STUDY OF THE Fe-75%Ag SYSTEM OBTAINED BY MECHANICAL ALLOYING

D. Bonyuet¹, G. González², J. Ochoa², F. Gonzalez-Jimenez³, L. D´Onofrio³

1. IIBCA, Universidad de Oriente, Cumaná 6101, Venezuela. E-mail: dickar@cumana.sucre.udo.edu.ve.
2. Laboratorio de Materiales, Centro Tecnológico, IVIC, Apdo. 21827, Caracas 1020A, Venezuela. E-mail: gemagonz@ivic.ivic.ve.
3. Escuela de Física, Facultad de Ciencias, Universidad Central de Venezuela, Caracas, Venezuela

Resumen
Aleación Mecánica (AM) es una técnica de procesamiento de polvos, la cual nos permite inducir reacciones de estado sólido en sistemas binarios inmiscibles en el equilibrio. Fe y Ag tienen una naturaleza mutuamente repulsiva que las hace completamente inmiscibles bajo condiciones termodinámicamente estables. El proceso de molienda por bolas, siendo una técnica fuera del equilibrio, parece prometedor para obtener al menos una solución sólida parcial en este sistema. Mezclas de polvos de Fe y Ag con 75 % en peso de Ag fueron estudiados mediante difracción de rayos x (DRX), microscopía electrónica de barrido (MEB), y microscopía electrónica de transmisión (MET). Hemos encontrado que es posible obtener una pequeña solución sólida parcial en este sistema mediante AM. Esto es confirmado mediante espectroscopia Mössbauer.

Palabras clave: aleación mecánica, sistemas inmiscibles

Abstract
Mechanical alloying (MA) is a powder processing technique, which allows us to induce solid state reactions in binary systems immiscible in equilibrium. Fe and Ag have a mutually repulsive nature that makes them completely immiscible under thermodynamically stable conditions. The ball milling process being a non-equilibrium technique seems promising in obtaining at least a partial solid solution in this system. Mixtures of Fe and Ag powders with 75 wt% Ag were studied by x-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). We have found that it is possible to obtain a small mutual solid solution in this system by MA. This is also confirmed by Mössbauer spectroscopy.

Keywords: mechanical alloying, immiscible systems

1. Introduction
In the last ten years there has been much interest in the use of mechanical alloying as a non-equilibrium technique to prepare solid solutions in binary systems immiscible in equilibrium, which are characterized by a large positive heat of mixing. In alloy systems with negative heat of mixing, the phase formation is explained by an interdiffusional reaction occurring during milling. On the other hand, in the case of systems with positive heat of mixing, phase formation is still subject of controversy, since in these systems a diffusional reaction results in decomposition of the alloy. However, formation of partial solid solutions in several systems with positive heat of mixing, including Fe-Cu [1-6], Co-Cu [7], Cu-Ta [8,9], Cu-W [10], and Ag-Cu [11,12], have been observed. In contrast, for the system Ag-Fe this is still a matter of discussion. Some authors, Nasu et al. [13,14], Kuyama et al. [15], claimed that they had found the formation of solid solution in the system Fe-Ag, whereas, Angiolini et al. [16], Cohen et al. [17], Ma et al. [18], reported incomplete or minimal alloying, even after prolonged milling. The fact that Ag and Fe do not form extended solid solution remains unexplained [19]. It seems that there is some critical value of the enthalpy of mixing such that for high values a very limited mutual solubility can be only observed even in the liquid state, and cannot be mixed at atomic level by MA.
In this work, the Fe-Ag system in the composition of 75 wt.% Ag has been studied, and the phase formation under ball milling from 10 to 40 h investigated. It is known, from equilibrium diagram, that mutual solubility of Ag and Fe is very low in both the solid and liquid sates. However, we found from Mössbauer spectroscopy that MA produces a mutual dispersion of the Ag and Fe.

2. Experimental method
Elemental powders of Ag and Fe with 99.9% purity and initial particle size less than 50 mm were blended at the composition of 75 wt.% Ag. The milling was carried out using a SPEX 8000 mixer/mill with hardened steel balls and vial. The mixer/mill was modified in order to use a smaller vial, so the impact of the balls is more energetic. The ball-to-powder weight ratio was 9:1. In the milling procedure, 1 g of sample was used in every case and the vial was sealed in a nitrogen atmosphere and tightly clamped to prevent oxidation. The vial was processed for times of 10, 20 and 40 h, and the mill was operated by repeating a cycle of half hour period, interspersed with cooling down periods of also half hour. A new sample was used for each processing time in order to avoid possible air contamination due to the repeated opening of the vial.
The phase transformation in the ball-milled powders was examined by x-ray diffraction (XRD), transmission electron microscopy (TEM) and Mössbauer spectroscopy. The XRD patterns were carried out in a Siemens D5000 diffractometer using a Cu Ka (Ni filter) radiation and a Cu tube. The Scherrer formula was used to estimate the crystallite grain size after milling
Specimens for transmission electron microscopy (TEM) were prepared by embedding the powders in epoxy resin. The final sample thinning to electron transparency was performed by electropolishing and by taking thin sections by ultramicrotomy procedures. Observations have been made with a Philips CM12 microscope operating at 120 kV. Specimens for scanning electron microscopy (SEM) were prepared by embedding powders in epoxy resin, and further metallographically polishing by standard methods. The SEM observations were carried out on a Philips LX30 microscope.

3. Results and Discussion
Figure 1 shows the XRD patterns corresponding to 75 wt.% Ag for different milling times. We see that Ag peaks overlaps the Fe peaks, and only the Ag(111) and Ag(311) peaks can be observed separately.
Peak broadening and intensity decreasing with milling time is also observed, which indicate that the refinement of the grain size. In the early stages of milling, the grain sizes decrease rapidly to about 14 nm at 10 h of milling, and then reach a steady state with final values of about 12 nm after 20 h of milling. It is interesting to note that grain sizes of the resulting products depend upon the overall compositions of the powder mixtures, since in this case the final grain size is somewhat bigger that the final grains size for 10 wt% Ag (10 nm) and 30 wt% Ag (7 nm) [20].
The SEM images showed the Fe particles completely covered by Ag even after 10 h of milling. TEM results of the samples for the different milling times are shown in Figure 2. For 10 h, there are more Ag-rich particles than Fe-rich particles, clearly evidenced in the images by the higher contrast. For 20 h, the mixture between particles is more uniform.
Furthermore, the TEM images are in well agreement with our 57Fe Mössbauer spectroscopy results [21]. Herr et al. [22] also reported the formation of Fe-Ag alloys at the grain boundaries based on free volume and local strain at the interfaces.

Fig. 1 XRD patterns of Fe75%Ag milled for 0, 10, 20, 40 h

Fig. 2 TEM Images of Fe75%Ag milled for 10, 20, 40 h

Our result is in good agreement with Kuyama et al [15], who carried out high-resolution TEM experiments and also infered that there was a mutual mixing of elements at the atomic level at the interfaces.
The electron diffraction patterns only show reflections that Ag and Fe. For 40 h, there is a different situation, since the electron diffraction pattern shows reflections shifted with respect to those of the Ag and Fe reflections. In addition, in the bright field images, it can be seen a short-range order region formed at the particle interfaces. This could be an indication of Fe diffusion in the Ag interface particle.
Figure 3 shows the Mössbauer spectra for 10 and 20 h, at room temperature (RT) and 77 K. For 10 h of milling time, at both temperatures, we can see a dominant six-line component with Hhf = 333 kG at RT and 336 kG at 77 K, which are due to the ferromagnetic a-Fe.

Fig. 3. Mössbauer spectra for Fe-Ag 75 wt%Ag. (a) 10 h milling time, RT.(b) 10 h milling time, 77 K. (c) 20 h milling time, RT. (d) 20 h milling time, 77 K

There is also a weak broad six-line component with smaller hyperfine magnetic field, 274 kG at RT and 270 kG for 77 K, which is due to the change of the environment of Fe atoms because of the presence of the Ag atoms. This could indicate thatthere is a mutual mixing of elements at atomic level, which is taking place at the interface boundaries.
For 20 h of milling time, at both temperatures, we see also the dominant six-line component, with Hhf = 332 kG at RT and 338 kG at 77 K, which is originated from the ferromagnetic a-Fe. At RT there is also a weak broad six-line component which comes from the intermixing of the elements, and a broad doublet due probably the iron oxide which may be introduced after the sample being taken out the glove box and be absorbed by the surface of the particles.
This contamination is increased under milling time of 40 h. For 77 K, we can see two weak six-line components, with Hhf = 307 and 274 kG, due to the mutual atomic mixing of the elements.
Therefore, what we found from Mössbauer spectra is in well agreement with TEM images, in which the Fe particles are completely embedded in the Ag particles. Nasu et al. [14] draw to similar conclusions from Mössbauer experiments. Herr et al. [22] also reported the formation of Fe-Ag alloys, by co-sputtering, at the grain boundaries based on free volume and local strain at the interfaces. Our result is in agreement with Kuyama et al. [15] who carried out high-resolution TEM experiments and also infered that there was a mutual mixing of elements at the atomic level at the interfaces.

Conclusions
Uniform distribution of Fe particles of nanometer size embedded in Ag particles is achieved after periods as short as 10 h of ball milling. A mutual dispersion of Ag and Fe is obtained. A small diffusion of Fe into Ag seems to take place at the Ag. Mössbauer spectroscopy and DSC analysis confirm these results [21,23]. Further studies are being carried out in this system for a better understanding of the alloy formation by mechanical alloying.

Acknowledgments
The authors are very thankful to Ms. Losada for the careful work with the ultramicrotomy thin sections and to Miss U. Spadavecchia for the all the help with the paper editing.

References
1. K. Uenishi, K. F. Kobayashi, S. Nasu, H. Hatano, K. N. Ishihara, and P. H. Shingu, Z. Metallkd., 83, (1992) 2.         [ Links ]
2. A. R. Yavari, P. J. Desré, and T. Benameur, Phys. Rev. Lett., 68, (1992) 2235.         [ Links ]
3. J. Eckert, J. C. Holzer, C. E. Krill III, and W. L. Johnson, J. Appl. Phys., 73, (1993) 2794.         [ Links ]
4. E. Ma, M. Atzmon, and F. E. Pinkerton, J. Appl. Phys., 74, (1993) 955.         [ Links ]
5. J. Y. Huang, A. Q. He, Y. K. Wu, H. Q. Ye, and D. X. Li, J. Mater. Sci., 31, (1996) 4165.         [ Links ]
6. J. Z. Jiang, C. Gente, and R. Bormann, Mater. Sci. Eng. A 242, (1998) 268.         [ Links ]
7. C. Gente, M. Oehring, and R. Bormann, Phys. Rev. B, 48, (1993) 13244.         [ Links ]
8. G. Velt, B. Scholz, and H.-D. Kunze, Mater. Sci. Eng. A, 134, (1991) 1410.         [ Links ]
9. K. Sakurai, Y. Yamada, C. H. Lee, T. Fukunaga, and U. Mizutani, Mater. Sci. Eng. A, 134, (1991) 1414.         [ Links ]
10. E. Gaffet, C. Louison, M. Harmelin, and F. Faudot, Mater. Sci. Eng. A, 134. (1991) 1380.         [ Links ]
11. K. Uenishi, K. F. Kobayashi, K. N. Ishihara, and P. H. Shingu, Mater. Sci. Eng. A, 134, (1991) 1342.         [ Links ]
12. R. Najafabadi, D. J. Srolovitz, E. Ma, and M. Atzmon, J. Appl. Phys.. 74. (1993) 3144.         [ Links ]
13. S. Nasu, P. H. Shingu, K. N. Ishihara, and F. E. Fujita, Hyp. Int., 55, (1990) 1043.         [ Links ]
14. S. Nasu, S. Morimoto, H. Tanimoto, B. Huang, T. Tanaka, J. Kuyama, K. N. Ishihara, and P. H. Shingu, Hyp. Int., 67, (1991) 681.         [ Links ]
15. J. Kuyama, H. Inui, S. Imaoka, S. Nasu, K. N. Ishihara, and P. H. Shingu, Jpn. J. Appl. Phys., 30, (1991) L 854.         [ Links ]
16. M. Angiolini, A. Deriu, F. Malizia, G. Mazzone, A. Montone, F. Ronconi, M. Vittori-Antisari, and J. S. Pedersen, Mater. Sci Forum, 269-272, (1997) 397[         [ Links ]STANDARDIZEDENDPARAG]
17. N. S. Cohen, E. Ahlswede, J. D. Wicks, and Q. A. Pankhurst, J. Phys.: Condens. Matter., 9, (1997) 3259.         [ Links ]
18. E. Ma, J.-H. He, and P. J. Schilling, Phys. Rev. B, 55, (1997) 5542.         [ Links ]
19. R. B. Schwartz, Mater. Sci. Forum, 269-272, (1998) 665.         [ Links ]
20. D. Bonyuet, G. González, J. Ochoa, and C. Rojas. Rev. LatinAm. Met. Mat., 20, (2001) 85.         [ Links ]
21. D. Bonyuet, G. González, J. Ochoa, L. D´Onofrio, and F. González-Jiménez. In preparation.         [ Links ]
22. U. Herr, J. Jing, U. Gonser, H. Gleiter, Sol. St. Comm., 74, (1990) 197.         [ Links ]
23. D. Bonyuet, G. González, J. Ochoa, L. D´Onofrio, F. González-Jiménez, and C. Albano. In preparation         [ Links ]