percolating porosity is produced even in pure copper. Micro-cracks may form in metals, if the local maximum shear stress exceeds the shear yield stress. However, their growth and propagation is postponed till late in the deformation process owing to the ductility of metals, the hydrostatic component of the stress system and/or dynamic recovery/recrystallization. In other words, crack growth and propagation is present only when the scope for further deformation is highly restricted. Using this approach, the load required for equal channel angular pressing, the change in the slope of the Hall–Petch plot with decreasing grain size and the theoretical limit for the smallest grain size attainable in a metal in a severe plastic deformation process are predicted and validated by experimental results. Experimentally successful prevention of percolated crack formation by the superposition of a hydrostatic pressure is also accounted for using this model.

Keywords:

• severe plastic deformation
• dislocation
• grain boundary diffusion
• fracture

Acknowledgements

The authors thank Professor Dr. G. Gottstein for a stimulating discussion and J. Ribbe for help with the FIB investigation. KAP thanks the DFG for the provision of a Mercator Professorship. Financial support of the DFG (Deutsche Forschungsgemeinschaft) through a research grant is also acknowledged.

Notes

Notes

1. There is experimental evidence for the triggering of recovery processes at late stages in deformation (i.e. at large strains) when the grain size reaches close to its saturation value 52. They are important in metals such as Cu subjected to SPD.

2. Under some deformation conditions, e.g. at an elevated temperature, the saturation grain size may be increased by the presence of recovery processes (including dynamic recrystallization, grain growth). Then, the crack forming tendency will be less than at ambient temperatures. Under those conditions, influence of the hydrostatic pressure on the final grain size would be less.