Structure and strengthening of Al-Mg alloys during hot working
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STRENGTHENING
m e c h a n i s m s and m i c r o s t r u c t u r a l d e v e l o p m e n t d u r i n g hot d e f o r m a t i o n (T/Tm > 0.5) has b e e n the s u b j e c t of c o n s i d e r a b l e i n t e r e s t d u r i n g r e c e n t years.a-~l S t r e n g t h e n i n g has b e e n a t t r i b u t e d to the p r e s e n c e of long r a n g e i n t e r n a l s t r e s s and r a t e c o n t r o l l i n g glide p r o c e s s e s d u r i n g d e f o r m a t i o n . It has b e e n suggested that s t r e n g t h e n i n g in solid s o l u t i o n s is a r e s u l t of r e t a r d e d d y n a m i c r e c o v e r y which in s o m e c a s e s is due to the effect of the solute on the s t a c k i n g fault e n e r g y . ~ T h i s leads to an i n c r e a s e d d i s l o c a t i o n d e n s i t y and r e f i n e m e n t of the s u b g r a i n size.1 A n i n v e r s e r e l a t i o n s h i p between s u b g r a i n size and steady flow s t r e s s has been noted in m a n y alloy s y s t e m s .I,5,6, 9, i0
In contrast, earlier work by Cotner and Tegart i~ on hot deformation of Ai-Mg binary system over a range of temperature and magnesium levels, revealed that while the strength increased with greater concentration of magnesium, the size of the subgrains also increased for the same testing conditions. They found no systematic differences in subgrain misorientation as a function of the Mg content or test temperature to account for the higher elevated temperature strength of AI-Mg alloys compared to aluminum. McQueen and Jonas ~ later suggested that the strengthening could arise from differences in the subgrain boundary (dislocation) structure. Structural characterization of hot worked samples quenched after deformation has been questioned. 12 It has been argued that the time lapse between the termination of the test and subsequent water quenching, h o w e v e r s m a l l , could c a u s e l o s s in s t r u c t u r a l d e t a i l s . T h e extent of s t r u c t u r a l change d u r i n g this t i m e lapse can be e v a l u a t e d by i n t e r r u p t e d hot c o m p r e s s i o n t e s t ing.~-~5 McQueen and B e r g e r s o n 16 a t t e m p t e d to o v e r come this s h o r t c o m i n g by quenching the s p e c i m e n as the d e f o r m a t i o n is in p r o g r e s s , but these t e c h n i q u e s could cause a d d i t i o n a l w a r m or cold d e f o r m a t i o n and d i s t o r t the d y n a m i c s t r u c t u r e . A new t e c h n i q u e , t e r m e d c o n t r o l l e d t o r q u e quenching, was developed d u r i n g the p r e s e n t i n v e s t i g a t i o n e n a b l i n g q u e n c h i n g of the s p e c i m e n d u r i n g t e s t without any a d d i t i o n a l d e formation. T h e . p r e s e n t i n v e s t i g a t i o n was u n d e r t a k e n to p r o v i d e M. RAGHAVAN,formerly with Olin Metals Research Laboratories, is now Senior Research Scientist, with Exxon Research and Engineering, Linden, NJ. E. SHAPIRO is Group Supervisor, Olin Metals Research Laboratories, New Haven, CN 06511. Manuscript
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