Abstract of "Deformation Mechanisms in Magnesium Alloys at Elevated Temperature" by David E. Cipoletti, Ph.D., Brown University, May 2011The goals of this dissertation were to determine experimentally the plastic flow in pure magnesium and extruded magnesium AZ31 sheet during constant strain rate uniaxial extension at elevated temperatures, and to develop a model that is able to predict the influence of strain rate, temperature, grain size, and grain orientation on the deformation mechanisms and flow behavior of magnesium alloys. In pure magnesium, the strain rate sensitivity was found to be approximately 0.2 for deformation at strain rates from 0.03/s to 0.0001/s and temperatures from 350�C to 450�C, suggesting that dislocation creep is dominant within this range. For testing within the same range, the extruded magnesium alloy AZ31 sheet exhibited anisotropic behavior based on the orientation of the extruded direction with respect to the tensile axis due to the strong basal texture and elongated grain structure along the extruded direction. If loading was rotated from parallel to perpendicular to the extruded direction: the strain rate sensitivity increased from 0.3 to 0.5 at low strain rates, ductility increased, yield stress decreased, and the transition between the dislocation creep regime and grain boundary sliding regime increased to greater strain rates. To understand the experimental observations a finite element crystal plasticity model was developed that accounts for dislocation creep within the grains together with sliding and diffusion along the grain boundaries. The model was calibrated and validated using experimental data of multiple grain sized magnesium AZ31 sheet. The model was used to explain the origin of flow stress anisotropy in extruded AZ31 sheet. It was shown that two effects contribute to anisotropy - anisotropy of dislocation creep in the grains resulting from texture; and anisotropy of grain boundary sliding resulting from the elongated grain shape. The latter effect was shown to dominate the grain boundary sliding regime and former in the dislocation creep regime. Finite element simulations were also used to study the influence of heterogeneity in grain boundary sliding resistance on the creep response of aluminum alloy 5083 when deformed at 450�C.
Cipoletti, David E.,
"Deformation Mechanisms in Magnesium Alloys at Elevated Temperature"
(2011).
Mechanics of Solids Theses and Dissertations.
Brown Digital Repository. Brown University Library.
https://doi.org/10.7301/Z0MS3R18
