Y. HAYASHI: Ordering of NisMn and Diffusivity of Hydrogen in the Alloy 619

phys. stat. sol. (a) 107, 619 (1988)

Subject classification: 61.66 and 66.30; 51.2

Department of Iron and ateel Metallurgy, Faculty of Engineering, Kyushu Univerersity, F u h k d )

Ordering of N8Mn and Diffusivity of Hydrogen in the Alloy BY Y. HAYASHI Dedicated to Prof. Dr. S. AYELINCKX on the occasion of his 65th birthday

To study the effect of alloying on the diffusivity of hydrogen in metals, a Ni-Mn alloy with order and disorder transformation is examined. Ordering of Ni3Mn is studied by electron microscopic observation and differential thermal analysis. The diffusion coefficient of hydrogen in ordering Ni-Mn is measured by the permeation method. The relation between the arrangement of the alloying atoms and diffusivity of hydrogen in the alloy is discussed.

Um den LegierungseinfluB auf das Diffusionsvermogen von Wasserstoff in Metallen zu bestimmen, wird eine Ni-Mn-Legierung mit Ordnungs- und Fehlordnungsiibergang untersucht. Ordnungs- prozesse von Ni3Mn werden mittels elektronenmikroskopischen Beobachtungen und differentieller Thermoanalyse untersucht. Der Diffusionskoeffizient von Wasserstoff beim OrdnungsprozeB von Ni-Mn wird mit der Durchdringungsmethode gemessen. Die Beziehung zwischen der Anordnung der Legierungsatome und dem Diffusionsvermogen in der Legierung wird diskutiert.

1. Introduction It is of both theoretical and practical interest to study the influence of the structural change of alloys on the diffusivity of hydrogen in metals and alloys. I n alloys of dilute solid solutions, the alloying atoms are treated as trapping points for hydrogen diffusion [l]. At elevated temperatures diffusion jumps of hydrogen can be treated by classical rate theory; hydrogen atoms move between their stable positions by a thermal activation process. If the potential energies of a hydrogen atom at the stable and the saddle points in an alloy are determined by the arrangement of the alloying atoms around these points, diffusion studies of hydrogen in alloys with order-dis- order transformation are very interesting in discussing the alloying effects on the diffusivity of hydrogen. Many experiments have been carried out in ordering alloys, e.g. in Ni3Fe [2 to 61, N&Mn [6, 71, N&Pt [6], Cu3Pd [8], Cu3Au [4, 91, FeCo [lo], F%Al [I l l , etc., but the results are still not satisfactory. I n those alloys Ni3Mn shows a peculiar feature [6], and it is interesting to do further experiments on this alloy.

Theoretical work on the diffusivity of hydrogen in ordering alloys has been done by Krivoglatz and others [12]. They proposed a theory to calculate the diffusion coefficient of hydrogen in ordered and disordered alloys. The theory is based on the assumption that the potential energies of a hydrogen atom at stable points and saddle points are determined by the sum of the interatomic energies between the hydrogen atom and neighboring metal atoms around the stable and saddle points. It also assumes local equilibrium for hydrogen distribution and thermal averaging for calculation of the jump rate of the hydrogen atom.

The purpose of the present paper is to compare the results of the observation of the ordering process of Ni3Mn and the change of the diffusion coefficient of hydrogen during ordering, and to discuss the effect of the structural change of the alloy on the diffusivity of hydrogen referring to the theory by Krivoglatz and others.

l) Hakozaki, Fukuoka 812, Japan. 40'

620 Y. HAYASHI

2. Experimental

Alloys of Ni-Mn were obtained by melting the component metals with commercial purity in vacuum. Compositions of the alloys were Ni-23.5, 25.5, and 27.4 atyo Mn. For hydrogen permeation experiments thin disc specimens of 28 mm diameter and about 0.4 mm thick were prepared. The order and disorder structures were observed with an electron microscope, and the heat of ordering was measured by differential thermal analysis with specimens after various ordering annealing.

The diffusion coefficient of hydrogen in the alloys was measured by the time lag method of permeation. The apparatus consists of two vacuum chambers divided by a thin specimen; the one is a hydrogen loading and the other a detecting chamber. The permeation of hydrogen was measured by the pressure change in the detecting chamber either under closed or continuous pumping condition, after introducing hydrogen gas instantly to the loading chamber and keeping the pressure constant. The operating pressure in the detecting chamber was from to 1 Pa, and the loading hydrogen gas was purified by the permeation through a palladium membrane.

The applicability of the apparatus for measuring the diffusion coefficient of hydro- gen was examined by using pure nickel as a dummy specimen; it was proved to be applicable in the temperature range from 600 to 1100 K.

3. Experimental Results

As the order-disorder transformation temperature is reported to be 776 [13] or 753 K [14], ordering treatment was carried out a t 730 K. An electron microscopic observation of a 25.5% Mn specimen annealed for 1700 x loss (20d) a t 730K revealed a well ordered Ni,Mn structure as shown in Fig. 1.

Fig. 1. Electron microscopic observation of an ordered domain of Ni,Mn

Ordering of Ni,Mn and Diffusivity of Hydrogen in the Alloy

I---I 62 1

Fig. 2. Diffusion coefficient of hydrogen in Ni-Mn alloy. a) Temperature dependence of the dif- fusion coefficient, b) and c) change of diffusion coefficient in the course of ordering and disordering, respectively

The diffusion coefficient of hydrogen in Ni-Mn alloys was measured with changing temperature. The results are shown in Fig.2. First, the specimen was heated a t 940 K to make it disorder and quickly cooled to 730 K. Keeping the temperature at 730 K, the diffusion coefficient of hydrogen was measured. Change of diffusion coeffi- cient in the course of ordering is shown in Fig. 2 b. After holding the temperature for

a b C

d e f

Fig. 3. Electron microscopic observation of the growth of an ordered domain. a) t = 7.2 x 103, b) 21.6 x los, c) 43.2 x 105, d) 108 x 108, e) 346 x lo3, f ) 1730 X loss

622 Y. HAYASHI

v)

E

100 lo1 70 lo3 t ( 1 0 ~ ~ 1 --

Fig. 4. Growth of an ordered domain. Data obtained by X-ray diffraction (-.-) [15] and neutron diffraction (- - - -) [16] are also shown for comparison

180 x 103 s, the diffusion coefficient was measured with decreasing temperature, i.e. the temperature dependence of the diffusion coefficient of hydrogen in the ordered state of Ni3Mn. Then, the temperature was raised to 820 K and the time dependence of diffusion coefficient measured in the course of disordering, which is shown in Fig. 2 c. The temperature dependence of the diffusion coefficient in the disordered alloy was measured with changing temperature in the disordered region. The result shows that the diffusion coefficient of hydrogen in Ni-Mn alloy changes at the order- disorder transformation, it increases with ordering and decreases with disordering.

The ordering structure was observed by electron microscopy and diffraction. A specimen quenched from 940 K showed no superlattice diffraction spots, but by heating it for 7.2 x 10s s at 730 K superlattice diffraction spots appeared. Specimens heated at 730 K for various hours were observed, and the results are shown in Fig. 3. After 21.6 x 103 s annealing ordered domains could be observed in the dark field image. The growth of the ordered domain size is shown in Fig. 4. The results obtained by X-ray diffraction [15] and neutron diffraction [16] are also shown in the figure for comparison.

The ordering was also studied by differential thermal analysis. The heat of absorp- tion with disordering was measured for specimens with various degrees of ordering by changing the annealing time at 730 K. The result is shown in Fig. 5. After 180 x 108 s annealing the heat of absorption tends to saturation. , 1.0

1 p c 8 2 05 9 0

0 I I I 50 I00 - 150

t (10%) -- Fig. 5. Result of differential thermal analysis of ordering of Ni,Mn. Specimens are order-annealed for various hours at 730 K

Ordering of Ni,Mn and Diffusivity

Mn

of Hydrogen in the Alloy 623

Fig. 6. Structure of ordered Ni,Mn and sites of interstitial hydrogen: 0 is a stable position and P is a saddle point

4. Discussion

The diffusion coefficient of hydrogen in Ni-Mn alloy changes at the order4isorder transformation, In the course of ordering at 730 K the diffusion coefficient increases and attains a constant value after 18 x 103s. The state a t 18 x 103s annealing corresponds to the early stage of ordering from the view of the electron microscopic observation and the differential thermal analysis. Kirk and Cohen [15] showed a good correlation between domain size and the degree of long-range ordering. Referring to the correlation, the state where the diffusion coefficient attains a steady value corresponds to the state with a long-range order parameter about 0.35 and the domain size is several 100 nm. Patton and Baker [17] observed magnetization of Ni3Mn and showed the existence of the saturation of short-range order a t an early stage of ordering annealing.

The diffusion coefficient of hydrogen is, therefore, very much influenced by short- range ordering and is not sensitive to long-range ordering.

The L1, superstructure of Ni3Mn is illustrated in Fig. 6. The stable position of an interstitial hydrogen atom is the octahedral site of 0, or 0, type. If the saddle point for jumping is the intermediate position of the two adjacent octahedral sites, PI or P, will be the saddle point. The probability of occupation of metal atoms around those points is determined by the degree of ordering. If we assume the potential energies ( U , and Up) of a hydrogen atom at each position to be determined by the summation of the pair interaction energies of hydrogen and metal atoms, the values of U, and Up vary corresponding to the occupation of the metal atoms around a hydrogen atom at each point. We denote the energy for the m-th configuration as Ug,, and the prob- ability of the occupation of metal atoms for the m-th configuration as WED. Then, from statistic calculation the diffusion coefficient of hydrogen D is given by [lZ]

where Dh is a constant including vibrational frequency, entropy, and geometrical factor. Assuming a potential and distance relationship for each pair, the change of D with ordering will be explained [S]. The theory is based on the averaging jump rates

624 Y. HAYASHI: Ordering of Ni,Mn and Diffusivity of Hydrogen in the Alloy

of a hydrogen atom a t various energy sites, and no spatial correlation of the distri- bution of the different sit,es is taken into consideration.

The change of diffusion coefficient with order and disorder transformation of Ni,Mn can qualitatively be explained by (l), assuming st,ronger attractive interaction of hydrogen with manganese than with nickel [GI. The experimental results show that the diffusion coefficient of hydrogen is very much influenced by short-range ordering and not much by the degree of long range ordering. That means, the jump rate of hydrogen is mainly governed by the arrangement of metal atoms near-neighbour to the hydrogen atom a t stable and saddle points. The diffusion coefficient can be cal- culated by averaging the statistical jump rates of hydrogen, and spatial correlation of the distribution of varying sites has not a large effect in this system.

Acknowledgements

The author should like to express his thanks to Prof. Amelinckx for introducing him to the study of electron microscopy and phase transformation of alloys. The experience he has got in Mol forms the foundation of his present state. He also thanks Mr. Iwai for the experimental assistance.

References [l] J. VOLKL and G. ALEFELD, Diffusion in Solids Reaent Developments, Ed. A. S. NOWICK and

[2] R. Dus and M. SMIALOWSKI, Actametall. 15, 1611 (1967). [3] V. A. GOLTSOV, V. B. VYKHODETS, P. V. GELD, and Yu. P. SIMAKOV, Fiz. Khim. Mekh. Mat.

[a] V. B. VYKHODETS, V. A. GOLTSOV, and P. V. GELD, Ukr. fiz. Zh. 5, 107 (1970). [5] W. DRESLER and M. G. FROHBERO, J. Iron Steel Inst. 211, 298 (1973). [6] Y. HAYASHI, N. IWAI, and N. OHTANI, Z. phys. Chem., N. F. 114, 213 (1974). [7] Yu. P. SIMAKOV, P. V. GELD, M. M. SHTEYNBERG, and V. A. GOLTSOV, Fiz. Metallov i

[S] V. B. VYKHODETS, V. A. GOLTSOV, and P. V. GELD, Fiz. Metallov i Metallovedenie 26, 933

[9] V. A. GOLTSOV, V. B. VYKHODETS, P. V. GELD, and T. A. KRYLOVA, Fiz. Metallov i Metallo-

[lo] P. V. GELD, Yu. P. SIMAKOV, M. M. SHTEYNBERG, and V. A. GOLTSOV, Fiz. Metallov i

[ll] G. P. S ~ N O V A and L. L. KUNIN, Fiz. Metallov i Metallovedenie 23,737 (1967). [12] For example, M. A. KRIVWLATZ and A. A. SMIRNOV, The Theory of Order-Disorder in Alloys,

[13] M. HANSEN, Constitution of Binary Alloys, McGraw-Hill Publ. Co., New York 1958. [14] M. J. MARCINKOWSKI and N. BROWN, J. appl. Phys. 32,375 (1961). [15] M. KIRK and J. B. COHEN, Metallurg. Trans. 7A, 307 (1976). [IS] YE. Z. VINTAYKIN, D. F. LITVIN, and V. A. UDOVENKO, Fiz. Metallov i Metallovedenie 35,

[17] C. E. POTTON and G. L. BAKER, J. appl. Phys. 45, 3611 (1974).

J. J. BURTON, Academic Press, New York 1975.

5,597 (1969).

Metallovedenie SO, 524 (1965).

(1968).

vedenie 30, 657 (1970).

Metallovedenie 81, 148 (1966).

MacDonald, 1964.

215 (1973).

(Received March 29, 1988)