J.J. Bhattacharyya

Crystal Plasticity Modeling of the Dynamic Behavior of Magnesium Alloy, WE43-T5, Plate

Understanding the high strain rate behavior of Mg alloys is of interest for applications ranging from armor to automobile crash worthiness. Toward this end, the viscoplastic self-consistent (VPSC) polycrystal plasticity code, including the recently developed twinning-detwinning (TDT) model, is used to describe the homogeneous plastic flow of the rare earth element containing Mg alloy, WE43-T5, plate. The model accounts for the presence of an initial, moderate texture and its evolution as during deformation. It reveals that the moderate texture is responsible for the difference between the plate through-thickness and in-plane behaviors. The model also helps to reconcile why the in-plane response is nearly isotropic, despite the presence of orthotropic (not radially symmetric) texture. Note that a single set of parameters was used to fit the entire set of results, i.e. it is a model, which can describe all of the observed strength, strain, and strain hardening anisotropies and asymmetries.

Using the Crystal Plasticity Approach to Parse the Effects of Alloying and Aging on the Mechanical Behavior of Wrought Mg Alloys

Predicting the collective effects of solid solution alloying, precipitation, and grain size on the mechanical properties of a given alloy is one of the “holy grails” of mechanical metallurgy. For alloys with a cubic crystal structure, this is challenging enough to have been solved only in specific cases, and even there, predicting the anisotropies induced by crystallographic texture is very difficult. In the case of hexagonal close packed Mg alloys, this challenge is even greater and arguably more important. Research over the past decade has also highlighted the significant impact precipitate shape and orientation can have on the strength of Mg alloys. This review lecture will show, with examples from conventional and rare earth containing Mg alloys, that the crystal plasticity approach has enabled the level of our predictive capability to rapidly approach the level, which exists in other more mature alloys systems.

What is in a Strain Hardening “Plateau”?

A yield plateau occurs in some Mg alloys during compressive deformation that has been ascribed to the localization of twinning. In fine-grained AZ31, this plateau was explained by a region of large twin volume fraction nucleating in a small band and propagating, similar to a “Luders band” like phenomena. Once the band traverses the entire gauge length, the sample begins to strain harden. Similarly, ZK60 samples exhibit the same Luders like phenomena during extrusion direction compression as confirmed by digital image correlation. However, the band is not sufficient to fully explain the plateau in the stress-strain curve. Postmortem electron backscattered diffraction (EBSD) reveals the twin structure evolution through this plateau. It is found that twinning occurs in large elongated grains before spreading to the finer grains.

Texture Evolution During Grain Growth of Magnesium Alloy AZ31B

The strong basal texture which develops during the rolling of Mg alloy, AZ31B, sheet persists and can even increase during annealing. The present study was designed to elucidate the mechanism of texture evolution during grain growth. The grain growth kinetics were determined for temperatures between 260 and 450 °C revealing a grain growth exponent, n~4. The activation energy for grain growth was found to be close to the activation energy for grain boundary diffusion. Abnormal grain growth was observed as a broadening of the normalized grain size distribution at 400 and 450 °C. However, no temporal correlation was found between the peak in texture intensity and the peak in the width of the normalized distribution. Rather, the abnormal grain growth appears to correlate with the coarsening of second phase particles, which are present in the microstructure. The texture evolution is speculated to be due to anisotropy of grain boundary energy and mobility.

The Relative Contributions of Deformation Modes to AZ31 Rolling Textures in Different Temperature Regimes

Key differences in the textures of cold-, warm-, and hot-rolled Mg alloy AZ31 sheets and plates are identified. It is shown that incorporation of compression twinning within Visco-Plastic Self-Consistent (VPSC) polycrystal plasticity simulations reproduces key features of the cold-rolling texture that have not previously been predicted. Discussion of recent observations of recrystallization and grain growth provide explanations for the hot-rolled texture. Finally, it is demonstrated that starting with the correct initial texture is essential to produce observed features in all the rolling textures, including warm-rolling.