Cast microstructure and fatigue behavior of a high strength aluminum alloy (KO-1)
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Ko-I alloy is a high strength aluminum alloy extensively used in the production of premium quality castings. Its nominal composition in weight percent is: Cu: 4.7, Mg: 0.3, Mn: 0.3, Ag: 0.6, Ti: 0.2, Fe: less than 0.03, Si: less than 0.03, Al: balance. KO-I alloy exhibits, in the solution treated and artificially aged condition, average properties of 57,000 psi yield strength and 67,000 psi ultimate tensile strength with up to I0 pct elongation. Microstructurally, in the as-cast condition the alloy consists of aluminum-rich primary dendrites and a copperrich (CuAl2 containing some Ag, Mn, and Mg) interdendritic nonequilibrium secondary phase. The as-cast alloy also contains various amounts of insoluble secondary phase resulting from impurities (iron and silicon) or from alloying elements present in amounts exceeding their solubility (TiAI3). During solution heat treatment, the amount of copper-rich secondary phase is reduced considerably without, however, being completely eliminated under commercial solution heat treatment practices. The solution kinetics of such nonequilibrium secondary phases, and its dependence on solution time and temperature as well as dendrite arm spacing, have been investigated experimentally and analytically by Singh and F l e m i n g s 1 for a high p u r i t y w r o u g h t 7075 high s t r e n g t h a l u m i n u m a l l o y , and by B h a m b r i and K a t t a m i s for a M g Zn a l l o y g r a i n - r e f i n e d with z i r c o n i u m . 2'3 The i m p o r t a n t influence of s e c o n d a r y p h a s e s on m e c h a n i c a l p r o p e r t i e s of a l u m i n u m a l l o y s is f a i r l y w e l l known. A n t e s , L i p s o n , and R o s e n t h a l 4 have e s t a b l i s h e d that the e l i m i n a t i o n of s e c o n d a r y p h a s e p a r t i c l e s s u b s t a n t i a l l y i m p r o v e d the d u c t i l i t y of 7000 s e r i e s w r o u g h t a l u m i n u m a l l o y s . Singh and F l e m i n g s 5 have d e m o n s t r a t e d that both y i e l d and u l t i m a t e t e n s i l e s t r e n g t h s of a wrought 7075 high s t r e n g t h K-H. CH1EN is Graduate Student, Georgia Institute of Technology, Atlanta, Ga. T. Z. KATTAM1Sis Associate Professor, Department of Metallurgy, Institute of Materials Science, University of Connecticut, Storrs, Conn. 06268. F. R. MOLLARD is with lnstitut Battelle, Geneva, Switzerland. Manuscript submitted June 9, 1971. METALLURGICAL TRANSACTIONS
a l u m i n u m a l l o y i n c r e a s e d l i n e a r l y with d e c r e a s i n g a m o u n t of i n t e r d e n d r i t i c s e c o n d a r y p h a s e . A n u m b e r of s t u d i e s of the fatigue p r o p e r t i e s of v a r i ous a l u m i n u m a l l o y s have b e e n m a d e . 2'6-8 F a t i g u e c r a c k s have b e e n o b s e r v e d to i n i t i a t e a t s i t e s of high s t r e s s c o n c e n t r a t i o n such a s s e c o n d a r y p h a s e i n c l u s i o n s , both i n t e r m e t a l l i c and n o n m e t a l l i c , and m i c r o p o r e s . 2'6-8 E r h a r d t , P e l l o u
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