Defect structures and interactions in Al-Zn eutectoid alloys
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Defect s t r u c t u r e s in s u p e r p l a s t i c and n o n s u p e r p l a s t i c A1- Zn eutectoid alloys were studied by t r a n s m i s s i o n e l e c t r o n m i c r o s c o p y . Slowly cooled alloys develop a fine l a m e l l a r m i c r o s t r u c t u r e and are not s u p e r p l a s t i c . The equiaxed quenched and q u e n c h - a g e d a l l o y s , however, a r e s u p e r p l a s t i c u n d e r the p r o p e r conditions of t e m p e r a t u r e and s t r a i n r a t e . Q u e n c h - a g i n g p r o d u c e s subb o u n d a r i e s in the a l u m i n u m - r i c h ot phase and d i s l o c a t i o n loops in the z i n c - r i c h 13 phase. Subsequent r o o m t e m p e r a t u r e d e f o r m a t i o n c r e a t e s a typical c o l d - w o r k e d d i s l o c a t i o n s t r u c t u r e in the ot phase. At higher d e f o r m a t i o n t e m p e r a t u r e s , the c o l d - w o r k e d s t r u c t u r e is i n c r e a s i n g l y r e placed by a r e c o v e r e d s t r u c t u r e . At the s a m e t i m e , the d i s l o c a t i o n loop d e n s i t y in the /3 phase d e c r e a s e s to lower v a l u e s . D u r i n g s u p e r p l a s t i c d e f o r m a t i o n at 250~ the d i s l o c a t i o n loops in the /3 phase are annihilated by interactions with glide dislocations. In the (~ phase, a continuous, or steady-state, dynamic recovery process appears to operate. A completely recovered structure is maintained with dislocation-free subgrains. N U M E R O U S s t u d i e s of s u p e r p l a s t i c i t y 1-9 have off e r e d a v a r i e t y of m e c h a n i s m s , as well as a f a i r l y complete p h e n o m e n o l o g i c a l d e s c r i p t i o n , of the s u p e r p l a s t i c d e f o r m a t i o n p r o c e s s . The p o s t u l a t e d m e c h a n i s m s are based on boundary sliding,b~ boundary migration,3 grain rotation,~diffusional creep, %5 dislocation climb,e dislocation motion,7'8 or recrystallization3 during deformation. In these reports, more emphasis has been placed on the deformational and microstructural behavior of the alloys than on the role of the defects accompanying the process. Consequently, it has been concluded ~~that none of the proposed mechanisms can fully explain the superplastic deformation process. Superplasticity has been studied extensively in the Ai-78 pct Zn eutectoid alloy,s-~6 primarily because of the pronounced superplastic effect and the relatively large background of experimental data. Only a limited number of investigations, however, have utilized thin foil transmission electron microscopy in this alloy.12-,e The findings generally confirmed the presence of dislocation-free subgrains,~2''3 subboundarie s, iz, 13and dislocations,m,is while pile-ups were not specifically noted except for one micrograph in one paper. 's Unambiguous evidence of dislocation loops and their interactions with dislocations was first documented in the A1-78 pct Zn alloy by Lee et al. ~2 Moreover, dislocation generation at boundaries, superplastic elongation of grains, and re
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