Publication: Research › Ph.D. thesis – Annual report year: 2012
|Publisher||Risø National Laboratory for Sustainable Energy|
|Number of pages||139|
The objective of this study is to explore and analyze the effect of heat treatments on the structure and properties of nanostructured metals processed by plastic deformation to high strains. Commercial purity aluminium was plastically deformed either by cold rolling or by accumulative roll bonding (ARB) and then annealed at various temperatures. A number of techniques, including Vickers hardness tests, electron backscattered diffraction (EBSD), transmission electron microscopy (TEM), and positron annihilation lifetime spectroscopy (PALS), were used to characterize the structural changes on different length scales to identify possible recovery mechanisms.
In this study, triple junctions in a deformed lamellar nanostructure are classified into three categories (Y-junctions, H-junctions, and r-junctions) based on the structural morphology, and a series of relationships is formulated between the density of triple junctions and the boundary spacing. Based on TEM and EBSD observations, thermally-activated Y-junction motion is identified as the key process during recovery of highly-strained aluminium, leading to removal of thin lamellae and coarsening of microstructure. A mechanism for recovery by Y-junction motion is proposed, which can underpin the general observation that a lamellar structure formed by plastic deformation coarsens into a more equiaxed structure during recovery annealing.
Y-junction motion operates in a wide temperature range, even at room temperature, in highly-strained aluminium. However, during annealing below 100°C, Y-junction motion is limited. At such low temperatures, annihilation of zigzagged dislocations is found to be the dominating recovery mechanism whereas other mechanisms, such as subgrain coalescence and boundary migration, are of minor importance.
A model is proposed to analyze the recovery kinetics based on hardness measurements, a model which allows the activation energy of recovery to be estimated. During annealing of highly-strained commercial purity aluminium at 140°C and above, the apparent activation energy is found to increase as recovery proceeds and approach ~190 kJ/mol at the end of recovery, pointing at this stage to an effect of solute drag during recovery as in recrystallization.