Dynamic strain aging and serrated flow in MnO
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T. A. Bloom Department ofMetallurgical and Materials Engineering, Illinois Institute of Technology, Chicago, Illinois 60616 (Received 14 October 1985; accepted 17 December 1985) The effects of aging on the upper yield stress r up and serrated flow have been studied in MnO single crystals at 900 °C for oxygen partial pressurespOi of 1 0 ~ n and 10 ~ 7 Pa. Aging initially increases r up as a consequence of segregation of aliovalent impurities to dislocations for bothjpc,2 values. For long aging times and/?O2 = 10~ n Pa, serrated flow accompanied by solute softening is observed. The data fit predictions of a Portevin-Le Chatelier model for serrations, but with impurity atmospheres causing softening instead of hardening. This is believed to result from changes in local defect equilibria caused by segregation of impurities with valences greater than two to dislocations.
I. INTRODUCTION Serrated flow has been observed during high-temperature deformation of single crystals of binary oxides such as A i P j , 1 MgO,2'3 CoO,4 and NiO. 5 In general, the serrations have been attributed to solute-dislocation interactions such as the Portevin-Le Chatelier effect (PLE). Although many systematic studies of serrated flow and the effects of aging have been conducted for metals (see, for example, Ref. 6), few such investigations have been made for ceramics. Brown7 studied static-aging effects in NaCl crystals containing 2X 10~ 4 Cd ions. He found that the stress increment upon aging, Ar a , increased linearly with strain e up to about 7%, after which Ar a saturated. Srinivasan and Stoebe2 examined dynamic strain aging of MgO at temperatures between 600 and 900 °C. They found that Aro increased linearly with both strain (up to 6%) and aging time ta. Increased aging temperatures also increased Ar a , although overaging was observed at 900 "C. Dominguez-Rodriguez and Castaing5 studied dynamic strain aging and serrated flow in NiO for T
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