Invited Lecture
Multiscale modelling of precipitation hardening in metallic alloys

Precipitation hardening is one of the most efficient mechanisms to increase the yield strength of metallic alloys but accurate quantitative models for this phenomenon are still lacking. Two different multiscale approaches, based on atomistic simulations and discrete dislocation dynamics, are presented to address this problem and validated by comparison with experiments.

Atomistic simulations were used to determine the interaction between Guinier-Preston zones and dislocations in an Al-Cu alloy. The rate at which dislocations sheared the precipitates (determined by means of molecular dynamics) was controlled by the activation free energy, in agreement with the postulates of the transition state theory. An estimation of the initial shear flow stress as a function of temperature was carried out from the thermodynamic data provided by the atomistic simulations.

In the case of large precipitates that cannot be sheared by dislocations (such as θ’ precipitates in Al-Cu alloy), the dislocations overcome the precipitates by the formation of an Orowan loop. The mechanisms of dislocation/precipitate interaction were studied by means of discrete dislocation dynamics using the discrete-continuous method in combination with a fast Fourier transform solver to compute the mechanical fields. Simulations took into account the effect of precipitate shape, orientation and volume fraction as well the elastic mismatch between the matrix and the precipitate, the stress-free transformation strain around the precipitate and the dislocation character as well as dislocation cross-slip. The simulation predictions were compared with experimental results of the initial yield strength in an Al-Cu alloy overaged at high temperature.