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Structure and deformation characteristics of beta'AuZn Causey, Allan Robert

Abstract

The deformation behaviour of the intermetallic CsCl-structure compound β'AuZn has been investigated over a wide range of metallurgical variables. The plastic deformation, specifically the stress-strain relation, has been characterized in terms of the effects of composition, temperature, strain-rate, and grain size. The constitutional defect structure was investigated using lattice parameter measurements and annealing experiments. Polycrystalline AuZn was found to behave in a ductile manner, over the temperature range 77 to 533°K and composition range 48.0 to 52.0 a/o Au. In addition to slip on the planes of the zone, slip on planes of the zone was also suggested by the slip trace analyses. This may account for the observed ductility and high work hardening rate. The composition dependence of the yield stress exhibited the following behaviour; 1) a minimum at the stoichiometric composition, 2) a linear dependence of hardening on the deviation from stoichiometry, 3) approximately equal hardening due to both excess Au and Zn atoms, and 4) a temperature independent hardening-composition slope (except at 77°K where a hardening minimum was observed at 50.5 a/o Au). The existing order strengthening theories and solid solution hardening mechanisms were found to be unsatisfactory for AuZn. The temperature dependence of the yield stress was similar to that observed for bcc metals and alloys with a rapid increase in yield stress at temperatures below 200°K. The excess Au atoms decreased the temperature dependence significantly in the composition range 50.1 to 51.6 a/o Au The temperature dependence of the yield stress was investigated using the thermally activated flow parameter analyses. The magnitude of the activation volume, activation energy, frequency factor and shear stress extrapolated to 0°K were consistent with the predictions of either the thermally activated cross-slip mechanism developed by Escaig or the Peierls-Nabarro force mechanism proposed by Rajnak and Dorn. The Peierls mechanism was found to provide a more satisfactory explanation of the solid solution softening phenomena. The defect structure of AuZn was determined to be antistructural. The larger Au atom expanded the lattice more than the smaller Zn atom constricted it. The as-extruded Zn-rich wire exhibited an anomalous increase in resistivity during annealing experiments. The increase and subsequent decrease were attributed to a variation in the degree of long-range order.

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