On the characteristics of the threshold stress for superplastic flow in Zn-22 Pct Al
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~'o/G =/30 exp (Qo/RT)
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where G is the shear modulus,/30 is a constant, Q0 is an activation energy term, T is the absolute temperature, and R is the gas constant [=8.3144 J / ( m o l - K ) ] . Very recently, the effect on region I behavior of a selected impurity was studied. ~~ In that study, three grades of Zn-22 pct A1, A, B, and C, which were prepared from high-purity A1 and Zn and which contained Fe levels of 423, 125, and 40 ppm, respectively, were creep tested under identical experimental conditions. In addition to showing the presence of a typical sigmoidal relationship between stress and strain rate, the data on the three grades have revealed that region I behavior is SUSAN T. YANG, formerly Graduate Research Assistant, Department of Chemical and Biochemical Engineering, Materials Science and Engineering, University California-lrvine, is with Woon-Hawk Engineering Plastics, Inc., Costa Mesa, CA 92626. FARGHALLI A. MOHAMED, Professor, is with the Department of Chemical and Biochemical Engineering, Materials Science and Engineering, University California-lrvine, lrvine, CA 92717. Manuscript submitted June 30, 1994. METALLURGICAL AND MATERIALS TRANSACTIONS A
sensitive to Fe level. It has been demonstrated that such a sensitivity can be explained in terms of a threshold stress for creep. While these findings are similar to those reported earlier t7,81 for other grades of Zn-22 pct A1 containing different levels of various impurities, consideration of the values of the threshold stresses, r0, estimated from the data have raised the possibility that with increasing Fe level, r0 appears to approach a limiting value for Fe concentrations higher than 120 ppm and probably close to 400 ppm. This possibility has motivated the work reported here, in which the threshold stress behavior in Zn-22 pct A1 containing 1460 ppm of Fe has been investigated and compared to that characterizing grades A, B, and C. Zn-22 pct AI (grade 0) used in the present investigation was supplied by Johnson Matthey Electronics, Spokane, WA. The alloy was prepared by melting Zn and A1 of high purity (99.999 pct); the preparation procedure was reported previously. I7,8,~~ The purity of the starting materials, Zn and A1, and the level of impurities are given in Table I. Double-shear specimens of shape and dimensions described elsewhere t~l were machined from the above grade. Prior to testing, all specimens were solution treated in an argon atmosphere at 633 K for 15 hours, quenched to 273 K in an ice-water bath, and then annealed in an argon atmosphere at 533 K for 13.5 hours to produce an equiaxed grain size of 2.5 0.2 /zm. The experimental procedure was given in full detail elsewhere. ~4,7,~~ It is only important to mention the following points: (a) the tests were conducted in the temperature range of 409 to 493 K, using a constant-stress creep machine or an Instron machine operating at constant strain rate; (b) on the creep machine, uninterrupted and stress-change tests were performed, whereas on the Instron machine, constant strain rate a
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