A study of fracture in high purity 7075 aluminum alloys
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range in size from 5 to 30 ~m. The d i s p e r s o i d p a r t i cles p r e c i p i t a t e from solid solution during the homogenizing solution t r e a t m e n t of the ingot. These p a r t i c l e s a r e , for example, c h r o m i u m or z i r c o n i u m - r i c h , ranging in size f r o m 0.03 to 0.3 p m . The hardening p a r t i c l e s a r e t h e r m o d y n a m i c a l l y the l e a s t stable and they can p r e c i p i t a t e from the solid solution at r o o m t e m p e r a t u r e . They range in size f r o m 50A (5 nm) to 500A (50 nm), having e i t h e r coherent or incoherent i n t e r f a c e s . The high s t r e n g t h s exhibited by the 7000 s e r i e s alloys for example, a r e produced by z i n c - r i c h p r e c i p i t a t e s upon w a r m aging. In so far as the effect of constituent p a r t i c l e s on toughness is concerned, previous work 4 has demons t r a t e d that high purity 7000 s e r i e s aluminum alloys have a d r a m a t i c a l l y improved strength v s toughness trend over those of c o m m e r c i a l l y pure 7000 s e r i e s aluminum a l l o y s . Both the amount (volume fraction) and size of the constituent p a r t i c l e s d e t e r m i n e the p u r i t y of an alloy. F o r example, by specifying higher p u r i t y a l l o y s in forgings such as 7175 over 7075, cost p e n a l t i e s a r e i n c u r r e d . The level of p u r i t y above which f r a c t u r e toughness cannot be i m p r o v e d by this t e c h n i c a l a p p r o a c h has economic consequences. F o r the d i s p e r s o i d p a r t i c l e s , studies have shown 5 r e p l a c i n g the n o r m a l chromium d i s p e r s o i d with z i r conium in 707 5 has improved f r a c t u r e toughness with no loss of strength. The aluminum alloy 7050 was d e veloped b a s e d on the o b s e r v a t i o n that z i r c o n i u m i m p r o v e s toughness. However, the c a u s e s for this i m p r o v e m e n t a r e not c o m p l e t e l y understood. The effect of the hardening p a r t i c l e s on f r a c t u r e toughness has r e c e i v e d l e s s attention. This may be due to the g e n e r a l e m p i r i c a l o b s e r v a t i o n that by inc r e a s i n g the strength of an alloy through heat t r e a t ment, its f r a c t u r e toughness is s i m u l t a n e o u s l y de-
JOSEPH S. SANTNERis Project Engineer, Metals and Ceramics Division,Air Force MaterialsLaboratory, Wright-PattersonAir Force Base, OH 45433. Manuscript submitted March 24, 1977. ISSN 0360-2133/7810612-0769500.7510 METALLURGICALTRANSACTIONSA 9 1978AMERICANSOCIETYFOR METALSAND THE METALLURGICALSOCIETYOF AIME
VOLUME 9A, JUNE 1978-769
Table I. Compositions of Experimental and Commercial Alloys
Composition Zn Mg Cu Cr Fe Si Mn Ti Others (Total) Composition Zn Mg Cu Zr Fe Si Mn Ti Cr Other (Total)
7475
A
B
C
D
E
5.2 to 6.2 5.89 5.91 5.93 5.93 5.94 1.9 to 2.6 2.19 2.42 2.39 2.35 2.36 1.2 to 1.9 1.50 1.58 1.60 1.64 1.63 0.18 to 0.25 0.21 0.22 0.21 0.21 0.21 0.12 0.02 0.05 0.08 0.12 0.20 0.10 0.01 0.02 0.04 0.06 0.i 1 0.06
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