Morphology of small-angle boundary precipitates in BCC Fe-8 Pct Mo and Fe-4 Pct Au
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Table II. Stress-Etch Calibration Constants and Threshold Strains
CalibraUon Constants, Threshold ThresholdStress, Material Strain,e0 So = eoE , ksl 4340-A6 0.0011 D6-AC 0.00115 1045 (~0.003)*
32 28 85
So/Syzeld
0.18 0.12 0.85
lne = C + g/d C g, cm
-7 27 -7.16 -5.98
0.018 0.0079 0.0049
Boundary Precipitates in BCC Fe-8 Pct Mo and Fe-4 Pct Au H. B. AARON AND H. I. AARONSON
*Estimated. W H E N the crystal structures of the matrix and precipitate are different, precipitates formed at small-angle boundaries in an fcc matrix are normally primary sideplates. I* The one exception so
*Primarysideplatesgrowdirectlyfrom the grainboundaryinto the interiorof the matrixgrain,whereassecondarysideplatesdevelopfroma baselayerof anothermorphology,usuallygrainboundaryallotriomorphs2
s t r e s s r e g i o n beyond the p l a n e - s t r e s s y i e l d zone i s c l e a r l y etched. The furrow spacing c o r r e s p o n d s to a r e s i d u a l t e n s i l e s t r e s s ~61 k s i . The s p a t i a l r e s o l u t i o n of r e s i d u a l s t r e s s f i e l d s , which can be a c c o m p l i s h e d on a fine s c a l e a s shown in F i g s . 4 and 5, can be a v a l u a b l e l a b o r a t o r y tool. The technique i s p a r t i c u l a r l y c o n v e n i e n t b e c a u s e no s p e c i a l s u r f a c e p r e p a r a t i o n s n o r aging t r e a t m e n t s a r e r e q u i r e d . Even a s - m a c h i n e d c r o s s - s e c t i o n s can be d i r e c t l y etched and e x a m i n e d for t h r o u g h - t h e - t h i c k n e s s r e s i d u a l s t r e s s e s . Other p o t e n t i a l a p p l i c a t i o n s include: the study of p r o g r a m m e d loading effects on fatigue c r a c k p r o p a g a t i o n ; p r e s t r e s s effects on f r a c t u r e toughn e s s ; and the d e s i g n and d e v e l o p m e n t of c a s t i n g and f o r m i n g p r o c e s s e s . Its u s e in the m e a s u r e m e n t of s t r e s s e s in b r i d g e s t r u c t u r e s might be p a r t i c u l a r l y a d v a n t a g e o u s b e c a u s e of the difficulty of o t h e r w i s e d e t e r m i n i n g loads due to shifts in footings c a u s e d by e a r t h q u a k e s , floods, or s e t t l i n g and r e s i d u a l s t r e s s e s i n t r o d u c e d d u r i n g the e r e c t i o n p r o c e s s . T h i s work was supported by A r m y M a t e r i a l s and Mechanics Research Center, Watertown, Mass.
far found to this r u l e is A 1 - Z n - M g alloys 3-5 w h e r e i n the c o m p l e x and f i n e - s c a l e r e a c t i o n s e q u e n c e may have affected the r e s u l t s . When the m a t r i x has a bcc c r y s t a l s t r u c t u r e , on the other hand, a d i f f e r e n t p i c t u r e has begun to e m e r g e . In an A l n i c o alloy, 6 a s i n g l e m i c r o g r a p h showed p r i m a r y s i d e p l a t e s at s o m e - s m a l l angle b o u n d a r i e s , but p r i m a r y sawteeth and g r a i n b o u n d a r y a l l o t r i o m o r p h s at o t h e r s ] O r i e n t e d b i c r y s t a l s of an F e - S i alloy y i
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