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|Title:||NUMERICAL STUDY OF DEFORMATION MECHANISMS IN HCP METALS|
|Abstract:||The operative deformation mechanisms which include both dislocation slip and twinning have a significant impact on the mechanical response of hexagonal close-packed (HCP) metals. Twinning plays an important role in accommodating plastic deformation due to the limited number of independent slip systems in HCP metals. The objective of this research is to study the deformation mechanisms associated with twinning in HCP metals (magnesium and zirconium alloys). Heat treatments are often involved in the manufacturing of zirconium alloys. These alloys exhibit a strong thermal anisotropy with a thermal expansion coefficient along the c-axis nearly two times of that along a-direction. Therefore, residual stresses/strains are generated during the heat treatment process which influences the mechanical response (e.g. lattice evolution) under subsequent loading. The elastic viscoplastic self-consistent (EVPSC) model has been improved which includes thermal strain to study the behavior of a Zircaloy-2 slab under moderately large strains. Various self-consistent schemes (SCSs) of the EVPSC model are evaluated in terms of the deformation behavior of the material under different uniaxial strain paths. Numerical results show that the Affine and Meff=0.1 self-consistent models give much better performance for the Zircaloy-2 slab than the Secant and Tangent models. The EVPSC-TDT model has been employed to mimic the twinning and detwinning behavior of extruded Mg alloy ZK60A under monotonic and cyclic loading. The model differentiates between the stress required to initiate twinning and that required to grow (thicken) existing twins. This enables the model to simulate the unusual sharp yielding behavior during twinning as well as the gradual yielding associated with detwinning. It is demonstrated that this model gives a good prediction of the strength anisotropy, strength asymmetry, and strain hardening behavior along different directions, for cases in which the contribution of twinning is large, small and intermediate. For the first time, the lattice strain evolution is well predicted in an extruded magnesium alloy under cyclic loading which involves twinning and detwinning. In all polycrystal models, an empirical equation for the termination of twinning in a grain is required. A new physics-based empirical equation for describing this phenomenon in magnesium alloys has been proposed in this study. It should be noted that the popular empirical equation currently used in all polycrystal models is applied at the grain level, while the new empirical equation is introduced at the twinning system level. The new description is represented by a single parameter while the commonly used empirical equation depends on two parameters. It is demonstrated that the proposed empirical equation is easily calibrated with the single parameter and is able to accurately simulate the experimentally observed rapid hardening associated with twinning exhaustion.|
|Appears in Collections:||Open Access Dissertations and Theses|
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