Doctoral Dissertations

Date of Award

12-2009

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Materials Science and Engineering

Major Professor

Peter K. Liaw

Committee Members

Tai G. Nieh, Hahn Choo, John D. Landes

Abstract

The mechanical behaviors and the associated deformation mechanisms with a focus on extension twinning under monotonic and cyclic loadings are investigated using neutron and synchrotron diffractions in the wrought magnesium alloys, ZK60A and AZ31B. It has been demonstrated that the extension twinning plays significant roles in the mechanical behaviors.

The significant tension-compression asymmetries and high anisotropies are observed. The tension-compression asymmetries are related to the twinning activation in one direction but not in the opposite direction. The high anisotropies are correlated with the initial texture distinction relative to the loading directions. The similar yielding stresses are noted irrespective of the strain direction and strain sign if the deformation is dominated by twinning, while they are usually different with respect to the loading conditions if the dislocation slip is dominant. The extension twinning under tension exhibits a similar behavior to that under compression, presumably due to the same Schmid stress introduced on the twinning plane along the twinning shear direction. However, the distribution of basal poles within the twins under tension is closely related to the initial texture, while they are always aligned with the compressive axis under compression.

The low-cycle fatigue resistances follow the empirical Basquin and Coffin-Manson relationships with the texture dependency observed, related to the different activation sequences of twinning and detwinning involved, respectively, under tension and compression determined by the initial texture. Specifically, the post-detwinning deformation characteristics may be an important factor in understanding the texture dependency. The extension twinning is readily activated if an applied stress/strain supports c-axis extension of the hexagonal-close-packed (hcp) structure. The unique reorientation of the twins facilitates detwinning in the twinned areas during the subsequent strain/stress reversal. Therefore, the cyclic plastic deformation is dominated by the alternating twinning and detwinning, and the initial texture is recovered once detwinning capability is exhausted, concurrent with the disappearance of twin bands. In particular, detwinning occurs almost immediately in the twinned grains upon unloading, which is driven by the local tensile stress along their c-axes as a result of the stress redistribution between the soft- and hard-grain orientations due to the plastic anisotropy.

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