The Woodruff School

SUMMARY

In response to environmental and economic challenges, light-weight metals such as magnesium and its alloys have received a lot of attention in the last years. However, the incomplete understanding of the deformation mechanisms associated with their crystalline structure has led to manufacturing and industrial difficulties preventing their wider use. Consequently, the main objective of this thesis is to develop comprehensive modelling capabilities enabling to gain a better understanding of the various deformation mechanisms pertaining to strain hardening, especially in the case of low-symmetry materials such as hexagonal close-packed magnesium and its alloys, and that should be capable of addressing the questions of slip-driven plasticity, slip-twin interactions, and slip-precipitates interactions. To this end, modelling approaches addressing the limitations of current techniques are developed at two length scales: 1) At fine scale, a discrete dislocation dynamics (DDD) tool enabling simulations on low-symmetry crystals is developed to study the collective effect of dislocation-dislocation interactions and quantify their strength and effect on latent-hardening, especially in the case of pure Mg. Further, a transmission mechanism is implemented in the DDD framework so as to investigate the effects of dislocation-twin interactions. In addition, a novel DDD approach based on a Fast Fourier Transform (FFT) technique is developed: the DDD-FFT. Specifically, the DDD-FFT approach allows for the efficient treatment of anisotropic and heterogeneous elasticity, thereby paving the way towards performing DDD simulations in low-symmetry polycrystals. With this, dislocation-precipitate interactions are investigated. 2) At the macroscale, a new constitutive framework capable of receiving information from lower scales and establishing a direct connection with DDD simulations is developed and highlights the significant role of mobile dislocations whose evolution is generally not accounted for. Finally, it is expected that new constitutive laws suitable for complex loadings could be further developed such as to directly account for local mechanisms and effects extracted from this work, thereby delineating paths towards achieving scale transition between mesoscale and macroscale.

SUBJECT:	Ph.D. Dissertation Defense

BY:	Nicolas Bertin

TIME:	Friday, September 11, 2015, 12:00 p.m.

PLACE:	Love Building, 210

TITLE:	On the role of lattice defects interactions on strain hardening: a study from discrete dislocation dynamics to crystal plasticity modelling

COMMITTEE:	Dr. Laurent Capolungo, Chair (ME) Dr. David McDowell (ME) Dr. Surya Kilidindi (MSE) Dr. Hamid Garmestani (MSE) Dr. Carlos Tomé (Los Alamos National Laboratory)

SUMMARY In response to environmental and economic challenges, light-weight metals such as magnesium and its alloys have received a lot of attention in the last years. However, the incomplete understanding of the deformation mechanisms associated with their crystalline structure has led to manufacturing and industrial difficulties preventing their wider use. Consequently, the main objective of this thesis is to develop comprehensive modelling capabilities enabling to gain a better understanding of the various deformation mechanisms pertaining to strain hardening, especially in the case of low-symmetry materials such as hexagonal close-packed magnesium and its alloys, and that should be capable of addressing the questions of slip-driven plasticity, slip-twin interactions, and slip-precipitates interactions. To this end, modelling approaches addressing the limitations of current techniques are developed at two length scales: 1) At fine scale, a discrete dislocation dynamics (DDD) tool enabling simulations on low-symmetry crystals is developed to study the collective effect of dislocation-dislocation interactions and quantify their strength and effect on latent-hardening, especially in the case of pure Mg. Further, a transmission mechanism is implemented in the DDD framework so as to investigate the effects of dislocation-twin interactions. In addition, a novel DDD approach based on a Fast Fourier Transform (FFT) technique is developed: the DDD-FFT. Specifically, the DDD-FFT approach allows for the efficient treatment of anisotropic and heterogeneous elasticity, thereby paving the way towards performing DDD simulations in low-symmetry polycrystals. With this, dislocation-precipitate interactions are investigated. 2) At the macroscale, a new constitutive framework capable of receiving information from lower scales and establishing a direct connection with DDD simulations is developed and highlights the significant role of mobile dislocations whose evolution is generally not accounted for. Finally, it is expected that new constitutive laws suitable for complex loadings could be further developed such as to directly account for local mechanisms and effects extracted from this work, thereby delineating paths towards achieving scale transition between mesoscale and macroscale.