Creep and ductility in an Al-Cu solid-solution alloy
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I.
INTRODUCTION
I N creep analysis, the stress dependence of creep rate is generally deduced from plotting the steady-state creep rate as a function of the applied stress and applying the relation: n
_ l0n ~
01nr
r
[I]
where n is the stress exponent measured at constant temperature, T, ~, is the steady-state shear creep rate, and r is the applied shear stress. Earlier creep experiments ~-4 revealed the presence of two limiting values of the stress exponent, n , in solid-solution alloys. The upper limiting value is close to 5 and is observed in alloys whose creep behavior, like that of pure metals, exhibits the characteristics of climb control: 1'2'3 extensive primary creep, subgrain formation, and dependence of the creep rate on the stacking fault energy of the alloy. The lower limiting value is 3 and is noted in alloys whose creep behavior exhibits the characteristics of viscous glide control: 1'2~3 brief primary creep, random distribution of dislocations, and apparent insensitivity of the creep rate to changes in stacking fault energy. It was suggested 2-5 that the creep behavior of a solidsolution alloy may be controlled by the sequential processes of dislocation climb and viscous glide. On the basis of this suggestion, it was predicted 2'3'5 that under a favorable combination of material parameters (such as the atom misfit ratio) and experimental variables (such as stress), the creep behavior of an alloy would exhibit a transition from climb-controlled behavior at low stresses to viscous glidecontrolled behavior at intermediate stresses. Several sets of experimental results 5-1~ that support the predicted transition from dislocation climb to viscous glide, with increasing stress, are now available. These include
PRABIR K. CHAUDHURY, a Graduate Research Assistant, and FARGHALLI A. MOHAMED, Professor, are with the Department of Mechanical Engineering, University of Califorma-lrvine, Irvine, CA 92717. Manuscript submitted January 26, 1987. METALLURGICAL TRANSACTIONS A
(a) a change in the stress exponent for creep in A1-Mg alloys 6'9-n and an A1-21 wt pct Zn alloy ~2from a value of 4.5 at low stresses to a value of about 3 at intermediate stresses; this change in stress exponent is in agreement with the mechanical aspect of the prediction; and (b) a corresponding change in the creep substructure of two A1-Mg alloys 9'1~ from that typical of dislocation climb (significant tendency to form well-developed subgrains) to that typical of viscous glide (random distribution of dislocations) with increasing stress; this finding is in accordance with the substructural aspect of the prediction. In addition to providing evidence in support of the predicted transition from dislocation climb to viscous glide, the experimental data on AI-Mg alloys II and an A121 wt pct Zn alloy 12 revealed the presence of another transition at high stresses. This transition is manifested by a change in stress exponent from about 3 (viscous glide) to higher values and has been attributed to dislocation breakaway from a solute-atom atmosph
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