Materials Science and Engineering, 12 (1973) 87-93 87 (C" Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands Dynamic Strain Aging in Magnesium Oxide Single Crystals M. SRINIVASAN and T. G. STOEBE* Division of Metallurgical Engineering, University of Washington, Seattle, Wash. 98195 (U.S.A.) (Received October 2, 1972) Summary** Dynamic strain aging characteristics have been observed in MgO single crystals when deformed above 600° C. Serratedflow has been identified, which has been shown to depend on the deformation rate and temperature. The extent of agin9 has been found to be directly proportional to the time of ayin9 at a 9iven strain and temperature; t 2/a law which has been found to be applicable in b.c.c, iron and C-doped Ni is not observed in MgO. Secondary load drops are observed under certain conditions during strain rate change experiments. Dynamic strain aging in MgO seems to be due to Cottrell type impurity atmospheres formed around dislocation lines, but the identity of the impurity responsible for this behavior is not yet known. INTRODUCTION Strain aging, the hardening of a material caused by aging after plastic deformation, is a well known phenomenon that occurs in many materials. It is usually explained in terms of the formation of im- purity atmospheres around dislocations and hence is a time- and temperature-dependent process. Dynamic strain aging (DSA) occurs when a con- siderable amount of diffusion of impurities occurs at elevated deformation temperatures concurrently with the motion of the dislocations. This was first reported in iron and steep and then in many materials, including Zr 2, carburized Ni alloys3,4, Ti 5 and a 5005 A1 aUoy6. In ionic solids DSA has not been studied ex- * Currently on leave at the Atomic Energy Institute, University of Sao Paulo, Sao Paulo, Brazil. ** R6sum6 en fran~ais gtla fin de l'article. Deutsche Zusammenfassung am SchluB des Artikels. tensively. In LiF, Johnston 7 observed yield drops at room temperature after aging at 115°C, while Fotedar and Stoebe s have recently reported the results of detailed DSA studies. Stokes 9 observed yield point phenomena in MgO crystals while annealing above 600°C and testing at room tem- perature, where mechanical tests conducted above 600°C always showed yield points and jerky flow owing to continuous aging of dislocations partici- pating in the deformation. Several investi- gators 1°-14 have observed that the flow stress- temperature curve for MgO has a maximum near 800°C. Moon and Pratt 14 suggested the possibility of a Portevin-LeChatelier effect in MgO when they observed jerky flow in the temperature range 600°C - ll00°C in strain rate change experiments. The results of a more detailed investigation of the dynamic strain aging behavior of MgO are reported in this paper. EXPERIMENTS Single crystals of MgO were obtained from the Oak Ridge National Laboratories. A spectrographic analysis of these crystals showed the following impurity composition: Fe, 0.005~0; Si, 0.007~; Mn, 0.001~o; A1, 0.007%; Cu, 0.0005% and Ca, 0.005~, where the analysis is reported as oxides of the elements indicated. These crystals were sawn roughly to size, then cleaved to the final sample dimensions of approximately 8 x 2.5 x 2.5 mm 3. The sides of the specimens were polished smooth and flat and the ends were cleaved. During the entire specimen preparation procedure, care was exercised to introduce minimum surface damage to the crystals. The crystals were then heated at 2000° C for 2h in a graphite element furnace with an argon atmosphere. This treatment ensures that most of the iron in these crystals is in the +2 valence state 15