Erik Nes

Modelling of work hardening and stress saturation in FCC metals

Abstract A work hardening theory has been developed based on a microstructural concept comprising three elements; the cell/subgrain size, δ, the dislocation density inside the cells, ρi, and the cell boundary dislocation density or the sub boundary misorientation, ρb or ϕ. The theory is based on a statistical approach to the storage of dislocations. This approach predicts that the slip length, L, scales with the inverse square root of the stored dislocation density, ρ−1/2, and also, predicts a substructure evolution which is consistent with the concept of microstructural scaling (similitude) at zero degree Kelvin, at stress τ

Modelling recrystallization after hot deformation of aluminium

Abstract A physically based model for predicting recrystallization microstructures and textures after hot deformation of aluminium is presented. The modelling approach taken differs from similar models developed for steels. The present model is based on recent experimental investigations directed towards identifying the nature of the nucleation sites for recrystallized grains of different crystallographic orientations. Particle stimulated nucleation, nucleation from cube bands and nucleation from grain boundary regions have been incorporated in the model. The model has been applied to predictions of recrystallized grain sizes and textures of a hot deformed AlMgMn alloy and a commercial purity aluminium alloy. The model responds in a correct way to variations in strain, Zener-Hollomon parameter, initial grain size, initial cube fraction and precipitation. The good agreement between model predictions and experimental results confirms that the recrystallized cube grains are nucleated from “old” cube grains that were present in the starting material and survived the deformation.

The development of recrystallization microstructures studied experimentally and by computer simulation

Abstract The development of recrystallization microstructures has been investigated experimentally and theoretically. The theoretical microstructures are generated through computer simulation using different nucleation and growth models including inhomogeneous nucleation and growth. The results which are presented in the form of size distributions of grain section areas are compared to experimental size distributions of partially as well as fully transformed grain structures in commercial aluminium alloys. While the experimental distributions all are close to being log-normal, none of the theoretical ones reproduce this shape in full, the latter all having a more or less clear skewness to the right. The theoretical model which gave the best correspondence to the experimental observations was one with nuclei divided into classes associated with different decreasing growth rates. The discrepancy observed between theoretical and experimental results may be ascribed to the complexity of the recrystallization process which is hardly possible to include in an idealized computer model. In our opinion computer simulation seems at present to be of limited value to give detailed insight into the mechanism of recrystallization.

On the mechanisms of dynamic recovery

Abstract The annihilation of dislocations in dynamic recovery is analyzed in terms of reactions between mobile dislocations and dislocations stored in a Frank network. An extended model is presented comprising spontaneous dislocation collapse reactions and annihilation by dipole climb collapse.

Effect of precipitation on the evolution of cube recrystallisation texture

A study of the evolution of recrystallised structure and texture in the surface of a cold rolled twin roll cast AlFeSi alloy is presented. Annealing of such alloys often results in an abnormally coarse grained recrystallised surface structure with a strong cube texture. The evolution of this structure depends on the annealing procedures, that is, the precipitation state. Increased amounts of precipitating particles increase the grain size and the fraction of cube texture. The oriented growth theory does not offer any plausible interpretation of this precipitation effect. A recrystallisation model that incorporates the differences in Zener drag between different annealing procedures has shown that the evolution of a strong cube texture and coarse grains is the result of a preferential nucleation of cube oriented grains. Precipitation increases the critical nucleation diameter and the resulting grain size. Cube oriented subgrains have a size advantage compared to other potential nucleation sites and are therefore not so affected by precipitation.

Modeling recrystallization kinetics, grain sizes, and textures during multipass hot rolling

A physically based model for the evolution of recrystallization microstructures and textures during hot rolling of aluminum is presented. The approach taken differs from similar models developed for steels. The present model is based on recent experimental investigations directed toward identifying the nature of the nucleation sites for recrystallized grains of different crystallographic orientations. Particle stimulated nucleation (PSN) and nucleation from cube bands and grain boundary regions have been incorporated in the model. The multipass aspect complicates the modeling due to partial recrystallization between the rolling passes. Two different approaches have been suggested to handle this. The model has been applied to predictions of recrystallization kinetics, recrystallized grain sizes, and recrystallization textures during multipass hot rolling of aluminum. The predictions are reasonable compared to experimental results.

Work Hardening Behaviour of Heat-Treatable Al-Mg-Si-Alloys

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.

Modelling the evolution in microstructure and properties during processing of aluminium alloys

Abstract A new work-hardening model for f.c.c. metals and alloys has recently been developed [Prog. Mater. Sci. 41 (1998) 129; Mater. Sci. Forum 331–337 (2000) 1231; Mater. Sci. Technol., 17 (2001) 376–388; K. Marthinsen, E. Nes, K. Nord-Varhaug, B. Ronning, in: J.B. Bilde-Sorensen, et al. (Eds.), Proceedings of the 20th Riso International Symposium on Materials Science: Deformation-induced Microstructures, Analysis and Relation to Properties, Riso National Laboratory, Roskilde, Denmark, 1999, pp. 405–410; Mater. Sci. Eng., submitted for publication]. In the present work this model will be applied to the processing of aluminium alloys, covering conditions typical of both hot deformation (rolling and extrusion) and cold rolling, the emphasis, however, will be on hot deformation. Alloying effects due to solid solution (multi-component systems) as well as particles (constituent particles and dispersoids) will be considered.

A unified microstructural metal plasticity model applied in testing, processing, and forming of aluminium alloys

Abstract Over the last seven years a collection of models has been developed and put together by Nes, Marthinsen and coworkers in what here will be referred to as the Microstructure-based Metal Plasticity model, or in short as the MMP model. An overview of the most important modelling aspects will be given here. The basic mechanisms are related to the way the dislocations are stored and recovered in the lattice and how they affect the flow stress during deformation. The model at its current state is able to predict the microstructure evolution and the corresponding flow stress for the entire temperature range and for large strain rates as well as creep behaviour. The inherited processing-related quantities, such as grain size, solute content of alloying elements, and the texture, are taken into account, including a model for dynamic strain ageing. Anisotropy of the stress tensor is related mainly to the coupling to a texture model accounting for lattice rotations of the grains. However, a new and novel mo...

The effect of boundary spacing on substructure strengthening

Abstract The subgrain size and the spacing of high angle boundaries are important parameters used to describe the microstructure in metals deformed to large strains. How the flow stress depends on the boundary spacing is discussed here and it is argued that the best way this is treated is in the work hardening model developed by Nes and co-workers (Progr. Mater. Sci., 1998, 41, 129 – 193; Mater. Sci. Tech., 2001, 17, 376 – 387; Mater. Sci. Eng., 2002, A 322, 176 – 193). The theoretical arguments given are supported by experimental observations.