Precipitation in Al-Cu alloys: a multiscale analysis based on first principles simulations

Javier Llorca, Universidad Politécnica de Madrid, Getafe, Madrid, Spain

Precipitation of intermetallic phases from supersaturated solid solutions by means of high temperature ageing is the most common strategy to increase the strength of metallic alloys. Precipitation strengthening is particularly efficient in metallic systems, such as Al and Ni, in which precipitation leads to the nucleation and growth of one or more metastable phases, which may co-exist with the thermodynamically stable forms of the phases. The overall strength of the alloy depends on the size, shape, spatial distribution and volume fraction of the different intermetallic phases and the optimum heat treatments and alloy compositions have been obtained as a result of a detailed understanding of the precipitation processes and a lot of “trial and error” iterations. Nevertheless, multiscale strategies that integrate chemistry and mechanics can be used to address the complex sequence of precipitation of the different metastable phases and, eventually, provide quantitative information that can be used to optimize the microstructure as a function of the aging conditions and alloy composition. An example of these multiscale strategies based on first principle calculations is presented to study stability, precipitate nucleation and growth in Al-Cu alloys aged at high temperature [1, 2]. The formation enthalpy of the different phases (θ'', θ' and θ) can be obtained by means of ab initio simulations from supercell calculations, and the Helmholtz free energy can be determined as a function of temperature by including the vibrational entropic contribution. This information is used to assess the thermodynamic stability of the different phases as a function of temperature.

In addition, the critical size for the nucleation of each phase can be computed from the Gibbs free energy contribution corresponding to the chemical free energy, the elastic strain energy associated to the transformation strain and the interface energies, which can be determined from first principles simulations for each interface. Homogeneous and heterogeneous nucleation can be assessed by means of the elastic interaction energy between defects (dislocations) and the transformation strain of each precipitate. Finally, precipitate growth during aging is determined by means of mesoscale phase-field model, in which all the model parameters that control the free energy are determined from computational thermodynamics and first principle simulations. The simulation results in terms on homogeneous and heterogeneous nucleation and of the equilibrium precipitate shape are compared with experimental results [3].

References
[1] H. Liu, B. Bellón, J. LLorca. Acta Materialia, 132, 611–626, 2017.
[2] H. Liu, I. Papadimitriou, J. LLorca. Submitted for publication.
[3] A. Rodríguez-Veiga, B. Bellón, I. Papadimitriou, G. Esteban-Manzanares, I. Sabirov, J. LLorca. Journal of Alloys and Compounds, 757, 504-519, 2018.

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Session T2: Tuesday, 26 June 2018

Start: 02:15 p.m.
End: 03:00 p.m.