UC Santa Barbara

Twinning can be an essential deformation mechanism in hexagonal close packed (HCP) materials and when it occurs, can significantly change plastic behavior, causing, for instance, noticeable tension and compression asymmetry. In this thesis research, crystal plasticity models, such as elasto-plastic self-consistent (EPSC) model, viscoplastic self-consistent (VPSC) model, and elasto-viscoplastic fast Fourier transformation model, are applied to investigate twin-related hardening mechanisms in HCP alloys. A shared feature of these models is that all models are coupled with dislocation density-based hardening laws, which can predict the underlying dislocation slip activity. How twinning is represented is different among these models. In EPSC and VPSC, a composite grain model is employed to account for twinning, while in EVP-FFT, the intragranular twin domain is model explicitly within the single crystal. Based on their respective advantages, the VPSC and EPSC models are selected to first research dislocation-twin interactions in polycrystalline in commercially pure titanium (CP-Ti) and Mg-Y alloys, and later, the EVP-FFT model is applied for twin-twin interactions within Mg alloy crystals.As mentioned, in the first part of this research, the EPSC model was employed to study the interaction between slip and twinning in CP-Ti under cyclic loading. There are a variety of active mechanisms in CP-Ti at room temperature, including prismatic ⟨a⟩, basal ⟨a⟩, pyramidal slip ⟨c+a⟩, {101 ̅2} tensile twin, and {112 ̅2} compressive twin. A model for the development of a “slip-system-level” backstress due to dislocation density accumulation is included in the EPSC model to advance the model to treat cyclic loading and in particular predict the Bauschinger effect. Material parameters associated with the slip strengths for the three HCP slip modes are determined and newly reported. The model identifies the few systems within the pyramidal slip mode responsible for developing the most backstress among the three slip modes. The analysis also indicates that the backstress that developed in the forward loading path promotes pyramidal slip in the reversal loading path. In addition, reverse loading negligibly changes the relative slip mode contributions from monotonic loading, but it strongly affects the twinning-detwinning behavior. The novel conclusions are that pyramidal slip mode activity and detwinning and their interactions are largely responsible for the strong Bauschinger effect in CP-Ti. Second, dislocation-twin interactions in Mg-Y alloys are investigated using the VPSC model. The research is motivated by experimental tests on alloys performed elsewhere. The model is applied to understand tests in which the alloys were deformed in tension and compression in the rolling direction and in compression in the normal direction to invoke distinct proportions of slip and twin mechanisms with each test. Within the single-crystal hardening model used in VPSC polycrystal modeling, a slip-twin interaction law is introduced to account for dislocation density reductions due to dislocation absorption during twin boundary migration. To obtain a more comprehensive understanding, Mg alloys with four different Y concentrations, including Mg 0.2 wt%, Mg 0.6 wt%, Mg 1.0 wt%, and Mg 3.0 wt %, are researched. For each alloy, the model identifies a single set of material parameters that successfully reproduces all measured stress-strain curves and achieves agreement with measured deformation textures and twin area fractions. During deformation, the plastic anisotropy in yield stress, tension-compression asymmetry, and amount of {101 ̅2}⟨1 ̅011⟩ twinning is found to decrease with increasing Y. The model interpretation of the flow responses suggests that increased concentrations of Y increase the critical resolved shear stress for basal slip but have negligible effects on the other slip modes. The observed reductions in plastic anisotropy with increases in Y is explained by a concomitant decrease in the prismatic-to-pyramidal slip critical resolved shear stress ratio. The calculations suggest migrating twin boundaries in Mg–Y with different concentrations have the same dislocation absorption rate. Therefore, the chief finding identifies differences in slip strengths among the HCP slip modes as a critical link to macroscale plastic anisotropy. Finally, the EVP-FFT model was applied to research {1 ̅012} twin-twin interactions in Mg alloys. In the simulation, two non-parallel intersecting twins with the same zone axis are modeled explicitly within a grain (within a polycrystal). These two twins form a co-zone twin-twin junction structure (TTJ). The study reveals important effects the TTJ has on the growth of the impinging twin (IT) and the recipient (RT) twin, as well as the ability of the IT to apparently cross the RT. In addition, the effect of the relative twin thickness between the IT and RT on these twinning activities is investigated. These TTJ simulations are also performed on three different Mg alloys to reveal the effect of alloy additions on the local stress field produced by the TTJ. The results show that increasing the RT thickness does not affect IT growth, while increasing the IT thickness promotes the reformation of the IT on the other side of the RT, giving the appearance of twin-crossing. The latter apparent crossing is not automatic and requires additional loading to occur. Alloy additions are shown to not significantly alter twin growth mechanisms around the TTJ. However, the formation of non-co-zone twins is more likely to happen in an alloy with low plastic anisotropy.