ELSEVIER Materials Science and Engineering Al 91 (1995) 223-231

The oxidation of Ni,Si-base alloys

N. Sukidi”, C.C. Koch”, C.T. Liub a North Carolina State University, Department of Materials Science and Engineering, Raleigh, NC 27695, USA

bMetals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA

Received 15 April 1994; in revised form 24 June 1994

Abstract

The isothermal oxidation behavior in air of Ni-19at.%Si and Ni-19at.%Si-8at.%Cr alloys, which are based on the L 1 I structure of Ni&, was studied in the oxidation temperature range from 500 “C to 900 “C for exposures up to 20 h. The maximum oxidation rate for Ni- 19at.%Si was observed at 700 “C. The oxides which formed in this alloy at temperatures up to 700 “C consisted of external NiO layers and internal oxides that preferentially formed in the two-phase (Liz + f.c.c.) regions of the microstructure adjacent to the sample surface. The increase in oxidation rate with temperature below 700 “C is attributed to these oxides. For oxidation temperatures above 700 “C a sharp decrease in oxidation rate is observed and is attributed to the formation of a continuous SiOz protective oxide film at the base of the NiO layers. The addition of 8 at.% Cr significantly modified the oxidation behavior of the alloy with a monotonic increase in weight gain observed as a function of oxidation temperature. The formation of Crz03 appears to dominate the oxidation behavior of the Ni-19at.%Si+3at.%Cr alloys. At temperatures below 800 “C, the oxidation rates for the Cr-containing alloy were found to be significantly lower than those for the Ni-19at.%Si base alloy. However, at temperatures of 800 “C and above the oxidation rates were larger than in the base alloy, suggesting that the Cr,O, scale is less protective than the SiOz scale which forms in the base alloy in this temperature range. The formation of a continuous Ni,Si layer by Cr depletion that is separated from the external CrzO, by an Ni-rich oxide region is observed.

Keywords: Nickel; Silicon; Alloys; Oxidation

1. Introduction

An alloy development program [l] has been con- ducted recently at Oak Ridge National Laboratory (ORNL) on a series of Ni,Si-base alloys which iden- tified several alloys with very high strength, super- plastic formability, reasonable ambient temperature ductility and corrosion resistance. In addition to adequate strength and ductility, high temperature inter- metallics have to resist the effects of service environ- ments. Although studies have been done to characterize an environmental embrittlement effect in Ni,Si-base alloys [2-51 at ambient temperatures, there are limited oxidation data available for the alloys at elevated temperatures [6,7].

There have been several studies of the effects of Si additions on the oxidation of f.c.c. Ni-rich alloys. Douglass et al. [8] studied the oxidation of Ni-4.2at.%Si (2.05 wt.%) and Ni-8.87at.%Si (4.45 wt.%) in oxygen in the temperature range 600-

1000 “C. The 4.2 at.% Si alloy exhibited extensive internal oxidation while the 8.87 at.% Si alloy did not. The oxidation kinetics of the 4.2. wt.% Si alloy was normal in the sense that oxidation, as measured by mass gain, increased as the oxidizing temperature increased. However, the oxidation kinetics did not exhibit a parabolic rate but, rather, a power law of the form (AW)” = kt with values of n from 2.7 to 4.9. At 600 “C, and especially 700 “C, the 8.87 at.% Si alloy showed a larger mass gain than the 4.2 at.% Si alloy, and, at 700 “C, oxidation weight gain was comparable with that of pure Ni. The only oxide identified by X-ray diffraction for both alloys was NiO. NiO is the most stable oxide of N&O, and NiO, [9] and it appears alone in the oxidation of nickel in air or oxygen. At oxidation temperatures of 800 “C and higher, a thin protective layer of silica was believed to form on the Ni- 8.87at.%Si alloy. This was attributed to the Si diffusion becoming sufficiently rapid to form a silica film. Stott et al. [lo] also studied the oxidation behavior of several

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224 N. Sukidi et al. / Materials Science and Engineering A191 (1995) 223-231

Ni-Si alloys, but only at 1000°C. They observed a duplex NiO layer and, at higher silicon content alloys, a continuous amorphous SiOZ layer at the base of the NiO scale, which reduced the oxidation rate. However, the SiO, layers develop defects, possibly microcracks from oxide growth stresses, and are unable to prevent some continued transport of Ni2+ ions into the NiO scale and oxygen into the alloy. The SiO, layers are brittle; thus, they can provide planes of weakness and can cause spalling during cooling. The structure of the oxide layers inferred from the above studies [8,10] consists of an outer NiO layer separated from an inner NiO layer rich in silicon and of formation of an SiOZ layer that is incorporated into the inner part of the duplex NiO scale. As one would expect from thermo- dynamic considerations, it was also found that the silicon content of NiO decreased with increasing oxida- tion temperature. Increased diffusivity, as a result of increased temperature, may significantly contribute to the formation of the more stable oxide, i.e. SiO,. Chaze and Coddet [ 1 l] showed that the presence of silicon, in Ti-Si alloys (0.25, 0.5, and 1 wt.% Si), leads to a decrease in oxidation rate and the decrease in oxidation is even more evident when the temperature is raised and the silicon content is increased.

The purpose of the present work was to determine the oxidation behavior of Ni-Si alloys with a higher Si concentration than previously studied. The alloy selected for study was Ni-19at.%Si, the base alloy composition previously used in the alloy development program for Ni,Si at OFWL [l]. Oxidation studies were conducted in air in the expected range of service temperatures of 500-900 “C. A ternary alloy, Ni-1 Bat.%Si-Sat.%Cr, which was designed for better mechanical behavior than the base alloy [ 11, was also investigated in the same temperature range.

2. Experimental procedures

The alloys used in this study were arc melted, from high purity metals, at least four times. The button was turned over after each melt. Each button was then melted again and drop cast into a water-cooled, copper mold. Homogenization was done at 1100 “C for 10 h followed by 16 h at 950 “C in vacuum prior to fabrica- tion. At this point the finished product was hot rolled to plates at 1050 “C, using a double, type 304 stainless steel sheath around the sample. Samples were obtained for the oxidation study by punching disks from these plates using a 7.9 mm die. The disks were then ground through 600 silicon carbide paper to a thick- ness of about 0.5 mm and polished with 0.3 mm alumina. Oxidation measurements were carried out in a Perk&Elmer TGA-7 thermogravimetric analyzer.

Mass change measurements as a function of time were continuously recorded in flowing air (20 cm3 min- ‘) for a maximum duration of 20 h. The data were nor- malized in mass gain per unit area (milligrams per square centimeter). The accuracy of the results was verified using a Mettler microbalance with a resolution of 20 pg. This was done by oxidizing a sample at a given temperature for one specific time and comparing the mass change of this sample against the recorded value on the thermogravimetric analyzer for the same time and temperature.

X-ray analysis, using a Rigaku X-ray diffractometer, was performed after oxidation for phase identification, followed by X-ray photoelectron spectroscopy in some cases. Metallography included polishing the sample at an angle of 5” 44’ to view the oxides formed at the surface. This technique widens the region of interest to be observed on an optical microscope. A field emis- sion scanning electron microscope equipped with energy-dispersive spectroscopy equipment was also used for qualitative phase analysis. The composition of the oxide scale was studied using an electron micro- probe (JEOL Superprobe 733).

3. Results

3.1. Isothermal oxidation

Ni-19at.%Si was oxidized isothermally in the tem- perature range 500-900 “C in flowing air (20 cm3 min ‘) for a maximum duration of 20 h. Fig. 1 shows that, for this base alloy, the mass gain during oxidation increases as the temperature increases from 500 “C to 700 “C; however, the mass gain decreases from 700 “C to 900 “C. In our analysis of data, we assume a relation of the form

(AR’)“= kt

0 2 4 6 a IO 12 14 16 18 20

Time(h)

Fig. 1. Mass change of Ni-19at.%Si as a function of time in air for various oxidation temperatures.

N. Sukidi et al. 1 Materials Science and Engineering A191 (1995) 223-231 225

where AW (mg cm-‘) is the change in mass per unit surface area, t (h) is the exposure time, and y1 is the oxidation exponent. The parabolic rate law requires the oxidation exponent II to have a value of 2 (II = 2). Assuming the parabolic rate law (PI = 2), the parabolic rate constant k (mg2 cm -4 h- ’ ) may be calculated from the slope of the curve of the mass gain as a function of square root of time. True parabolic curve kinetics would appear as a straight line. Table 1 shows the calculated parabolic rate constants. The parabolic rate law is obeyed at 500 “C and at 600 “C after a 20 h exposure. At 700, 800, and 900 “C the parabolic rate law is obeyed for the first 6 h after which time the parabolic rate law breaks down. At these temperatures, the mass gains as a function of time become very small and approach a constant value (k becomes quite small). Different parabolic rate constants could be approxi- mated for this temperature range for exposures longer than 6 h as shown in Table 1.

The addition of 8 at.% Cr to Ni-19at.%Si decreases the overall mass gains of this alloy, as illustrated in Fig. 2. The oxidation of Ni-19at.%Si-8at.%Cr was done using the same test condition as above. This alloy exhibits a normal oxidation behavior; that is, its oxida- tion rate increases with increasing temperature. This is

Table 1 Parabolic rate constants calculated for Ni-lYat.%Si in the range 500-900 “C

Time (h)

Parabolic rateconstant (X lo-” mg* cme4 h-‘)

500 “C 600 “C 700 “C 800 “C 900 “C

<6 10.81 14.38 43.91 4.64 2.26 >6 10.81 14.38 2.21 1.01 0.39

- 0e20 t

___ I

, 6OQ'C I I I I I

_.__

0 2 4 6 8 10 12 14 16 18 20

Time (h) Fig. 2. Mass change of Ni-19at.%Si-8at.%Cr as a function of time in air for various oxidation temperatures.

in contrast to the base alloy which showed a decrease in oxidation rate above 700 “C. Table 2 shows the parabolic rate constants of the alloy for several con- ditions. We observe that the parabolic rate constant increases from 600 to 900 “C after a 20 h exposure. The parabolic rate law is obeyed at temperatures below 800 “C for the entire exposure time. Above 800 “C the parabolic rate law is satisfied mainly for the first 6 h of the exposure; for longer exposure AW approaches a constant value and k becomes small. Different parabolic rate constants are approximated at 800°C and above for exposures longer than 6 h as shown in Table 2.

3.2. X-ray difraction

The phase determination by X-ray diffraction was consistent with the published Ni-Si binary phase dia- gram [12] in Fig. 3. We obtained X-ray spectra on sample surfaces after exposure at various temperatures and time. Fig. 4 shows X-ray spectra of Ni-19at.%Si for different conditions. We have identified the super- lattice lines (Ni,Si phase) in addition to the nickel solid solution lines on Ni-19at.%Si. The only oxide iden-

Table 2 Parabolic rate constants calculated for Ni-19at.“/oSi-8at.%Cr in the range 600-900 “C

Time Parabolic rate constant ( x 10-‘mg’ cm-l h-l) (h)

600 “C 700 “C 780 “C 800 “C 820 “C 900°C

<6 0.27 0.72 0.82 1.78 4.91 10.06 >6 0.27 0.72 0.82 0.68 1.67 4.34

Concentration of Si (at %) Fig. 3. Ni-rich portion of the Ni-Si phase diagram. Ni-19at.%Si base alloy is indicated. a is Ni-rich f.c.c. solution, /3, is Ni,Si ordered f.c.c., L 1 2 structure.

The solid

226 N. Sukidi et al. 1 Materials Science and Engineering Al 91 (1995) 223-231

20 40 60 60 100 120

2 9 (degrees)

Fig. 4. X-ray diffraction spectra for Ni-19at.%Si before oxida- tion and after oxidation for 20 h at 700 “C or 900 “C.

tified after oxidation was NiO. The NiO scale was found to form externally. We observed that the inten- sity of the superlattice lines of Ni,Si decreased remark- ably after oxidation of the base alloy. Also, the NiO intensities appear to decrease above 700°C. The amorphous SiO,, which was assumed to form during oxidation at temperatures above 700 “C, is apparently not able to be resolved by X-ray diffraction measurements.

After addition of 8 at.% Cr, no clear Ni,Si super- structure lines are visible. However, these lines are observed in the alloy containing less than 3.26 at.% Cr. We have identified the presence of CrzO, on oxidized Ni-19at.%Si-8at.%Cr at all temperatures. Although X-ray diffraction did not detect the superlattice lines on the Cr-containing alloy (8 at.% Cr), these lines were observed after oxidation of the alloy in the temperature range used.

3.3. Morphologies and compositions

The base alloy used in this study had a significant volume fraction of Ni,Si compared with the previous oxidation studies on f.c.c. Ni(Si) solid solution [8,10]. The microstructure consisted of the minor phase of Ni- rich f.c.c. solid solution and the main phase of the Ni,Si Ll, structure intermetallic. Fig. 5(a) shows the micro- structure of Ni-19at.%Si with the Ll, Ni,Si phase (bright) and a two-phase region with Ni,Si as precipi- tates in the Ni-rich f.c.c. solid solution. The identifica- tion of the phases was done using an electron microprobe. Fig. 5(b) shows the two-phase region imaged with a field emission scarming electron micro- scope.

Fig. 5. (a) Optical micrograph of Ni-19at.%Si showing Liz phase (bright) and two-phase Ll, + f.c.c. regions. (b) Scanning electron microscopy (SEM) micrograph of two-phase Ll, +f.c.c. region. Bright contrast from Ll z particles.

Metallography performed by using an angle-lapping technique, i.e. grinding at an inclined angle of 5” 44’ to the specimen surface, revealed an increase in internal oxide particle size in the temperature range 500-700 “C and a decrease in the temperature range 700-900 “C. This is consistent with the oxidation rate trend seen in Fig. 1 where the maximum mass gain is observed around 700 “C. The oxides can be related to the mass gain during oxidation. Polished and etched angle-lapped samples revealed that oxides formed mainly along grain boundaries. A cross-sectional micrograph (Fig. 6(a)) of Ni-19at.%Si oxidized at 700°C for 20 h shows external oxide formation on a region of internal oxides. After etching it becomes apparent that the internal oxides preferentially formed in the two-phase regions of the alloy (Fig. 6(b)).

Using electron microprobe analyses (Fig. 7), we clearly see the internal oxides which formed in the two- phase field and we also observe thicker NiO layers that formed selectively on oxides in the Si-depleted two- phase region. This was observed both after 2 and 20 h exposure at 700 “C. The external oxide (NiO) on the Si-rich region (Ni,Si) is found to be much thinner than the external oxide (NiO) on the two-phase field region.

N. Sukidi et al. I Materials Science and Engineering A191 (1995) 223-231 227

Fig. 6. Cross-section of Ni-19at.%Si oxidized for 20 h at 700 “C. (a) &etched sample micrograph exhibiting external NiO layers which are thicker adjacent to internal oxides; (b) etched sample showing the internal oxides formed preferentially on the two- phase regions of the alloy.

Fig. 7. SEM electron backscattered image of Ni-19at.%Si oxidized for 20 h at 700°C. External NiO layers and internal oxides clearly shown.

The external oxide (NiO) scale appears to be the only continuous oxide scale formed at 700 “C and below. After oxidation at 700 “C the internal oxides have been determined to contain both silicon and nickel. Their structure was not identified. The electron microprobe detected only nickel and silicon in weight per cent con- centration; oxygen could not be directly measured. In the internal oxide region, the relative compositions of nickel and silicon were found to be 62 wt.% Ni and 14 wt.% Si by electron microprobe analysis. The differ- ence in total weight per cent accounts for the presence of the oxygen in addition to other minor elements. Fig. 8 shows the observed change in elemental concentra- tions through the oxide scales formed. The oxygen

"0 3 6 9 12 15

Scan distance (pm)

Fig. 8. Relative elemental weight per cent variation as a function of scan distance determined by electron microprobe analysis. Ni-19at.%Si oxidized for 20 h at 700 “C.

concentration is assumed to be the difference of 100 wt.% minus the total weight per cent concentration of nickel and silicon as determined by the electron micro- probe. Across the internal oxide (two-phase) regions (Fig. 8), we see an increase in silicon and oxygen con- centration relative to the bulk and a decrease in the concentration of nickel. The external oxides mainly contained nickel. The concentration of silicon decreases markedly in the external oxide region. After 900 “C, after 20 h of exposure, the formation of inter- nal oxides was not observed. We have instead identified the formation of a silica protective scale below the external NiO scale. Fig. 9 shows the SiO, layer that forms below a thin external oxide layer of NiO. The oxide separation from the bulk (Fig. 9) may be a result of stresses induced by cracking during cooling. This results in obvious scale spallation after 20 h exposure while no evidence of spalling was observed after 4 h oxidation at 900 “C.

The Ni-19at.%Si-8at.%Cr alloy showed smaller mass gain than the base alloy below 800 “C (Fig. 10). The oxide scale of this alloy, after 2 h exposure at 700 “C, was too thin for imaging and characterization. NiO was not detected either by X-ray diffraction or by the electron microprobe. At 900 “C after 30 h expo- sure, we observed the formation of three distinct layers (Fig. 1 1 ), consisting of an external Cr,O, layer, a continuous Ni,Si layer, and an Ni-rich oxide between the CrzO, and Ni,Si layers. No evidence of internal or external SiO, scale is observed or detected in this alloy by microscopy or electron microprobe analysis even after exposure of over 150 h. Chromium oxide is the most evident oxide scale formed under this condition, as illustrated in Fig. 11. Separation between the oxide and the bulk was evident after cooling the sample; however, cracks were less pronounced than in the case of the base alloy (Ni- 19at.%Si).

228 N. Sukidi et al. / Materials Science and Engineering Al 91 (1995) 223-231

Fig. 9. Cross-sectional micrographs of Ni-19at.%Si oxidized at 900 “C for 20 h. Continuous Si02 layer formed below a thin external oxide layer of NiO.

0.18 , 1

0.16

- “E 0.14 s 0.12

.E. 0.10

'5 0.08 c7 E 0.06 .cn p) 0.04 3

0.02

12h 8h 4h

400 500 600 700 800 900 1000

Temperature (‘T)

Fig. 10. Mass change as a function of oxidation temperature for Ni-19at.%Si and Ni-19at.%Cr for times of 4,8, and 12 h.

4. Discussion

Uncoated high temperature alloys are required to be self-protective in their operating environment. For self- protection, the formation of a stable external oxide which will drastically reduce further deterioration of the alloy is desired. However, internal oxide and/or

Fig. 11. (a) Cross-sectional micrograph of Ni-19at.%Si-8at.%Cr oxidized at 900 “C for 30 h showing three layers formed on the surface consisting of a Cr,O, layer, an Ni-rich oxide region, and an Ni,Si continuous layer; (b) chromium elemental mapping.

buried oxide formation is often the result of initial reactions prior to the formation of a well-protective scale on the metal or alloy. These phenomena often explain the higher rates seen initialy in oxidation pro- cesses. In general, additions of silicon to nickel reduce the oxidation rate of nickel except possibly for small additions of silicon (up to 1 wt.%) when we may actually see an increase in oxidation rate at 900 “C [ 131 or at 1000 “C [14]. This was attributed to doping of the NiO scale by the higher valent Si4 + ions [ 131 and it may also be attributed to internal oxide formation. How- ever, higher concentrations of silicon can significantly lower the oxidation rate of nickel, especially at tem- peratures greater than or equal to 800 “C. These trends are illustrated in Fig. 12 in which mass gain due to oxidation is plotted against the temperature for oxida-

N. Sukidi et al. / Materials Science and Engineering A191 (1995) 225231 229

2.5 t Posi I

d /- s----o--____ 19 Si 0.0 I I I ,0-76 -- -_ I

500 600 700 800 900 1000

Oxidation Temperature (“C) for 18 h

Fig. 12. Mass gain vs. oxidation temperature for an exposure time of 1X h for Ni-Si alloys. The Ni, ‘Ni-4.2at.%Si, and Ni-8.9at.%Si alloys [S] were oxidized in static oxygen; the Ni- 1 Yat.%Si alloy was oxidized in flowing air.

tion times of 18 h. The data for pure Ni, Ni-4.2at.%Si, and Ni-8.9at.%Si are from Ref. [8], where oxidation was carried out in static oxygen. The data for Ni- 19at.%Si, from the present work, were obtained by oxidation in flowing air. Therefore a direct quantitative comparison of these data may not be valid, although the qualitative trends are instructive. It is clear that silicon has a beneficial effect in terms of decreasing the oxidation rate of nickel and this effect becomes signifi- cant with increasing silicon concentration and for temperatures greater than 700 “C. Similarly, Chaze and Coddet [ 111 showed that the presence of silicon in Ti-Si alloys (0.25, 0.5, and 1 wt.% Si) leads to a decrease in oxidation rate of the metal and the decrease in oxida- tion is even more evident when the temperature is raised and the silicon content is increased.

The increases both in silicon content and tempera- ture of oxidation favor the formation of a protective SiOz. scale. The increase in oxidation temperature provides for increased silicon diffusivity which is needed to form the SiO, film. In the Ni-&9at.%Si alloy [8], for example, a duplex NiO scale is formed with the interface between the two layers fairly parallel and coincident with the original surface. For temperatures up to 700 “C, the mass gains, though smaller, are com- parable with those in pure nickel (Fig. 12). Significant improvement in oxidation resistance is particularly observed at the higher temperatures (above 700 “C) where silicon diffusivity is high enough to form a protective film. The Ni-lBat.%Si alloy of the present study did not show evidence of SiO, formation for oxidizing temperatures up to 700 “C. However, the internal oxides in the (f.c.c. + Ll,) two-phase regions

Table 3 Lattice parameter variation as a function of oxidation temperature

Temperature Lattice parameter (“C) (nm)

0 0.350 97 600 0.350 66 700 0.350 64 900 0.350 62

did exhibit a significant enrichment in silicon, as measured by the electron microprobe (Fig. 8). At higher temperatures (800 “C and above) the SiO, film (Fig. 9) forms, but it is not detectable by X-ray diffrac- tion because it is a thin amorphous layer. With the lack of internal oxides seen at temperatures below 700 “C, the SiO, scale may be related to the sharp decrease in oxidation rate observed above 700°C (Fig. 10). The marked decrease in oxidation rate at temperatures above 700 “C may be explained by the formation of an SiO, film which exhibits low diffusivity for both cations and anions and is a highly stable oxide [ 151. Since the SiO, film may be slowing the diffusion of nickel ions to the surface the external NiO scales formed at these temperatures are thinner than those formed at temper- atures lower than 700 “C. This is illustrated by the X-ray spectra (Fig. 4) whereby the NiO peak intensities decrease with increasing oxidation temperature (e.g. 900 “C) compared with lower temperatures (e.g. 700 “C). A significant decrease in the intensities of the superlattice peaks of Ni,Si was also observed with increasing oxidation temperature. Any explanation of this behavior may have to consider the disordering of the superlattice along the surface because silicon has a higher affinity for oxygen than nickel; thus, a change in the actual composition of the metal lattice may occur thereby shifting the lattice parameter of Ni,Si. The shift in lattice parameter as a function of temperature was observed and is shown in Table 3.

The ternary alloy, Ni-19at.%Si-8at.%Cr, exhibited oxidation behavior that was apparently dominated by the formation of Cr,O,. The more rapid diffusivity of chromium than silicon in the nickel matrix allowed for the preferential formation of a protective Cr,O, film even at the lower oxidation temperatures. As illus- trated in Fig. 10 a significantly lower oxidation rate at temperatures below 800 “C is observed for the Cr- containing alloy as compared with the Ni- 19at.%Si alloy. At these temperatures the formation of protec- tive Cr-rich oxide scales decreases the oxidation rate by a factor of as much as 50. The protection mecha- nism is presumed to be that a Cr,O, scale acts as a barrier to outward diffusion of Ni2+ to form NiO. This

230 N. Sukidi et al. 1 Materials Science and Engineering AI 91 (1995) 223-231

results in a thin Ni-rich oxide region below the Cr,O, (Fig. 1 l(a)). The decrease in silicon mobility due to the presence of chromium at these temperatures also affects the mechanism of the alloy protection. As a result, a continuous Ni,Si layer that is depleted of chromium forms on the surface. The formation of the ordered Ni,Si superlattice lines is also evident on the X-ray diffraction spectra, whereas the intensity of superlattice lines is not apparent before oxidation suggesting that the addition of 8 at.% Cr effectively disordered the Ll,Ni,Si compound. This is not the case for alloys containing less than 3.26 at.% Cr. The absence of SiO,, a highly stable oxide phase, in this alloy even at higher temperature (e.g. 900 “C) as com- pared with the base alloy (Fig. 11) is another indication of the dominance of Cr as compared with Si mobility. No evidence of internal or external SiO, scale is observed or detected in this alloy by microscopy or electron microprobe analysis. Because of the lack of SiO,, which is a more stable oxide than Cr203, it may be expected that protection at higher temperatures will deteriorate so that the mass gains increase with increased temperature and exceed the mass gain on the base alloy at higher temperatures. Therefore at higher oxidation temperatures (above 800 “C) the oxidation rate of the Cr-containing alloy is greater than that of the base alloy, as seen in Fig. 10. Thermodynamically, one would expect the more stable oxide of SiO, to form underneath the less stable oxide of Cr,O,. Since this is not seen experimentally, kinetic effects may be overwhelming the thermodynamics and SiO, is, thus, not observed. However, there is apparently disaccom- modation between the Cr,O, scale and the bulk alloy such that the thermal stresses generated on cooling lead to fracture and separation along the Cr,O,-bulk interface. This disaccommodation is, however, less pronounced than in the base alloy system between NiO and the bulk. The Cr oxide is apparently less protec- tive, less resistant to cation and anion diffusion, at the higher temperatures compared with the SiO, scale in the base alloy.

The limited experiments to determine the possibility of spalling of the oxide scales were conducted on the Ni-19at.%Si base alloy and on the Cr-containing alloy. Spalling on the base alloy was only observed after cooling from oxidation temperatures in excess of 700 “C where the SiO, film forms beneath the external NiO scale. There is apparent adhesion between the SiO, film and the crystalline NiO scale (Fig. 9). However, because of the thermal expansion mismatch between the layered structures, separation may occur between the scales or from the substrate. In the pres- ence of oxide growth stresses, cracking or interlayer separation is the likely consequence of cooling or cyclic oxidation. Failure within the SiO, layer was reported

[lo] for Ni-Si alloys with somewhat lower silicon concentrations (up to 13.6 at.% Si). Stress-free and imperfection-free SiO, scales are more tolerant of the stresses imposed during cooling against cracking. Therefore, preoxidation of alloys to form an SiOz diffusion barrier, as done by Meschter [ 161, is possible and many studies [ 17,181 observed SiO, scales un- damaged by thermal cycling. When the substrate is a multiphase alloy, the oxide layer may experience various thermal expansion mismatches depending on the thermal expansion of the adjacent substructure (e.g. in our case f.c.c. or Ll,). The variation in thermal expansion mismatch on the oxide layer, as a result of multiple phases, may induce shearing stresses on the oxide layer resulting, if shear is large enough, in the fracture of the oxide layer along phase boundaries. Care in sample preparation is essential in order to differentiate this fracture phenomenon from the random fracture of the surface layer that may occur during cross-sectional sample preparation. No spalling was observed for samples oxidized at temperatures up to 700 “C even though the NiO layers were thicker than at higher temperatures.

In the Cr-containing alloy, evidence of separation is apparent on cooling between the Cr,O, and the thin Ni-rich oxide region. Separation in this case is less pronounced than in the case of the base alloy.

A simple parabolic law (A W)” = kt cannot be fit to the oxidation curves (Figs. 1 and 2) for the entire 20 h of exposure time. Douglass et al. [8] used a computer program for regression analysis of their data fit to (AW)n = kt and obtained values for y1 which ranged from 2.7 to 4.9 for Ni-4.2at.%Si and from 3.0 to 6.9 for Ni-8.9at.%Si. These values suggest that the initial mass gain is rapid and that the rate decreases at longer times. The Ni-19at.%Si and Ni-19at.%Si-8at.%Cr samples of the present study behaved similarly. The parabolic rate law (n = 2) could only be fitted for approximately the first 6 h of exposure at oxidation tempertures of 700 “C and above. At longer times the mass gains decreased and the parabolic rate constant became small. This behavior of an initial increase in mass followed by a long period during which further mass gain is very small is typical of low temperature oxidation exhibiting logarithmic dependences which are not well understood. While the parabolic rate law is not clearly obeyed for these alloys, the parabolic rate constants k, calculated from the data fit to this law over the temperature intervals given in Table 1 for the base alloy have approximately the same magnitude ((1-5)X lo-” mg* cm -4 h- ‘) as that for SiO, at 800-900 “C and are much smaller than the value for NiO [ 191. This again suggests that SiO, is controlling the oxidation behavior at these temperatures in the base alloy.

N. Sukidi et al. / Materials Science and Engineering Al 91 (199-i) 223-231 231

5. Summary

The isothermal oxidation behavior of Ni-19at.%Si and Ni- 1 Bat.%Si-8at.%Cr was determined in the tem- perature range 500-900°C. The major results of this study are as follows. (1)

(2)

(3)

The oxidation rates of Ni-19at.%Si are higher at 500-700 “C than at 700-900 “C. The larger oxida- tion rates at lower temperatures are due to the formation of internal oxides and growth of thick external NiO layers on the alloy two-phase region. The sharp decrease in oxidation rate above 700 “C is attributed to the formation of protective con- tinuous SiO, layers below the NiO scale. Alloying with 8 at.% Cr changes the oxidation behavior of the base alloy from a maximum mass gain at 700 “C for the base alloy to a continuous increase in oxidation rate with temperature. The addition of 8 at.% Cr significantly lowers the oxida- tion rates of the alloy below 800 “C in comparison with the Ni- 19at.%Si alloy. The smaller mass gains observed at temperatures below 800 “C are attrib- uted to the formation of Cr,O, that acts as a barrier to outwards diffusion of Ni*+ to form NiO. The removal of Cr from the (NiCr)Si surface to form Cr,O, leads to the formation of a continuous Ni,Si external layer after oxidation. No evidence of internal or external SiO, scale is seen on the Cr- containing alloy. Thus, at higher temperatures (e.g. 900 “C) Cr,O, layers appear to be less protective than the SiO, formed on the base alloy in this tem- perature range.

Acknowledgments

The research was supported by the Assistant Secre- tary for Conservation and Renewable Energy, Office of

Industrial Technologies, Advanced Industrial Concepts (AK) Materials Program, Department of Energy, and Subcontract 19X-SLO47V from Martin Marietta Energy Systems Inc. N.S. was supported by a fellow- ship from the Advanced Industrial Concepts Materials program which is administered by Oak Ridge Institute for Science and Education for the US Department of Energy.

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