Doctoral Dissertations

Date of Award

5-2007

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Materials Science and Engineering

Major Professor

Peter K. Liaw

Committee Members

Joseph E. Spruiell, Hahn Choo, John D. Landes

Abstract

Equal Channel-Angular Processing (ECAP), as a severe plastic deformation of metals and composites, is analyzed both theoretically - to describe the ECAP macromechanics - and experimentally - to obtain ultrafine-grained materials with new thermo-mechanical properties - with a focus on hexagonal-closed-packed (HCP) structures such as Mg alloys. Due to their obvious similarity to ECAP, the slip-line–field theories developed for orthogonal cutting are applied to the ECAP deformation for predicting the shear-strain spatial heterogeneities. A theoretical model for predicting the plastic-deformation zone in an ECAP-ed billet with a free surface is provided, and is validated experimentally. A shear-strain-mapping procedure was developed by decomposing the large deformation process into fine steps, and, by analyzing the partially-deformed billets, the strain maps captured the spatio-temporal evolutions of the ECAP-induced plastic shear strains. This approach was later generalized for studying the local behavior of different material parameters, such as textures (texture mapping).

The mechanical testing of the as-received and ECAP-deformed Mg-alloys (ZK60 and AZ31) was performed in monotonic and cyclic tests, for three loading orientations. The ECAP-ed samples demonstrate: (a) a good grain refinement from 50 - 70 μm down to 2.5 - 7 μm), (b) a superplastic ZK60 alloy, with an elongation to failure of 371 % at 3500C and the strain rate of 10-2 s-1, and (c) a longer fatigue life for the AZ31 alloy, relative to the as-received material.

The starting and ECAP-deformed materials were characterized by optical microscopy, Xray diffraction using both soft and hard X-rays, and neutron diffraction. The grain sizes, the textures, the coherent-domain sizes, the elastic microstrains, and the dislocation densities were determined for the samples deformed by rolling, extrusion, and ECAP. The synchrotron radiation measurements allowed monitoring the lattice rotation induced by the ECAP deformation in Mg alloys. The grain-orientation dependent deformation is studied relative to the deformation history, and its influence on the mechanical behavior is analyzed relative to the twinning contribution.

The results of the present work constitute a valuable benchmark for the understanding and modeling of the deformation mechanisms, such as the dislocations slip, twinning, recovery, or recrystallization in HCP structures.

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