[SY-C4] Atomistic and continuum approaches to analyse 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 approaches, based on atomistic simulations and discrete dislocation dynamics, are presented to address this problem.
Atomistic simulations, in combination with the transition state theory, were used to determine the interaction between Guinier-Preston zones and dislocations in an Al-Cu alloy and between Mg17Al12 precipitates and basal dislocations in an Mg-Al alloy. The rate at which dislocations sheared the precipitates (determined by means of molecular dynamics) was controlled by the activation Gibbs free energy, in agreement with the postulates of the transition state theory. However, harmonic TST does not hold for this interaction. In addition, the activation enthalpy energy and the activation volume were determined and 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 q’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. It was found that the influence of the precipitate aspect ratio and orientation were 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.
Atomistic simulations, in combination with the transition state theory, were used to determine the interaction between Guinier-Preston zones and dislocations in an Al-Cu alloy and between Mg17Al12 precipitates and basal dislocations in an Mg-Al alloy. The rate at which dislocations sheared the precipitates (determined by means of molecular dynamics) was controlled by the activation Gibbs free energy, in agreement with the postulates of the transition state theory. However, harmonic TST does not hold for this interaction. In addition, the activation enthalpy energy and the activation volume were determined and 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 q’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. It was found that the influence of the precipitate aspect ratio and orientation were 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.