The relation between grain-boundary orientation and intergranular cracking
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THE p r e s e n c e of grain boundaries in a m e t a l exerts a profound influence on many of its properties. For e x a m p l e , these internal interfaces can interact strongly with impurities and lattice defects,1 can provide a p r e f erential site for precipitation,2,3 can impede the motion of dislocations,4 can serve a s a source of dislocations, and perhaps m o s t importantly, in t e r m s of m a t e r i a l integrity, can serve a s a p r e f e r r e d path for c r a c k nucleation and propagation,s'6 An understanding of this l a s t phenomenon requires a systematic study of the magnitude of these effects a s a function of the specific orientation of the g r a i n boundary and of the relative orientation of the adjoining grains. P r i o r studies have b e e n extremely limited and incomplete. The problems of sufficient g r a i n boundary sampling and of simple but meaningful representation of the orientation effects have precluded significant activity in this a r e a . The procedures to be p r e s e n t e d have obviated m o s t of these difficulties. It h a s b e e n shown that significant grain-boundary cracking can be produced in purified iron by cathodically charging with hydrogen a t room t e m p e r a t u r e , in the a b s e n c e of a n applied e x t e r n a l s t r e s s .6 For e x a m p l e , it is a simple m a t t e r to prepare s a m p l e s with 20 p c t of the grain-boundary a r e a exhibiting welldefined intergranular c r a c k s . This represents a n i d e a l system in w h i c h to study the orientation dependence of such c r a c k s . It h a s b e e n suggested by one of the authors,7 continuing an argument f i r s t detailed by Li,8'9 that grain-boundary properties are more often dependent directly on the relative atomic misfit a t the boundary structure ( d e s c r i b e d by the t e r m , "grain-boundary porosity") t h a n on the specific grain-boundary energy. With the data obtained in this investigation, it is possible to demonstrate semiquantitatively the effects of the relative orientation of grains and g r a i n boundaries. In o r d e r to show this graphically, it w a s n e c e s s a r y to devise a simple m e a n s of representing the five d e g r e e s of freedom r e B. B. RATH and I. M. BERNSTEIN are with the Edgar C. Bain Laboratory for Fundamental Research, U.S. SteelCorp., Research Center, Monroeville, Pc. Manuscript submittedMarch 18, 1971. METALLURGICAL
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quired to describe completely the boundary orientation.
E X P E R I M E N T A L P R O C E D U R E T h e following m a t e r i a l s w e r e studied: i) A s - r e c e i v e d F e r r o v a c - E i r o n w i t h a n interstitial c o n t e n t of 50 p p m C , 2 0 p p m O , a n d I 0 p p m N . 2) Wet-hydrogen-decarburized F e r r o v a c - E iron with a n interstitial c o n t e n t of I 0 to 15 p p m of c a r b o n a n d of nitrogen and 50 p p m o x y g e n . 3) V a c u u m - m e l t e d P l a s t i r o n to w h i c h 0.15 w t pct T i w a s a d d e d to " g e t t e r " f r e e interstitials.* *Ferrovac-E andPlast
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