Effect of stress reductions on the stress exponent and subgrain size in an Al-Zn alloy
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I.
INTRODUCTION
A s a result of many investigations on the high-temperature creep of pure metals and solid-solution alloys,~-5 two relations are well established. The first relation describes the dependence of the steady-state creep rate, 5', on the applied stress, r, at large grain sizes and intermediate stresses (10 -5 G < ~- < 10-3 G, where G is the shear modulus) and can be expressed as h2'3 = BO-/C) o
If]
where B is a function of temperature and n is the stress exponent; for pure metals and alloys of the metal class,* n is close to 5 whereas for alloys of the alloy class,* n is about *"Alloy" class and "metal" class correspond to Sherby and Burke's 4 "class I" and "class II", respectively.
3. The second relation describes the dependence of the average subgrain size, 6, measured during the steady-state creep of metals and alloys of the metal class on the applied stress and is represented b y ~'z ~/b = 10(~-/C)-'
[2]
where b is the Burgers vector. Equation [2] was originally suggested by Bird et al. ~ and was based on an analysis of the data of a number of materials, with the exception of AI whose polarized light optical data gave subgrain sizes larger than those predicted from the equation. However, a very recent investigation on A1,6 using both TEM and EP, demonstrates that optical techniques tend to overestimate subgrain size and that the data, when taken from representative TEM measurements, are consistent with Eq. [2]. Equations [1] and [2] imply that under steady-state creep conditions both 3; and 6 are not dependent on the loading
ANURAG GOEL, formerly Graduate Student, University of California, Irvine, CA 92717, is now with Amada Engineering, La Mirada, CA 90638. TIMOTHY ]. GINTER, Graduate Research Assistant, and FARGHALLI A. MOHAMED, Associate Professor, are with the Department of Mechanical Engineering, University of California, Irvine, CA 92717. Manuscript submitted April 12, 1983. METALLURGICAL TRANSACTIONS A
history of the specimen being tested but are uniquely determined by the value of the applied stress. On the basis of this implication, it is expected that if the applied stress is rapidly changed either by an increase or by a decrease, both 3) and 6, following a period of transient creep, will reach new values that are appropriate to the new stress level. A recent investigation on an AI-5 wt pct Zn alloy 7 verified the above expectation in the case of stress increase tests but not in the case of stress reduction tests. According to the results of that investigation, 7 the final creep rate after a stress reduction is slower than the steadystate rate that would be determined from an uninterrupted test at the reduced stress--a finding which leads to a higher stress exponent--and the subgrain size does not coarsen even after creep strains of more than 15 pct were allowed at the reduced stress. The results of the investigation on A1-5 pct Zn 7 are in conflict with those of most recent creep investigations on NaC18 and AI9.t~ which show clearly that there is no difference between the subgrain
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