Fracture-microstructure observations in the SEM
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The c o r r e l a t i o n b e t w e e n the m i c r o s t r u c t u r e of a f r a c t u r e d m e t a l and f e a t u r e s of the f r a c t u r e s u r f a c e i s done m a i n l y by o p t i c a l m i c r o s c o p y . The f r a c t u r e d p a r t i s cut p e r p e n d i c u l a r to the f r a c t u r e s u r f a c e , then mounted, p o l i s h e d and etched. This technique has i t s l i m i t a t i o n s . One i s the r e l a t i v e l y low r e s o l v i n g p o w e r of the o p t i c a l m i c r o s c o p e but m o r e i m p o r t a n t i s the f a c t that only the c r o s s s e c t i o n of the f r a c t u r e is o b s e r v e d , m a k i n g c r y s t a l l o g r a p h i c c o n c l u s i o n s u b j e c t to doubt. O b s e r v a t i o n s of f r a c t u r e s u r f a c e s by s c a n n i n g e l e c t r o n m i c r o s c o p y , on the o t h e r hand, l a c k m i c r o structural information. The t e c h n i q u e d e s c r i b e d h e r e i s d e s i g n e d to o v e r c o m e t h e s e d e f e c t s and to p r o d u c e high r e s o l u t i o n , t h r e e d i m e n s i o n a l m i c r o g r a p h s of both the f r a c t u r e s u r f a c e and t h e m i c r o s t r u c t u r a l f e a t u r e s p e r p e n d i c u -
D. SHECHTMAN, formerly a visiting scientist in the Metallurgy and Ceramics Research Laboratory, Aerospace Research Laboratories, Wright-Patterson AFB, Ohio, is now with the Department of Materials Engineering, Technion, Haifa, Israel. Manuscript submitted June 2, 1975.
l a r to it. The a r e a on the f r a c t u r e s u r f a c e c h o s e n to be p r e s e r v e d for o b s e r v a t i o n i s c o a t e d with a l a y e r of s t o p - o f f l a c q u e r and the s p e c i m e n i s then e l e c t r o p o l ished. Due to the l o c a l l y h i g h e r c u r r e n t d e n s i t y n e a r the edge of the l a c q u e r p r o t e c t i v e coating, a r e l a t i v e l y d e e p g r o o v e is f o r m e d , one s i d e of which i s p e r p e n d i c u l a r to the fracture surface. Fig, 1 shows the formation of such a groove in a fractured specimen of processed Ti-6AI-2Sn-4Zr2Mo fractured in a brittle mode under triaxial stress. Fig. 2 is a detailed stereo micrograph of this Specimen, on which the correlation between the microstructure (~ and thin fl plates) and the fracture can be made. The electropolishing of this specimen was performed in a solution of 3 g succinic acid and 3 cc concentrated sulfuric acid in i000 cc methanol. The polishing was done at a temperature of i0 to 15~ and voltage of 15 to 20 volts for a few minutes.I The a phase is polished off and fl plates are not effected. Ultrasonic cleaning of the specimen in acetone is needed in order to remove the overlapping/3 leaves. When the electropolishing does not reveal details of the microstructure, additional chemical etching may be required. Fig. 3 demonstrates such a case. The specimen is Ti-29.8 pct Al, isothermally forged and broken in tension at room temperature. The specimen was electropolished as previously described and et
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