| Popis: |
Early metallographic studies of the deformation of h.c.p. metals were confined to cadmium and zinc. More recently magnesium, titanium, zirconium and beryllium have been required for the nuclear and aircraft industries and this has led to increased demand for a more fundamental understanding of the plastic deformation in h.c.p. metals. These metals differ in two important respects from the cubic metals. Firstly only three coplanar shear vectors are normally available for slip and secondly mechanical twinning is a dominant deformation mode. The results of a metallographic study of deformation in h.c.p. metals is presented in this thesis. Particular attention has been paid to the behaviour of deformation twins, because there is a dearth of information on this aspect in the literature. The thesis may be divided roughly into four parts. In the first part the crystallography and deformation modes in h.c.p. metals is reviewed. Certain errors and confusions in the literature are discussed and where appropriate, particular emphasis is placed on magnesium and titanium, since they are typical of the technologically important h.c.p. metals. The second part describes the behaviour of twins in these metals. It is shown that twin growth and contraction differs from the ideal depicted by crystallographic models of twinning. The third part is concerned with the effect of cyclic stresses. The first evidence for slip band extrusions in fatigued h.c.p. metals is presented. The distribution of the extrusions is related to the dislocation structure observed and the results are compared with those obtained in cubic metals. The fragmentation of twins under cyclic stresses and fatigue crack nucleation at twin boundaries is also demonstrated. Finally the role of twinning in the deformation of h.c.p. metals is considered. A Schmid factor criterion is used to compare the ease of slip before and after twinning on {1012} and {1121} systems. Basal and prism slip systems are considered in parent and twinned crystal. It is found that pre-existing {1012} twins greatly enhance prism slip compared with basal, while {1121} twins also favour basal slip. When dislocation twin interactions are considered however, slip can be nucleated more readily by {1012} twins when prism slip occurs. Similarly for {1121} twinning, only prism slip can give rise to slip. These results may explain the relationship between ductility and slip and twinning modes in h.c.p. metals. |
