Date of Completion

12-8-2014

Embargo Period

11-30-2017

Keywords

Al-Mg alloy, ultrafine grained, bimodal microstructure, finite element analysis, crystal plasticity, grain boundary modeling

Major Advisor

Leila Ladani

Associate Advisor

Michael Accorsi

Associate Advisor

Baki Cetegen

Associate Advisor

George Lykotrafitis

Associate Advisor

David Pierce

Field of Study

Mechanical Engineering

Degree

Doctor of Philosophy

Open Access

Open Access

Abstract

Grain size reduction has been known as a strengthening mechanism in most metals. The improvements in strength are at the cost of ductility in Al 5083, so a microstructure with a bimodal grain size distribution consisting of coarse grained (CG) and ultrafine grained (UFG) phases was developed. This creates a complex, inhomogeneous microstructure that can be difficut to predict and analyze. In this work this material is studied through a combination of experimetnal work and finite element simulations.

A full-factorial experimental design is developed for tensile tests under a variety of experiemental condtions using a custom devloped small scale specimen design. These tests followed by microstructural analysis examine the effects of temperature, anisotropy, strain rate, and CG ratio on the elastic-plastic constitutive behavior and failure of the material. Temperature is found to moduluate many of the observed phenomena. A major finding is that while the UFG material exhibits significantly improved strength at room temperature, its strength quickly degrades with increasing temperature. Eventually, around 493 K, its refined grain size becomes detrimental to its strength. A proposed explanation for this is the activation of grain boundary mediated plasticity effects such as grain boundary sliding. Additionally, changes in fracture texture are noted at different temperatures and between loading orientations.

To further investigate some of these findings, a multiscale simulation approach is developed. These simulations study the deformation and failure of the material at a microstructural level, incorporating crystal plasticity and grain boundary modeling techniques in procedurally generated finite element models to represent emergent effects at the grain level. The models are used to extract from the experimental data the appropriate crystal plasticity material constants for both the UFG and CG phases at two temperatures. These models showed crack initiation at the CG/UFG interface with lateral crack propagation through the matrix. At higher temperatures, these sites moved into the UFG matrix. Grain boundary activity can be quantified through these techniques and the simulations show that grain boundary sliding becomes more active at higher temperatures, while grain rotation is predominant at lower temperatures.

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