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Effect of boron distribution on grain-goundaries in Ni3Al
Scripta M E T A L L U R G I C A et ~ T E R I A L I A
Vo!.
2~, pp. 1857-1860, 1990 Printed in the U.S.A.
Pergamon Press plc All rights reserved
EFFECT OF BORO~ DI.qTI:HBUTION ON GI~AIN--DOU?iDARIES IN NiaAl
Song Shenhua , Yuan Zhexi , Xu Tingd,mg , and Yu gongSen= Department of Materials, Wuhan Iron and Steel University, Wtthan Hube[, China ~t Department of Materials Physics, The university of science and technology 8eijing, Bei j ing, China ( R e c e i v e d J u n e 5, ( R e v i s e d J u l y 17,
1990) 1990)
Introduction Ni~kl with the LI, ordered crystal structure below the peritectic temperature (1395"C) [1] is know to have good high temperature properties [2,3]. However , the major difficulty with ~i)A1 as a high t~perature structure material is its low ductility and brittleness in polycrystalline forms [4,5]. This brittleness effectively precludes its fabrication into useful structural components. Aoki an,] [zumi [6] first discovered that the addition of bor)n into polycrystalline NisA1 increases the ductility and modifies the failure mechanism to that of transgranular fracture. Recently, liu et al. [7,8], by refining the thermomechanical process, have brought room temperature ductility up to 50~ with a completely transgranular fracture surface and also have defined the alloy composition range over which boron is effective. Presently, the reason for the improvement of ductility by the addition of boron into NisAI is generally interpreted to be boron segregation to grain botmdaries [8--10]. Therefore, if the segreation of boron to grain botmdaries is responsible for the improved ductility of B-doped Ni3A1, the role of boron in increasing the ductility of Ni3A1 will disappear above a certain temperature because of minimal segregation of boron to grain botmdaries. However, up to now, investigations of this aspect have been sparse. The purpose of the present work is to make clear whether the segregation of boron to grain boundaries is largely responsible for the improv~aent of ductility of a given B--doped ~isA1. Exper imental procedure A O. 065wtg B--doped ~i~A1 was prepared by vacuum induction melting and precision casting from high purity nickel, aluminium and a ~aster alloy of Ni--O.75wth'B. The size of ingot 'eas 65:30xl80~. The alloy composition is shown in Table 1. Table 1. Alloy composition (wtS) .
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
Ni 86.52
.
.
.
.
.
.
.
.
.
.
.
.
A1 12.75
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
S P 0.009 0.006
.
.
.
.
.
.
.
B 0.065
611 the ingots were homogenized for 5--h at IO00"C in argon and then air---cooled to room temperature. ~ext, some of ingots homogenized were heated to 700"C anti 850"C, respectively, and held for 2--h in argon, and then air---c.ooled to room temperature . All the ingots heat--treated as above ~ r e fractured in a bend test. The fracture surfaces were examined by a S--570 scanning electron microscope (SEN). In the work, the tensile properties were not examined because the tensile elongation of the B--doped NisAI "~s low in the cast state. Structural features in the B--doped hi3At ~ r e examined in a H--800 analytical transmission electron microscope. Thin foils for electron microscopic investigation were prepared by means of a standard dtml--jet electro--polishing technique. The B--doped ~i~A1 *~s electrochemically thinned in a solution of 95N methyl alcohol and 55 perchloric acid at --10"C.
1857 0036-9748/90 $3.00 + .00 Copyright (c) 1990 Pergamon Press
plc
858
BORON DISTRIBUTION
IN Ni3AI
Vol.
2~, No.
The distrib,ttions of boron in the samples "~re detected by Particle Tracking kutoradi----ography (PTk)[11]. Three--acetic acid fiber foils were ~tsed as detecting foils, whose detecting sensitivity and the spatial resolution to boron are, respectively, about l ppm and 2 uz. ~n inte~a! flux of hot neutron radiation of l.O:- !11~ cm~ was chosen in the w~)rk. The detecting foils "~re etched for 20 rain at 50"C' ~ith an aqt~e,_nus soluti,:~n of 7.58 NaOlt. The surface of each etched foil ~as coated with chr,miu~m and observed by optical microscopy. Besu!ts and Discussion The distributions of boron in the samples air---cooled frnm different temperatures are given in Fig. 1. ~."eno [12] reported that, in the PTLthe continuous etched pit belt on the etched fc,ils represents bor~-~n segregation, ,'bile the etched pit clustering represents boride precipitation. According to l.leno's study, Fig. 1 sho~s that a very small amount of boron grain--boundary segre~tion and no apparent bor[,te precipitation appear for the sample air--~:ooled from I0110"C; a relatively large amount of boron segregation and no apparent boride precipitation appear for the samples air--cooled from 850 and 700 "C- Noreover, Fig. t also shows that boron segregation decreases ~;ith increasing cooling starting temperature, in agreement with the equitihrh~ segregat ion theory [ 13] Figure 2 shows l'g~4 photographs of the B--doped .~i~Al air~:ooled from 700"C where (a) depicts a m,~rphotog5 showing no precipitation of second phase in grains or along grain boundaries , and (b) shows a selected--area--diffraction (S,~[I) pattern of the matrix which characterizes the LI2 ordered crystal structure, indicating that the boron additions do not affect the long--range ordered crystal slructure in NigAI. Figure 2 shows further that no apparent precipitation of horide~ appears in any of the samples heat--treated as above, consistent with the results of Liu el al. [7] The fracture behavior of the ~ijAl sa~,ples, broken hy bend tests, ,,as studied by scanning electron microscopy. Figure .-3 shows SE~,! fractographs of the sample air--cooled fro~ 700 "C,~here (a) is a h:,',~magnification vie~' showing transgranular fracture, and (b) is a high magnification vie, showing fracture facets, possibly corresponding to crystallographic planes. The SE!,~ fractographs of the samples air---~:ooled from 850 and 11100 "C are given in Fig. 4. ~,hich shows that the sample cooled from 8511 "C exhibits transgranular fracture [Fig. ,1 (a)], ~'hereas the sample cooled from IO00"C exhibits [ntergranMur fracture [Fig.4 (b)]. The results of the fracto~aphy are in agreement Mth the ~ r k of ,'houdhury [1.l] who reported that slow cooling from 1323k promoted [ntergranular segregation of b~-,ron in g-~toped ~ii~tl "~hile rapid quenching retained the low level attained at I.~23k;such variations in boron segregation due to variat ions in thermal history manifested themselves in the fra,.'tnre ;r,orpholog2,' of Ni)A1 and probably in the :echanical properties. ~'e conclude from Fig. l, .'3., and 4 that ( 1 ) t h e grain--boundary cohesion of the samp]e air---~enched f r ~ 7110 "C or 850 "C2 was considerably enhanced; i.e., the ductility of the sample was obviously improved, a,hereas the grain--boundary cohesion of the sample quenched fro, 101111"C,as nnt considerably enhanced; i.e., the dialilit> of the sample was not obviously improved~ (2) the segregation of boron to grain boundaries occurs for the B--doped .'Ii3AI;(3) above some of temperature bet~,een 8511 and IO00"C, the role of boron in iaprvving the d~ctility of the B--doped ~itA1 disappears because the segregation of boron to grain boundaries is very smal I. The effects of segregation solutes on grain boundary cohesion have been the subject of n~erous experimental and theoretical [nvestigations~ however, the vast majority of such inquiries have dealt with embrittling solates[15] and relatively few address beneficial effects of segregating solutes [16, 17]. Recently Eberhart and Vvedensky [18] proposed an electron[,.: modeI to explain the effect of boron on grain--boundary cohesion in Ni~r~l..~ccording to their model, boron ,,ill have a P electron count of between 1 and 2 when segregated to a metallic grain boundary. Thus, boron will be stab/e, in a /ahn--l'eller sense [19], in all but high syra.~,etry local environments. For this reason it is considered that boron is a ch~ically acc~.~odating species, capable of forming bonds in a variety of low symmetry environments. TMs, boron possesses an electronic configuration which syr~etrs, arguments dictate can stabilize the non-~:rystallographic bonding geometries found near grain boundaries. ~c)reover, gcmxl bond formation will be promoted when the orbital energy, orbital eIectronegativity, of the segregant matches the orbital energy of the grain boundary. In order to raise the local yieht strength of a brittle boun~ry, bond formation through the directional P--orbitaIs of boron must be promoted. Thus, boron-assisted b o n d formation across the grain boundaries is most effective when the P--~grbital electronegativity of N3ron matches the electronegati~;ity ( i.e.,
I0
Vol
*'
No
I0
BORON
DISTRIBUTION
IN N i . A I
1859
O
the Feral level, to a f i r s t apprc~,.i~ation) of the p o i y c r y s t a l l i n e environment. .~ large e l e c t r o n e g a t i , , i t y difference evidently inhibits bond bridging by promoting charge ,'ransfer b e r g e n the zegregant and the ~ai~.s, leading to charge l o c a l i z a t i o n in ionic h,)nds. .~ccording to Eberhart and V:.edensk)"s cah:ulations, the P---orbital e l e c t r o n e g a t b , i t y of borc~v is close to the Fermi e n e r ~ ,-ff polycr?.,stalline N[~•~.t. Consequently, horon should be a c,.,he~ive enhancer for ~[~ AI. t.
OflC t ti~ ionb ,.
!
(1) ~'hea the O.065~t,~iB----doped 313:~I ,as held at 700 or 8.--_,OT_" and air--cooled, a r~:a~t,.e.~, large a~,-,unt of boron segregation ~o grain bounlaries appeared f,;r r.he altt:~.inide, ,~'hereas a very s~all amount of boron s ~,,,;~.x, . . . . . ~-'-' ioa appeared ~hen the a l t v i n i d e ~'as hehl at [000C a~,:l a i r - - < on I ed. (2) The segregation of boron to grain bmndaries is Iar~ely responsible f , r the [~@r,?ve.~!ent of d u c t i l i t y of the O.065wt,',iB---doped ~i~AI. ~'hen the zagnitude of boron segregation is larger.. the al,uzinide exhibits comptete!y transgranutar fracture; '~hen the boron segreff~ti;m i:; s~aller, the ahtc~inide exhibits al2ost co,~pletely intergramtlar fracture. (:3) Above a c r i t i c a l temperature that lies bet~'een 850 and 1000"C, the role of b,:,ron in improving the d u c t i l i t y of the B--doped Ni3.~I disappears be,Tattse the ,.:egregMion of hor,~ at grain boundaries becomes v e r y s~at:. ~eferences 1. ~,,'.Haosen. G)ns!itution of Binary ~.Iloss. PP. 119, ~-!,gra;.--H~i!, .~e¢: York (195°!. 2. P.H. l'horton, ff.G. Davis and T.L. Johnson, !,,~!all. Tra,q:< i, 207 (1970). L 3 . ,P...A Flinn, Trans. T.'~..,. ° --,,L~.'E 218. 145 (19@). I. R. ~,~ skw[ h, J. ~uter, ~sci. i;. i!],']i (1-078). 5. A.V. %ybolt and J.tl. Westbrook. Acta ,~4etall. 12, 449 (1964). 6, K. Ac~ki and 0 I z ~ i , ..Nippon Kinzoku Gald<.aishi 43, 1190(I.e79). 7. C.T. Liu and C.C,. Koch, Technical .~spects of c:riti,:at Vaterial used by the steel [ [',.d~tstry, VOL. [!B L,B.,IR 83--2679--9. ~ational Bttrean of Standards (1::,.,.3). ,3. - C.T. L[u, c'! .... ~h[te and J.A. Hortom A c t a ,' e t a t l . .:,:,, ~. . . . . ,:,.:, . . ' (i,q85) 9. CL. ~'hite, ILL Padgett. ('.T.t.t:t S ~ 'faI[~o'ce, ~crfpta ~4etaIl. 18, I417 (1!),°.4",. . ' ' and ...~,. IU, J.A. fforton and i~.I(. "" ~lller, ,,~cta ~'etal .. .:,o.133 ( 1!~87:'. I. "-" ii. He LL. and (:hu 'f.Y., J. phys. D; 3ppl. phys. iR
IL 13. 14. t5. [6. 17. 18. 1~.
It,15 (]98,3).
,'~'askatsa l.!eno and Tohru Inoue, Trans. iron Steel rns~. Jao. 13, 7'10 (1973). :~.P.... eah and E.[I. Hondros, Inst. ,,a: e t . ltev. -,o-, 2~2 ,~, ,-9- ~ ,,. Ashok Ch,:udhtu'y, ~ r i p t a ~letaI!. 20, 1tl61 (tt?8~J). [I.F. Stein and L.L ,lteldt, i n t e r r a c i a l ¢.;egr~gati._--n ( e d i l e d hy W.¢, Jc~hnson J.~;~.B!akeb'), PP. 2:39--260. U.;E ~etal Park (1977). '~',..~. ~'hite; R. ~.. Clausing and L. Heatherly, "-',~eta,'l. Trans. IOL 683 (197!?). C.L. ~,'hite, J.ff. ,Kei'.;er, and [ . : . Braski, ,,;~afI. ",' Trans. 12-'., [.185 (1981) t ,g,J ._i . ~ ! ] ,, Dc.G ~.E. Eherhart and O.O. t,...er~.-,r~, Scril:~ta i,:~: a , , . ~22, i!~:2 ~I ;~o). ,'.!.E. Eberhart, R.Y. Latanision and l<.It. Johnson, n~.ta ' . . . . . .. . ~ t a t ; .~' , ,.... 3 J', 176fJ (1985).
arid
1860
BORON DISTRIBUTION
IN Ni3AI
Vol. 24, No. I0
Fig. 1 The distributions of boron In samples air-quenched f r o m (a) looo°c, (b) 85o°c.
(c) 7oo° c.
o*OoO-o e¢OOQ*O 6
0
•
Fig.2 TEMphotographs of the sample air--quenched from 700"C, showing (a) morpholo~ showing no apparent precipitation of second phase and (b) selected--area--diffraction (SAD) pattern of the matrix charcterizing the LI~ ordered crystal structure.
Fig.,3 SE~! fractographs of the sample air--quenched frr~ 700"C, (a) low magnification view showing transgranular fracture, and (h) high magnification view showing fracture facets.
Fig.4 SEM fractographs of the s~ples air-quenched from (a) 850"C and (b) IOOO'C, showing (a) transgranuiar fracture and (b) intergranular fracture.
Vo!.
2~, pp. 1857-1860, 1990 Printed in the U.S.A.
Pergamon Press plc All rights reserved
EFFECT OF BORO~ DI.qTI:HBUTION ON GI~AIN--DOU?iDARIES IN NiaAl
Song Shenhua , Yuan Zhexi , Xu Tingd,mg , and Yu gongSen= Department of Materials, Wuhan Iron and Steel University, Wtthan Hube[, China ~t Department of Materials Physics, The university of science and technology 8eijing, Bei j ing, China ( R e c e i v e d J u n e 5, ( R e v i s e d J u l y 17,
1990) 1990)
Introduction Ni~kl with the LI, ordered crystal structure below the peritectic temperature (1395"C) [1] is know to have good high temperature properties [2,3]. However , the major difficulty with ~i)A1 as a high t~perature structure material is its low ductility and brittleness in polycrystalline forms [4,5]. This brittleness effectively precludes its fabrication into useful structural components. Aoki an,] [zumi [6] first discovered that the addition of bor)n into polycrystalline NisA1 increases the ductility and modifies the failure mechanism to that of transgranular fracture. Recently, liu et al. [7,8], by refining the thermomechanical process, have brought room temperature ductility up to 50~ with a completely transgranular fracture surface and also have defined the alloy composition range over which boron is effective. Presently, the reason for the improvement of ductility by the addition of boron into NisAI is generally interpreted to be boron segregation to grain botmdaries [8--10]. Therefore, if the segreation of boron to grain botmdaries is responsible for the improved ductility of B-doped Ni3A1, the role of boron in increasing the ductility of Ni3A1 will disappear above a certain temperature because of minimal segregation of boron to grain botmdaries. However, up to now, investigations of this aspect have been sparse. The purpose of the present work is to make clear whether the segregation of boron to grain boundaries is largely responsible for the improv~aent of ductility of a given B--doped ~isA1. Exper imental procedure A O. 065wtg B--doped ~i~A1 was prepared by vacuum induction melting and precision casting from high purity nickel, aluminium and a ~aster alloy of Ni--O.75wth'B. The size of ingot 'eas 65:30xl80~. The alloy composition is shown in Table 1. Table 1. Alloy composition (wtS) .
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
Ni 86.52
.
.
.
.
.
.
.
.
.
.
.
.
A1 12.75
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
S P 0.009 0.006
.
.
.
.
.
.
.
B 0.065
611 the ingots were homogenized for 5--h at IO00"C in argon and then air---cooled to room temperature. ~ext, some of ingots homogenized were heated to 700"C anti 850"C, respectively, and held for 2--h in argon, and then air---c.ooled to room temperature . All the ingots heat--treated as above ~ r e fractured in a bend test. The fracture surfaces were examined by a S--570 scanning electron microscope (SEN). In the work, the tensile properties were not examined because the tensile elongation of the B--doped NisAI "~s low in the cast state. Structural features in the B--doped hi3At ~ r e examined in a H--800 analytical transmission electron microscope. Thin foils for electron microscopic investigation were prepared by means of a standard dtml--jet electro--polishing technique. The B--doped ~i~A1 *~s electrochemically thinned in a solution of 95N methyl alcohol and 55 perchloric acid at --10"C.
1857 0036-9748/90 $3.00 + .00 Copyright (c) 1990 Pergamon Press
plc
858
BORON DISTRIBUTION
IN Ni3AI
Vol.
2~, No.
The distrib,ttions of boron in the samples "~re detected by Particle Tracking kutoradi----ography (PTk)[11]. Three--acetic acid fiber foils were ~tsed as detecting foils, whose detecting sensitivity and the spatial resolution to boron are, respectively, about l ppm and 2 uz. ~n inte~a! flux of hot neutron radiation of l.O:- !11~ cm~ was chosen in the w~)rk. The detecting foils "~re etched for 20 rain at 50"C' ~ith an aqt~e,_nus soluti,:~n of 7.58 NaOlt. The surface of each etched foil ~as coated with chr,miu~m and observed by optical microscopy. Besu!ts and Discussion The distributions of boron in the samples air---cooled frnm different temperatures are given in Fig. 1. ~."eno [12] reported that, in the PTLthe continuous etched pit belt on the etched fc,ils represents bor~-~n segregation, ,'bile the etched pit clustering represents boride precipitation. According to l.leno's study, Fig. 1 sho~s that a very small amount of boron grain--boundary segre~tion and no apparent bor[,te precipitation appear for the sample air--~:ooled from I0110"C; a relatively large amount of boron segregation and no apparent boride precipitation appear for the samples air--cooled from 850 and 700 "C- Noreover, Fig. t also shows that boron segregation decreases ~;ith increasing cooling starting temperature, in agreement with the equitihrh~ segregat ion theory [ 13] Figure 2 shows l'g~4 photographs of the B--doped .~i~Al air~:ooled from 700"C where (a) depicts a m,~rphotog5 showing no precipitation of second phase in grains or along grain boundaries , and (b) shows a selected--area--diffraction (S,~[I) pattern of the matrix which characterizes the LI2 ordered crystal structure, indicating that the boron additions do not affect the long--range ordered crystal slructure in NigAI. Figure 2 shows further that no apparent precipitation of horide~ appears in any of the samples heat--treated as above, consistent with the results of Liu el al. [7] The fracture behavior of the ~ijAl sa~,ples, broken hy bend tests, ,,as studied by scanning electron microscopy. Figure .-3 shows SE~,! fractographs of the sample air--cooled fro~ 700 "C,~here (a) is a h:,',~magnification vie~' showing transgranular fracture, and (b) is a high magnification vie, showing fracture facets, possibly corresponding to crystallographic planes. The SE!,~ fractographs of the samples air---~:ooled from 850 and 11100 "C are given in Fig. 4. ~,hich shows that the sample cooled from 8511 "C exhibits transgranular fracture [Fig. ,1 (a)], ~'hereas the sample cooled from IO00"C exhibits [ntergranMur fracture [Fig.4 (b)]. The results of the fracto~aphy are in agreement Mth the ~ r k of ,'houdhury [1.l] who reported that slow cooling from 1323k promoted [ntergranular segregation of b~-,ron in g-~toped ~ii~tl "~hile rapid quenching retained the low level attained at I.~23k;such variations in boron segregation due to variat ions in thermal history manifested themselves in the fra,.'tnre ;r,orpholog2,' of Ni)A1 and probably in the :echanical properties. ~'e conclude from Fig. l, .'3., and 4 that ( 1 ) t h e grain--boundary cohesion of the samp]e air---~enched f r ~ 7110 "C or 850 "C2 was considerably enhanced; i.e., the ductility of the sample was obviously improved, a,hereas the grain--boundary cohesion of the sample quenched fro, 101111"C,as nnt considerably enhanced; i.e., the dialilit> of the sample was not obviously improved~ (2) the segregation of boron to grain boundaries occurs for the B--doped .'Ii3AI;(3) above some of temperature bet~,een 8511 and IO00"C, the role of boron in iaprvving the d~ctility of the B--doped ~itA1 disappears because the segregation of boron to grain boundaries is very smal I. The effects of segregation solutes on grain boundary cohesion have been the subject of n~erous experimental and theoretical [nvestigations~ however, the vast majority of such inquiries have dealt with embrittling solates[15] and relatively few address beneficial effects of segregating solutes [16, 17]. Recently Eberhart and Vvedensky [18] proposed an electron[,.: modeI to explain the effect of boron on grain--boundary cohesion in Ni~r~l..~ccording to their model, boron ,,ill have a P electron count of between 1 and 2 when segregated to a metallic grain boundary. Thus, boron will be stab/e, in a /ahn--l'eller sense [19], in all but high syra.~,etry local environments. For this reason it is considered that boron is a ch~ically acc~.~odating species, capable of forming bonds in a variety of low symmetry environments. TMs, boron possesses an electronic configuration which syr~etrs, arguments dictate can stabilize the non-~:rystallographic bonding geometries found near grain boundaries. ~c)reover, gcmxl bond formation will be promoted when the orbital energy, orbital eIectronegativity, of the segregant matches the orbital energy of the grain boundary. In order to raise the local yieht strength of a brittle boun~ry, bond formation through the directional P--orbitaIs of boron must be promoted. Thus, boron-assisted b o n d formation across the grain boundaries is most effective when the P--~grbital electronegativity of N3ron matches the electronegati~;ity ( i.e.,
I0
Vol
*'
No
I0
BORON
DISTRIBUTION
IN N i . A I
1859
O
the Feral level, to a f i r s t apprc~,.i~ation) of the p o i y c r y s t a l l i n e environment. .~ large e l e c t r o n e g a t i , , i t y difference evidently inhibits bond bridging by promoting charge ,'ransfer b e r g e n the zegregant and the ~ai~.s, leading to charge l o c a l i z a t i o n in ionic h,)nds. .~ccording to Eberhart and V:.edensk)"s cah:ulations, the P---orbital e l e c t r o n e g a t b , i t y of borc~v is close to the Fermi e n e r ~ ,-ff polycr?.,stalline N[~•~.t. Consequently, horon should be a c,.,he~ive enhancer for ~[~ AI. t.
OflC t ti~ ionb ,.
!
(1) ~'hea the O.065~t,~iB----doped 313:~I ,as held at 700 or 8.--_,OT_" and air--cooled, a r~:a~t,.e.~, large a~,-,unt of boron segregation ~o grain bounlaries appeared f,;r r.he altt:~.inide, ,~'hereas a very s~all amount of boron s ~,,,;~.x, . . . . . ~-'-' ioa appeared ~hen the a l t v i n i d e ~'as hehl at [000C a~,:l a i r - - < on I ed. (2) The segregation of boron to grain bmndaries is Iar~ely responsible f , r the [~@r,?ve.~!ent of d u c t i l i t y of the O.065wt,',iB---doped ~i~AI. ~'hen the zagnitude of boron segregation is larger.. the al,uzinide exhibits comptete!y transgranutar fracture; '~hen the boron segreff~ti;m i:; s~aller, the ahtc~inide exhibits al2ost co,~pletely intergramtlar fracture. (:3) Above a c r i t i c a l temperature that lies bet~'een 850 and 1000"C, the role of b,:,ron in improving the d u c t i l i t y of the B--doped Ni3.~I disappears be,Tattse the ,.:egregMion of hor,~ at grain boundaries becomes v e r y s~at:. ~eferences 1. ~,,'.Haosen. G)ns!itution of Binary ~.Iloss. PP. 119, ~-!,gra;.--H~i!, .~e¢: York (195°!. 2. P.H. l'horton, ff.G. Davis and T.L. Johnson, !,,~!all. Tra,q:< i, 207 (1970). L 3 . ,P...A Flinn, Trans. T.'~..,. ° --,,L~.'E 218. 145 (19@). I. R. ~,~ skw[ h, J. ~uter, ~sci. i;. i!],']i (1-078). 5. A.V. %ybolt and J.tl. Westbrook. Acta ,~4etall. 12, 449 (1964). 6, K. Ac~ki and 0 I z ~ i , ..Nippon Kinzoku Gald<.aishi 43, 1190(I.e79). 7. C.T. Liu and C.C,. Koch, Technical .~spects of c:riti,:at Vaterial used by the steel [ [',.d~tstry, VOL. [!B L,B.,IR 83--2679--9. ~ational Bttrean of Standards (1::,.,.3). ,3. - C.T. L[u, c'! .... ~h[te and J.A. Hortom A c t a ,' e t a t l . .:,:,, ~. . . . . ,:,.:, . . ' (i,q85) 9. CL. ~'hite, ILL Padgett. ('.T.t.t:t S ~ 'faI[~o'ce, ~crfpta ~4etaIl. 18, I417 (1!),°.4",. . ' ' and ...~,. IU, J.A. fforton and i~.I(. "" ~lller, ,,~cta ~'etal .. .:,o.133 ( 1!~87:'. I. "-" ii. He LL. and (:hu 'f.Y., J. phys. D; 3ppl. phys. iR
IL 13. 14. t5. [6. 17. 18. 1~.
It,15 (]98,3).
,'~'askatsa l.!eno and Tohru Inoue, Trans. iron Steel rns~. Jao. 13, 7'10 (1973). :~.P.... eah and E.[I. Hondros, Inst. ,,a: e t . ltev. -,o-, 2~2 ,~, ,-9- ~ ,,. Ashok Ch,:udhtu'y, ~ r i p t a ~letaI!. 20, 1tl61 (tt?8~J). [I.F. Stein and L.L ,lteldt, i n t e r r a c i a l ¢.;egr~gati._--n ( e d i l e d hy W.¢, Jc~hnson J.~;~.B!akeb'), PP. 2:39--260. U.;E ~etal Park (1977). '~',..~. ~'hite; R. ~.. Clausing and L. Heatherly, "-',~eta,'l. Trans. IOL 683 (197!?). C.L. ~,'hite, J.ff. ,Kei'.;er, and [ . : . Braski, ,,;~afI. ",' Trans. 12-'., [.185 (1981) t ,g,J ._i . ~ ! ] ,, Dc.G ~.E. Eherhart and O.O. t,...er~.-,r~, Scril:~ta i,:~: a , , . ~22, i!~:2 ~I ;~o). ,'.!.E. Eberhart, R.Y. Latanision and l<.It. Johnson, n~.ta ' . . . . . .. . ~ t a t ; .~' , ,.... 3 J', 176fJ (1985).
arid
1860
BORON DISTRIBUTION
IN Ni3AI
Vol. 24, No. I0
Fig. 1 The distributions of boron In samples air-quenched f r o m (a) looo°c, (b) 85o°c.
(c) 7oo° c.
o*OoO-o e¢OOQ*O 6
0
•
Fig.2 TEMphotographs of the sample air--quenched from 700"C, showing (a) morpholo~ showing no apparent precipitation of second phase and (b) selected--area--diffraction (SAD) pattern of the matrix charcterizing the LI~ ordered crystal structure.
Fig.,3 SE~! fractographs of the sample air--quenched frr~ 700"C, (a) low magnification view showing transgranular fracture, and (h) high magnification view showing fracture facets.
Fig.4 SEM fractographs of the s~ples air-quenched from (a) 850"C and (b) IOOO'C, showing (a) transgranuiar fracture and (b) intergranular fracture.