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ORDERING OF ATOMS IN γ’ PARTICLES OF Ni-15at%Al ALLOY BY FIM OBSERVATION
T. Suzuki, A. Tachikake, Y. Miyagawa, Y. Nishi, E. Yajima
To cite this version:
T. Suzuki, A. Tachikake, Y. Miyagawa, Y. Nishi, E. Yajima. ORDERING OF ATOMS INγ’ PARTI- CLES OF Ni-15at%Al ALLOY BY FIM OBSERVATION. Journal de Physique Colloques, 1986, 47 (C2), pp.C2-221-C2-225. �10.1051/jphyscol:1986233�. �jpa-00225666�
JOURNAL DE PHYSIQUE
Colloque C2, suppl6ment au n03, Tome 47, mars 1986 page c2-221
ORDERING OF ATOMS IN Y' PARTICLES OF Ni-15at%Al ALLOY BY FIM OBSERVATION
T. SUZUKI, A. TACHIKAKE, Y. MIYAGAWAf, Y. N I S H I t + and E. YAJIMA"
Department of Physics, Tokai University, 1117 Kitakaname, ~ i r a t s u k a , Japan
'Graduate student of Tokai University, Hiratsuka, Japan
++Department of Materials Science, Tokai University, Hiratsuka, Japan
Abstract - The purpose of the present work is to study the ordering of atoms
-- in the Y' particles of a Ni-15at%Al alloys by FIM observation. The specimens are prepared by a liquid-quenched method in an argon atmosphere.
There are three stages in the precipitation and the growth process of the 7' particles. The sphere-like Y ' particles are observed as small dark spots in the second stage by FIM observation. While the cube-like Y'pmticles are found in the third stage. By FIM observation the y 1 particles is deduced to be higher than that in the sphere-like particles.
I - INTRODUCTION
There are many studies for growth process of Y' particles in the Ni-A1 alloys /1,2,3/. However, those studies are mainly described by the later growth process of y 1 particle. A few works are used by FIM and APFIM/L+,~,~/. Hill and Ralph /5/
have investigated in Ni-lL+at%Al alloy. They have suggested that 7' phase in an alloy forms by spinodal decomposition. While, Wendt and Haansen /6/ have described that the y 1 phase in the Ni-l&at%Al is decomposed by nucleation process. In our, previous paper /7/ the precipitation have been investigated for the 7' particles.
The concentration of the matrix has been measured by magnetic mass balance for Ni- 15 at%Al alloy aged at 973 and 873 K. The structure changes for Y' particle have been observed by means of transmission electron microscopy. In the study of the coasening process of the Ni-A1 supersaturated solid solution, A. J. Ardell /2/ has derived a following equation.
Here C is the equilibrium solute concentration of matrix, C is the concentration at themaging time t is the rate constant. Figure 1 is plots of the t
concentratio . In the first stage, the A1 concentration dose not represent t-P/3sr~le. On the other hand, this rule is applied in the 2nd and 3rd stages. Two straight lines are represented in these stages. By the TEM
observations the Y' particles in the 2nd stage are sphere-like in shape and have the L12 type ordering phase. While in the 3rd stage, the Y' particles are the cube-like in shape and the interface, which are parallel to the (100) planes. We found that there are three stages for the precipitations and coarsening of the Y1 particles. Thus, the present investigation describes such a study of the imaging changes for the T f particles during aging by means of FIM.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986233
JOURNAL DE PHYSIQUE
Fig. 1 - Change in A1 atomic fraction with t-'l3 in the previous work /7/.
11.0
I1 - EXPERIMENTAL PROCEDURE
1st stage 2nd stage 3rd stage
p
1 1 1 1 1 1 1 1 1 1 1 1 1 1 ~ 1 1 ( ~ ~ ~ ~The Ni-15at%Al alloys were melted by arc-furnace in an argon atmosphere.
Using the pure metals ; 99.9% Ni y d 99.99% Al. The Ni-15at%Al alloys were
homogenized at 1473 K for 2.2 x 10 s In an argon atmosphere. The specimens of Ni- A1 alloys were prepared as the sheets by piston-anvil type apparatus in an argon atmosp ere. The thickness of the sheet is about 0.2 mm. The cooling rate is about 1 x 10 k K/s at 1273 K. For the aging process, the specimens were heated in an electric resistance furnace under an argon atmosph~~e. The field ion microscopy were made of stainless steel. A vacuum of 3 x 10- torr could be obtained by pumping with a 32 l/s sputter ion and a Ti getter pump after baking for several hours. The FIM photographes were obtained by phosphorus screen using H imaging
gas at about 78 K. 2
1.0 2.0 30 4.0
I11 - RESULTS
Figure 2 shows FIM image of the specimen aged for 600 s at 973 K, with the dark field micrograph /7/. Figures 2(b) and ( c ) are obta~ned with the condition that the tip voltage is about 5% more than best image voltage and at overexposure.
The precipitated particles are observed as many small dark spots with the sphere- like in shape (see Fig. 2(b)). Fig. 2(c) shows the FIM image of the same tip after field evaporation. The parts of small dark spots are resolved, the new dark spots appeare by the field evaporation. Fig. 2(d) shows the FIM image that is obtained immideately after photographing of Fig. 2(c) at the best image voltage and right ex2osuc-e. It is supposed that the 7' particles of the 2nd stage are observed as dark small spots by FIM.
On the other hand, Fig. 3 is FIM image of the specimen aged for 3600 s at 973 K. The FIM image 5s obtained with best image voltage. The y 1 particle is observed as step ring cluster(indicated by the arrow) on the (110) superlattice plane and are cube-like in shape by FIM. The (110) planes of the 7' particle are constructed by layers of Ni atoms and layers of mixture of Ni and A1 atoms. Therefore, the Y' particles are exclusively observed at the layer of Mi atoms as step ring cluster.
Fig. 2 - Hydrogen ion and dark field electron micrographs of Ni-ISat%Al alloy specimens aged for 600 s at 973 K. (a) Dark field micrograph of 7 ' particles in the matrix /7/. (b) Hydrogen ion micrograph observing? particles as small dark spots (marked by allows). (c) Micrograph of the same specimen after field evaporation. (d) Hydrogen ion micrograph of the same specimen observing with the best image voltage and right exposure.
Fig. 3 - Hydrogen ion
micrograph of Ni-ISat%Al alloy aged for 3600 s at 973 K. 7 '
particles are observed as step rings (indicated by the arrow).
JOURNAL DE PHYSIQUE
IV - DISCUSSION
1) Fxamination for Y r particle of 2nd stage
The Y r particles are observed as small dark spots in matrix by FIM
observation. It is assumed that the 7' particles are decreased at the degree of long range order. Under that condition, the Y t particle contains vacancies or A1 atoms more than those in the matrix. Thus, the y t particle is observed as a hollow on the matrix by evaporation. The volume fraction for the small dark spots is estimated by Fig. 2(d). The area for the small dark spots is measured in the part of the rectangle (indicated by enclosed area). The obtained volume fraction is about 0.2. On the other hand, the volume fraction of yr particles for the same aged specimen is estimated using A1 content in the matrix /7/. The volume fraction is about 0.230. The volume fraction of small dark spots agrees aproximately with this. Then the Y r particles are observed as the small sphere-like dark spots by FIM, because the degree of order is low within these particles.
2) The ordering of atoms in the y t particles on the 3rd stage
The rr particles of the 3rd stage are observed as step ring clusters with cube-like on (110) plane by FIM. We estimate the ordering of atmos in the r t particles using the bright points of step ring that is constructed by Ni atoms.
The K represents the ordering of atoms. For the same step ring around (110) plane, the K is expressed as follows.
K = (the number of bright points) / (the number of total points) (2) The K is about 0.90 (see the step ring in Fig. 3). While the long range order parameter S is expressed as follows.
= (PA1-XA1) / (l-xAl) = (pNi-xNi) / ( l-xKi) (3) Here PA1 is probability that atom sites are filled with Ni atoms, and X is the atomic concentration of A1 atoms. If the concentration of the Y ' partige is 23 at%Al, the S at the maximum order is 0.90. Aoki and Izumi have shown that the experimental value of S is 0.89 for the 23 at%Al alloy /8/. They have described that all A1 atoms occupy cub5-corner sites. While Ni atoms occupy all face center sites and vacant cube corner sites of the unite cell, respectively. The (110) planes of the y t particles with the maximum order phase are constructed with the layers of Ni atoms ana the mixed layers of A1 and Ni atoms. Thus, the K is assumed to be about 1.0 for the Ni-layers. However, the estimated K is about 0.85 of the observed layers as the step ring. The deviation of these K values may be concerned with two influences. one'is the interaction between the imaging hydrogen gas and the atoms of the tip. another is that the y t particle is not equally maximum order. However, we conclude that the FIM images are clearly different for the Y t
particle between 2nd and 3rd stages.
V - CONCLUSIONS
There are three stages for the precipitation and growth process of the 7'
particles in Ni-15at%Al alloy. This investigation is observed by field ion microscopy for 2nd and 3rd stages o f ? ' particles during isothermal aging at 973 K.
These results are as follows :
(1) At the 2nd stage, the Yr particles are observed as small dark spots of sphere- like in shape by FIM. This result shows that the ordering of atoms is low within these particles.
(2) The step ring cluster in the cube-like Y t phase at 3rd stge is observed by FIM.
The ordering degree within the y t particle is deduced to be higher than that of 2nd stage.
(3) Increase in the ordering degree is found during the Y ' precipitation.
VI - ACKNOWLEDGEMENT
The authors would like to thank Profs. S. Nakamura, S. Nakajima, Y. Kimura and H. Tsuruoka of Tokai university for their continuous encouragement during this study.
VII - REFERENCES
/I/ Ardrell, A. J. and Nicholson, R. B., J. Phys. Ghem. Solids 2 (1966) 1793.
/2/ Ardell, A. J., Acta Met. 16 (1968) 511.
/3/ Kirkwood, D. H., Acta Met. 3 (1970) 563.
/4/ Faullmer, R. G. and Ralph, B. , Acta Met. 20 (1 972) 703.
/5/ Hill, S. A. and Ralph, B., Acta Met. 3 n982) 2219.
/6/ Wendt, H. and Haasen, P., Acta Met. 2 (1983) 1649.
/7/ Tachikake, A., Suzuki, T., Niahi, Y., Yajima, E., J. Japan Inst. Met. to be published.
/ 8 / Aoki, K. and Isumi, O., Phys. Stat. Sol. W2 (1975) 657.
