Abstract |
소성변형에 대한 著者들의 이론을 초소성합금(Zn-Al eutectoid, A1-Cu, Pb-Sn, Sn-Bi, Mg-Al eutectics)에 적용하였다. 그 결과 초소성합금의 소성변형은 두 개의 grain boundary流動單位의 平行連結로 나타낼 수 있었다. 이 두 개의 流動單位는 流動式에 나타나는 parameter Xgj/아gj와 βgj(j=1 혹온 2)로 表現할 수 있으며 이들을 實驗的으로 求할 수 있었다. 著者들의 流動式은 實驗과 잘 一致하였다. Strain rate sensitivity 對 -In(strain rate) 곡선을 이론으로 구한 결과 유동단위수만큼의 peak가 βgj(j=1 or 2) 값에 따라 분리되어 나타났고 초소성의 조건도 βgj 값에 의하여 결정됨을 알았다. βgj값의 粒子크기 依存性을 구하였고 온도변화에 따른 βgj값 변화로부터 각 流動單位의 활성화엔탈피, ΔHgj도 구하였다. 그 결과 ΔHgj는 재료성분원소들의 grain boundary 자체확산에 의한 활성화엔탈피와 같이 나타났고 또 이들은 粒子 크기 증가에 마라 증가함을 보였다. 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/αgj and βgj (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 (=δIn f/δIn 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 βg1 and βg2. The condition of superplasticity is also determined by βgj, which satisfies Smj≤1.53/βgj [Eq.(13)], where Smj is the s of the jth unit at the peak. The grain size dependence of βgj is described by ln(βgj)-1=alnχ+b [Eq. (16)], where χ 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 βgj and ΔHgj are related, we obtained the conclusion that superplasticity occurs in the system having small ΔHgj 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 ΔHgj value increases with grain size as expected from Eq. (16). |
