Molecular Theory of Superplastic Deformation

[摘要] The authors? theory for plastic deformation was applied to superplastic alloys (Zn-Al eutectoid, Al-Cu, Pb-Sn, Sn-Bi, Mg-Al eutectics). The plastic deformation of the superplastic alloys could be described by two Maxwell models connected in parallel which represent two grain boundary flow units. The flow units are characterized by the two parameters Xgj/¥ágj and ¥âgj (j=l or 2, g signifies the grain boundary) the values of which were obtained by applying our flow equation [Eq. (5)] to experiment. We confirmed that our flow equation describes the superplasticity very well. The curve of strain rate sensitivity m (=¥äIn f/¥äIn s) vs. -In s, where f and s are stress and strain rate, respectively, showed two peaks corresponding to flow unit gl and g2, the separation of the two peaks is determined by the difference between ¥âg1 and ¥âg2. The condition of superplasticity is also determined by ¥âgj, which satisfies Smj¡Â1.53/¥âgj [Eq.(13)], where Smj is the s of the jth unit at the peak. The grain size dependence of ¥âgj is described by ln(¥âgj)-1=aln¥ö+b [Eq. (16)], where ¥ö is the grain size, and a and b are constants. The activation enthalpy for each flow unit, AH& was also determined from the temperature dependence of pgj which is proportional to the relaxation time of the j th unit. Since the superplasticity is determined by Eq. (13), and since ¥âgj and ¥ÄHgj are related, we obtained the conclusion that superplasticity occurs in the system having small ¥ÄHgj values. The Aej values were equal to the activation enthalpies of grain boundary self-diffusion of the component atoms of the alloys, this accords with our proposed flow mechanism. The ¥ÄHgj value increases with grain size as expected from Eq. (16).

[学科分类] 化学（综合）