Date
5-1-2025
Department
School of Engineering
Degree
Doctor of Philosophy in Engineering (PhD)
Chair
Marcos Lugo
Keywords
Magnesium, Twinning, Material Modeling, Internal State Variables
Disciplines
Mechanical Engineering
Recommended Citation
Kenney, Daniel S., "A Novel Macroscale Model for Deformation Twinning in Magnesium Alloys" (2025). Doctoral Dissertations and Projects. 6899.
https://digitalcommons.liberty.edu/doctoral/6899
Abstract
The characterization of deformation twinning in magnesium alloys is evaluated with macroscale modeling to predict material behavior and microstructure evolution with Internal State Variables (ISVs). Twinning in magnesium presents key challenges with constitutive modeling at the macroscale where the large number of grains with individual slip/twin must be generalized. This research proposes the Kearns factor (Kearns, 1965) as a method for quantifying the crystallographic texture associated with twinning and shows how the change in Kearns factors is associated with tension-compression asymmetry, anisotropic yielding and hardening, and anisotropic lateral strains. The material model developed in this work proposes a second rank tensor similar to the Kearns factors in order to represent the distribution of (0002) basal texture. The ISV model expands on the Baamann plasticity model (Bammann, 1990), which characterizes temperature and strain rate dependent deformation by kinematic and isotropic slip deformation. Twinning is added to the model as a separate yield surface, representing the twinning as a mechanism distinct from slip with hardening laws dependent only on basal texture. Twinning is shown to be temperature independent, with experiments ranging from 20 °C to 300 °C. The model assumes twinning to be temperature (within 20 °C to 300 °C) and strain rate independent, though all experiments were conducted at quasistatic rates. The model was initially developed to calibrate deformation behavior of a rolled AZ31 plate, with uniaxial loading in compression in the Transverse, Rolled, and Normal Directions (TD, RD, and ND respectively), and tension in the TD and RD. Subsequent tests were conducted on a cast ZK60 alloy which was pre-compressed to 4.9-5.2% strain. Tension and compression samples were machined from this pre-compressed material and tested at 0°, 45°, and 90° from the initial compression direction, along with the raw cast material. For both AZ31 and ZK60 tests, interrupted compression tests were carried out to inspect the microstructure and quantify twinning throughout early deformation (<6% strain). This model is shown to characterize the complex behavior of magnesium deformation due to twinning by only incorporating 3 additional calibration constants to the model. Though simple, this model provides a sound framework for future work to predict twin-related phenomenon like cyclic hardening due to twinning and detwinning, twin related damage, and twin induced recrystallization.
