THE ROLE OF DISLOCATIONS IN HIGH-STRAIN-RATE SUPERPLASTICITY OF AN AL-NI-MISCH METAL ALLOY

Plastic deformation, microstructure and deformation substructure in an A1-14 wt% Ni-14 wt% Mm (misch metal) alloy produced from amorphous po wders were examined focusing on the effects of the initial average gra in size d(0), the grain boundary sliding and the motion of dislocation s on the superplasticity. The as-received and annealed specimens with d(0)=0.8, 1.6 and 2.5 mu m deformed at 573 K at an initial strain rate of 1/s. The specimen with d(0)=0.8 mu m showed high-strain-rate super plasticity accompanied by low flow stress and 350% elongation, while t he flow stress increased and elongation decreased with greater grain s ize. Grain refinement occurred and the frequency of small angle bounda ries showed a complicated change during superplastic deformation. On t he other hand, in the specimen with d(0)=1.6 mu m, the grains coarsene d rapidly. A large number of;dislocations were observed in grains even in the superplastically deformed specimen. The high-strain-rate super plasticity of A1-14 wt% Ni-14 wt% Mm alloy is controlled by the grain boundary sliding but the motion of dislocations also plays an importan t role in accommodating the highly strained region near triple junctio ns of the grain boundaries. (C) 1998 Acta Metallurgica Inc.. Published by Elsevier Science Ltd. All rights reserved.