Papers by Author: Erik Nes

Abstract: Subgrain structures are frequently characterized by the electron backscatter diffraction (EBSD) method, which is both accurate and provides good statistics. This is essential to better understand the subgrain growth mechanisms and e.g. establish the driving forces and motilities for comparison with physically based models. However, there is no commercially available software which can provide adequate subgrain boundary maps necessary for e.g. size and misorientation analysis. Here, a method that produces such maps utilizing only commercially available software is presented. The clue is to provide the EBSD-software with a parameter that can be used to identify all subgrains. By combining various maps exported from the EBSD-software into photo editing software, a new map is made in which all subgrain boundaries are identified. Missing and incomplete boundaries are traced manually before a reconstructed subgrain map is generated and imported back into the EBSD-software. With this method, the built-in algorithms in the EBSD-software can be readily used to e.g. characterize subgrain growth in aluminium with respect to orientation, size and misorientation.

Abstract: Motivated by improving current softening models for recycle friendly alloys, softening was investigated in high purity and commercial purity aluminium alloys. Utilizing the electron backscatter diffraction (EBSD) technique, orientation dependent sub-grain growth was characterized with respect to grain size and average boundary misorientation. In the high purity alloys, small additions of Mn in solid solution slowed down the recovery kinetics. The recovery mechanisms were however not altered, but recovery kinetics were found to be orientation dependent. The presence of high angle grain boundaries or transition bands, i.e. large and sharp orientation gradients, seemed to change the growth from slow and continuous to a faster and discontinuous process. This was typical for Cube and Goss, while weak, short and long range orientations gradients observed in Copper, S and Brass, did not alter growth which was slow and continuous. Before detailed studies of recovery of the commercial purity alloy were initiated, a rather slow recovery was observed and further investigated. Preliminary results indicate that iron in solid solution is dramatically slowing down the kinetics but can form clusters by an intermediate annealing in order to speed up recovery.

Abstract: A recent work hardening model developed by Nes and co-workers at NTNU, Trondheim provides a unified theory for warm and cold stress-strain behaviour which in principle accounts for alloy aspects such as effect of dispersoids (size and number density) and solute content, including dynamic strain aging for Mg containing aluminium alloys. In the present paper the applicability and predictive power of the model are tested for multicomponent alloys to account for the combined effect of different solute elements in solid solution and dispersoids, with a special focus on hot deformation of a range of Al-Mg-Mn alloys. It is demonstrated that the model, without any re-tuning, only accounting for the variations in alloy chemistry and deformation conditions is capable of predicting the stress-strain for a range of compositions, strain rates and temperatures.

Abstract: The work hardening of alloys hardened by precipitate heat treatments depends on the distribution of the precipitate sizes and the solute level left in the metal matrix. A mean field theory for precipitation is first applied for the ageing and subsequently it is coupled to a work hardening model to study the stress-strain responses of age hardened conditions of AA6xxx alloys. The predictions are compared to mechanical experiments and to TEM characterisations.

Abstract: The starting material, the deformed state, as well as the recrystallised microstructure and texture have been analysed as a function of Zener-Hollomon parameter and strain for two differently heat-treated AlMgSi alloys, deformed in torsion. An interesting and somewhat surprising observation is that the grain size was always higher in the material heat treated to form large Mg2Si particles. Moreover, no indications of PSN effects were observed in any of the materials, even at the highest Zener-Hollomon parameters. This observation was quite unexpected as the highest Zener-Hollomon parameters were well above the typical critical level for which significant PSN effects have been observed in similar alloys. The results have been carefully analysed and possible explanations are discussed.

Abstract: The microstructure in heavily deformed metals can be characterized as a complex “mixture” of low and high angle boundaries. By careful annealing of such cold deformed conditions, ultra-fine grained materials can be obtained. This phenomenon has been known for long and utilised in the production of special aluminium sheet qualities, and has received new interest with the emergence of the equal channel angular pressing (ECAP) technique. This work reviews the mechanical properties resulting from plastic deformation and annealing of aluminium, looking at alloys which prior to annealing was subjected to both severe plastic deformation (ECAP) and more conventional deformation by cold rolling. The effect of the resulting microstructures on the subsequent work hardening properties are model, applying the new microstructural metal plasticity model (MMP-model) developed in Trondheim over the last decade.

Abstract: In the present work a detailed characterisation of a cold rolled and annealed AA3103- alloy has been carried out. The effect of different concentrations of Mn in supersaturated solid solution on the deformation and subsequent annealing behaviour has been studied in detail. A physically based microstructure model has been used to predict the evolution in microstructure and flow stress during annealing of the given alloy, with particular focus on the effect of concurrent precipitation on the softening behaviour.