Effect of intense pulsed ion beams irradiation on the oxidation behavior of γ ′-based superalloy

Effect of intense pulsed ion beams irradiation on theoxidation behavior of c0-based superalloy

Hongtao Zhang a, Tianmin Wang a,*, Cong Wang a, Baoxi Han b, Sha Yan b,Weijiang Zhao b, Yafang Han c

a Center of Material Physics and Chemistry, School of Science, Beijing University of Aeronautics and Astronautics, Beijing 100083, Chinab Institute of Heavy Ion Physics, Peking University, Beijing 100871, China

c Beijing Institute of Aeronautical Materials, Beijing 100095, China

Received 16 October 2001; received in revised form 26 June 2002

Abstract

Intense pulsed ion beams (IPIB) with three different power densities (25, 37.5 and 50 MW/cm2) are employed for the

surface treatment of c0-based superalloy IC6. The influence of IPIB irradiation on the oxidation behavior of IC6 at 1100

�C for up to 100 h is investigated. It is found that the phase states of IC6 are dramatically changed after IPIB irradiationand the oxidation behavior of the irradiated coupons depends greatly on the power density of IPIB. IPIB irradiation

with a power density of 25 or 37.5 MW/cm2 significantly reduces the oxidation rate with respect to the unirradiated

coupon. The improvement of the oxidation resistance can be attributed to a change in the oxidation products from a

three-layered scale of Ni-rich oxides for the unirradiated coupon to a two-layered scale of Mo- and Al-rich oxides. In

contrast, IPIB irradiation with a power density of 50 MW/cm2 proves to be detrimental, causing a higher oxidation rate.

The oxidation mechanism for IPIB irradiated coupons is discussed.

� 2002 Elsevier Science B.V. All rights reserved.

Keywords: Intense pulsed ion beams; Irradiation; Alloy IC6; Short-circuit diffusion paths

1. Introduction

Intense pulsed ion beams (IPIB) have been de-

veloped over the last two decades as a prospective

technique for surface modification of materials [1–

3]. When irradiated by IPIB, the near-surface layer

of targets undergoes a rapid melt and solidification

with heating and cooling rates of 108–1010 K/s.

Such rates are high enough to result in phase and

microstructure changes, such as the formation ofamorphous phase and nonequilibrium micro-

structures [4,5]. These changes are responsible for

the property modification of target materials. For

example, both wear resistance and surface hard-

ness of AISI-4620 steel and O-1 tool steel have

been considerably improved after exposure to

IPIB [3,6]. Since oxidation resistance is one of the

most important surface-related properties forhigh-temperature alloys, it should be expected that

IPIB irradiation can affect this property of the

alloys. However, investigations dealing with the

Nuclear Instruments and Methods in Physics Research B 197 (2002) 83–93

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*Corresponding author. Tel.: +86-10-8231-7931; fax: +86-

10-8231-5933.

E-mail addresses: [email protected] (H. Zhang),

[email protected] (T. Wang).

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effects of IPIB irradiation on oxidation behavior

have not yet been fully undertaken.

In the present work the possibility of improving

the oxidation resistance of Ni3Al based superalloyby IPIB irradiation was studied. The directionally

solidified (DS) c0-based superalloy IC6 is selected

as the object of investigation because of its ex-

ceptional properties, such as high incipient melting

temperature (1315 �C), good ductility from room

temperature to 1100 �C, excellent high tempera-

ture rupture strength, etc. [7]. The isothermal ox-

idation tests of IC6 unirradiated and irradiated byIPIB were performed at 1100 �C, which is the

operating temperature for IC6 [8].

2. Experimental

The master alloy with the chemical composition

of Ni–(7.5–8.5)Al–(13–15)Mo–(0.02–0.1)B (inwt%) was first prepared in a vacuum induction

furnace. Then the columnar grain rods were pro-

duced by rapid solidification technique in a com-

mercial DS vacuum induction furnace. The as-cast

rods were homogenized at 1260 �C for 10 h fol-

lowed by oil quenching in order to minimize the

segregation in the as-cast condition and to obtain

optimum mechanical properties. To obtain targetcoupons, the heat-treated rods were cut trans-

versely into 15� 10� 2 mm3 plates along the di-

rection of solidification growth. Then the coupons

were mechanically polished to Ra � 0:1 lm. Irra-diations were performed on the TEMP techno-

logical accelerator from the Nuclear Physics

Institute, Tomsk Polytechnic University in Russia

[9,10]. The beam consisted of carbon ions (70%)and protons (30%). The coupons were irradiated

respectively with three kinds of maximum ion

current density (J) and pulse number (n) at the

same maximum accelerating voltage (V ¼ 250 kV)

and pulse duration (s ¼ 60 ns, full width at half-

maximum), i.e. J1 ¼ 100 A/cm2, n1 ¼ 24, P1 ¼VJ1 ¼ 25 MW/cm2; J2 ¼ 150 A/cm2, n2 ¼ 16,

P2 ¼ VJ2 ¼ 37:5 MW/cm2; J3 ¼ 200 A/cm2, n3 ¼12, P3 ¼ VJ3 ¼ 50 MW/cm2. The distance from

beam generation to irradiated coupons is about 20

cm. The reproducibility of pulse power from shot

to shot is better than 20%. Only the two large faces

of the coupons were irradiated, accounting for

about 75–80% of the total surface area. In order to

study the effect of the IPIB power density solely,

the ion doses for three kinds of the irradiatedcoupons were controlled at approximately same

level (1� 1014 ions/cm2).

Prior to oxidation tests, the phase states of the

unirradiated and irradiated coupons were in-

spected by X-ray diffraction (XRD) with Cu-Karadiation at 40 kV and 40 mA. Then all coupons

were cleaned ultrasonically in ethanol. Isothermal

oxidation tests were performed at 1100 �C in staticair for up to 100 h. The coupons were quickly

inserted into the heat zone of the furnace at the

beginning of tests. The mass changes of coupons

were monitored continuously by a balance with a

resolution of 0.1 mg.

Following oxidation, the oxidation products

were characterized by XRD, scanning electron

microscopy (SEM) and energy dispersive X-rayanalysis (EDXA).

3. Results

3.1. XRD analysis of as-irradiated coupons

Fig. 1 shows the XRD patterns of unirradiatedand irradiated coupons. As might be expected, the

Fig. 1. XRD spectra of the unirradiated and irradiated cou-

pons.

84 H. Zhang et al. / Nucl. Instr. and Meth. in Phys. Res. B 197 (2002) 83–93

unirradiated coupon consisted mainly of c0-Ni3(Al, Mo). There were only two peaks of (2 0 0) and

(1 0 0) from c0 phase for the coupon irradiated by

IPIB with a low power density P1 (hereafter re-ferred as ‘‘P1 coupon’’). The relative peak heightratio of (2 0 0) to (1 0 0) was great. It is obvious

that the preferred orientation was present in the

surface layer of P1 coupon. For the coupon irra-diated by IPIB with a moderate power density P2(hereafter referred as ‘‘P2 coupon’’), only the (2 0 0)peak from c0 phase remained. So the preferred

orientation still existed. Moreover, a slight rise andbroadening of the background over a 2h range 45–55� suggests the formation of minor amorphous c0

phase on the surface of P2 coupon. A new phase,

besides c0, appeared on the surface region of the

coupon irradiated by IPIB with a high power

density P3 (hereafter referred as ‘‘P3 coupon’’).

However, the number of diffraction peaks was too

limited to ascertain what the new phase was. Per-haps it was Ni2Al3.

3.2. Kinetic study

The isothermal oxidation kinetics for the unir-

radiated and irradiated coupons at 1100 �C for

100 h are plotted in Fig. 2. It is obvious that

P2 coupon exhibited the best oxidation resistanceamong all coupons. P1 coupon also displayed

better oxidation resistance than the unirradiated

coupon. Contrarily, compared with the unirradi-

ated coupon, the oxidation resistance of P3 coupondecreased.

All kinetic curves showed the similar trend.

That is to say, each curve could be divided intothree stages. In the first stage, the mass increased

quickly with the increasing time. But the mass loss

predominated over the mass gain in the second

stage. Then the mass increased slowly with ex-

tended oxidation in the third stage. Unlike the

conventional or pure Ni3Al alloys where the para-

bolic rate law is obeyed for the isothermal oxida-

tion kinetics [11–16], the complex kinetic curvesshown in Fig. 2 are associated with the presence of

molybdenum in IC6 [17,18], as described below.

3.3. Phase analysis of the oxide scales

Fig. 3(a)–(d) shows the XRD spectra of the

oxide scales formed on the unirradiated and irra-

diated coupons after oxidation at 1100 �C for100 h, respectively. Fig. 3(a) is the XRD result of

oxide scales on the unirradiated coupon, which

was characterized by high peaks from NiO and

low peaks from NiAl2O4, NiMoO4, MoO2 and a-Al2O3. Fig. 3(b) shows the XRD profile of oxide

scales on P1 coupon. The main oxides were

NiAl2O4 and NiMoO4. Small peaks from MoO2

and a-Al2O3 could be detected. The oxide scales onP2 coupon consisted of a large amount of a-Al2O3

and NiAl2O4 and relatively small amount of Ni-

MoO4 and MoO2, as shown in Fig. 3(c). Fig. 3(d)

shows the XRD chart of oxidation products on P3coupon. The high peaks from NiO and NiAl2O4

and low peaks from NiMoO4, a-Al2O3 and MoO2

could be identified. The phase composition of

oxide scales did not show a significant differencebetween the unirradiated and P3 coupon. More-over, Ni3Mo phase could be found in Fig. 3(b) and

(c).

3.4. Morphology of oxide scales

3.4.1. Unirradiated coupon

Fig. 4 shows the surface morphology and cross-sectional microstructure of the unirradiated cou-

pon after exposure at 1100 �C for 100 h. The

surface (Fig. 4(a)) was characterized by the thick

and bright nodules which were disintegrated by theFig. 2. Mass change of the unirradiated and irradiated coupons

during isothermal oxidation at 1100 �C in air.

H. Zhang et al. / Nucl. Instr. and Meth. in Phys. Res. B 197 (2002) 83–93 85

dark islands in some regions. EDXA results indi-

cated that the bright nodules were NiO and the

dark islands were a mixture of nickel and alumi-

num oxides. A magnified examination of bright

nodules revealed that the average size of NiO

grains was about 5 lm (Fig. 4(b)). Cross-sectional

investigations (Fig. 4(c)) revealed that the oxida-tion products consisted of three layers. From

EDXA and XRD results it was found that the

outer layer contained NiO and the intermediate

layer contained a large amount of the spinel phase

(NiAl2O4) and a small amount of NiMoO4 and

MoO2. An Al-rich continuous layer formed in the

inner layer consisted of a-Al2O3. A diffusion layer

enriched in Mo and depleted in Al was devel-

oped beneath the oxide scales, irrespective of IPIB

irradiation.

3.4.2. P1 and P2 coupon

Both surface and cross-sectional examinations

revealed that the microstructure of oxide scales

formed on P1 coupon was similar to that formed

Fig. 3. XRD spectra of oxide scales formed at 1100 �C for

100 h: (a) unirradiated coupon; (b) P1 coupon; (c) P2 coupon;(d) P3 coupon.

Fig. 4. Scale pattern of the unirradiated coupon after oxidation

at 1100 �C for 100 h: (a) overview of surface morphology; (b) a

high magnification of bright area in (a); (c) cross-sectional mi-

crostructure.


on P2 coupon after isothermal oxidation at 1100�C for 100 h. So only the scale morphologies of P2coupon are shown here (Figs. 5 and 8). Fig. 5 is the

surface morphology of P2 coupon after oxidationat 1100 �C for 100 h. The white and dark grey

regions continuously and randomly distributed on

the outer surface of P1 and P2 coupon, as shown inFig. 5(a). The darkest spots corresponded to the

craters, which often occur in the surface mor-phology for IPIB irradiated coupons [19]. The

presence of craters does not affect the oxidation

behavior because no difference in composition

between zone containing craters and those not

containing craters can be detected through EDXA.

No apparent spallation of the oxide scales could be

found. A magnified examination of the surface

(Fig. 5(b)) illustrated that the outer part of thescales was covered with small white granular par-

ticles of about 0.5–1.0 lm in size (area A) and

much finer grey network oxides (area B). The con-

glomeration of granular particles was also present.

In spite of the surface morphology resemblance

between P1 and P2 coupon, the compositions of the

oxidation products of P1 coupon significantly dif-fered from those of P2 coupon. This is illustrated inFigs. 6 and 7. Fig. 6(a) and (b) shows the EDXA

spectra of the granular particles and finer networkoxides on P1 coupon after 100 h isothermal

Fig. 5. Surface morphology of P2 (or P1) coupon after oxida-tion at 1100 �C for 100 h: (a) overview; (b) a high magnificationin (a).

Fig. 6. Analysis of the composition of surface scale formed on

P1 coupon after oxidation at 1100 �C for 100 h: (a) EDAX

spectrum of area A in Fig. 5(b); (b) EDAX spectrum of area B

in Fig. 5(b).


oxidation at 1100 �C, respectively. The carbon

peak came from carbon deposited on the coupon.

It is evident that for P1 coupon, molybdenum was

richer in the granular particles than in the network

oxides, while the opposite concentration distribu-

tion for aluminum was formed on the surface of

oxide scales. Accordingly, the granular particles

contained mainly NiMoO4 and MoO2, and the

network oxides contained a mixed oxide enriched

in aluminum, i.e. NiAl2O4 and a-Al2O3, which wasconsistent with the XRD results. In comparison

with the unirradiated coupon, appreciable amount

of molybdenum appeared on the outer surface of

P1 coupon after exposure at 1100 �C for 100 h. It

was evident that the outward diffusion of molyb-

denum occurred in P1 coupon during oxidation.Fig. 7(a) and (b) shows the EDXA spectra for

the granular particles and network oxides on P2coupon after isothermal oxidation at 1100 �C for

100 h, respectively. Considering that both granular

particles and network oxides were enriched in

aluminum, it may be that Al-containing oxides on

the outer layer of the scale were more plentiful for

P2 coupon than for P1 coupon, which was consis-tent with the XRD results mentioned above. In

addition, molybdenum was richer in the granularparticles than in the network oxides. Compared

with composition of the outer scales in the unir-

radiated and P1 coupon, it was obvious that notonly molybdenum, but also a large amount of

aluminum appeared on the outmost surface of

oxide scales in P2 coupon through outward diffu-sion. Combined EDXA with XRD results, it could

Fig. 7. Analysis of the composition of surface scale formed on

P2 coupon after oxidation at 1100 �C for 100 h: (a) EDAX

spectrum of area A in Fig. 5(b); (b) EDAX spectrum of area B

in Fig. 5(b).

Fig. 8. Cross-sections in P2 (or P1) coupon after oxidation in1100 �C for 100 h.


be deduced that the granular particles were com-

posed of NiAl2O4 and NiMoO4, while the network

oxides contained mainly a mixture of a-Al2O3 and

NiAl2O4.

Fig. 8 shows cross-sections in P2 coupon afteroxidation in 1100 �C for 100 h. In contrast to

the case of the unirradiated coupon where well-

developed NiO, NiAl2O4 þNiMoO4, and a-Al2O3

layers were formed, the scales formed on P2 or P1coupon consisted of two layers. This can be clearly

illustrated by energy line scans starting from the

outer layer of scales to the substrate metal for P2coupon, as Fig. 9 shows. It indicated that the outer

layer was a mixture of NiMoO4, NiAl2O4 and a-Al2O3. The inner continuous layer consisted of a-Al2O3. The scale formed on P1 and P2 coupon wasrelatively porous and was more so near the outer/

inner layer interface.

3.4.3. P3 coupon

Fig. 10(a) and (b) shows the surface morphology

and cross-section of P3 coupon after oxidation in1100 �C for 100 h, respectively. Severe spallation

took place during cooling. At sites where no spal-lation occurred (upper part of Fig. 10(a)), the

coarse-grained NiO was found, while a fine-grained

structure, which was a mixture of nickel and alu-

minum oxides, was present in the spalled zone.

Fig. 9. Energy line scans of the scales formed on P2 couponafter oxidation in 1100 �C for 100 h (A: complex oxides; B: a-Al2O3; C: diffusion layer; D: substrate).

Fig. 10. Surface morphology (a) and cross-section (b) of P3coupon after oxidation in 1100 �C for 100 h. Fig. 11. A crack beneath the irradiated surface of P3 coupon.


Since the outer NiO layer was largely spalled dur-

ing cooling, only the intermediate and inner layer

could be observed in Fig. 10(b). Therefore, the

scales formed on P3 coupon still exhibited a three-layer structure, which was the same as those

formed on the unirradiated coupon. Beneath the

lost outer NiO layer, the intermediate layer was

predominantly a mixture of nickel and aluminum

oxides. Small amount of NiMoO4 was also present.

In the inner part of the scale there was a continuous

a-Al2O3 layer. Additionally, a deep crack under the

irradiated surface could be found in somewhere ofP3 coupon (Fig. 11). The crack caused by extremelyhigh stress during IPIB irradiation made more

areas oxidized, which also contributed to the over-

all mass gain after oxidation for 100 h.

4. Discussion

4.1. Phase response of IC6 targets to irradiation

with IPIB

Fig. 1 shows that phase states of the targets

change significantly under IPIB irradiation. The

preferred orientation formed in P1 coupon suggeststhe occurrence of intense plastic deformation

during the interaction between IPIB and targets,which arises from extremely high stress. In general,

the great stress is introduced in the surface layer of

the irradiated targets as a result of recoil impulse,

high temperature gradients and implantation of

the accelerated ions [5]. For example, Korotaev

et al. studied the structural transformation of

Ni3Al single crystals upon IPIB irradiation with

power density P � 50 MW/cm2. It was estimatedthat the stress could reach 3–5 GPa in the surface

layer [20]. Consequently, the preferred orientation

appeared in P1 coupon is the deformation texture.Such a surface deformation is of great importance

for the oxidation behavior of the irradiated cou-

pons, which will be discussed in the following

sections. With increasing power density of IPIB,

the heating and cooling rates at the surface oftargets increase. The formation of amorphous and

new phases on P2 and P3 coupon, which is commonfor the irradiated materials, can be attributed to

rapid solidification [4,5].

4.2. Oxidation behavior of the unirradiated coupon

The oxidation behavior of Ni3Al intermetalliccompounds has been extensively studied [11–16].

Only the most relevant aspects will be retrospected

here. The Gibbs free energies of formation of the

oxides show that a-Al2O3 is a more thermody-

namically favored oxide than NiO. But the growth

rates are of the ratio of NiO: a-Al2O3 ¼ 1: 10�3 �10�4 [21]. As a result, NiO is the faster growing

oxide. After oxidation for a long time, the spinelphase NiAl2O4 is formed through a solid-state

reaction between the outer NiO and the inner a-Al2O3 layers. Thus the scales formed in the tem-

perature range of 900–1100 �C for a long time

usually consist of an outer layer of NiO, an in-

termediate layer of NiAl2O4 and an inner layer of

a-Al2O3.

In comparison with conventional or pure Ni3Alalloys, IC6 exhibits a few differences in the oxi-

dation behaviors because of a high Mo content in

IC6. These differences lie in the oxidation kinetics

and the scale pattern [17,18]. Molybdenum is ad-

ded into IC6 as a solid-solution strengthener [7].

But with respect to the oxidation resistance, Mo is

undesirable because MoO3 tends to volatilize or

melt even at moderate temperature [22]. The dif-fusivity of Mo is much smaller than that of Ni and

Al due to a large atom size of Mo (1.39 �AA in ra-

dius). Meanwhile, as far as the affinity for oxygen

is concerned, aluminum is much higher than mo-

lybdenum. So the formation of MoO2 and MoO3

lags behind that of NiO and Al2O3. Ye et al. in-

vestigated the oxidation behavior of IC6 at 1100

�C in air for 300 h [17]. They found that the iso-thermal oxidation kinetic curve of IC6 at 1100 �Cshowed three stages, which coincides with the re-

sult in this paper (Fig. 2). In the first stage, the

oxidation of Ni, Al and Mo happens simulta-

neously, but NiO is formed more quickly. With

the increasing time, oxygen can diffuse inwardly

through less protective NiO scales and the out-

ward diffusion of molybdenum enhances. Thus thevolatileness of MoO3 is intensified. When the mass

loss resulted from the volatileness of MoO3 pre-

dominates over the mass gain, the second stage

in kinetic curve appears. With further oxidation,

NiMoO4, besides NiAl2O4, forms through a solid-


state reaction between Mo and NiO [23]. The

continuous layer of a-Al2O3 is developed as well.

On the one hand, the healing layer of a-Al2O3 can

hinder the migration of molybdenum. On theother hand, the supply of Mo via the outward

diffusion can not keep pace with the volatileness of

MoO3 because of a low diffusivity of Mo. Hence

the volatileness of MoO3 is gradually retarded and

the oxidation rate increases slowly, as the case

in the third stage of the kinetic curve. Beneath the

healing layer of a-Al2O3 there is a diffusion layer

(i.e. Al-depleted zone). With progressive oxida-tion, the outward diffusion of nickel and alumi-

num makes molybdenum so supersaturated in the

diffusion layer that the new phase Ni3Mo is

formed by phase transformation [24], as found in

Fig. 3.

4.3. Oxidation behavior of the irradiated coupons

Both the mass change measurements and anal-

ysis about the composition and microstructure of

oxide scales reveal that IPIB irradiation not only

alters the oxidation rate, but also induces a sig-

nificant change in the oxidation mechanism. Since

the chemical compositions of the IC6 alloy are

fixed even after exposure to IPIB, the change of

phase state and microstructure induced by IPIBirradiation should be taken into consideration to

account for the oxidation behavior of the irradi-

ated coupons. Compared with the unirradiated

coupon, the crystalline defects (point defects and

dislocations) remarkably increase in the surface

layer of the irradiated coupons owing to rapid

solidification during irradiation [5,25]. When the

coupon is irradiated by IPIB with a low powerdensity P1, the formation of deformation texture inP1 coupon indicates that intense plastic deforma-tion has taken place and a large strain energy has

been stored in alloy IC6. Such a high stored energy

can give rise to recrystallization when P1 coupon isheated to the test temperature [26]. Subsequently,

numerous grain boundaries can be produced on

the deformed surface of the alloy during oxidation.Investigations by Giggins and Pettit on Ni–Cr

alloys found that the fine-grained structures ap-

peared on the cold-worked alloy surface via re-

crystallization when the specimens were tested at

900/1100 �C [27]. All kinds of defects in the form

of grain boundaries, point defects and dislocations

can serve as preferential sites for nucleation of

oxides or as short-circuit diffusion paths by locallyfavoring diffusion of the metal [28]. The selec-

tive oxidation of chromium could be promoted.

Accordingly, the oxidation resistance of Ni–Cr

alloys was improved after cold working. The

beneficial effect caused by the fine-grain structure

is also demonstrated by many investigations. For

example, Wang et al. studied the oxidation be-

havior of the sputtered microcrystalline coatings[29,30]. They found that the oxidation products for

microcrystalline Ni3Al at 1000–1100 �C consisted

of a-Al2O3 and NiAl2O4, in contrast to the case of

the coarse-grain Ni3Al alloy where Ni-rich oxides

were formed. The improvement of oxidation re-

sistance of microcrystalline Ni3Al was corre-

spondingly attributed to the change of the oxide

scale pattern. Their results are therefore in goodaccordance with the results observed here. There is

no doubt that IPIB irradiation can induce a deeper

deformed zone and a higher energy deposition in

the surface of coupons than general cold work.

This can be given evidences by the appearance of

deformation texture in P1 coupon, amorphous c0

phase in P2 coupon and new phase in P3 coupon.Thus short-circuit diffusion, which results from thenumerous defects induced by IPIB irradiation as

well as abundant grain boundaries induced by re-

crystallization on the deformed surface, plays an

important role in the oxidation behavior of the

irradiated coupons. In spite of low diffusivity of

molybdenum, the outward diffusion of molybde-

num is dramatically enhanced, let alone the case of

nickel and aluminum. The formation of NiAl2O4

and NiMoO4 by the solid-state reaction and the

establishment of a healing layer of a-Al2O3 are

facilitated by the enhanced diffusion of cations. It

is well known that NiMoO4 has a good stability up

to 1150 �C [23], and the spinel (NiAl2O4) structure

is comparatively free of defects when compared to

the simple structure of NiO [14]. The better oxida-

tion resistance of P1 coupon arises from the changeof scale pattern from Ni-rich oxides in unirradi-

ated coupon to Mo- and Al-rich oxides.

With the increasing power density of IPIB from

P1 to P2, minor amorphous c0 phase is formed on


the surface of P2 coupon. The best oxidation re-sistance of P2 coupon results from the increasing

amount of Al-rich oxides in the scales. The for-

mation of Al-rich oxides, i.e. the selective oxidationof aluminum, should be correlated not only with

the deformed surface, but also with the formation

of the amorphous c0 phase. Hampikian et al.

studied the effects of yttrium ion implantation on

the oxidation behavior of Ni–Cr and Co–Cr alloys

[31–33]. They found that both alloys contained

an amorphous surface phase as a result of suffi-

cient dose implantation of yttrium. Microstruc-ture analysis via transmission electron microscopy

showed that recrystallization of the amorphous

phase took place quickly in the first few minutes of

oxidation, which resulted in the fine-grain recrys-

tallized NiCr phase in Ni–Cr alloys and inter-

metallic phase for Co–Cr alloys. The improved

oxidation resistance of yttrium implanted Ni–20Cr

was ascribed to the fact that fine-grain NiCr viarecrystallization provides fast-diffusion paths for

chromium. Thus the selective oxidation of chro-

mium occurred. It is reasonable to assume that in

present experiment recrystallization of the amor-

phous phase occurs immediately after P2 coupon isinserted into the heat zone of the furnace. As a

result, the effect of short-circuit diffusion on the

oxidation behavior of P2 coupon becomes morepronounced because fine-grain structure via re-

crystallization can be synergeticly induced not only

by the deformed surface, as the case in P1 coupon,but also by the formation of the amorphous phase.

It is worth noting that the grain size of the scale

formed on P1 and P2 coupon is much smaller thanthat of the scale on the unirradiated coupon (cf.

Figs. 4(b) and 5(b)). This is also associated withthe increasing nucleation sites for the oxides by

means of short-circuit diffusion. A similar phe-

nomenon has been observed for a lot of alloy

systems in the previous work no matter the nu-

cleation sites for the oxides originate from the

dispersed rare-earth oxide phase [34] or from the

fine-grained structure of alloys [35,36].

The voids within the oxide scales for all irradi-ated coupons were more conspicuous than those in

the unirradiated coupon. In view of the fact that

the mobility of molybdenum in the irradiated

coupons is significantly enhanced during oxidation

by means of short-circuit diffusion paths, these

voids may be associated with the violent vola-

tileness of MoO3, which is in conformity with

the mass loss in oxidation kinetic (Fig. 2). Oncethe healing layer of a-Al2O3 is fully developed, the

volatileness of MoO3 is inhibited.

When the coupon is irradiated by IPIB with a

high power density P3, a new phase as well as the

crack appears in P3 coupon. The worst oxidationresistance of P3 coupon is attributed to the for-

mation of Ni-rich oxides as well as the extra mass

gain around the crack zone. The oxidation be-havior of P3 coupon is affected not only by the

effect of short-circuit diffusion, but also by the

appearance of this new phase. Unfortunately,

the effect of new phase on the oxidation behav-

ior of P3 coupon is still unclear because of uncer-tainty of new phase. But it can be concluded that

the IPIB irradiation with an extremely high power

density is harmful to the oxidation resistance ofalloy IC6.

5. Conclusion

The major conclusions from this study can be

summarized as follows:

(1) The change of the phase states for the irradi-

ated coupons depends on the power density

of IPIB. With the increasing power density

from 25 to 50 MW/cm2, the deformation tex-

tures from c0 phase, minor amorphous c0 phase

and new phase appear in turn.

(2) Compared with the unirradiated coupon, the

intense effects of short-circuit diffusion inducedby the change of the microstructures and phase

states after IPIB irradiation occur in the irradi-

ated coupons during oxidation at 1100 �C. Thescale pattern in the coupons irradiated by IPIB

with a power density of 25 and 37.5 MW/cm2

is characterized by two layers, consisting of

an outer layer of Mo- and Al-rich oxides and

an inner layer of a-Al2O3. The change in theoxidation products from a three-layered scale

of Ni-rich oxides for the unirradiated coupon

to such a two-layered scale of Mo- and Al-rich

oxides is responsible for the improved oxida-


tion resistance. However, the IPIB irradiation

with an extremely high power density (50

MW/cm2) is harmful to the oxidation resis-tance of alloy IC6.

Acknowledgements

This work was sponsored by the Open Labo-

ratory of Solid Lubrication in Lanzhou Institute of

Chemical Physics under contract no. 9903, and byBeijing University of Aeronautics and Astronau-

tics for the basic research.

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Effect of intense pulsed ion beams irradiation on the oxidation behavior of γ ′-based superalloy

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