Papers by Author: Jesper Friis

Abstract: Using a first-principles methodology we have investigated the interfacial and bonding characteristics of the Al(001)/Fe (0-11) interface. The Al/Fe interface model was developed using a face-to-face matching method. Among many possible interface structures, the Al (001)/ Fe(0-11) orientation relation gave the minimum lattice misfit along the a and b directions (a=b= -0.47%). Hence, this interface structure provided the minimum energy value and was used for this study. To predict the interface strength and stability, the work of separation and interfacial energy were calculated. Here, all systems were calculated under exactly the same conditions (k-point mesh, cutoff energy, lateral lattice strain etc). In order to predict the bonding nature at the interface, charge density difference plot was evaluated, which showed charge gain at the interface. The aim of this study is to describe the adhesive behavior between Al and Fe, provide some insights about strength and stability of this interface structure for galling, and provide reference interface system for Al/Fe welding.

Abstract: The properties and surface appearance of aluminium extrusion are critically dependent on the microstructure and texture of the extruded profiles, and the requirements with respect to these aspects may vary with applications. Moreover it is often a challenge to produce extrusions with a consistent and homogenous grain structure and texture along as well as through the cross section of the profiles. It is thus vital to understand and be able to predict (model) how different microstructures and textures are formed and how they evolve during and after extrusion. In the present work a model framework has been implemented which includes a FEM model to account for the strain, strain rate and temperature along a set of particle paths during extrusion. From these the deformation texture and grain structure are calculated with an appropriate deformation texture model and a sub-structure evolution model, respectively. The sub-structure model have in the present work been coupled to a crystal plasticity model to provide an orientation dependent subgrain size and dislocation density during deformation which provides the driving force for the post-extrusion recovery and possible recrystallization behaviour. The post-extrusion microstructure and texture evolution is calculated with a recovery and recrystallization model, which is accompanied by a recrystallization texture model. The framework and its constituent models and their interplay are presented, and some preliminary results when applying this modelling framework to Al-Mg-Si extrusions are presented and discussed in view of corresponding experimental results.

Abstract: In the structures of all metastable precipitates in Al-Mg-Cu and Al-Mg-Si alloys, we find that column surrounding of an element column in the needle/lath direction order according to simple principles. Advanced transmission electron microscopy and DFT calculations support the principles originate with a line defect, which is a segment of a <100>Al column shifted to interstitial positions. We propose the defect aids solute decomposition by partitioning the FCC matrix locally into columns of fewer and higher number of nearest neighbours, which suit smaller and larger size solute atoms, respectively. The defect explains how <100> directionality of the precipitates can arise in a cluster. Ordering of a few defects leads naturally to GPB zones in Al-Mg-Cu and to β'' in Al-Mg-Si.

Abstract: Small angle grain boundaries have been grown in a small Bridgman furnace, using seeded growth method, at three different pulling rates i.e. 3 μm/s, 13 μm/s and 40 μm/s. In order to assess segregation mechanisms of impurities towards the central grain boundary, melt has been polluted by 50ppma of either copper or indium. Secondary ion mass spectrometry (SIMS) local analyses have been performed to investigate the impact of solid state diffusion and limited rejection of solute at the grain boundary for each growth rate. The results are discussed in connection with an atomistic model built on Vienna Ab-initio Simulation Package (VASP).

Abstract: A dedicated diffusion controlled precipitation model for AlMnFeSi-alloys, based on classical nucleation and growth theory, has been implemented and coupled to a phenomenological softening model accounting for the combined effect of recovery and recrystallization during annealing after cold rolling. The result is a fully coupled precipitation and softening model which in principle is capable of accounting for variations in solute levels and size and volume fraction of dispersoids and their interaction with the softening behavior during annealing.

Abstract: The basic equations and mathematical framework of a mean-field model for recovery and recrystallization, the latter based on the Johnson-Mehl-Avrami-Kolmogorov (JMAK) approach, capable of handling time-dependent nucleation of recrystallization, is presented. Different approaches to account for time-dependent nucleation are discussed. A physically-based nucleation model where “nucleation” of recrystallization is brought about by “abnormal” subgrain growth seems most appealing, in terms of realism and mathematical convenience. Its implementation and effects on the recrystallization behavior are demonstrated through an example of back-annealing after cold deformation of a generic aluminium alloy case

Abstract: The microstructure evolution in commercial AlMgSi alloys during and after extrusion of a simple U-shaped profile has been modelled. The strain, strain rate and temperature along a set of particle paths are taken from FE-HyperXtrude simulations and used as input to the work hardening model ALFLOW, to predict the evolution of the subgrain size and dislocation density during deformation. As soon as the profile leaves the die, the subsequent recovery and recrystallization behaviour is modelled with the softening model ALSOFT. This procedure enables the modelling of recrystallization profiles, i.e. the fraction recrystallized through the wall thickness of the extruded profile. The sensitivity to chemistry (alloy composition), profile deflection and the cooling rate at the die exit has been investigated by means of a set of generic modelling cases.

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.