Effect of preoxidation and grain size on ductility of a boron-doped Ni3Al at elevated temperatures

wx%6160!89 13.00 + 0.00 Copyright ‘s 1989 Fkgamon Pms pk

.&co med. Vol. 37. No. IO. pp. 2681-2688, 1989 Pnntcd in Great Brimtn. All rights rcscrwd

EFFECT OF PREOXIDATION AND GRAIN SIZE ON DUCTILITY OF A BORON-DOPED Ni,Al AT ELEVATED TEMPERATURES M. TAKEYAMA andC.T.LIU Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6116, U.S.A. (Receked 29 Septern~~ 1988; in r~~ed~rrn Abstract-The

ductility of a preoxidized Ni,Ai (Ni-23Al-O.SHf5.2B.

IS hfarch 1989) at.%) alloy with various grain sizes

(I 7-193 hrn) was evaluated by means of tensile tests at 600 and 760‘C in vacuum. The preoxidation does not affect the ductility of the finest-grained material at either temperature. whereas it causes severe embrittiement in the largest-grained material. especially at 760°C. Auger studies revealed very little oxygen penetration along grain boundaries in the finest-graincd material but substantial oxygen penetration in the iarg~t-grainy one. A ~ntinuous, thin Al-rich oxide layer which forms on the fine-grained samples protects the underlying alloy from oxygen penetration. preventing any loss of ductility, whereas the nickel-rich oxide which forms on the large-gtsined samples allows oxygen to penetrate along grain boundaries, causing severe embrittlement. The grain boundaries act as short-circuit paths for rapid diffusion of aluminum atoms from the bulk to the surfaces, and this is responsible for the change in oxidation product from Ni-rich to Al-rich oxide with decreasing grain size. R&m&-La ductiiitc dun a&age preoxydi de NitAl (Ni.-23Al-~,S~if5,23 en % atomiques) de ditfercntn taillcs de grains (entre 17 et 193 pm) a fte determinCe d partir d’essais de traction sous vide j 600 et 760 ‘C. Quelie quc soit la temperature, la preoxydation nhffecte pas la ductiliti des mattriaux de petitcs taillcs de grains. alors qu’cllc provoque une fragilisation importante dam ies materiaux de grandcs taillcs dc grains, particuliercmcnt ; 76O’C. Les analysesAuger mettent en evidence une p&tration minime de I’oxygene le long des joints de grains dans le materiau aux grains ies plus petits. mais tres importantc dans Ic meter&t aux grains les plus gros. La couche mince continue d’oxyde riche en aiuminium qui se forme sur ies tchentiiions a petits grains emp&zhe l’oxygene de p&trer dans l’aliiage sous-jacent. alors que i’oxydc rich& cn nickel qui se forme sur les ~hantiilons ;1gros grains laisse I’oxyglne p&n&rer le long dcs joints de grains. cc qui provoque un fragiii~ti(~n importante. Les joints de grains se comportent comme dcs trajets de court-circuit pour la diffusion rapide des atomes d’rluminium de I’intericur vcrs Its surfaces. cc qui cst la cause de la moditication du produit d’oxydation, oxyde &he en Ni - oxyde richc en Al, lorsque la taille dc grains diminuc. Zurammenfttssung-Die Duktilitit einer voroxidierten Ni,Al-Legicrung (Ni-23Ai5.5Hf5,2R, in At.-%) wird in Abh~ngigkeit der Korngr6~ (I7 bis 193pm) in Zugversuchen bei 600 und 760°C in Vakuum ermittelt. Die Voroxidation intuit die Duktilitiit des feink6~i~ten Materials bei keiner dcr beidcn Tempcraturen. insbesondere nicht bei 765 ‘C. Auger-Untersuchungen zeigten tin nur pringcs Eindringen dcs LuerstotTes in das feinkornigstc Material, jedoch betriichtliches Eindringen in des grobkiimige Material. Ein durchgehender diinner Al-rcicher Oxidhlm bildet sich an der Obcrtlfche des feinkiimigsten Materials und schtitzt die darunter liegende Legierung vor der Oxidation, wohingegcn sich auf dem grobkomigcn Material tin Ni-reiches Oxid biidet. welches die Diffusion des Sauerstoffs entlang van Korngrenxen ermiiglicht und damit starke Verspriidung verursacht. Die Korngrenzen wirken als KuR~hlu~wege fir die wsche Diffusion der Aluminiumatome aus dem Volumen an die Qberfhiche, dieser ProzeD ist verantwortlich fur die chemische Anderung der Oxidschicht von Ni-r&h zu Al-reich mit abnchmender KorngriiOe.

I. INTRODUCTION

The ductility of nickel atuminides at elevated tempcralures is affcctcd by both test environment and grain size [l-5]. Liu er ul. [2] studied the c&cc of oxidizing cnvironmcnt on the ductility of Ni,AI alloys at 600°C and revealed a sharp drop in ductility from 50to 3% when the test environment was changed from vacuum to air. They also found that preoxidation did not affctt the ductility of those samples that were tested in vacuum. Based on these findings, they attributed the loss in ductility IO dynamic embrittlement. The grain size of spccimcns used in their study, however,

was relatively small (about 20pm) and a grain size effect was not specifically investigated. Takeyama er a!. [4]. on the other hand, systematically investigated the grain size dependence of ductility of a borondoped N&AI from room temperature to IOOO”Cin a carefully controlled vacuum (< IO-‘Pa), and revcaled that the ductility is insensitive to grain size bctwcen I5 and i 73 ~1m at temperatures below SOO”C, whereas above 800°C it depends strongly on grain size. Rcccntly. they also demonstrated the severe embrittlemcnt in the large-grained material (170 pm) tested at 76O‘C in vacuum as a result of oxygen pcnctration during long-term heat treatment in

2682

TAKEYAMA

and LIU:

DUCTILITY

OF N&AI AT ELEVATED

evacuated quartz capsules [6]. indicating that the ductility of large-grained materials is possibly more sensitive to oxidizing environments than that of small-graincd ones. Although the individual effects of environment and grain size on ductility of nickel aluminides has been investigated extensively. a detailed investigation of their combined effects is lacking. This paper focuses on the ductility of a preoxidized N&AI alloy as a function of grain size. The results are compared with those obtained in previous work [4]. and discussed in conjunction with its oxidation behavior. involving oxide formation on the surfaces and oxygen penetration along grain boundaries.

TEMPERATURES

Fig. I. Change in ductility with grain size of bare and preoxidized specimens tested at @JOand 760 C in vacuum. The data here also include the results obtained previously [4].

2. ESPERIMESTAL The alloy used in this study has the same composition (Ni-23Al4UHf-O.2B. at.%) as the one used previously. and the dctailcd alloy and speeimen preparations wcrc described elsewhere 141. Different grain sizes (as dctermincd by the linear intercept method) wcrc ohtaincd by heat-treating for various times at IO00 and IO50 C in a dynamic vacuum furnace ( < IO ’ Pa). as summarized in Tahlc 1. Prior to the tcnsilc test. specimens were oxidized at IO00 C for IO min in air. Some of specimens wcrc tcstcd without the prcoxidation trcatmcnt (hcrcafter dcsignatcd as bare spocimcn). Tcnsilc tests wcrc conducted on an lnstron testing machine at a nominal strain rate of 3.3 x IO Is ’ in vacuum at 600 and 760 C. The vacuum was very carcfully controlled and held below I x IO ’ Pa during testing. The fracture surfaces wcrc examined by scanning clcytron microscopy (SEM). Auger clcctron spcctroropy (AES) was used to analyze fracture surfaces of spccimcns. Auger samples cut from the gage section of fractured tsnsilc spccimcns wcrc V-notched, surface ground and elr~tropolishcd. followed by hydrogen charging and copper plating to promote grain-boundary fracturc. The samples were impact-fractured in the Auger chamber at ambient temperatures under a high vacuum of 5.0 x IO -’ Pa. Dctailcd Auger specimen preparations has been dcscribcd elsewhere (71. The analysts wcrc pcrformcd under a primary beam energy of IO kcV and beam current of 0.02-0.2pA. Oxidation samples with a size of approximately 7 x IO x 0.7 mm wcrc cut from the rolled sheets and anncalcd in the vacuum furnace to produce a grain size of 17 or 193 pm. The sample surfaces wcrc mechanically polished with SIC papers and electropolished in a solution of 13% H$O, and 87% Tahk

I. Grain site and heat treatments or rpccimcns used in thls study

Grain sire (rm)

Heat treatment

1-l 64 I93 246

1000 C. 3l min loo0 Ci76 h 1050 C/7d + 1000 C/Id IOSO C/I?d + IO(w)C/l d

CH,OH by parts to remove the surface tarnish and the worked layers that were produced by annealing and mechanically grinding. respectively. Cyclic oxidation tests were conducted in a vertical furnace at IOOQ’C in air for up to I5 min. During exposure, the samples were held by a platinum wire in the middle of the furnace whcrc the tcmpcrature was monitored by a Pt/Pt-IO% Rh thcrmoeouplc. The oxidized samples wcrc wcighcd to an accuracy of I .O x IO ’ g. to detcrrninc oxidation behavior in terms of weight change. AW AW=(W,-

W&A,,

whcrc W, is the weight after oxidation for I minutes. W, the original weight and A, the original surfice arca. Surface morphology of oxidized samples WJS cxamincd by SEM. An clcctron microprobe cquippcd with a wave-length dispersive spcctromctcr (WDS) was also used to qualitatively analyze the composition of surface oxide scales. 3. RESULTS

3.1. Tensile elongation and fracture hchior Figure I shows the change in ductility with grain size of bare and preoxidized specimens tested in vacuum, where the data include the results obtained previously [4]. The ductility of bare spccimcns remains unchanged at a level of 40 and 24% at 600 and 760 C, rcspcctively, although at 760 C thcrc is a tcndcncy for the ductility to dLwrcascfor the grain sizer larger than I50 pm. The ductility of preoxidized spccimcns with the finest grain size is almost identical to that of the bare spccimcn at both tcmpcraturcs; howcvcr. it decreases steadily to 29 and I% at 600 and 760 C. rcspcctivcly. as the grain size increases to I93 pm. The grain size dcpcndcnce of the ductility of preoxidizcd spccimcns is more pronounced at 760 ‘C. Figure 2 compares the fracture surfacesof bare and prcoxidizcd spccimcns having a grain size of I93 pm tested at 760 C. The bare specimen shows ductile grain-boundary facets with large and shallow dimples. togcthcr with regions fractured transgranularly along certain crystallographic planes [Fig. Z(a)]. This

and LIU:

TAKEYAMA

mode

report4

previously

dir4

intergranular

with

the results

for ths bare spccimcn

the prcoxi-

oxygen peak was observed for the preoxidizcd

consistent

exhibits

liacturc.

almost

completely

brittle

with only very few transgranu-

lar regions in the specimen center. In addition. small brightly boundary with

that both bare and prcoxidizcd

a grltin

sirs of

ductile transgranular grain-boundary prcoxidizcd fracture

193 /irn

cxhibitcd

fracture. although

facets

were

also

at 600 C

transgmnular

grain-boundary prcviousiy

fracture

fracture

at

spccimcns is due to contamination

from

used in this study. Intergranular

system

the spccimcn center were also analyzed the oxygen concentrations

in

the

in the

specimens that is.

A small amount

the Augur

by AES.

detected

whereas

the external

760 C.

men

reported

[4],

large

gradient

in Table

peak-height-ratios.

amounts

exists

(7.5 at.%).

of oxygen

specimen

surface (12.8 at.%)

center

the

and their scnsi-

near the surfaccs of the bare

in the preoxidizcd

at 600 C and ductile

and

from

of oxygen (I .8 at.%)

only at grain boundaries spccimcn,

spcci-

regions near

were estimated

Z together with the oxygen/nickel O/Ni.t

a large

peak obscrvcd for both

tivity factors [S]. The results arc summarized

observed

as

whcrcas

essentially

some smooth

prcoxidizcd

men [Fig. 3(b)]. The carbon

3(a)],

relative peak heights of the ekmcnts

spccimcn. There was no diffcrcncc

modes of bare and

[Fig.

specimens

with the finest grain si/e at both tempcraturcs; ductile

some

imaged particles arc seen on the grain-

facets [Fig. S,(b)]. Fracture stud&

rcvcalcd

2683

TEMPER4TURES

[J]. On the contrary,

is fully

spccimcn

AT ELEVATED

of (a) bare and (b) prconidizcd clwtron microgr+s rhowing the frwture surkws spwimrns having a grain size of 193pm tcstcd at 76U C.

Fig. 2. Scanning

fracture

OF Ni,AI

DUCTILITY

were

not only near

but also in the spcci-

indicating

a concentration

from surface to ccntcr. The same analyses

pcrformcd

for both bare and prcoxidizcd

specimens

with the finest grain size rcvcalcd no oxygen on the Auger

spectra from grain-boundary

the cxtcrnal

regions near

surfaces of the bare and

prcoxidizcd

grain-boundary amount

regions,

of oxygen

(<

except

that

spccimcns with a grain sire of 193 /Im tested at 760 C

tcctcd from some of the grain-boundary

arc shown in Fig. 3. In addition

the

and boron paks.

- - -- . _~-tA snrtill amount of

a small oxypn

to nickel. aluminum peak was dctectcd

.___. -_ _^-.- ..--_.___

oxjgcn (0, Ni _ 0. I) wasdctwted in the regions l’wturcd transgranulsrly of the wmplos examincd. and the oxygen was rccognizcd to come from the Auger syctcm. Then. the conccntrakn was evaluntcd by suhtractmg the amount of oxygen detected in thr tmnsgrunular rc+ns.

surfaces

of

the

results demonstrate tant

only

a small

I at.%) was occasionally prcoxidizcd

de-

facets near

specimen.

These

that grain size plays an impor-

role in oxygen

pcnstration

in the prcoxidizcd

spccimcns.

Table total

3 summarizes

oxidation

time.

the weight change (AR’) The

large-graincd

with

(193 pm)

2684

TAKEYAMA

lo/

’

and LIU:

DUCTILITY

OF Ni,Al AT ELEVATED TEMPERATURES labk

I

3. Weight shangc of the fine-graincd (17pm) and largegrained (193~m) samples with oxidation time Total oxidation time (mint 2 5 10 IS

Ni

Ni Ni 0 Ni

I

I

300600

I 900

I t2m

I i!m

4

ELECTRON ENERGY (a’.‘)

Fig. 3. Auger spectra from intergranular regions of (a) bare and (b) preonidized specimens having a grain size of 193 pm tested at 760 C and then refractured in the Auger system.

sample after each period of oxidation. Note that the lint-graincd sample shows a distinctly lower oxidation rate (AM’,‘r) than that the large-grained one does. Figure 4 compares the morphologics of surface oxide scales for both samples after oxidation for 2 and I5 min. The fine-graincd sample shows some white or white-gray patches on the dark-gray background area after 2 min exposure [Fig. 4(a)]. and the patches appears to slightly extend after oxidation for 15 min [Fig. 4(b)]. On the other hand, a totally different surface morphology was observed on the large-graincd sample; white-gray regions already dominate the surface after 2 min exposure [Fig. 4(c)], and the entire surface was covered by a white-gray layer after I5 min exposure [Fig. 4(d)]. The thickness of the white-gray oxide scale appears to vary from grain to grain [Fig. 4(c)], indicating the anisotropy of the oxidation rate 191. Surface oxide layers formed after oxidation for IS min were examined with an electron microprobe. Elemental X-ray maps of Ni. Al. 0 and the correTable 2. The oxygeninictcl vak-height ratios (O/Ni PHR) and the calculated oxygen concentr~rion on grzsin-boundary regions for the bare and prcoxidited rpmmcns having a grain size of 193 pm tcstcd at 760 C and then rcfr~ctored in Auger system Specimen Bare Preoridued

Analyzed position’ Edge center we cenler

0 Ni PHR

0: (al.?:)

0.06

1.8 12.x 7.5

0.57 0.30

‘Edge: within 100 pm from rhc cn~emal surfaces;center: 300_350/1m from the external surfacc~. -NOI detected.

AW (m&cm’) x 10: Fine grained Large-graincd (17pm) (193pm) 4.70 5.96 6.96 7.55

19.3 20.0 24.7 29.9

sponding backscattered electron images (BEI) of cross sections near the external surfaces for the fine and large-grained samples are shown in Figs 5 and 6, respectively. In the fine-grained sample, a thin (_ I pm). continuous layer is formed on the specimen surface where aluminum and oxygen are enriched and nickel is depleted (Fig. 5). indicating that the layer, corresponding to the dark area in Fig. 4, is an Al-rich oxide, possibly AlrO,. In the large-grained sample, the surface oxide scale is much thicker (3-4 pm), and it consists of two layers: an outer layer (2-3pm) enriched in nickel and depleted in aluminum, and an inner layer (C I pm) enriched in aluminum and depleted in nickel (Fig. 6). Thus, the outer layer is N&rich oxide and the inner layer is Al-rich oxide. However, the BE1 in Fig. 6 shows some dark spots in the outer layer and white regions in the inner layer, indicating that the surface Ni- and Al-rich oxide layers are inhomogcncous. Interestingly, aluminum is depleted just below the Al-rich oxide layer for both samples; the depicted tone appears to be wider in the large-grained sample than ths fine-grained one. Note that the aluminum-containing regions observed above the surface oxide scales of both samples come from the polishing processes which use alumina powder (and has nothing to do with the surface oxide scale). 4. DlSClJSSION

Prcoxidation does not affect the ductility of the finest-grained specimen at 600 and 76WC, consistent with the previous results [2]. However, the present results clearly demonstrate that, unlike the bare specimens, the ductility of the preoxidized specimens is very sensitive to grain size (Fig. 1). In particular, preoxidation of the largest-grained specimen causes the ductility drop from 15 to I% at 76O.C. with the corresponding fracture mode changing from ductile to brittle grain-boundary fracture (Fig. 2). Auger studies revealed a large amount of oxygen in the intergranular regions of this specimen (Fig. 3). Thus. there is no doubt that oxygen penetration along the grain boundaries occurring during the preoxidation treatment contributes to the loss is ductility of largegrained specimens. Our recent studies [2.6] have identified oxygen as an embrittling agent which wcakens grain-boundary cohesion and thus promotes the initiation and propagation of microcracks along the boundaries. leading to brittle grain-boundary fracture.

TAKEYAMA

fig.

4

and LX!:

Scanning circtron

DUCTILITY

microgr~phs

OF

showing the

of 17pm (a, b) anlf t93 pm (c, d) after

annealing appears to allow a small amount of oxygen to penetrate along grain boundaries even in a drnamir furtwe (see Table 2).

AT ELEVATED

surbcc morph~)ll~~y

oxidlttion

It is possible that the brightly imaged particles, probably hafnium oxide [h]. seen on the grain boundaries [Fig. I?(b)] contribute to the loss in ductility. as is ~~~rnrn~~~ly observed in Ni-base superalloys [IO]. However, reexamination of the specimen with a grain size of I73 )tm tested previously at 850 C in vacuum, of which the fracture mode was brittle grain-boundary fracture [4]. rcvcalcd almost the same number of brightly imaged particles on the grainboundary facets as observed in Fig. 2(b). This result indicates that most of the particles were formed either during the alloy preparations or the long termanncalingt. and the formation of the particles is not a major reason for the loss in ductility of the prroxidizcd specimens. The interesting point here is that the Occurrence of oxygen penetration depends strongly on grain size. This grain size dcpendcnce seems to be associated with the oxidation products formed on the specimen surfaces. A continuous. thin Al-rich oxide layer was fcxmcd on the external surfaces of the fine-grained sample (Fig. 5). whereas a thick Ni-rich oxide scale was extensively formed on the surfaces of the large+A long-lerm

Nifl

TEMPERATURES

26a5

of samples having a grain size

for 2 min (n. c) and fS min fb. d) at IMU C.

grained one [Fig. 4(c) and (d)]. Since the oxidation rate of the fine-grained sample is much smaller than that of the large-grained one, oxygen transport through the Al-rich oxide is much slower than that through the Ni-rich oxide Sale. Conscyuently, the Al-rich oxide film is more ctfective in protecting the underlying alloy from oxygen penetration, whereas the Ni-rich oxide scale allows oxygen to penetrate into the grain boundaries. Thus, a variation in grain size. which influence both the oxidation product and the ease of oxygen penetration. drasticaily affects the mechanical properties of prcoxidized specimens at elevated temperatures. The change in the main oxidation products from Ni-rich to Al-rich oxide with decreasing grain size suggests that the grain boundaries would act as short-circuit paths for rapid diffusion of Ai atoms from the bulk to the surfaces. The smaller the grain size, the larger the total grain-boundary area in the bulk. which in turn implies a shorter diffusion distance on average from a grain interior to the nearest boundary. This results in more aluminum atoms reaching the surfaces through grain boundaries. Moreover. the smaller the grain size, the more the grain boundaries intersect the surfaces, which would preferentially act as nucleation sites for Al-rich oxide. This also yields a shorter distance for lateral growth

TAKEYAMA

and LIU:

DUCTILITY

OF

Ni,Ai

Fig. 5. B;lckscaftercd eMron imrrge (BEI) and elemental a sample having r grain sil.o of 17pm alirr

AT ELEVATED

TEMPERATURES

X-ray images (h’i. Al. 0) of a cross section of oxidation for IS min at IOWJ C.

Fig. 6. Backscattcrcd elcctwn image (BEI) and elemental X-my imaps (Ni. Al, 0) of ;t cross action a sample having a grain size of 193 pm after oxidation for 15 min at INWC.

of

TAKEYAMA and LIU:

DUCTILITY OF N&Al AT ELEVATED TEMPERATURES

of Al-rich oxide to cover the grain surface. In other words. a shorter time is required to form a COtiik~Uous Al-rich oxide film on the surfaces. A similar effect of grain size on oxidation behavior has been observed in Ni-Cr alloys [I I] and stainless steels [l2], in which the increased resistance to oxidation due to the grain refinement was attributed to the rapid diffusion of chromium along grain boundaries to form a thin Cr,O, film on the surfaces. Those observations are consistent with the present results in the sense that short-circuit diffusion of reactive elements through grain boundaries promotes the formation of a protective oxide scale on the surfaces of fine-grained materials. The oxidation behavior depending on grain size can be rationalized by considering the flux of aluminum available to the specimen surfaces. According to Pettit’s oxidation study on nickel-aluminum alloys [ 131.Al,O, is stable and is the only oxide observed on specimen surfaces if the flux of aluminum from the bulk to the alloy/oxide interface, JtiI, is larger than that consumed to develop surface oxide, Jolldr.In the case that J,,,Dris smaller than Jvlld.. on the other hand. nonprotective oxides of NiAI,O, and NiO cover the surfaces. However. we suggest that the flux of aluminum from bulk 10 intcrfacc contains two components: flux through grain boundaries, JG, and through the grain interior, JH J r~oy =

JG+ J,.

is dcpcndcnt on grain size and it should be higher for the fine-graincd specimen than for the largegrained one, whereas J, is a bulk property and insensitive 10 grain size. Then. J,,,oy in the fine-grained specimen is large enough to form a continuous Al-rich oxide layer on the surfaces in the early stages of oxidation. Jalluy in the large-grained specimen, on the other hand, would be relatively small so that the Ni-rich oxide is predominantly formed on the surfdces. However, the oxide scale in the large-grained specimen consist of two layers containing probably a different type of oxide particles (Fig. 6). Then, it is assumed that in the beginning of oxidation, Ni-rich and Al-rich oxides both nucleate on the surfaces; then, because of the relatively slow growth of the Al-rich oxide due to the small value of J,,,,,y. rapid growth of Ni-rich oxides causes the formation of a thick Ni-rich oxide layer. The subsequent development of Al-rich oxide layer underneath the Ni-rich oxide layer is probably due to a decreased activity of oxygen [6.9]. Further study would be helpful in understanding the detailed formation of surface oxide scales on spccimcns with various grain sizes. Thus. flux of aluminum plays an important role in controlling the oxidation behavior of the alloys. Kucnzly and Douglass [I41 revealed that the oxide scale on NilAl changes from NiO to Al,O, as the oxidation temperature increases from 900 to 12OO’C, and they also attributed it to the increased flux of aluminum from the bulk to the surfaces with increasJ(;

ing temperature. Since a variation in grain size was

foudd to strongly affect the aluminum flux. in addition to the oxidation temperature, oxygen activity in the environment, and the initial bulk Al concentration, grain size also has to be taken into account in order to understand the oxidation mechanism of nickel aluminides.

5. SIJM‘MARY The study of the tensile ductility of a preoxidized N&AI (Ni-23 Ala.5 Hf-0.2 B, at.%) as a function of grain size draws the following conclusions: I. Preoxidation does not affect the ductility of the finest-grained material, whereas it causes severe embrittlement in the largest-grained one, especially at 760°C. 2. Preoxidation does not affect the fracture mode of the finest-grained materials; however, it drastically changes the fracture mode of the largest-grained materials from ductile to brittle grain-boundary fracture at 760°C. 3. Substantial oxygen pcnctration along grain boundaries occurs only for the large-graincd materials during the preoxidation treatment. which mainly contributes to the loss in ductility. 4. A continuous, thin Al-rich oxide layer is formed on the surfaces of the fine-graincd materials. whcrcas a thick Ni-rich oxide layer covers the external surfaces of the large-graincd ones. 5. The Al-rich oxide is prolcctivc enough to prevent oxygen pcnctration into the underlying alloy; however. the Ni-rich oxide allows oxygen to penetrate into the grain boundaries. 6. Grain boundaries act as short-circuit paths for rapid diflusion of Al atoms from the bulk to the surfaces, and this is responsible for changing the oxidation products from Ni- to Al-rich oxide with decreasing grain size. Acknowle&emenfs-The authors wish 10 thank E. P. Georgeand J. V. Cathcart for reviewing the manuscript and helpful discussions. We also thank T. J. Henson. R. A. Padgeltand B. C. Leslie for technical assistance. and C. L. Dowkcr and G. M. Sims for manuscript preparation. This research was sponsored by the Division of Materials Sciences. U.S. Depanmenl of Energy. under contract DE-ACOS-84OR21400 with Martin Marietta Energy Systems. Inc. REFERENCES I. C. T. Liu. C. L. White and E. H. Lee. Scripra nwrd. 19, 1247 (1985,. 2. C. T. Liu and C. L. White. Acru merd. 35.643 (1987). 3. A. I. Taub. K.-M. Chang and C. T. Liu. Scripro mefoil. 20, 1618 (1986). 4. M. Takeyama and C. T. Liu. Acra meroll. 36, 1241 (1988). 5. T. P. Weihs, V. Zinovicv, D. V. Viens and E. M. Schulson, Arm meroll. 35, I I09 (1987). 6. M. Takeyama and C. T. Liu, unpublished results. Oak Ridge National Laboratory (1988).

268%

TAKEYAMA and LIU:

DUCTILITY OF N&AI AT ELEVATED TEMPERATURES

7. A. Choudhury.C. L. White and C. R. Brooks. Scripta mefaK 20, 1061 ( 1986). 8. L. E. Davis. h’. C. MacDonald, P. W. Palmberg. G. E. Riach and R. E. Weber. Handbook of Auger Electron Spectroscopy. Perkin-Elmer. Eden Prairie, Minn (1978). 9. J. V. Cathcart. Proc. S_rmp.High-Temperature Ordewd Inte~meta~~icAllow Vol. 39. p. 445, MRS (1985). 10. D. A. Woodford and R. H. Bricknell, in Treatise on Materials Science and Technology (edited by C. L.

Briant and S. K. Bane& Voi. 25, p. 157. Academic Press,New York (1983). II. C. S. Giggins and F. S. Pcttit, TM.&A.I.M.E. 24!!. 2509 (1969). 12. G. J. Yurek. D. E&n and A, Garratt-Reed, Metoll. Trans. A 13A. 473 (1982). 13. F. S. Pet&, T.M.S.-A.I.M.E. u9, 12% (1967). 14. J. D. Kuenzlyand 13. L. Douglass, UxiaYationof Met& s, 139 (1974).