Multiscale modelling of precipitation hardening in Al-Cu alloys
| Dates: | 1 February 2018 |
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| Times: | 14:00 - 16:00 |
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| What is it: | Seminar |
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| Organiser: | Department of Materials |
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| How much: | Free |
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| Speaker: | Prof Javier Llorca |
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A multiscale modelling roadmap is presented to study precipitation hardening in Al-Cu alloys. In the first step, the homogeneous and heterogeneous formation of precipitates during high temperature ageing in Al-Cu alloys is simulated
using the phase-field model in which the model parameters that determine the different energy contributions (chemical free energy, interfacial energy, lattice parameters, elastic constants) were obtained from either computational
thermodynamics databases or from first-principles density functional theory. From the information, the evolution and equilibrium morphology of precipitates in 3D was obtained. The model was able to reproduce the equilibrium shape
of the different orientation variants of precipitates during homogeneous nucleation as well as the heterogeneous nucleation on dislocations, leading to the formation of precipitate arrays.
From this information, two different strategies were used to determine the strengthening provided by either Guinier-Preston zones or q’precipitates. Guinier-Preston zones are formed by monolayers disks of Cu atoms of a
few nm in diameter on {100} planes of the FCC Al lattice. The GP zones are overcome by the dislocations through a thermally-activated process and the energy barrier was determined using transition state theory. Under these
conditions, the rate at which the dislocations overcome the GP zone is given by an Arrhenius equation with a pre-exponential factor and an activation energy, that can be obtained by means of molecular dynamics simulations
and the nudged elastic band method, respectively.
In the case of q’precipitates, the dislocations overcome the precipitates by the formation of an Orowan loop and the mechanisms of dislocation/precipitate interaction were studied by means of discrete dislocation dynamics.
It was found that the elastic mismatch has a negligible influence on the dislocation/precipitate interaction in the Al-Cu system while the influence of the precipitate aspect ratio and orientation was reasonably well captured by
the simple Orowan model in the absence of the stress-free transformation strain. Nevertheless, the introduction of the stress-free transformation strain led to dramatic changes in the dislocation/precipitate interaction and in the
critical resolved shear stress to overcome the precipitate, particularly in the case of precipitates with small aspect ratio.
The new methodology to study the dislocation/precipitate interactions opens the possibility to obtain quantitative estimations of the strengthening provided by precipitates in metallic alloys using multiscale modelling
strategies based in first principles calculations.
Speaker
Prof Javier Llorca
Role: Scientific Director
Organisation: IMDEA Materials Institute and Polytechnic University of Madrid
Travel and Contact Information
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C16
MSS Tower
Manchester
