INTERACTION BETWEEN IRON AND LIQUID GALLIUM IN THE COURSE OF INTENSIVE MECHANICAL ACTIVATION

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1 INTERACTION BETWEEN IRON AND LIQUID GALLIUM IN THE COURSE OF INTENSIVE MECHANICAL ACTIVATION Kiseleva 1 T.Yu., Levin 1 E.E., Novakova 1 A.A., Kovaleva 2 S.A., Grigoreva 3 T.F., Barinova 3 A.P., Lyakhov 3 N.Z. 1 Moscow M.V. Lomonosov State University, Leninskie Gory, Russia, Kiseleva.TYu@gmail.com 2 The Joint Institute of Mechanical Engineering, NAS, Minsk, Belarus 3 Institute of Solid State Chemistry and Mechanochemistry, SB RAS, Kutateladze str., Novosibirsk, Russia. Grig@solid.nsc.ru Abstract. Mössbauer spectroscopy, X-ray diffraction, Transmission electron and Atomic Force microscopy have been performed on Fe(8)Ga(2) powder samples milled within different duration in order to study the peculiarities of structure transformation in the course of solid-liquid metal mechanochemical interaction. Introduction. The modern industry demands expansion of a constructional materials variety and improvements of their properties towards to spatial sizes reduce. Mechanosynthesized composite powder materials used as precursors for different functional materials can satisfy these requirements supporting special structure and properties achieved at the stage of their preparing. Thus mechanical alloying is one of the power methods to produce powders for perspective applications in preparation of diffusion welding, sensors and actuators, fillers or reinforsers for magnetically oriented composites [1-2] due to simplicity and efficiency of nanocrystalline state achievement. The possibility of absolutely diverse classes of chemical compounds connecting has been demonstrated with the aim to generate a composit material in which nonequilibrium and metastable phases are created [3]. The ability of taking the advantage of particular properties of the constituent materials to meet specific demands is the most important motivation for the development of composites. The processes of interaction of different solid metals are well studied as their milling in liquids and surfactants [1]. The peculiarities of mechanochemical interaction of a solid metal iron with easily melting metal like indium, tin, bismuth, aluminium have been studied in our several work [1, 3-6]. The present investigation has been undertaken to provide additional evidence and sequence of mechanochemical interaction of iron with metal namely gallium which actually is at once in a liquid state belonging to the low melting point (29 C). For the Fe-Ga system equilibrium phase diagram [7] bcc ordered B2 (FeGa) and DO 3 (Fe 3 Ga) phases are formed in the Fe-rich side, while fcc ordered L1 2 (Fe 3 Ga) is stable at low temperature below 58 C in high composition region over about 25 % Ga. The strong correlation of functional properties and microstructure of Fe-Ga materials is a motivation of detailed study of structure evolution towards to nanocomposite materials formation by Fe and Ga mechanosynthesis. Experimental. Mechanical milling of Fe powders with Ga in relative proportions 8:2 has been realized with an AGO-2 planetary mill sealed under argon. Vial volume was 2 1

2 cm 3. Steel balls diameters and mass were 5 mm and 2 g, respectively. The speed drum rotation was ~1 rpm. Subsequent samples have been prepared by milling for periods from 4 sec to 12 min. At different stages of 1 g Fe:Ga (8:2) powder mixture mechanochemical interaction, obtained composite powders have been studied by Mössbauer spectrocopy, X-ray diffraction, transmission and electron microscopy. Mössbauer spectra were obtained at room temperature using constant acceleration spectrometer with 57 Co(Rh) radiation source. The phase composition determined from the spectra implied the same recoil fraction f for components. The powder diffraction data were collected in step scan mode at room temperature using RIGAKU D/max RC powder diffractometer equipped with a scintillation detector (Bragg-Brentano geometry (θ-2θscan), CuKα radiation, flat graphite analyser crystal). The samples were pasted onto a self-made zerobackground sample holder. It is essential to use this type of holder in order to improve signal-to-noise ratio and to avoid sample displacement and transparency errors. The transmission electron microscopy (TEM) was performed on LEO 912 AB Omega. Morphology and surface thopography have been studied by Atomic Force microscope NT-26 (Microtestmachines, Gomel) with standart commercial V-type sonde NSC11 (Mikromasch) in contact mode. Results and discussion. Interaction between iron and gallium gives rise to exothermic reaction with the enthalpy of formation H = 21 kj/mol [8]. Due to a low exothermicity of this reaction, the system does not demonstrate selfpropagating combustion behavior. However, exothermic character of the reaction probably enhanced the reaction rate of mechanochemical synthesis of the intermetallics phase. In addition, gallium exhibits several important physical properties such as low vapor pressure and high thermal reactivity that are beneficial for mechanochemical synthesis. Since gallium melts during the mechanochemical treatment (melting point ~29 C), this improves the reactive contact between reagents. This plays an important role in enhancing the nucleation of new Fe Ga phase during the high-energy mechanochemical process. 1. X-ray diffraction results. X-ray diffraction data for the samples at different stages of mechanochemical interaction are shown in Fig. 1. Presented patterns showed peaks that correspond to the reflections associated with bcc Fe. In addition, the pattern for the sample milled for the shortest period also showed the presence of several small peaks near 34 and 41 that correspond to elemental gallium. FeGa 3 reflexes observed just after 2 min of milling became wider at 12 min that testify grain size reduction. The reflexes of FeGa 3 fully dissapear after 2 min of milling. For milling times of 6 min or greater, the X-ray patterns look like a single-phase bcc structure. Analysis of x-ray diffraction data line profile by Le Beil method allowed specifying the lattice parameter of formed phases. The lattice parameter of bcc Fe (2.87 ±.1 Å at 2 min) doesn t change up to 3 min within accuracy of.1 Å. At prolonged time of milling (6 and 12 min) good fit of line profile by Rietveld analysis has been recieved for superposition of two phase bcc Fe and Fe 3 Ga (ICDD PDF2 [18436]). The general 2

3 Fe3Ga(21) Fe3Ga(31) Fe3Ga(111) Fe3Ga(21) Fe3Ga(21) Fe3Ga(111) FeGa3(111) FeGa3(21) FeGa3(112) FeGa3(212) FeGa3(22) FeGa3(4) FeGa3(51) FeGa3(111) FeGa3(112) FeGa3(21) Fe(2) Fe(211) Fe(22) FeGa3(212) FeGa3(22) FeGa3(4) FeGa3(51) Fe(11) B FeGa3(111) FeGa3(21) FeGa3(112) FeGa3(22) FeGa3(212) FeGa3(4) FeGa3(211) B Ga(131) Ga(33) Ga(41) Ga(4) Ga(15) Ga(132) Ga(22) Ga(42) Ga(42) Fe(2) Ga(481) Fe(211) Fe(22) Fe(11) tendency is traced: at increase in grinding time the bcc-fe maintenance decreases opposite to Fe 3 Ga increase a b c FeGa3(11) FeGa3(11) d e f Figure 1. X-ray diffraction patterns of sample milled for 4 sec (a), 2 min (b), 12 min (c), 2 min (d), 6 min (e), 12 min (f) 3

4 2. Mössbauer spectroscopy results. Room temperature 57 Fe Mössbauer spectra for the studied samples as a function of milling time are illustrated in Fig. 2. The basic component of Mössbauer spectrum of the sample milled within 4 sec (not presented in Fig. 2) reveals sextet with parameters characteristic to bcc α-fe. The spectrum contained also weak singlet component (~3 %) with isomer shift δ =.37 ±.1 mm/s which corresponds to the local atomic distribution of dilute Fe in Ga [9]. This fact proves to be true the data of X-ray diffraction Fig. 1 a. The components, representing a set of subspectra with the values of effective magnetic hyperfine fields in thea range from H eff = 325 кoe to H eff = 29 кoe is caused by formation on the surface of iron particles the defective area in which formation of disordered intermetallics compounds or local surroundings like solid solutions of gallium in α-fe occurs. This components as well as α-fe subspectra are clearly resolved in the spectra up to 2 min of milling. Mossbauer spectra analysis shows that serious changes of phase structure up to 2 minutes of activation don't occur. Weak increase of a disorder condition on the iron particles surface is observed: desordered component are slihgtly rises together with α-fe particle reduce. Quantitative phase analysis derived from analytical spectra fit for samples after 2, 8, 12 min of mechanical activation (Fig. 2 b-d) shows that the basic result of interaction at these durations is the FeGa 3 intermetallics occurence (3 % - at 2 minutes of activation), and increase of its quantity up to 6 % at 12 min. The Mössbauer spectrum of this phase is presented on the whole experimental spectrum as a doublet (shown by dark color). This is accompanied by reduction of bcc α-fe quantity from 76 to 7 %. The Mössbauer spectrum of the sample after 2 min of milling shows (Fig. 2 d) that at almost invariable FeGa 3 content there is a redistribution of a local α-fe surrounding: slight reduction of disordered component is accompanied allocation of an environment in the particles with structure close to Fe 3 Ga. Spectrum of this phase has hyperfine magnetic spitting. Three times increase in time of activation till 6 min leads to the sharp change of phase structure. The spectrum analysis shows a total disappearance of phase FeGa 3. There is the decrease of pure α-fe environment. Asymmetry of α-fe Mössbauer subspectrum reflects Ga appearing in the nearest environment of Fe atoms and formation of bcc solid solution Fe(Ga) [1-11]. At the same time bcc Fe reflexes on X-ray diffraction pattern (Fig. 1) became wider and move towards smaller diffraction angles supporting this fact. Observed asymmetry of these maxima is characteristic to occurrence of solid solutions with different concentration, or to superposition with reflexes of disordered Fe 3 Ga. Thus at this stage of mechanochemical interactions in sample we observe substantial growth of a component, connected with a disordered solid solution of gallium in iron. The analysis of the relative intensity of the corresponding component reflects considerable hanging of gallium concentration in a solid solution. It is necessary to notice that at this stage essential contribution to Mössbauer spectrum is given by sextets with parameters, characteristic to intermetallics phases Fe(Ga), distorted DO 3 and Fe 3 Ga. 4

5 Fe(Ga)disord Fe(Ga)disord Fe(Ga)disord S% Fe(Ga)disord 1, 1 8 Fe,98 6 4,96 a 2 Ga(Fe),94 1, B 1 8 Fe,98 6 4,96 b 2 FeGa 3, ,, Fe,96 4,94 c 2 FeGa 3 1, B 1 8 Fe,98 6 d 4,96 2 FeGa 3 Fe 3 Ga,94 1,1 1,, e Fe Fe(Ga)+DO 3 Fe 3 Ga,98 2 1,1 1,, H Fe(Ga)+DO 3 Fe 3 Ga,98 f 4 2 Fe mm/s 5

6 Figure 2. Mössbauer spectra of Fe-Ga samples milled within 2 min (a), 8 min (b), 12 min (c), 2 min (d), 6 (e) and 12 min (f) Mössbauer spectrum of 12 min milled sample (Fig. 2) has a hyperfine fields destribution form with most probable values of H eff corresponds to solid solution α- Fe(Ga) and Fe 3 Ga. The increase in Fe 3 Ga content we observe local environments with structure like DO 3 (increases to 54 %) which is sharply decreases the component of pure iron (up to 18 %). This fact is completely supported observed shift of structural maxima towards to Fe 3 Ga positions and decrease of α-fe reflexes intensity. 3. Transmission Electron and Atomic Force Microscopy results The particle agglomerate surface thopography shown on Fig. 3 reveals that at the early mechanoactivation stage (4 sec) extended composite units are formed by separate particles of sized below 2 nm. The units consist of different phases. a) b) c) d) Figure 3. AFM image of Fe-Ga samples milled within 4 sec (а), 12 min (b), 6 min (c) and 12 min (d) 6

7 a) b) c) d) e) f) g) h) 7

8 Figure 4. Dark field image obtained from the sample at different stages of the mechanochemical interaction process: a - 2 min, c - 12 min, e - 6 min, g - 12 min. Nanometric crystals can be seen. Corresponding diffraction patterns (b, d, f, h) At whole TEM pictures of the 2 min milled sample a wide distribution of the particles sizes from small (~1-15 nm) to large (2 m) have been observed. Dark field image (Fig 4) allows visualizing the morphology of intermetallics phase formation. As seen from the Fig 4 the surface of the large extended particles (2 nanometers) is covered by other phase (light layer) and small (5-6 nm) round inclusions (light stains). The analysis of electron diffraction patterns (Fig. 4 b), received from this fraction, have shown the presence of single-crystal fraction reflexion and weak diffuse rings from disordered structure of the sample. Two phases - α-fe and FeGa 3 - have been identified by electronic diffraction maxima pattern. The particle size distribution at the stage of 12 min milling (Fig. 4 c) lies in the range of 1-5 nm. The surface of the particle (2 nm) became covered by nm inclusions with roundish form. α-fe and FeGa 3 have been identified by electron diffraction. AFM image (Fig 3 b) demonstrates that at this stage of interaction particles became roundish. The size of units lies in the range of.7-2 µm. At units there are particles of two phases: 2-4 nm with tetragonal "facet" (light), the second - more dark gray. TEM image of the sample milled within 6 minutes (Fig 4 e) shows slight reduction of the particles sizes articles to 15 nm. This is accompanied by intermetallics inclusions size reduction on the particle surface. At the dark images (Fig 4.) the quantity of small fraction with sizes of 5-7 nanometers (light stains) became significant. At the same time polycrystalline rings on the electron diffraction became more resolved having characteristic width. Separate single-crystal reflections also observed. AFM image (Fig. 3 d) reflects that 12 min activation leads to the particles reduction in units to the sizes lower than 2 nm. The size of units reaches 2 µm. There are particles of different phases at the unit. Dark image of the sample milled within 12 min (Fig. 4 g) shows that the particle (2 nm) is almost completely covered by small particles with size of 3-5 nm. Intensive and well resolved polycrystalline rings of α-fe (Ga) and Fe 3 Ga have been derived. The microstructure of the particle represents a homogeneous distributed one-dimensional intermetallic inclusion in the whole particle matrix. Discussion The initial Fe : Ga ratio input in powder blend corresponds to the relation for formation of Fe 3 Ga and Fe(Ga) solid solution according equilibrium phase diagram [7]. But non-equilibrium conditions realized in mechanoactivation process causes the features of phase synthesis reactions and nanophase composite structure formation. Mechanical treatment increases the reactivity of the used solid reactant through the generation of extended crystal defects, new surfaces, and lattice distortions. At the same time gallium is practically at once in a liquid state (melting point ~29 C), this improves the reactive contact between reagents and speed up the diffusion rates. In addition, gallium exhibits several important physical properties such as low vapor pressure and high thermal reactivity that are beneficial for mechanochemical synthesis. Interaction between iron and gallium gives rise to exothermic reaction with the enthalpy of formation ΔH = 21 kj/mol [8]. Due to a low exothermicity of this 8

9 reaction, the system does not demonstrate selfpropagating combustion behavior. But our study reveals that interaction of Fe and Ga occurs already at the early stages of milling. Besides formation of solid component fresh surface spreading of the liquid metal takes place. Intensive mechanical influence on a liquid phase can essentially change its driving force. Simultaneously with gallium spreading other physical and chemical processes proceed, namely dissolution of a solid in liquids, diffusive penetration of atoms of a liquid into the volume of solid particles through structure defects, chemical reactions etc. The role of a liquid and defects interactions can to be great enough. The analysis of a structural condition of investigated sample at early stages of mechanical activation (from 2 mines to 2 mines) has shown formation of intermetallics FeGa 3 and absence of a solid solution Fe(Ga). This process occurs, apparently, directly on the reacting metals interface. There is an opinion that atoms of a solid phase don't pass in contact with the melt of other metal until on its particle surface formation of other metal layer thickness isn't riched. Diffusion of second component atoms from a liquid phase reveals the solid solution or intermetallics became in balance with the melt. As a result of such interaction intermediate layers of alloys can be formed. Formation of intermetallics FeGa 3 from solid solution of Ga(Fe) observed by Mössbauer spectroscopy support, most likely, such mechanism. Disordered local environment on the iron particles with Ga up to 2 min activation as derived from the spectra testifies a considerable role of interaction of liquid metal with defects. However the absence of the Fe(Ga) solid solution formation up to 6 minutes of mechanical activation was surprising. The individual phases lattice parameters changes show that while bcc Fe parameter slightly increases from 6 min and reaches values 2.95 Å at 12 min, Fe 3 Ga parameters decreases towards standard value at increase in time of a grinding (from ±.3 Å (2 min) to 5.89 ±.5 Å (12 min). It is possible to assume that this fact is connected with Fe 3 Ga phase stoicheometry change. One of possible explanations is gallium washing away formation of strict sсtoichiometric Fe 3 Ga compound with tabular lattice parameter. Sharp changes of phase structure after 6 min are possibly caused by iron grain size achievement of the critical size at which interaction is possible, and also by destruction of FeGa 3 («gallium washing away») lattice at interaction with iron particles or with crushing balls. At this moment the quantity of a solid solution α-fe (Ga) decreases sharply. Then at activation time increase the local ordering of the solid solution with formation of an iron atoms environment with locally ordered solid solution and Fe 3 Ga recrystallization have been observed. Local pure bcc-fe local surrounding still presented in at long activation duration may be caused by iron appearing from the grinding balls. Finally, after two-hour mechanical activation of the sample we have composite particles consisted of superposition of α-fe (Ga) solid solution and Fe 3 Ga. Summary The solid Fe particles and liquid Ga interaction in the course of high energy mechanical activation have been investigated. The analysis of structural transformations at consecutive stages of Fe and Ga mechanosynthesis has allowed showing the formation mechanism of intermetallics nanocomposite in which the disorder matrix of particles with the size of 2 nm contains intermetallics monodimensional 3-5 nanometers inclusions. 9

10 The work is carried out under the Integration Project of SB RAS No. 138 and BRFFI Т9СО-14 «Development of Fundamental Basis of the Action of Activation on Regulation of the Processes of Interaction of Solid Metals and Their Comopunds with Metal Melts for the Purpose of Obtaining Functional Materials with Required Structure and Properties». REFERENCES 1. Mechanocomposites precursors for creation of materials with new properties. Novosibirsk, SB RAS, Eliseev А.А., Lukashin A.V. Functional nanomaterials. Moscow, Fizmatlit, Grigorieva T.F., Barinova A.P., Lyakhov N.Z. Mechanochemical synthesis of intermetallic compounds. Russian Chem. Rev (1) Kiseleva T.Yu., Novakova A.A., Grigorieva T.F., Barinova A.P. Iron and Indium interactions during mechanical attrition. Journal of Alloys and Compounds Novakova A.A., Grigoreva T.F., Barinova A.P., Kiseleva T.Yu., Lyakchov N.Z. Fe(In) Solid Solution Formation During Mechanical Attrition. Journal of Alloys and compounds Grigor eva T.F., Barinova A.P., Lyakhov N.Z. Initial Stages of Mechanical Alloying in Metallic Systems with a Low-Melting Component. Doklady Chemistry (4-6) Okamoto H.: "Fe-Ga (Iron-Gallium)", Binary Alloy Phase Diagrams, 2nd Ed., Ed. T.B. Massalski, ASM International, Bakker H. Enthalpies in Alloys: Miedema's Semi-empirical Model. Zurich, Trans Tech Publication, Gaudet J.M., Hatchard T.D., Farrell S.P., Dunlap R.A. Properties of Fe-Ga based powders prepared by mechanical alloying. Journal of Magnetism and Magnetic Materials (6) Dunlap R.A., McGraw J.D., Farrell S.P.: Mössbauer effect study of structural ordering in rapidly quenched Fe-Ga alloys. Journal of Magnetism and Magnetic Materials (2) Newkirk L.R., Tsuei C.C., Keck W. M. Mössbauer Study of Hyperfine Magnetic Interactions in Fe-Ga Solid Solution. Physical review (11)