The role of grain boundary solutes and structure on the yielding and intergranular cracking of iron
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boundary and the abilityof such complexes to react with hydrogen.
T H E R E have been many studies which have supported the contention that the concentration and d i s t r i b u t i o n of the i n t e r s t i t i a l solutes, carbon and nitrogen, can have a profound effect on the m e c h a n i c a l p r o p e r t i e s of iron. It has been r e p o r t e d that these i n t e r s t i t i a l s can control both the incidence of c r a c k i n g and the r e s u l t a n t c r a c k path in l o w - t e m p e r a t u r e f r a c t u r e of iron, and the magnitude of the Petch p a r a m e t e r s during yielding, t-s Recently it has been suggested that they can also cont r o l the frequency of hydrogen-induced g r a i n - b o u n d a r y c r a c k i n g in purified iron. 4 The r e l a t i o n between these effects and i n t e r s t i t i a l d i s t r i b u t i o n has been clouded by c l a i m s that the e x p e r i m e n t a l technique used in some of the studies is suspect, s In p a r t i c u l a r , the d i s t r i b u tion is usually e s t a b l i s h e d by control of cooling r a t e a f t e r a h i g h - t e m p e r a t u r e anneal. Rapid cooling can produce d i s l o c a t i o n s and it has been postulated that t h e i r p r e s e n c e could change the Perch p a r a m e t e r s . 5 One p u r p o s e of this study was to d e m o n s t r a t e that the o b s e r v e d changes in the Petch p a r a m e t e r s t r u l y r e f l e c t the d i r e c t influence of carbon and nitrogen. The second p u r p o s e of this investigation was to e s tablish the effect of i n t e r s t i t i a l s on the strength of grain boundaries, i.e., whether making a boundary r e s i s t a n t to the p a s s a g e of d i s l o c a t i o n s also made it b r i t tle. The effects of v a r y i n g the concentration and d i s tribution of carbon and nitrogen on the Petch p a r a m e t e r s and on the frequency of hydrogen-induced c r a c k ing w e r e examined. In addition, the r e l a t i o n between these foregoing p a r a m e t e r s and grain boundary s t r u c ture was a l s o evaluated. By g r a i n boundary s t r u c t u r e , we do not mean the i n t r i n s i c g r a i n boundary d i s l o c a tions which make up, in p a r t , the e q u i l i b r i u m s t r u c t u r e of the boundary. Instead we a r e c o n s i d e r i n g f e a t u r e s which we 6 and o t h e r s 7 have l a b e l l e d e x t r i n s i c grain boundary d i s l o c a t i o n s or ledges. These a r e f e a t u r e s which do not contribute d i r e c t l y to e i t h e r the i n t r i n s i c boundary s t r u c t u r e or to the g r o s s m i s o r i e n t a t i o n b e tween adjoining g r a i n s . It has been postulated that such s t r u c t u r e is capable of acting as dislocation s o u r c e s . I. M. BERNSTEIN is Assistant Professor, Department of Metallurgy and Materials Science, Carnegie-Mellon University, Pittsburgh, Pa. 15213. B. B. RATH is Scientist, McDonnell-Douglas Research Laboratory, St. Louis, Mo. 63166. Manuscript submitted May 4, 1972. METALLURG
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