Mechanisms of deformation in beta-brass single crystals
The investigation reported in this thesis is an extension of an earlier investigation of the stress/strain behaviour of β-brass single crystals under static and dynamic loading. It was aimed at elucidating the mechanisms of deformation, using optical microscopy to examine slip habits and transmission electron microscopy to examine dislocation substructures. Marked differences were observed in the stress/strain response, slip behaviour, and the substructural rearrangements of single crystals of β-brass when widely different rates of uninterrupted compressive loading were used. Under static loading (strain rates = 10-4 sec-1) the crystals workhardened in a parabolic manner, slip being confined to either of the two (101) [111] or (211) [111] systems. Cross-slip in the statically deformed crystals was evidenced by the wavy appearance of the slip traces. Under dynamic loading (strain rates > 103 sec-1) the crystals exhibited large yield drops followed by a linear rate of work-hardening which was lower than the static work-hardening rate. Slip on the four (-101) [111], (-110) [111], (101) [-11l] and (110) [-111] systems occurred simultaneously. The traces on any one dynamically deformed specimen were both coarser and finer than those observed on specimens statically deformed. The coarse traces belonging to a given {110} <111> slip system always bounded the finer traces of another {110} <111> system. No wavy slip was observed on the dynamically deformed specimens. The substructures present in the statically deformed crystals were consistent with a single mode of slip and exhibited features indicative of high rates of hardening on the active slip planes. The important features of the statically induced substructures were as follows:- (a) There was a predominance of dislocations with Burgers vectors of a[111]. Many of these lay in non-screw high-energy orientations, consistent with the occurrence of cross-slip and the observed high hardening rates of the crystals. (b) Strong tangling developed on the slip planes as a result of a[111] edge segments interacting with other a[111] segments and with loop and dipole debris produced on the slip planes. (c) A partially developed cell structure resulted from this tangling. The dynamically induced substructures were consistent with the simultaneous operation of the four {110} <111> slip systems. It was inferred from the substructural investigations that the lower rates of hardening in the dynamically deformed crystals arose from the distribution of slip over the four systems with a consequent reduction in hardening on each individual system. The interactions between dislocations on intersecting slip planes appeared to be less important in contributing to hardening than the influence of the four systems in reducing the hardening on any one system. A series of interrupted tests indicated that behaviour under staticstatic, dynamic-dynamic and dynamic-static loading sequences was consistent with the uninterrupted static and dynamic behaviour, but that the dynamic reload stress/strain and slip behaviour, and the associated substructural changes were greatly modified by static preloading. The crystals deformed by a "static" type slip mechanism, but exhibited "dynamic" type stress/strain behaviour during the dynamic reload in this sequence.
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