On the Portevin-Le Chatelier Effect Due to Snoek Strain Aging in the Niobium Oxygen System
- PDF / 649,766 Bytes
- 7 Pages / 603.28 x 788 pts Page_size
- 64 Downloads / 216 Views
I.
INTRODUCTION
DYNAMICstrain aging in bcc interstitial alloys may be manifested by serrated yielding, abnormally high work hardening rates, a negative strain rate sensitivity, as well as other phenomena. It is generally accepted that in bcc alloys these effects arise from interactions between dislocations and interstitial solute atoms that occur during deformation. The mechanism of dislocation-interstitial atom interaction may involve either a SnoekLSchoeck-Seeger2 ordering of the interstitials about the dislocations or the formation of Cottrell3 atmospheres around the dislocations. It was previously observed by Bradford and Carlson4 that in the vanadium-oxygen system serrations may occur within two distinctly different temperature ranges. A strong effect of oxygen concentration was also noted. Thus, for specimens containing between 47 and 265 ppm of oxygen well defined serrations were observed only between 623 and 723 K. However, specimens containing higher oxygen concentrations, i.e., 955 and 1800 ppm, also showed serrations in a lower temperature range between 423 and 448 K, with the largest serrations appearing in the specimens with the higher oxygen concentration. They interpreted the high temperature serrations as due to Cottrell dynamic strain aging but were not able to identify a mechanism for the lower temperature serrations. It is now believed that the lower temperature serrations were due to Snoek dynamic strain aging. 5"6The results of Bradford and Carlson also suggest that a high solute concentration is needed to make the Snoek serrations apparent. In spite of the large number of papers dealing with Cottrell dynamic strain aging, relatively few have dealt with the related Snoek phenomena. Among these there is one by Evans and Douthwaite7 who give computer calculations for the binding energy between a dislocation and its equilibrium Snoek atmosphere and the stress required to free a dislocation from its atmosphere. Another by Reed-Hills concerns the kinetics of the formation of the Snoek atmosphere
S.C. PARK, Graduate Student, and R. E. REED-HILL, Professor, are both with the Department of Materials Science and Engineering, University of Florida, Gainesville, FL 32611. L.P. BECKERMAN, Metallurgist, is with the Metals and Ceramics Division, Materials Laboratories, AFWAL/MLLS, Wright-Patterson Air Force Base, OH 45433. Manuscript submitted May 24, 1982.
METALLURGICALTRANSACTIONS A
around a dislocation. Most research on dislocation interstitial atom interaction, however, has involved internal friction studies of the cold work peak. For example, see the recent set of papers. 9-13These latter unfortunately give little information of direct bearing to strain aging phenomena because the cold work peak phenomena normally occur several hundred degrees above the temperatures associated with dynamic strain aging. At the cold work peak temperature the interstitial solute atoms are extremely mobile. Consequently, dislocation motion is expected to be coupled with interstitial atom movement. In dynamic strain agin
Data Loading...
