The effect of sample size on the steady state creep characteristics of Ni-6 pct W
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is--A
exp(- Qcreep RT )
Ill
where ~s ts the steady state strain rate, r is the true applied s t r e s s , Qereep is the apparent activation energy for creep, R is the gas constant, n is the s t r e s s exponent, T is the absolute temperature, and A Is a constant. The results of these Investigations are that Qereep = Q self diffusion and n ~ 5. Both of these o b s e r vations are consistent with the hypothesis that creep is controlled by the climb of dislocations. However, these investigations usually exclude the effects of grain size by considering steady state strain rates that are independent of grain size. s Often, high temperature applications require the use of material in the form of thin sheets. In these thin sheet materials, it is easy to obtain grain structures with only a few grains a c r o s s the sheet thickness. How-
D. K. MATLOCK, formerly Graduate Student, Department of Materials Science and Engineering, Stanford University is now Assistant Professor, Department of Metallurgical Engineering, Colorado School of Mines, Golden, Colorado, 80401. W. D. NIX is Professor of Materials Science and Engineering, Stanford University, Stanford California, 94305. Manuscript submitted August 27, 1973. METALLURGICAL TRANSACTIONS
ever, for design criteria, one must rely on data obtained on " n o r m a l " creep samples as these are often the only data available. Therefore, it is important to extend our investigations of high temperature creep to include fundamental studies on samples which do not have many grains a c r o s s the specimen thickness. This paper r e p r e s e n t s such a study. The possible consequences of using sample configurations which are different from " n o r m a l " samples can be understood by considering the interaction of grain matrix deformation with grain boundary sliding during high temperature creep. The following discussion is based on the concept that grain boundary sliding and grain deformation are separate but interacting deformation modes for high temperature deformation of polycrystalline samples. In " n o r m a l " creep studies, sliding on interior grain boundaries must be accompanied by accommodation deformation in adjacent grains. This accommodation deformation can a r i s e from slip in grains adjacent to the grain boundaries due to s t r e s s concentrations at triple points. 7'8 A schematic diagram of this type of sliding with associated accommodation is illustrated in Fig. l(a) which shows a c r o s s sectional view of a " n o r m a l " polycrystalline sample in which sliding occurs on some of the interior grain boundaries. The associated accommodation is shown by the dotted lines which indicate slip. In contrast to sliding on interior grain boundaries, grain boundaries which intersect the surface of " n o r m a l " polycrystalline samples or t r a v e r s e the specimen thickness in thin samples, should slide with a c commodation requirements which are different from those for " n o r m a l " interior grain boundaries. Fig. l(b) shows sliding on grain boundaries which
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