- Email: [email protected]
Effect of γ and γ ′ former doping on ductility of Ni3Al
Scripta METALLURGICA et MATERIALIA
Vol. 25, pp. 303-307, 1991 Printed in the U.S.A.
EFFECT OF 7 A.
Institute f o r
AND
Pergamon Press plc All rights reserved
7 ' FORMER DOPING ON DUCTILITY OF NI3AI
Chlba*
, S.
Hanada
and S.
Watanabe
Materials Research,Tohoku Unlverslty,Sendai
980,Japan
* On leave from Faculty of Engineering, Department of Metallurgy, Iwate University, Morloka 020,Japan (Received October (Revised November
19, 1990) 13, 1990)
Introduction
It has been recently revealed from an ALCHEMI investigation (1) that Pd has a strong tendency to substitute for the NI site independent of the composition of the host elements in NIHAI. This means that the occupation site for Pd both In N i 7 5 A 1 2 3 P d 2 , , a ~ , NITsAI2sPd 2 Is exclusively the Ni site. Subsequent investigations r~,~°J concerning-the ductility improvement of NIsAI macroalloyed with Pd, has shown that recrystalllzed NITsAI23Pd 2 exhibits an elongation to fracture of 114, whereas recrystalllzed NITsAI25P~ 2 remains brittle. The ductility of NI3AI macroalloyed wlth s u b s ~ u ~ g n a l ~ernary elements has
doped NI3A1 with an A1 poor composition is attributed to degradation of the ordering energy, caused by the addition of Pd and the decrease In Al content. In addition, it has been suggested that the stronger the tendency for a ternary element ,~9 ~ s t l t u t e for the Ni site. the more the ordering energy of Ni3A 1 d e g r a d e s t J , t ~ . Therefore, there may be ternary elements other than Pd which can improve the ductility of NI3AI. Recently, Jia et al (8) showed that when a ternary X element tends to subs t i t u t e for the A1 site in NIRAI, the addition of the X element stabilizes the 7 ' phase (i.e., It is a 7 ' former), whereas when a X element tends to s u b s t i t u t e for the Ni site, the 7 phase is stabilized by the addition of the X element (i.e., it Is a 7 former). They showed a figure dividing the ternary addltlo~% elements to NIRAI by a solid llne Into two categories, 7 ' and 7 formers ~°~, as seen In FIg?l. The purpose of this study is to confirm the above expectation as to whether or not the addition of 7 former elements improves the ductility of NiRAI. Therefore, ternary elements listed In Table 1 are selected from Flg.1 as typical 7 and 7 ' formers. Experimental The nominal compositions of specimens used In thls investigation are Ni75A123X 2 and Ni74Al~sXz. The ternary elements, X, selected as ~ and ~ ' former, are llsted-fn TKbIe i. Alloy buttons were prepared by are-meltlng and remelted four times by turning over to attain chemical homogeneity. Plates sliced from the cast buttons doped with 7 formers, about 1 to 4 mm in thickness x lOmm In width x 50... in length, were cold-rolled by i0 ~ 40 reduction, encapsulated in a vacuum of i0 -° Pa with a sponge Zr getter, and annealed at 1323K for 24 ~ 48h for homogenization and recrystalllzatlon. They
0036-9748/91 Copyright (c) 1991
303 $3.00 + .00 Pergamon Press
plc
504
D U C T I L I T Y OF Ni3AI
Vol.
25, No.2
were again cold-rolled by up to 65~ reduction, encapsulated Into a vacuum quartz tube and subjected to recrystalllzatlon annealing. On the other hand, since plates sliced f r o m the button containing a ~ ' former could not be coldrolled, the plates with the size of imm in thickness x 10mm in width x 40... in length, were annealed at 1323K for 2~h for homogenization. Tensile specimens having gauge section of 1 x 2 x 16 mmo were spark-machlned from the cast and the recrystalllzed specimens. After the surface layer was removed by electropollshlng, tensile tests were performed at ambient temperature in a vacuum o~ 70 -0 Pa u s i n g an Instron-type machine at an initial strain rate of 5.2 x 10--s -~. An optical and a scanning electron microscope were employed to examine the mlcrostructures and f r a c t u r e d surfaces of tensile specimens, respectively. Results As-cast specimens doped with ~ formers were successfully rolled by i0 40~ reduction without cracking, whereas as-cast specimens doped with 7 ' formers could not be entirely rolled. Figure 2 shows the nominal stress-straln curves for as-cast specimens doped with 7 formers. As shown i n t h i s f i g u r e , 4 9% e l o n g a t i o n t o f r a c t u r e is obtained in the respective specimens, indicating that ductility o f NiaA1 c a n b e i m p r o v e d by t h e a d d i t i o n o f 7 f o r m e r s , a l t h o u g h o n e must e x a m i n e ~ a r e f u l l y the presence of second phase 7 w h i c h may lead to ductllization. On t h e o t h e r h a n d , s p e c i m e n s d o p e d w i t h ~ ' f o r m e r s a r e found not to exhibit any elongation. Figure 3 (a), (b) a n d ( c ) show o p t i c a l m i c r o g r a p h s o f Pd, Cu an d Co d o p e d recrystallized tensile specimens, respectively. The a v e r a g e g r a i n sizes in Fig.3 (a), (b) and (c) are 53, 29 an d 58 # m, respectively. In these micrographs, e q u l - a x e d g r a i n s c a n b e s e e n an d t h e second phase 7 is not detected at grain boundaries or within grains. In addition, twin boundaries are frequently found. F i g u r e 4 shows t h e n o m i n a l s t r e s s - s t r a i n curves for recrystalllzed specimens d o p e d w i t h ~ f o r m e r s . As c a n b e s e e n i n t h i s f i g u r e , 11% e l o n g a t i o n t o f r a c ture for the Pd-doped specimen, 6% f o r Cu, 5% f o r P t a n d 4% f o r Co a r e obtained, w h i l e b o t h F e a n d Cr d o p e d s p e c i m e n s f r a c t u r e d b e f o r e y i e l d i n g . On the other hand, t h e s p e c i m e n s d o p e d w i t h 7 ' f o r m e r s e x h i b i t e d no e l o n g a t i o n . According to Fig.l, Pd i s s i t u a t e d farthest above the solid line, meaning that Pd i s t h e m o s t e f f e c t i v e i n d e g r a d i n g t h e o r d e r i n g e n e r g y o f NiaA1, i.e., it is the strongest ~ former. On t h e o t h e r h a n d , C r , l y i n g n e a r t h e s o l i d l i n e , is considered to be ineffective in degrading the ordering energy o f NI3A1, i.e., it is the weakest ~ former. Therefore, i t i s e x p e c t e d t h a t Pd i s t h e most effective in improving the ductility, w h e r e a s Cr i s t h e m o s t i n e f f e c t i v e . The p r e s e n t r e s u l t s support the above expectation. That is, as described above, the s p e c i m e n d o p e d w i t h Pd w h i c h i s t h e s t r o n g e s t 7 former exhibits maximum e l o n g a t i o n t o f r a c t u r e o f 11%, w h e r e a s s p e c i m e n s d o p e d w i t h C r , which is the weakest ~ former, e x h i b i t no e l o n g a t i o n . In addition, the specimens doped with Fe, belonging to the ~ forming category, do n o t exhibit an y elongation; therefore, t h e e x t e n t o f d e g r a d a t i o n o f o r d e r i n g e n e r g y o f Ni3A1 by Fe i s t h o u g h t t o b e s m a l l c o m p a r e d w i t h Cu, w h i c h i s a t t h e same d i s t a n c e a s Fe f r o m t h e s o l i d l i n e i n F i g . 1. Figure 5 (a), (b) a n d ( c ) show s c a n n i n g e l e c t r o n m l c r o g r a p h s o f t h e f r a c tured surface of tensile s p e c i m e n s d o p e d w i t h P d , C u an d Co, r e s p e c t i v e l y . Although lntergranularly fractured facets are seen predominantly in these specimens, transgranularly fractured grains are also observe~^~ These features are characteristic o f s p e c i m e n s w h o se d u c t i l i t y i s e n h a n c e d ~°p • On t h e o t h e r hand, the fractured s u r f a c e o f s p e c i m e n s d o p e d w i t h ~ ' f o r m e r s was f o u n d to be c o m p l e t e l y l n t e r g r a n u l a r .
Discussion I n a p r e v i o u s p a p e r ( 3 ) , we d i s c u s s e d t h e c o r r e l a t i o n between ductility an d ordering e n e r g y o f N13A1 a n d c o n c l u d e d t h a t d e g r a d a t i o n o f o r d e r i n g e n e r g y i s responsible for improvement in ductility o f NI3A1. P r e s e n t r e s u l t s a r e t h o u g h t to support further the above conclusion.
VOl. 25, No. 2
DUCTILITY OF Ni3AI
505
Recently, A c k l a n d a n d V i t e k (10) h a v e c a l c u l a t e d t h e g r a i n b o u n d a r y s t r u c ture i n LZ2 o r d e r e d a l l o y s . They showed t h a t s t r o n g l y o r d e r e d a l l o y s s u c h a s NI3A1 h a v e ~olumns o f p a r t i a l vacancies in the grain boundaries, whereas weikly ordered alloys s u c h a s Cu3Au h a v e much more h o m o g e n e o u s b o u n d a r y structures. From t h e s e r e s u l t s , the~ discussed the intrinsic brittleness of Ni3AI a n d p o i n t e d o u t t h a t t h e c a v i t i e s m i g h t s e r v e a s s u i t a b l e n u c l e i f o r i n tergranular cracks. In addition, Kruisman et al. (ll) have investigated the atomic structure of stoichiometric and non-stolchlometric grain boundaries in the LI 2 o r d e r e d c r y s t a l structure. They s u g g e s t e d t h a t b o u n d a r i e s i n Ar i c h ( w i T h r e s p e c t t o AsB s t o i c h l o m e t r y ) L12 compounds were much l e s s l i k e l y t o break in a brittle man~er than stoichiome~rlc or B-rich compounds; in the former c a s e t h e y became a n A - r i c h compound a n d t h u s were relatively undistorted, while in the latter case they possessed stolchiometrlc, highly distorted structures. Accordingly, since the local resistance to shear and/or to the transmission of s l i p t h r o u g h g r a i n b o u n d a r i e s i s much h i g h e r i n t h e distorted than in the undistorted boundary structures, the boundaries possessi n g d i s t o r t e d s t r u c t u r e s a r e more s u s c e p t i b l e t o b r i t t l e fracture. As m e n t i o n e d a b o v e , it appears to be acceptable that the structure of grain boundaries is affected by variation in the ordering energy and deviation from t h e s t o i c h i o m e t r y o f c o m p o u n d s . However, t h e most c o m p l i c a t e d p r o b l e m t o b e s o l v e d i s how t h e v a r i a t i o n i n t h e s t r u c t u r e o f g r a i n b o u n d a r i e s l e a d s t o macroscopic lntergranular fractures. At t h i s p o i n t , i t s h o u l d be n o t e d t h a t a l though the macroscopic ductility of polycrystalline N13A1 i s l o w , significant plastic ,~A~o~lon has been observed near the lntergranularly fractured surfaces ~'J-~j. This suggests that grain boundary fracture in polycrystall i n e NI3A1 i s a s s o c i a t e d ~ d i s l o c a t i o n movement, n a m e l y , d l s l o c a t ~ , r ~ tions at grain boundaries ~°~, motions of grain boundary dislocations ~°j-~x°j and dislocation em~ssions from the crack tip in the vicinity of the grain boundaries (I9)'(2OT. If ordering energy is degraded, dislocation reactions at grain boundaries and the motion of,~#l~,~gundary dislocations are ~R~oted, resulting in ductility i m p r o v e m e n t ~ x ° j - ~ x ° J . F u r t h e r m o r e , Hack e t a l . ~ ° ~ s u g gested t h a t i f t h e APB e n e r g y i s h i g h , an additional energy term is required for emissions of a dislocation from a c r a c k tip. Therefore, high ordering energy suppresses dislocation emissions from the tip, resulting in crack initiation and propagation in a brittle manner. Consequently, it is considered that degradation of ordering energy is an important factor for ductllization o f NI3A1. Summary On t h e b a s i s o f t h e a s s u m p t i o n t h a t degradation of ordering energy is responsible for ductility i m p r o v e m e n t o f m a c r o a l l o y e d Nl3A1, tensile behavior has been investigated using the specimens with the composition of NiTsA12sX 2 ( X : t h e t e r n a r y e l e m e n t ) . The a d d i t i o n o f a 7 f o r m e r s u c h a s P t , Cu, a n d C o - i s w e l l a s Pd, w h i c h d e g r a d e s t h e o r d e r i n g e n e r g y , l s e f f e c t i v e in improving the ductility o f NIsA1, w h e r e a s a ~ ' f o r m e r s u c h a s S i , T l , V a n d Nb, w h i c h e n hances the ordeFlng energy, is ineffective. In addition, motion and/or react i o n s Of d i s l o c a t i o n s in the vicinity of grain boundaries and/or at grain boundaries play an important role In lntergranular failures. Acknowledgements One o f t h e a u t h o r s ( A . C h i b a ) w o u l d l i k e t o t h a n k P r o f . K . T a n o s a k i , Associate Prof.N.FuJlta and Mr.K.Nonaka, of Iwate University, for their encouragement. References ( 1 ) A . C h l b a , D . S h l n d o a n d S . H a n a d a , A c t a Met. t o b e p u b l i s h e d . ( 2 ) A . C h i b a , S . H a n a d a a n d S . W a t a n a b e , S u b m l t t e d t o A c t a Met. (3)A.Chiba,S.Hanada and S.Watanabe,Mater.Trans.,JIM.,31,824(1990). (4)T.Takasugl and 0.Izumi,Acta Met.33,1247(198S). (5)T.Takasugi,O.Izumi and N.Masahashi,Acta Met.,33,1259(1985).
DUCTILITY OF Ni3AI
306
Vol. 25, No. 2
(6)A.I.Taub and C.L.Brlant,Acta Met., 85,1597(1987). ( 7 ) N . E n o m o t o and H . H a r a d a , M e t a l i . T r a n s . A , 2 0 A , 6 4 9 ( 1 9 8 9 ) . (8)C.-C.Jia,Doctoral Thests,Tohoku University(1990). (9)C.T.Llu,C.L.White, and J.A.Horton, Acta Net.,83,213(1985). ( 1 0 ) G . J . A c k l a n d and V . V l t e k , p p . 1 0 5 , M a t . R e s . S y m o . P r o c . , 1 3 8 ( 1 9 8 9 ) . (11)J.J.Krulsman,V.Vltek and J.Th.M.De Hosson,Acta Met., 86,2729(1988). (12)E.M.Schulson,D.L.Davidson and D.Vlens,Metall.Trans.A,14A, 1528(1988). (13)T.0gura,S.Hanada,T.Masumoto and O.Izuml,Netall.Trans.A,16A, 441(1985). (14)S.Hanada,T.Ogura,S.Watanabe,O.Izumt and T.Masumoto,Acta Met.,34,18 (1986). (15)A.H.King and N.H.Yoo,Scrlpta Met.,21,1115(1987). (16)E.N.Schulson,T.P.Weihs,I.Baker,H.J.Frost and J.A.Horton,Acta Met.,34, 1895(1986). ( 1 7 ) I . B a k e r , E . A . S c h u l s o n and J . A . H o r t o n , A c t a N e t . , 3 5 , 1 5 3 8 ( 1 9 8 7 ) . (18)H.J.Frost,Acta Het.,36,2199(1987). ( 1 9 ) J . E . H a c k , D . J . S r o l o v i t z and S . P . C h e n , S c r i p t a M e t . , 2 0 , 1 6 9 9 ( 1 9 8 6 ) . (20)J.E.Haek,S.P.Chen and D . J . S r o l o v i t z , A e t a M e t . , 8 7 , 1 9 5 7 ( 1 9 8 9 ) . I
I
!
50
I A~jf.]
i
AucI Pdo
0
~
i
/
/
/
/
- ~ . . , ~ c p Cu / /
Mn~IrHUaJ~oO F e / ~ "
Q Pt
Mn~ ~r~o ~ " / / / oV
E -50 \ -100
/
Z
/
/
//Tio TQ~Nb
-150 -200
/
o ~'-former
///~o
m T-former
p
'&~
I
I
'
I
-200 -150 -100 -50 V~ / kJ'm~ ~
0
Fig.l Ternary elements X In NIsAI divided Into ~ and 7 ' former, sltuat~d a~@ye and b e l o w t h e solid l i n e , r e s p e c t i v e l y ~ ° J . VNI X and VXA1 a r e i n t e r a c t i o n prameters ealculated-~T M l e d e m a ° s f o r m u l a , f o r N1-X and X-A1 bond p a i r s , r e s p e c t t v e i y .
T a b l e 1 T e r n a r y e l e m e n t s i n NIsA1 selected as 7 and 7 ' formers
from Flg.l. 7" former Pd, Pt.Cu,Co,Feand Cr "Y" former Nb, V, Ti and Si
Fig.3 Optical mlcrogTaphs of doped wlth (a)Pd,(b)Cu and (c)Co.
DUCTILITY OF NisAI
Vol. Z5, No. Z
307
300
200
| ,oo
!
I
,
I i
I
I
,
Strain
F i g . 2 Nominal s t r e s s - s t r a i n curves for as-cast specimens doped wlth 7 formers.
500 Recrystallized X=P d / Ni-23AI-2X / 400 o 0.
2%
d=5y / Pt
%
~ 300 ut
200 Fe
Cx
TT
100
Tensile siren Fig. 4 Nomlnal stress-straln curves for recrystalllzed specimens doped wlth 7 formers wlth the composition of NI75AI23Pd2(XfPd,Pt,Co,Fe and Cr).
Fig. 5 Scanning electron ml-crographs of the fractured surface of recrystalllzed tensile specimens doped with (a)Pd,(b) Cu an d ( c ) C o .
Vol. 25, pp. 303-307, 1991 Printed in the U.S.A.
EFFECT OF 7 A.
Institute f o r
AND
Pergamon Press plc All rights reserved
7 ' FORMER DOPING ON DUCTILITY OF NI3AI
Chlba*
, S.
Hanada
and S.
Watanabe
Materials Research,Tohoku Unlverslty,Sendai
980,Japan
* On leave from Faculty of Engineering, Department of Metallurgy, Iwate University, Morloka 020,Japan (Received October (Revised November
19, 1990) 13, 1990)
Introduction
It has been recently revealed from an ALCHEMI investigation (1) that Pd has a strong tendency to substitute for the NI site independent of the composition of the host elements in NIHAI. This means that the occupation site for Pd both In N i 7 5 A 1 2 3 P d 2 , , a ~ , NITsAI2sPd 2 Is exclusively the Ni site. Subsequent investigations r~,~°J concerning-the ductility improvement of NIsAI macroalloyed with Pd, has shown that recrystalllzed NITsAI23Pd 2 exhibits an elongation to fracture of 114, whereas recrystalllzed NITsAI25P~ 2 remains brittle. The ductility of NI3AI macroalloyed wlth s u b s ~ u ~ g n a l ~ernary elements has
doped NI3A1 with an A1 poor composition is attributed to degradation of the ordering energy, caused by the addition of Pd and the decrease In Al content. In addition, it has been suggested that the stronger the tendency for a ternary element ,~9 ~ s t l t u t e for the Ni site. the more the ordering energy of Ni3A 1 d e g r a d e s t J , t ~ . Therefore, there may be ternary elements other than Pd which can improve the ductility of NI3AI. Recently, Jia et al (8) showed that when a ternary X element tends to subs t i t u t e for the A1 site in NIRAI, the addition of the X element stabilizes the 7 ' phase (i.e., It is a 7 ' former), whereas when a X element tends to s u b s t i t u t e for the Ni site, the 7 phase is stabilized by the addition of the X element (i.e., it Is a 7 former). They showed a figure dividing the ternary addltlo~% elements to NIRAI by a solid llne Into two categories, 7 ' and 7 formers ~°~, as seen In FIg?l. The purpose of this study is to confirm the above expectation as to whether or not the addition of 7 former elements improves the ductility of NiRAI. Therefore, ternary elements listed In Table 1 are selected from Flg.1 as typical 7 and 7 ' formers. Experimental The nominal compositions of specimens used In thls investigation are Ni75A123X 2 and Ni74Al~sXz. The ternary elements, X, selected as ~ and ~ ' former, are llsted-fn TKbIe i. Alloy buttons were prepared by are-meltlng and remelted four times by turning over to attain chemical homogeneity. Plates sliced from the cast buttons doped with 7 formers, about 1 to 4 mm in thickness x lOmm In width x 50... in length, were cold-rolled by i0 ~ 40 reduction, encapsulated in a vacuum of i0 -° Pa with a sponge Zr getter, and annealed at 1323K for 24 ~ 48h for homogenization and recrystalllzatlon. They
0036-9748/91 Copyright (c) 1991
303 $3.00 + .00 Pergamon Press
plc
504
D U C T I L I T Y OF Ni3AI
Vol.
25, No.2
were again cold-rolled by up to 65~ reduction, encapsulated Into a vacuum quartz tube and subjected to recrystalllzatlon annealing. On the other hand, since plates sliced f r o m the button containing a ~ ' former could not be coldrolled, the plates with the size of imm in thickness x 10mm in width x 40... in length, were annealed at 1323K for 2~h for homogenization. Tensile specimens having gauge section of 1 x 2 x 16 mmo were spark-machlned from the cast and the recrystalllzed specimens. After the surface layer was removed by electropollshlng, tensile tests were performed at ambient temperature in a vacuum o~ 70 -0 Pa u s i n g an Instron-type machine at an initial strain rate of 5.2 x 10--s -~. An optical and a scanning electron microscope were employed to examine the mlcrostructures and f r a c t u r e d surfaces of tensile specimens, respectively. Results As-cast specimens doped with ~ formers were successfully rolled by i0 40~ reduction without cracking, whereas as-cast specimens doped with 7 ' formers could not be entirely rolled. Figure 2 shows the nominal stress-straln curves for as-cast specimens doped with 7 formers. As shown i n t h i s f i g u r e , 4 9% e l o n g a t i o n t o f r a c t u r e is obtained in the respective specimens, indicating that ductility o f NiaA1 c a n b e i m p r o v e d by t h e a d d i t i o n o f 7 f o r m e r s , a l t h o u g h o n e must e x a m i n e ~ a r e f u l l y the presence of second phase 7 w h i c h may lead to ductllization. On t h e o t h e r h a n d , s p e c i m e n s d o p e d w i t h ~ ' f o r m e r s a r e found not to exhibit any elongation. Figure 3 (a), (b) a n d ( c ) show o p t i c a l m i c r o g r a p h s o f Pd, Cu an d Co d o p e d recrystallized tensile specimens, respectively. The a v e r a g e g r a i n sizes in Fig.3 (a), (b) and (c) are 53, 29 an d 58 # m, respectively. In these micrographs, e q u l - a x e d g r a i n s c a n b e s e e n an d t h e second phase 7 is not detected at grain boundaries or within grains. In addition, twin boundaries are frequently found. F i g u r e 4 shows t h e n o m i n a l s t r e s s - s t r a i n curves for recrystalllzed specimens d o p e d w i t h ~ f o r m e r s . As c a n b e s e e n i n t h i s f i g u r e , 11% e l o n g a t i o n t o f r a c ture for the Pd-doped specimen, 6% f o r Cu, 5% f o r P t a n d 4% f o r Co a r e obtained, w h i l e b o t h F e a n d Cr d o p e d s p e c i m e n s f r a c t u r e d b e f o r e y i e l d i n g . On the other hand, t h e s p e c i m e n s d o p e d w i t h 7 ' f o r m e r s e x h i b i t e d no e l o n g a t i o n . According to Fig.l, Pd i s s i t u a t e d farthest above the solid line, meaning that Pd i s t h e m o s t e f f e c t i v e i n d e g r a d i n g t h e o r d e r i n g e n e r g y o f NiaA1, i.e., it is the strongest ~ former. On t h e o t h e r h a n d , C r , l y i n g n e a r t h e s o l i d l i n e , is considered to be ineffective in degrading the ordering energy o f NI3A1, i.e., it is the weakest ~ former. Therefore, i t i s e x p e c t e d t h a t Pd i s t h e most effective in improving the ductility, w h e r e a s Cr i s t h e m o s t i n e f f e c t i v e . The p r e s e n t r e s u l t s support the above expectation. That is, as described above, the s p e c i m e n d o p e d w i t h Pd w h i c h i s t h e s t r o n g e s t 7 former exhibits maximum e l o n g a t i o n t o f r a c t u r e o f 11%, w h e r e a s s p e c i m e n s d o p e d w i t h C r , which is the weakest ~ former, e x h i b i t no e l o n g a t i o n . In addition, the specimens doped with Fe, belonging to the ~ forming category, do n o t exhibit an y elongation; therefore, t h e e x t e n t o f d e g r a d a t i o n o f o r d e r i n g e n e r g y o f Ni3A1 by Fe i s t h o u g h t t o b e s m a l l c o m p a r e d w i t h Cu, w h i c h i s a t t h e same d i s t a n c e a s Fe f r o m t h e s o l i d l i n e i n F i g . 1. Figure 5 (a), (b) a n d ( c ) show s c a n n i n g e l e c t r o n m l c r o g r a p h s o f t h e f r a c tured surface of tensile s p e c i m e n s d o p e d w i t h P d , C u an d Co, r e s p e c t i v e l y . Although lntergranularly fractured facets are seen predominantly in these specimens, transgranularly fractured grains are also observe~^~ These features are characteristic o f s p e c i m e n s w h o se d u c t i l i t y i s e n h a n c e d ~°p • On t h e o t h e r hand, the fractured s u r f a c e o f s p e c i m e n s d o p e d w i t h ~ ' f o r m e r s was f o u n d to be c o m p l e t e l y l n t e r g r a n u l a r .
Discussion I n a p r e v i o u s p a p e r ( 3 ) , we d i s c u s s e d t h e c o r r e l a t i o n between ductility an d ordering e n e r g y o f N13A1 a n d c o n c l u d e d t h a t d e g r a d a t i o n o f o r d e r i n g e n e r g y i s responsible for improvement in ductility o f NI3A1. P r e s e n t r e s u l t s a r e t h o u g h t to support further the above conclusion.
VOl. 25, No. 2
DUCTILITY OF Ni3AI
505
Recently, A c k l a n d a n d V i t e k (10) h a v e c a l c u l a t e d t h e g r a i n b o u n d a r y s t r u c ture i n LZ2 o r d e r e d a l l o y s . They showed t h a t s t r o n g l y o r d e r e d a l l o y s s u c h a s NI3A1 h a v e ~olumns o f p a r t i a l vacancies in the grain boundaries, whereas weikly ordered alloys s u c h a s Cu3Au h a v e much more h o m o g e n e o u s b o u n d a r y structures. From t h e s e r e s u l t s , the~ discussed the intrinsic brittleness of Ni3AI a n d p o i n t e d o u t t h a t t h e c a v i t i e s m i g h t s e r v e a s s u i t a b l e n u c l e i f o r i n tergranular cracks. In addition, Kruisman et al. (ll) have investigated the atomic structure of stoichiometric and non-stolchlometric grain boundaries in the LI 2 o r d e r e d c r y s t a l structure. They s u g g e s t e d t h a t b o u n d a r i e s i n Ar i c h ( w i T h r e s p e c t t o AsB s t o i c h l o m e t r y ) L12 compounds were much l e s s l i k e l y t o break in a brittle man~er than stoichiome~rlc or B-rich compounds; in the former c a s e t h e y became a n A - r i c h compound a n d t h u s were relatively undistorted, while in the latter case they possessed stolchiometrlc, highly distorted structures. Accordingly, since the local resistance to shear and/or to the transmission of s l i p t h r o u g h g r a i n b o u n d a r i e s i s much h i g h e r i n t h e distorted than in the undistorted boundary structures, the boundaries possessi n g d i s t o r t e d s t r u c t u r e s a r e more s u s c e p t i b l e t o b r i t t l e fracture. As m e n t i o n e d a b o v e , it appears to be acceptable that the structure of grain boundaries is affected by variation in the ordering energy and deviation from t h e s t o i c h i o m e t r y o f c o m p o u n d s . However, t h e most c o m p l i c a t e d p r o b l e m t o b e s o l v e d i s how t h e v a r i a t i o n i n t h e s t r u c t u r e o f g r a i n b o u n d a r i e s l e a d s t o macroscopic lntergranular fractures. At t h i s p o i n t , i t s h o u l d be n o t e d t h a t a l though the macroscopic ductility of polycrystalline N13A1 i s l o w , significant plastic ,~A~o~lon has been observed near the lntergranularly fractured surfaces ~'J-~j. This suggests that grain boundary fracture in polycrystall i n e NI3A1 i s a s s o c i a t e d ~ d i s l o c a t i o n movement, n a m e l y , d l s l o c a t ~ , r ~ tions at grain boundaries ~°~, motions of grain boundary dislocations ~°j-~x°j and dislocation em~ssions from the crack tip in the vicinity of the grain boundaries (I9)'(2OT. If ordering energy is degraded, dislocation reactions at grain boundaries and the motion of,~#l~,~gundary dislocations are ~R~oted, resulting in ductility i m p r o v e m e n t ~ x ° j - ~ x ° J . F u r t h e r m o r e , Hack e t a l . ~ ° ~ s u g gested t h a t i f t h e APB e n e r g y i s h i g h , an additional energy term is required for emissions of a dislocation from a c r a c k tip. Therefore, high ordering energy suppresses dislocation emissions from the tip, resulting in crack initiation and propagation in a brittle manner. Consequently, it is considered that degradation of ordering energy is an important factor for ductllization o f NI3A1. Summary On t h e b a s i s o f t h e a s s u m p t i o n t h a t degradation of ordering energy is responsible for ductility i m p r o v e m e n t o f m a c r o a l l o y e d Nl3A1, tensile behavior has been investigated using the specimens with the composition of NiTsA12sX 2 ( X : t h e t e r n a r y e l e m e n t ) . The a d d i t i o n o f a 7 f o r m e r s u c h a s P t , Cu, a n d C o - i s w e l l a s Pd, w h i c h d e g r a d e s t h e o r d e r i n g e n e r g y , l s e f f e c t i v e in improving the ductility o f NIsA1, w h e r e a s a ~ ' f o r m e r s u c h a s S i , T l , V a n d Nb, w h i c h e n hances the ordeFlng energy, is ineffective. In addition, motion and/or react i o n s Of d i s l o c a t i o n s in the vicinity of grain boundaries and/or at grain boundaries play an important role In lntergranular failures. Acknowledgements One o f t h e a u t h o r s ( A . C h i b a ) w o u l d l i k e t o t h a n k P r o f . K . T a n o s a k i , Associate Prof.N.FuJlta and Mr.K.Nonaka, of Iwate University, for their encouragement. References ( 1 ) A . C h l b a , D . S h l n d o a n d S . H a n a d a , A c t a Met. t o b e p u b l i s h e d . ( 2 ) A . C h i b a , S . H a n a d a a n d S . W a t a n a b e , S u b m l t t e d t o A c t a Met. (3)A.Chiba,S.Hanada and S.Watanabe,Mater.Trans.,JIM.,31,824(1990). (4)T.Takasugl and 0.Izumi,Acta Met.33,1247(198S). (5)T.Takasugi,O.Izumi and N.Masahashi,Acta Met.,33,1259(1985).
DUCTILITY OF Ni3AI
306
Vol. 25, No. 2
(6)A.I.Taub and C.L.Brlant,Acta Met., 85,1597(1987). ( 7 ) N . E n o m o t o and H . H a r a d a , M e t a l i . T r a n s . A , 2 0 A , 6 4 9 ( 1 9 8 9 ) . (8)C.-C.Jia,Doctoral Thests,Tohoku University(1990). (9)C.T.Llu,C.L.White, and J.A.Horton, Acta Net.,83,213(1985). ( 1 0 ) G . J . A c k l a n d and V . V l t e k , p p . 1 0 5 , M a t . R e s . S y m o . P r o c . , 1 3 8 ( 1 9 8 9 ) . (11)J.J.Krulsman,V.Vltek and J.Th.M.De Hosson,Acta Met., 86,2729(1988). (12)E.M.Schulson,D.L.Davidson and D.Vlens,Metall.Trans.A,14A, 1528(1988). (13)T.0gura,S.Hanada,T.Masumoto and O.Izuml,Netall.Trans.A,16A, 441(1985). (14)S.Hanada,T.Ogura,S.Watanabe,O.Izumt and T.Masumoto,Acta Met.,34,18 (1986). (15)A.H.King and N.H.Yoo,Scrlpta Met.,21,1115(1987). (16)E.N.Schulson,T.P.Weihs,I.Baker,H.J.Frost and J.A.Horton,Acta Met.,34, 1895(1986). ( 1 7 ) I . B a k e r , E . A . S c h u l s o n and J . A . H o r t o n , A c t a N e t . , 3 5 , 1 5 3 8 ( 1 9 8 7 ) . (18)H.J.Frost,Acta Het.,36,2199(1987). ( 1 9 ) J . E . H a c k , D . J . S r o l o v i t z and S . P . C h e n , S c r i p t a M e t . , 2 0 , 1 6 9 9 ( 1 9 8 6 ) . (20)J.E.Haek,S.P.Chen and D . J . S r o l o v i t z , A e t a M e t . , 8 7 , 1 9 5 7 ( 1 9 8 9 ) . I
I
!
50
I A~jf.]
i
AucI Pdo
0
~
i
/
/
/
/
- ~ . . , ~ c p Cu / /
Mn~IrHUaJ~oO F e / ~ "
Q Pt
Mn~ ~r~o ~ " / / / oV
E -50 \ -100
/
Z
/
/
//Tio TQ~Nb
-150 -200
/
o ~'-former
///~o
m T-former
p
'&~
I
I
'
I
-200 -150 -100 -50 V~ / kJ'm~ ~
0
Fig.l Ternary elements X In NIsAI divided Into ~ and 7 ' former, sltuat~d a~@ye and b e l o w t h e solid l i n e , r e s p e c t i v e l y ~ ° J . VNI X and VXA1 a r e i n t e r a c t i o n prameters ealculated-~T M l e d e m a ° s f o r m u l a , f o r N1-X and X-A1 bond p a i r s , r e s p e c t t v e i y .
T a b l e 1 T e r n a r y e l e m e n t s i n NIsA1 selected as 7 and 7 ' formers
from Flg.l. 7" former Pd, Pt.Cu,Co,Feand Cr "Y" former Nb, V, Ti and Si
Fig.3 Optical mlcrogTaphs of doped wlth (a)Pd,(b)Cu and (c)Co.
DUCTILITY OF NisAI
Vol. Z5, No. Z
307
300
200
| ,oo
!
I
,
I i
I
I
,
Strain
F i g . 2 Nominal s t r e s s - s t r a i n curves for as-cast specimens doped wlth 7 formers.
500 Recrystallized X=P d / Ni-23AI-2X / 400 o 0.
2%
d=5y / Pt
%
~ 300 ut
200 Fe
Cx
TT
100
Tensile siren Fig. 4 Nomlnal stress-straln curves for recrystalllzed specimens doped wlth 7 formers wlth the composition of NI75AI23Pd2(XfPd,Pt,Co,Fe and Cr).
Fig. 5 Scanning electron ml-crographs of the fractured surface of recrystalllzed tensile specimens doped with (a)Pd,(b) Cu an d ( c ) C o .