The influence of subgrain boundaries on the rate controlling creep processes in Fe-3 Pct Si
- PDF / 1,001,892 Bytes
- 7 Pages / 612 x 792 pts (letter) Page_size
- 17 Downloads / 176 Views
O
NE of the basic questions that must be answered before a satisfactory theory of high temperature creep can be formulated concerns the influence of subgrain boundaries on the rate controlling creep process. There appear to be two distinct possibilities: 1) the rate-controlling creep process involves a dislocationsubgrain boundary interaction or 2) the creep rate is determined primarily by the motion of dislocations within subgrains. The latter proposition assumes subgrain boundaries are incidental to the creep process and merely a by-product of dislocation motion. There is ample evidence that subgrains are an integral part of the creep structure. 1-4 The subgrain network is generated during primary creep and its size remains unchanged during steady state creep. The subgrain size generally varies inversely with applied stress and is essentially independent of temperature. Changes in applied stress during a creep test result in appropriate changes in the observed subgrain size. Historically, most investigators have chosen to ignore the possibility that creep is controlled by dislocationsubgrain boundary interactions. The reasons for this are twofold. First the early metallographic observations of McLean and his coworkers implied that the misorientation across subgrain boundaries increases with strain even in the steady state region. 5-8 It would seem improbable for the subgrain boundary to be involved in the rate limiting steady state creep process if the nature of subgrain boundary continually changes with strain. The second observation suggesting that
lar creep kinetics, implying that the detailed structure of the subgrain boundary is unimportant to the rate limiting creep process.
subgrain boundaries are relatively unimportant re-
unchanged.
lates to general constancy of the stress and temperature dependencies of the creep rate for different materials. Metals with different crystal structures and widely different stacking fault energies exhibit simiR. G. STANG is Assistant Professor, Department of Mining, Metallurgical and Ceramic Engineering, University of Washington, Seattle, Wash. W. D. NIX and C. R. BARRETT are Professor and Associate Professor, respectively, Department of Materials Science and Engineering, Stanford University, Stanford CA 94305. Manuscript submitted November 6, 1974. METALLURGICAL TRANSACTIONS A
Correspondingly, steady state creep theories pro-
posed in the last fifteen years have generally involved 9-13 calculations of the motion of individual dislocations or concerned themselves with dislocation-dislocation interactions. 14-17
There is at present, however, considerable evidence suggesting that the reasons listed above for neglecting subgrain boundaries as an important part of creep substructures are based on incomplete or inconclusive evidence. For one thing, recent electron microscopy studies on creep substructures have shown that the average subgrain misorientation and subgrain shape
do not appear to change during steady state creep, thereby implying a constancy of the subgrain
Data Loading...
