Origin of brittle intergranular fatigue fracture in warm aged Al-3.6 wt pct Cu
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J. S. SANTNER is with U.S. Air Force Materials Laboratory, Metals & Ceramics Division, Wright-Patterson Air Force Base, Dayton, OH 45433. M. E. FINE is Walter P. Murphy Professor of Materials Science & Engineering, Northwestern University, Evanston, IL 60201. Manuscript submitted September 5, 1975. METALLURGICAL TRANSACTIONS A
(a)
(b)
(
(c) Fig. 1--TEM m i c r o g r a p h s of typical grain boundary a r e a for AI-3.6 wt pct Cu aged at 160~ for 48 h with a magnification of 20,000 t i m e s : (a) 0 deg tilt, (b) t deg 40 rain tilt, and (c) 4 deg 10 mia tilt. VOLUME 7A, APRIL 1976-601
Table I. Fracture Mode as a Function of z~tK and da/dN for AI-3.6 Wt Pct Cu Aged at 190~
Aging Time, 12 h
Pct lntergranular Fracture 0 33 50 67 100
Aging Time, 36 h
Aging Time, 54 h
z~rf
da/dN
AK
da/dN
AK
da/dN
6 to 10 I0 to 14 14 to 17 17 to 34 none
0.2 to 1.6 1.6 to 6.7 6.7 to 30 30 to none
4 to 10 I0 to t4 14 to 22
1.0 to 4.6 4.6 to 14 14 to 120
4 to 9
0.2 to 2.0
9 to 11
2.0 to 5.6
22 to 33
120 to "~
11 to 28
5.6 to r
AKvaluesare givenin units of MN/m~/2,valuesof crack propagation rates da/dN are in units of 10-2/am/cycle.
Table II. Mechanical and Microstmctural Pmpertias of Two AI-3.6 Wt Pet Cu Alloys
Heat Treatment Two days, 160~ 36 h, 190~
s, ParticleSpacing,/am 0.5 0.5
h, ParticleSize,/am
Ou MPa
Kc MPav~
0.3 0.5
299 289
33 30
Fig. 2--SEM fractograph of grain boundary failure in A1-3.6 wt pct Cu aged 48 h at 160~ magnification 780 times. This structure is identical for all areas exhibiting this fracture mode. b o u n d a r i e s a p p r o x i m a t e l y 0.3 ~ m in d i a m and s p a c e d a p p r o x i m a t e l y 0.5 Izm a p a r t . F i g . 2 shows a t y p i c a l high m a g n i f i c a t i o n SEM f r a c t o g r a p h of the b r i t t l e i n t e r g r a n u l a r f r a c t u r e s u r f a c e found in a fatigue c r a c k p r o p a g a t i o n s p e c i m e n aged two days at 160~ The two a d j a c e n t p l a n a r s u r f a c e s of the g r a i n s a r e a p p r o x i m a t e l y 45 deg with r e s p e c t to the plane of the photograph. N e t w o r k s of r i d g e s outlining pitted a r e a s a r e o b s e r v e d on both s i d e s of the b o u n d a r y . T h e s e a p p e a r as e q u i a x e d t e n s i l e d i m p l e s f o r m i n g a c e l l - l i k e s t r u c t u r e of 5 /~m d i a m on the r i g h t - h a n d s i d e and 0.5 /zm diana on the l e f t - h a n d s i d e . P r e s u m a b l y , the v o i d s a s s o c i a t e d with the d i m p l e f o r m a t i o n i n i t i a t e at the p r e c i p i t a t e s in the g r a i n boundary. The g r a i n boundary p r e c i p i t a t e s shown in the TEM m i c r o g r a p h s a r e a p p r o x i m a t e l y 0.5 ~zm a p a r t which is s m a l l e r than the d i a m e t e r of the c e l l - l i k e s t r u c t u r e on the r i g h t - h a n d side; h o w e v e r , the g r a i n on the l e f t - h a n d side of the boundary shows the p e r i o d i c i t y p r e d i c t e d f o r d u c t i l e void c o a l e s c e n c e 602-VOLUME 7A, APRIL
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