Volume 2, number 4A MATERIALS LETTERS March 1984

INTERDIFFUSION IN THE NICKEL-IRON SYSTEM

Vaidehi GANESAN, V. SEETHARAMAN and V.S. RAGHUNATHAN

Metallurgy Programme, Reactor Research Centre, Kalpakkam 603102, India

Received 9 January 1984

Interdiffusion behaviour between nickel and iron in the temperature range 1073-1373 K was investigated. The activa-

tion energy and frequency factor for the interdiffusion process were calculated in the concentration range lo-90 wt% Fe.

The analysis of the diffusion data indicated a vacancy-aided diffusion mechanism. The values of the interdiffusion coeffi-

cients corresponding to 1073 and 1148 K were found to be higher than those obtained by extrapolation of high-tempera-

ture data. This is possibly due to the presence of the 01 phase leading to diffusion in a two-phase field in the couples an-

nealed at temperatures below 1173 K.

1. Introduction

Interdifussion between dissimilar metals or alloys

constitutes an important process of considerable sig- nificance to engineering materials. Chemical diffusion data obtained from the analyses of concentration pro- files in sandwich-type diffusion couples have been very useful in predicting the kinetics of several phe- nomena such as precipitate coarsening, layer growth, diffusion bonding, degradation of protective coatings,

etc. Interdiffusion data corresponding to several iron-

based binary systems, e.g., Fe-MO [1,2], Fe-Ni [3,4], Fe-V [5], Fe-Al [5], Fe-Mn [6] are available in the literature. In this paper, we shall consider the in- terdiffusion process occurring in the nickel-iron sys-

tem in the temperature range 1073-1373 K.

2. Experimental procedure

Armco iron and pure nickel were used for prepar- ing the diffusion couples. The chemical compositions of these materials are shown in table 1. Sandwich- type diffusion couples of iron and nickel were pre- pared using the procedures described in detail else- where [7]. These couples were sealed in quartz cap- sules in a vacuum of lo-’ Torr and diffusion anneal- ed at different temperatures in the range 1073-1373

Table 1 Chemical compositions of the materials used, Wt%

Nickel Iron

c 0.012

co 0.045

Fe 0.004

cu 0.005

MnO.OO1

Ni balance

c 0.017

s 0.012

Fe balance

K. After annealing, the couples were sectioned and polished carefully. These were examined in a Cameca MS-46-R electron probe microanalyser to obtain con-

centration profiles of the elements Fe and Ni across the diffusion zone, Typical operating conditions of the microprobe analyser were: acceleration voltage,

20 kV; hpecimen current, 50 nA (in iron standard); take-off angle, 18’; beam diameter, 2 pm; beam cur- rent, 150 PA. The X-ray intensities corresponding to the Ka radiation of Fe and Ni were measured at inter- vals of 2 pm near the original interface where the in- tensity values were changing rapidly. At distances far from the interface, the X-ray intensity was measured at intervals of 10 m. For all measurements a constant counting time of 10 s was used. X-ray intensities

were also obtained from pure metal standards under identical conditions of analysis. The stability of the

0 167-577x/84/$ 03.00 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

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Volume 2, number 4A MATERIALS LETTERS March 1984

microprobe was checked periodically with the help of a reference spot in iron located far away from the dif-

fusion zone, by checking the specimen current and X-ray intensities at that location. The measured ra-

tios of intensities taken from the specimen and the standards were corrected for background, absorption,

fluorescence and atomic number effect by standard procedures using a modified version of the MAGIC IV

computer programme [ 81. The diffusion couples were examined in an optical

microscope after polishing and etching. A solution

containing 2% nital was used to reveal the structure

of iron and electrolytic etching with 10% phosphoric acid in water was successful in revealing the micro-

structure of the nickel regions.

3. Results

Fig. 1 shows a typical concentration-penetration

curve obtained by the electron probe microanalysis of the diffusion couples annealed at 1323 K for 50 h. The concentration-dependent interdiffusion coeffi-

cients were determined by a Boltzmann-Matano analysis [9,10] of the concentration data. In the pres- ent work the interdiffusion constants are given by the

expression:

$C') = - & (j$ s”’ xdC, C’ 0

(1)

where D(C’) is the interdiffusion coefficient corre- sponding to an iron concentration of C’, dx/dCI,, is the reciprocal of the slope of the concentration-pene- tration curve at the concentration d and t is the an- nealing time. The interdiffusion coefficients comput-

ed at different temperatures and concentrations of iron in the range lo-90 wt% are shown in fig. 2. It

can be seen that values of E(C’) increase with an in- crease in the iron concentration up to 30 wt% Fe and

- Matano 90 - - 90

80 - -80

70 - - 70

60 - -60

$ $

DISTANCE /pm

Fig. 1. Concentration-penetration curves of nickel and iron across the diffusion zone in a nickel-iron couple annealed at 1323 K

for 50 h.

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Volume 2, number 4A MATERIALS LETTERS March 1984

-1, 10

a T/K

A 1223

I I / i I / / I I I

10 20 30 40 50 60 70 80 90 100

CFe fwt %

WEIGHT PER CENT NICKEL ,673O 10 20 40 50 1

I I I 30 !

I I 60 / 70 I 80 I 90 1

lo73~~.~~~~:~. , , $, ,$$ / 0 10 20 30 40 50 60 70 80 90

ATOMIC PER CENT NICKEL

then decrease monotonically with an increase in the iron concentration.

Fig. 3 shows that the temperature dependence of the interdiffusion coefficients follows an Arrhenius behaviour, i.e. 3 = Do exp(-Q/RT), where, Do is the frequency factor, Q is the activation energy for diffu- sion, T the annealing temperature and R the universal gas constant. The values of Do and Q computed from the plots shown in fig. 3 and valid for the tempera-

Fig, 2. (a) Concentration dependence of the interd~fusion co- efficients. (bf Phase diagram of the nickel-iron system [ 131.

ture range 1223-1373 K are presented in table 2. It

is clear that both Q and Do decrease with an increase in iron concentration up to 30 wt% Fe and thereafter increase. It should, however, be pointed out that the value of the interdiffusion coefficients measured at 1073 and 1148 K do not fall on the straight lines shown in fig. 3. We shall consider the possible reasons for this anomaly at a later stage.

Zener [ II] has shown that the values of Do and Q

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Volume 2, number 4A MATERIALS LETTERS

-1f 10

wt % Fe l 10 x 20 v 30

a. LO

- 50

B 60

n 70

@ 80

8 90

V 6

I I 1 I

0 7.5 8.0 8.5 9.0 ! K lOLIT

March 1984

i

Fig. 3. Arrhenius plots of log 5 versus l/T for different concentrations of iron.

in a given system are related by the following expres- [ 1 l] that the activation entropy LW for the interdif-

sion: fusion process can be estimated using the following relation:

log Do = a(Q/T,) + b, (2) m = WQP,>, (3)

where a and b are constants and T, refers to the fusion where /J = -d(fi/po)/d(T/Ts) and cc and p. are the point of the alloy. The value of the constant a is ex- values of the elastic modulus at T and 0 K, respective- petted to be, 5.28 X 10W2 J-l mol K-l. Fig. 4 shows ly; X is a constant characteristic of the crystal struc-

that the plot of log Do versus Q/T slope of 5.74 X 10m2 J-’ mol K- . This suggests that f

is linear with a ture and the diffusion mechanism. The product Xp

can be calculated from the slope a of the log Do ver- an identical diffusion mechanism operates in the con- sus Q/T, plots: centration range of lo-90 wt% Fe. It has been shown

l$=Ra, (4)

Table 2 Variation of activation energy and frequency factor with iron concentration

Sample CFe (wt%) DO (m* s-l) Q (kJ mol-’ ) Q/T, (J mol-’ K-l) AS (J mol-’ K-l )

1 10 5.37 x 10-2 326.32 189.94 81.56 2 20 2.4 x lo-* 315.62 184.25 79.12 3 30 1.74 x 10-2 310.53 181.28 77.84 4 40 2.43 x lo-* 315.62 184.79 79.35 5 50 5.37 x 10-2 326.32 191.05 82.05 6 60 1.35 x 10-l 337.77 197.18 84.67 7 70 2.85 x 10-l 350.06 203.17 87.24 8 80 2.32 x 1O-2 320.89 184.63 79.28 9 90 2.85 x 10-l 350.06 198.56 85.26

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Volume 2, number 4A MATERIALS LETTERS March 1984

/ !l( 150 160 170 180 190 200 I

Q/T,/(J.rnol-‘.K-‘1

,

Fig. 4. Variation of log Do with Q/T,

Table 3

Experimental and extrapolated5 values at 1073 and 1148 K

CFe (Wt%) 1016 5 (m2 s-l) 1016 5 (m2 s-l)

(actual) (extrapolated)

1073 K 1148 K 1073 K 1148 K

where R is the universal gas constant. Using the data obtainable from fig. 4, Xp and A5’ for the Ni-Fe sys- tem have been estimated as 0.43 and 82 + 5 J mol-’ K-l, respectively.

4. Discussion

It is clear from fig. 2a that the interdiffusion co-

efficients 0” reach a maximum around 30 wt% Fe at all the temperatures investigated. Further, table 2 shows that the activation energy as well as the fre- quency factor exhibit a minimum at the same con-

centration. It is interesting to note that both the liquidus and the solidus curves for the Ni-Fe system exhibit a minimum at 30 wt% Fe (fig. 2b). This pro- vides evidence for the correlation between the diffu-

sion data and the phase diagram. This is consistent with the findings of Million et al. [3] and Kohn et al. [ 121 that s reaches a maximum around the stoichiom-

etry Ni,Fe [13]. The values of the interdiffusion coefficients cor-

responding to 1148 K (fig. 3) were found to be higher than those obtained by extrapolation of the high-tem- perature data. Similar anomalous behaviour of diffu- sion data was also noticed at 1073 K. A change in the

slope of log 0” versus l/T plots below 1173 K (fig. 3) illustrates these observasons. The extrapolated and experimental values of D for different concentration are compared in table 3. It is clear that the experimen- tal values of 0” obtained at 1073 K are significantly higher than those obtained by extrapolation. Fe-Ni alloys containing less than 10% Ni are likely to be in

1

10 2.9 0.70 0.19 1.1

20 1.2 0.74 0.20 1.3

30 1.6 0.88 0.26 1.6

40 2.7 2.40 0.20 1.3

50 1.4 2.70 0.19 1.1

60 1.5 2.10 0.15 0.86

70 1.1 2.50 0.11 0.62

80 1.1 1.80 0.14 0.80

90 4.3 2.60 0.11 0.62

the (Y + y phase field in the temperature range 1073- 1173 K. Thus the presence of the (11 phase in those regions of the diffusion zone which are highly en-

riched in iron may have contributed to this anomaly.

It has not been possible to verify this hypothesis ex- perimentally due to the following factors: (i) cooling

the microstructure containing a mixed (Y t y phase

to room temperature would have transformed it to

one containing the (Y phase alone; (ii) the width of the diffusion zone in which the mixed (Y t y phase exists at 1073 K is =lO p.

5. Conclusions

Diffusion annealing of the iron-nickel couples in the temperature range 1073-1373 K led to smooth

variations of the concentrations of iron and nickel across the interface. The interdiffusion coefficients

obtained at different temperatures in the range 1173 < T < 1373 K increased with an increase in the iron concentration up to 30 wt% Fe, and thereafter de- creased with an increase in the iron concentration. Analysis of the temperature dependence of the diffu- sion coefficients showed that both the activation ener- gy and the frequency factor exhibited a minimum at 30 wt% Fe. The values of the interdiffusion coeffi- cients measured at T < 1173 K were found to be sig- nificantly higher than those obtainable by extrapola- tion of the high-temperature data.

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Volume 2, number 4A MATERIALS LETTERS March 1984

Acknowledgement

The authors wish to thank Dr. P. Rodriguez, Head, Metallurgy Programme for his keen interest in this work and for his continued encouragement. They also

acknowledge the cooperation of Mr. A.L.E. Terrance

in the computation of the chemical compositions from the raw X-ray intensity data.

References

[l] R.D. Rawlings and C.W.A. Neway, J. Iron Steel Inst. 206 (1968) 723.

[2] C.P. Heirwegen and G.D. Rieck, Met. Sci. 8 (1974) 383. [ 31 B. Million, J. Ruzickova, J. Velisek and J. Vrestal, Mat.

Sci. Eng. 50 (1981) 43.

[4] E.A. Balakir, Diffusion and defect data, Vol. 11 (Trans. Tech Publication, Ohio, 1975).

[5] V.S. Raghunathan, Ph.D. Thesis, Indian Institute of Science, Bangalore, India (1976).

[6] K.K. Srivastava and J.S. Kirkaldy, Metall. Trans. 13A (1982) 2113.

[7] V. Ganesan, V. Seetharaman and V.S. Raghunathan, J. Nucl. Mat. 118 (1983) 313.

[8] J.W. Colby, in: Proceedings of the Symposium on the Electron Microprobe, ed. K.F.J. Heinrich (Wiley, New York, 1966) p. 95.

[9] C. Matano, Japan. J. Phys 8 (1933) 109. [lo] P.G. Shewmon, Diffusion in solids (McGraw-Hill,

New York, 1963) p. 28. [ 1 l] C. Zener, in: Imperfections in nearly perfect crystals,

ed. W. Schockley (Wiley, New York, 1959) p. 289. [12] A. Kohn, J. Levasseur, J. Philibert and M. Wanin, Acta

Metall. 18 (1970) 163. [ 131 M. Hansen, Constitution of binary alloys (McGraw-Hill,

New York, 1958) p. 678.

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