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Date of Award

12-1996

Degree Type

Thesis

Degree Name

Doctor of Philosophy (PhD)

Department

Materials Science and Engineering

Supervisor

J.D. Embury

Co-Supervisor

G.C. Weatherly

Abstract

The mechanical behaviour in uniaxial tension and compression of extruded and aged Mg-8.5wt%Al was studied. In-situ neutron diffraction was used to follow the elastic lattice strains under load in the matrix and the precipitates. The internal stresses determined from these measurements are highest in grains unfavourably oriented for both basal slip and {10Τ2} twinning, lowest in grains oriented favourably for both, and in between for grains oriented favourably for {10Τ2} twinning only. Most variations in scattered peak intensity are due to the lattice reorientation produced by {10Τ2} twinning. A critical resolved shear stress criterion is shown to apply for twinning. Intensity variations which cannot be explained by {10Τ2} twinning occurred in some grains during tensile loading. They are likely due to {10Τ1} twinning which produces c-axis compression, unlike {10Τ2} twinning. Transmission electron microscopy revealed the presence of c- and non-basal a-dislocations in the undeformed alloy. Basal slip is the most common slip system, though non-basal a-slip also occurs. Only {10Τ2} twinning was observed by TEM. Twins often traversed grains completely, despite the presence of the precipitates. Schmid factor considerations show that pure magnesium yields first by basal slip. The early portion of the stress-strain curve should thus be considered a region of rapid strain hardening due to basal dislocation pile-ups at grain boundaries. In compression {10Τ2} twinning can also occur at very low applied stress. Strengthening in the alloy before yield is explained using a Brown and Clarke mean-stress hardening model. Beyond yield, relaxation mechanisms reduce the mean stress contribution essentially to zero in tension. The mechanical and physical properties of the intermetallic were obtained from experiments on a single crystal. Property correlations have been used to estimate the fracture toughness and yield stress, assuming its behaviour is similar to that of a ceramic.

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