Deformation modes in NbAl 3

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L. A. JACOBSON

The DOz2 lattice of the NbA13 i n t e r m e t a l l i c compound shows v e r y l i m i t e d ductility at room t e m p e r a t u r e . In this study the slip and twinning s y s t e m s that a r e active during the d e f o r mation p r o c e s s were investigated. Evaluation of the p o s s i b l e d e f o r m a t i o n modes was p e r f o r m e d and c o n t r a s t a n a l y s i s in the t r a n s m i s s i o n e l e c t r o n m i c r o s c o p e r e v e a l e d both e x pected and unexpected deformation modes. Two types of d i s l o c a t i o n s w e r e found in the d e f o r m e d s t r u c t u r e , namely the a (110) s u p e r d i s l o c a t i o n on the {112} plane and loops of unidentified d i s l o c a t i o n s on the {010} plane. No evidence of (201) s u p e r d i s l o c a t i o n s was found, p r o b a b l y due to the fact that this type of dislocation is expected to move in groups of four. Twins of the {112} type w e r e found to play an i m p o r t a n t r o l e in the deformation p r o c e s s since they supply a component of s h e a r p e r p e n d i c u l a r to the (001) plane.

MANYo r d e r e d

i n t e r m e t a l l i c compounds a p p e a r to have g r e a t potential for application as h i g h - t e m p e r a ture s t r u c t u r a l m a t e r i a l s b e c a u s e of t h e i r high m e l t ing points and s t a b i l i t y in oxidizing e n v i r o n m e n t s . The t r a n s i t i o n - m e t a l a l u m i n i d e s (MAI~, where M is a group IV B or V B metal) have been examined in the past with r e g a r d to their s u i t a b i l i t y for h i g h - t e m p e r a t u r e a p p l i cation; and while they w e r e not found to be applicable to s t r u c t u r a l components, they f o r m e d the b a s i s for a v a r i e t y of oxidation p r o t e c t i o n coatings for r e f r a c t o r y m e t a l s . 1 Very little is known about the o p e r a t i v e p l a s tic deformation m e c h a n i s m in these aluminides, all of which have r e l a t e d c r y s t a l s t r u c t u r e s b a s e d upon close packing of atoms on planes which would be designated [111] in the cubic c r y s t a l s t r u c t u r e . The s t r u c t u r e s differ from one another in the o r d e r i n g a r r a n g e m e n t of atoms within the c l o s e - p a c k e d planes and in the stacking sequence of planes. The compound NbA13 was chosen for this study b e c a u s e of its high melting point (1605 9 10~ m o d e r a t e density (4.54 g/cc), and adequate h i g h - t e m p e r a t u r e oxidation r e s i s t a n c e . Det e r m i n a t i o n of o p e r a t i v e d e f o r m a t i o n modes was und e r t a k e n in o r d e r to a s c e r t a i n whether adequate duct i l i t y might be r e a l i z e d to suggest f e a s i b i l i t y of its eventual use in h i g h - t e m p e r a t u r e s t r u c t u r e s . NbA13 is one of the three i n t e r m e t a l l i c compounds f o r m e d in the aluminum-niobium s y s t e m . It has no detectable homogeneity range, 2 m e l t s congruently, and has the DO2

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