Transformation temperatures of martensite in \-phase nickel aluminide
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#-NIAL is a ~ e l e c t r o n compound having the typical B2 CsC1 s t r u c t u r e and e x i s t s o v e r the composition range of 43 to 70 at. pct Ni at 1670 K. 1 Unlike most compounds, NiAl has not been o b s e r v e d to t r a n s f o r m m a r t e n s i t l c a l l y at the s t o i c h i o m e t r i c composition, z However, n i c k e l - r i c h a l l o y s do decompose in this fashion. Maxwell and G r a l a r e p o r t e d m a r t e n s i t i c m i c r o s t r u c t u r e s for a quenched 68.5 at. pct Ni alloy, s w h e r e a s Ball produced s t r e s s - i n d u c e d m a r t e n s i t e in a 57 at. pet Ni alloy. 4 Other s t r e s s effects a s s o c i a t e d with the t r a n s f o r m a t i o n w e r e o b s e r v e d by Enami and Nenno in a 63.2 at. pct Ni alloy, s Specifically they obs e r v e d the s h a p e - m e m o r y effect in a specimen d e f o r m e d at r o o m t e m p e r a t u r e and heated to 543 K, while deformation at 553 K exhibited " s u p e r e l a s t i c " behavior. These types of b e h a v i o r a r e common for E-compound m a r t e n s i t e s and have been o b s e r v e d in CuZn, 6 TiNi, 7 AuCd, 8 and Cu-A1-Ni ~ a l l o y s . D i l a t o m e t r i c work by Lltvinov et al. showed that a volume expansion o c c u r r e d at 383 K when t r a n s f o r m i n g a 64 at. pct Ni alloy to m a r t e n s i t e . 1~ Rosen and Goebel o b s e r v e d m a r t e n s i t i c m i c r o s t r u c t u r e s at r o o m t e m p e r a t u r e for a l l o y s having g r e a t e r than 63.5 at. pct Ni. u F r o m X - r a y diffraction data they d e s c r i b e d the c r y s t a l s t r u c t u r e of a 66 at. pet Ni alloy as L16, a t e t r a g o n a l d i s t o r t i o n of the o r d e r e d CuAu I fcc s t r u c t u r e . The e x c e s s of nickel atoms were a c commodated on the (001) face c e n t e r s , where a random d i s t r i b u t i o n of nickel and aluminum a t o m s was p r o posed. Oxidation r e s i s t a n t coatings on n i c k e l - b a s e s u p e r a l l o y s c u r r e n t l y r e p r e s e n t the most widely used c o m m e r c i a l application for NiAl. Extended high t e m p e r a ture s e r v i c e , however, eventually d e g r a d e s the o x i d a tion r e s i s t a n c e of NiAl by diffusion of aluminum into the s u b s t r a t e and in the r e p e a t e d f o r m a t i o n and s p a l l ing of a p r o t e c t i v e AlzOs film. Recently m a r t e n s i t e was identified i n a NiAl coating on a n i c k e l - b a s e s u p e r alloy, which had been depleted of aluminum by 300 h of cyclic oxidation at 1365 K. ~ The s t r a i n s a s s o c i a t e d with the m a r t e n s i t i c r e a c t i o n might contribute to the spalling of A120s and thus to the degradation of the coating. The p r e s e n t work r e p o r t s Ms t e m p e r a t u r e s for the n i c k e l JAMES L. SMIALEK is Materials Engineer, Materials and Structure Division, NASA Lewis Research Center, Cleveland, Ohio 44135, and Graduate Student, Division of Metallurgy and Materials Science, Case Western Reserve University,
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