8 Revista Latinoamericana de Metalurgia y Materiales, Vol. 22, N° 1, 2002, 8 - 11

STUDY OF THE Fe-75%AgSYSTEMOBTAThffiDBYMEC~CALALLOTING

D. Bonyuet', G. González-, J. Ochoa-, F. Gonzalez-Jlmenez", L. D'Onofrio3

lIlBCA, Universidad de Oriente, Cumaná 6101, Venezuela. E-mail: dickar@cumana.sucre.udo.edu.ve.2Laboratorio de Materiales, Centro Tecnológico, IVIC, Apdo. 21827, Caracas 1020A, Venezuela. E-

mail: gemagonz@ivic.ivic.ve.3Escuela 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 sólidoen sistemas binarios inmiscibles en el equilibrio. Fe y Ag tienen una naturaleza mutuamente repulsiva que las hace completamenteinmiscibles bajo condiciones termodinámicamente estables. El proceso de molienda por bolas, siendo una técnica fuera delequilibrio, parece prometedor para obtener al menos una solución sólida parcial en este sistema. Mezclas de polvos de Fe y Agcon 75 % en peso de Ag fueron estudiados mediante difracción de rayos x (DRX), microscopía electrónica de barrido (MEB), ymicroscopía electrónica de transmisión (MET). Hemos encontrado que es posible obtener una pequeña solución sólida: parcialen este sistema mediante AM. Esto es confirmado mediante espectroscopia Mossbauer.

Palabras clave: aleación mecánica, sistemas inmiscibles

Abstract

Mechanical alloying (MA) is a powder processing technique, which allows us to induce solid state reactions in binarysystems immiscible in equilibrium. Fe and Ag have a mutually repulsive nature that makes them completely immiscible underthermodynamically stable conditions. The ball milling process being a non-equilibrium technique seems promising in obtainingat least a partial solid solution in this system. Mixtures ofFe 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 toobtain a small mutual solid solution in this system by MA. This is also confirmed by Mossbauer spectroscopy.

Keywords: mechanical alloying, immiscible systems

1. Introduction

In the last ten years there has been much interest in theuse of mechanical alloying as a non-equilibrium techniqueto prepare solid solutions in binary systems irnmiscible inequilibrium, which are characterized by a large positive heatof mixing. In alloy systems with negative heat of mixing, thephase formation is explained by an interdiffusional reactionoccurring during milling. On the other hand, in the case ofsystems with positive heat of mixing, phase formation is stillsubject of controversy, since in these systems a diffusionalreaction results in decomposition of the alloy. However,formation of partial solid solutions in several systems withpositive heat of mixing, including Fe-Cu [1-6], Co-Cu [7], Cu-Ta [8,9], Cu-W [10], andAg-Cu [11,12], have been observed.In contrast, for the system Ag-Fe this is still a matter ofdiscussion. Some authors, Nasu et al. [13,14], Kuyama et al.[15], claimed that they had found the formation of solidsolution in the system Fe-Ag, whereas, Angiolini et al. [16],Cohen et al.{17], Ma et al. [18], reported incomplete or minimalalloying, even after prolonged milling. The fact that Ag andFe do not form extended solid solution remains unexplained

[19]. It seems that there is some critical value ofthe enthalpyof mixing such that for high values a very limited mutualsolubility can be only observed even in the liquid state, andcannot be mixed at atomic level by MA.

In this work, the Fe-Ag system in the composition of 75wt.% Ag has been studied, and the phase formation underball milling from lOto 40 h investigated. It is known, fromequilibrium diagram, that mutual solubility of Ag and Fe isvery low in both the solid and liquid sates. However, wefound from Mossbauer spectroscopy that MA produces amutual dispersion of the Ag and Fe.

2. Experimental method

Elemental powders of Ag and Fe with 99.9% purity andinitial particle size less than 50 rnm were blended at thecomposition of75 wt.% Ag. The milling was carried out usinga SPEX 8000 mixer/milI with hardened steel balls and vial.The mixer/mill was modified in order to use a smaller vial, sothe impact ofthe balls is more energetic. The ball-to-powderweight ratio was 9: 1. In the milling procedure, 1 g of samplewas used in every case and the vial was sealed in a nitrogen

D. Bonyuet y col./Revista Latinoamericana de Metalurgia y Materiales

atmosphere and tightly c1amped to prevent oxidation. Thevial was processed for times of 10, 20 and 40 h, and the millwas operated by repeating a cyc1e of half hour period,interspersed with cooling down periods of also halfhour. Anew sample was used for each processing time in order toavoid possible air contamination due to the repeated openingofthe vial.

The phase transformation in the ball-rnilled powders wasexamined by x-ray diffraction (XRD), transmission electronmicroscopy (TEM) and Mossbauer spectroscopy. The XRDpatterns were carried out in a Siemens D5000 diffraetometerusing a Cu Ka (Ni filter) radiation and a Cu tube. The Seherrerformula was used to estímate the erystalIite grain size aftermilling

Speeimens for transmission eleetron mieroseopy (TEM)were prepared by embedding the powders in epoxy resin.The final sample thinning to eleetron transparency wasperformed by electropolishing and by taking thin seetionsby ultramierotomy procedures. Observations have been madewith a Philips CM12 mieroscope operating at 120 kv.Specimens for seanning eleetron mieroscopy (SEM) wereprepared by embedding powders in epoxy resin, and furthermetallographieally polishing by standard methods. The SEMobservations were carried out on a Philips LX30 mieroscope.

3. Results and Discussion

Fig. 1 shows the XRD patterns eorresponding to 75 wt.%Ag for different milling times. We see thatAg peaks overlapsthe Fe peaks, and only the Ag( 111) and Ag(311) peaks canbe observed separately.

Peak broadening and intensity deereasing with rnillingtime is also observed, whieh indieate that the refinement ofthe grain size. In the early stages of milling, the grain sizesdeerease rapidly to about 14 nm at 10 h of milling, and thenreach a steady state with final values of about 12 nm after 20h of milling. It is interesting to note that grain sizes of theresulting products depend upon the overalI compositions ofthe powder mixtures, since in this case the final grain size issomewhat bigger that the final grains size for 10 wt% Ag (10nm) and 30 wt% Ag (7 nm) [20].

The SEM images showed the Fe partic1es completelyeovered by Ag even after 10 h of milling. TEM results of theamples for the different milling times are shown in Fig. 2.

For 10 h, there are more Ag-rich partic1es than Fe-riehpartic1es, c1early evideneed in the images by the highercontrasto For 20 h, the mixture between partic1es is moreuniformo

Furthermore, the TEM images are in well agreement withour 57FeMossbauer spectroscopy results [21]. Herr et al.[_2] also reported the formatíon ofFe-Ag alloys at the grainboundaries based on free volume and loeal strain at the- terfaces.

9

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Fig. 1XRD pattems of Fe75%Ag milled for O, 10, 20, 40 h

Our result is in good agreement with Kuyama et al [15],who carried out high-resolution TEM experiments and alsoinfered that there was a mutual mixing of elements at theatomie level at the interfaees.

The electron diffraetion patterns only show reflectionsthat Ag and Fe. For 40 h, there is a different situation, sincethe eleetron diffraction pattem shows refleetions shifted withrespeet to those of the Ag and Fe refleetions. In addition, inthe bright field images, it can be seen a short-range arderregion formed at the particle interfaees. This eould be anindication of Fe diffusion in the Ag interface partic1e.

Figure 3 shows the Mossbauer spectra for 10 and 20 h, atroom temperature (RT) and 77 K. For 10 h of milIing time, atboth temperatures, we ean see a dominant six-line componentwith Hhf =333 kG at RT and 336 kG at 77 K, which are due tothe ferromagnetic a-Fe.

10 Revista Latinoamericana de Metalurgia y Materiales, Vol. 22, N° 1,2002.

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 atomsbecause of the presence of the Ag atoms. This couldindicate thatthere is a mutual mixing of elements at atomiclevel, which is taking place at the interface boundaries,

For 20 h of rnilling time, at both temperatures, we see alsothe dominant six-line component, with 1\r= 332 kG at RT and338 kG at 77 K, which is originated from the ferromagnetic a-Fe. At RT there is also a weak broad six-line componentwhich comes from the interrnixing of the elements, and abroad doublet due probably the iron oxide which may beintroduced after the sample being taken out the glove boxand be absorbed by the surface of the partic1es.

Fe75%Ag 10 h

Fe75%Ag40h

This contamination is increased under rnilling time of 40h. For 77 K, we can see two weak six-line components, withHhf= 307 and 274kG, due to themutual atomic rnixing oftheelements.

Therefore, what we found from Mossbauer spectra is inwell agreement with TEM images, in which the Fe partic1esare completely embedded in the Ag partic1es. Nasu et al.[14] draw to similar conc1usions from Mossbauer experiments.Herr et al. [22] also reported the formation of Fe-Ag alloys,by co-sputtering, at the grain boundaries based on freevolume and local strain at the interfaces. Our result is inagreement with Kuyama et al. [15] who carried out high-resolution TEM experiments and also infered that there wasamutual mixing of elements at the atomic level at the interfaces.

Fe75%Ag 20h

Fig.2 TEM Images ofFe75%Ag milled for 10, 20, 40 h

Fe75%Ag 40h detail

D. Bonyuet y col./Revista Latinoamericana de Metalurgia y Materiales 11

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

(e) 20 h milling time, RT. (d) 20 h milling time, 77 K

Conclusions

Uniform distribution of Fe particles of nanometer sizeembedded in Ag partic1es is achieved after periods as shortas 10 h of ball milling. A mutual dispersion of Ag and Fe isobtained. A small diffusion of Fe into Ag seems to takeplace at the Ag. Móssbauer spectroscopy and DSC analysisconfirrn these results [21,23]. Further studies are being carriedout in this system for a better understanding of the alloyforrnation by mechanical alloying.

Acknowledgments

The authors are very thankful to Ms. Losada for the carefulwork with the ultrarnicrotomy thin sections and to Miss U.Spadavecchia for the all the help with the paper editing.

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