Scripta METALLURGICA Vol. 25, pp. 1405-1408, 1991 Pergamon Press plc et MATERIALIA Printed in the U.S.A. All rights reserved

TRAPPING OF NITROGEN BY TITANIUM ATOMS IN a-Fe

Ye-Xin Chen, Jihua Zhang and Wu Ziliang Shanghai Institute of Metallurgy, Academia Sinica

Shanghai 200050, China

(Received February 7, 1991) (Revised March 28, 1991)

Introduction

Oriani developed the trapping theory for hydrogen and applied the theory to existing experimental data in Fe-Ti alloys[i]. In the light of this theory, Yang et al. [2] found that the interaction energy between the hydrogen and titanium atoms in ~-Fe is 33 kJ/mol. The study of the structure and thermodynamics of nitrided iron-titanium alloys has made great progress [3,4]. Particularly, the mechanism of nitrogen diffusion was studied by the internal friction method [5,6]. According to these experimental results, some methods of diffusion were supposed. On the basis of previous theory and experimental results, this work mainly attempts to apply the trapping theory to Fe-Ti-N alloy. The results obtained by the internal friction method are explained by the interaction energy between nitrogen and titanium.

Experimental

Fe-0.77at%Ti alloy was melted in a vacuum induction furnace, cast, forged and rolled to make specimens used for nitriding. The dimensions of the specimens are 20XSX0.8 nun . The specimens were decarburized and denitrided at 850°C for i0 h in a dry hydrogen atmosphere. They were nitrided at 673K, 773K and 873K, respectively, for 1 h in a 20NH3:80Fm atmosphere, and then cooled to room temperature in a nitrogen atmosphere in the furnace. The distribution of nitrogen atoms from the surface to the center of the specimens was investigated with a Auger Electron Spectrometer(AES).

Results and Discussion

After the specimens were nitrided, the distribution of concentration of nitrogen was shown from the surface to the center of the specimens in Fig.1. They were measured by AES. According to Fick's Law:

8C a2C =D (i)

8x 2

It is supposed that when nitriding temperature and nitrogen potential is kept unchanged, the concentration of nitrogen on the surface of the specimen is constant. Thus, the solution of equation(i) is,

C(x,t) = Co [l-erf(2~Dt) ] (2)

1405 0036-9748/91 $3.00 + .00

Copyright (c) 1991 Pergamon Press plc

1406 TRAPPING OF N IN Fe Vol. 25, No. 6

where Co is the concentration of nitrogen in the surface of the specimen, x is the distance from the surface, D is the diffusion coefficient of nitrogen, t is the times for nitriding and err(y) is the Gaussian error function. The relationship between diffusion coefficient, D and temperature, T is as follows:

D = Doexp(-Q/RT) (3)

From the data of Fig.1 and according to equation(2), the diffusion coefficient for nitrogen in Fe-0.77at%Ti alloy at 673K, 773K and 873K, respectively, was obtained. With these diffusion coefficients, we can make a plot of the lnD vs. l/T, which is shown in Fig.2. From Fig.2, we obtained the activation energy of diffusion for nitrogen in Fe-0.77at%Ti, which is 82.3 kJ/mol, and the Do equals to 0.0089 cm /s. Thus, we have the diffusion equation for nitrogen in the alloy, which is as follows,

Dt = 0.0089 exp(-82300/RT) (4)

Because the number of titanium atoms is very small in the alloy, nitrogen atoms are trapped by titanium at low occupancy. Thus, a simple model developed by Oriani [1] for the trapping of hydrogen was thought suitable for the trapping of nitrogen in Fe-Ti alloy. According to the model,we have

DiDt _ 1 + ~Nt exp(-AEt/RT) (5)

where Dl is the lattice diffusion coefficient of nitrogen in a-Fe without traps for nitrogen, Dt is the diffusion coefficient of nitrogen in one that contains the effect of existence of the traps, Nl and Nt represent, respectively, the number of normal and of extraordinary sites for nitrogen existing per unit volume, and AEt is the interaction energy between the nitrogen atoms and traps. Taking the logarithm of both sides of equation(5), we have:

DI in(-~ - 1 ) = ln( ~q-)Nt _ RTAEt (6)

The diffusion equation for nitrogen in pure =-Fe is [7],

Dl = 0.005 exp(-77000/RT) (7)

Substituting equation (4) and (7) into equation (6), and plotting ln[(Dl/Dt)-l] vs. l/T, we have the result shown in Fig.3. It was ascertained that interaction energy, AEt equals to 24046 J/mol and Nt/N; to 0.0064 from Fig. 3. Szabo-Miszenti [5] measured the activation energy, 104.6 and 129.7 kJ/mol for nitrogen diffusion in Fe-Ti alloy by the internal friction method, which corresponds to the peak temperature of -393K and 500K respectively. Zhang et al.[6] also found these internal friction peaks, which appear at temperatures of 393K, 513K and 653K, respectively, in Fe-Ti-N alloy. They believed that the 393K peak corresponds with the nitrogen atoms jumping from the TiN compound, the 513K peak with s-2i interaction internal friction with

Vol. 25, No. 6 TRAPPING OF N IN Fe 1407

one titanium atom and two nitrogen atoms, and the 653K peak with s-i interaction internal friction peak between one titanium and one nitrogen atom. Therefore, we think the 500K peak measured by Szabo-Miszenti is the same as the 513K peak. According to the trapping model[l] (see Fig.4), when supposing E'=0, we have the activation energy of nitrogen jumping from the trapping sits, E = Ea + AEt, which equals 101046 J/mol. This is in agreement with activation energy of 393K internal friction peak measured by Szabo-Miszenti. Considering that the titanium atom is scarce in the alloy used in this work and the technique conditions used, it is impossible for titanium atoms to gather and to precipitate. We think the 393K peak is probably due to s-3i interaction internal friction, i.e., three nitrogen atoms co-ordinated with one titanium atom, which agrees with the results observed by Szabo-Miszenti [5]. Thus, three internal friction peaks are considered to correspond to the s-i, s-2i and s-3i configuration, respectively. Although three configurations can all trap the nitrogen atoms, the nitrogen atoms are also the most likely to jump from the s-3i trapping site, by reason of energy. Equilibrium of nitrogen atoms jumping from and being trapped in s-3i trapping site during diffusion process is maintained. Therefore, we consider that there are three trapping patterns in Fe-Ti-N alloy, i.e., s-i, s-2i and s-3i. The difference in the three patterns is the difference of the interaction energy between titanium and nitrogen, i.e., AEt. The order of AEt from the large one to the small one is AEt(s-i) > AEt(s-2i) > AEt(s-3i) in the alloy. This is agreement with the result obtained by'internal friction measurement. Thus, the nitrogen diffusion in Fe-Ti-N alloy is mainly the diffusion of nitrogen in the s-3i state.

Conclusion

By means of the AES, the nitrogen concentration is measured in Fe-Ti-N alloy. The diffusion equation for nitrogen atoms in Fe-0.77at%Ti was obtained, which is D = 0.0089*exp(- 82300/RT ). By applying the trapping theory to this alloy, the interaction energy between the nitrogen and the titanium atoms in u-iron was found to be 24.046 kJ/mol.

References

i. R. A. Oriani, Acta Metallurgica, 18, 147 (1970). 2. K. Yang, M. Z. Cao, X. J. Wan and C. X. Shi, Script Metallurgica, 22, 1373

(1988). 3. D. H. Jack, Acta Metallurgica., 24, 137 (1976). 4. D. S. Rickerby, S. Henderson, A. Hendry and K. H. Jack, Acta Metallurgica,

34, 1687 (1986). 5. G. Szabo-Miszenti, Acta Metallurgica, 18, 477 (1970). 6. Zhang Jihua, Ling Li and Wu Ziliang, Proc. of the 9th Int. Conf. on

Internal Friction and Ultrasonic Attenuation in Solids,Beijing, China, July 17-20, 1989, edited by T. S. Ke, p. 461, International Academic Publisher, May, 1990.

7. M. Hillert, The Diffusion and Thermodynamics in the Alloy, p.122, Metallurgical Industry Publishing House, 1984, (in Chinese).

1408 TRAPPING OF N IN Fe Vol. 25, No. 6

I

LIJ

0 0 . I

\

\ \

A 675K o 773K • 875K

\ \

0.2 0.5

distance (ram)

Fig 1 The distribution of nitrogen in the specimen

- f (

-IE

-17

C3 _¢

- t8

-19

-2C[I

\

I I ! 12 1.5 1.4

IOOO/T(K -I) Fig 2 inD vs. I/T

\ I

1.5

-02

I ~- -I.I C~

Q 4.3 v v

_c

- l j

-L7 / - I . S - - , , ,

I.I L2 13 1.4 L5

IOO0/T (K") F i g 3 l n [ ( D , / D t ) - l ] v s . 1 / T

I£ DISTANCE

Fig 4 Model for trapping site[l]