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Plasticity of [Beta]'AuZn single crystals Schulson, Erland Maxwell

Abstract

Single crystals of the CsCl type intermetallic compound β’AuZn were prepared and tested in tension over a wide range of temperatures, strain rates and orientations for three compositions, Au-rich (51.0 at.% Au), stoichiometric and Zn-rich (51.0 at. % Zn). Slip surfaces are generally non-crystallographic planes in the zone of the slip direction [001], and are temperature, strain rate and orientation sensitive. A model based on thermally activated sessile-glissile transformations of screw dislocations has been proposed to explain non-crystallographic slip. Multi-stage work-hardening is observed over the temperature range 0.2 ⪝T/T𝘮⪝ O.35. In stage I the work-hardening rate is low[(formula omitted) /lOOO to (formula omitted)/5000] but rises sharply during stage II [Q₁₁ ~ (formula omitted)/5OO]. Stage III is characterized by a rapidly decreasing hardening rate coincident with the onset of profuse large-scale cross-slip. Surface slip line studies revealed that the end of easy glide is coincident with the onset of localized slip on non-crystallographic planes in the [100] zone. Thin foil electron microscopy was carried out on critically chosen crystallographic sections from annealed and deformed crystals. At the beginning of stage I clusters of edge dislocation dipoles were revealed, forming walls perpendicular to the glide plane. The dislocation density of the walls increases during easy glide. During testing at intermediate temperatures ( ~ .3 to .4 T𝘮 ) serrated yielding was detected in non-stoichiometric crystals and was attributed to dislocation-solute atom interactions. Under special testing conditions (77°K or near < OOl> orientations) slip occurs in directions. The associated work-hardening rates are very high and ductility is low. Thermal activation studies were made to determine the dislocation mechanism responsible for the temperature sensitivity of yield in stoichiometric crystals below 〜220°K. Activation volume measurements are consistent with both the Peierls-Nabarro and cross-slip mechanisms below 〜150°K.

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