ISSN 10628738, Bulletin of the Russian Academy of Sciences. Physics, 2011, Vol. 75, No. 9, pp. 1205–1208. © Allerton Press, Inc., 2011. Original Russian Text © E.B. Modin, O.V. Voitenko, A.P. Glukhov, A.V. Kirillov, E.V. Pustovalov, V.S. Plotnikov, B.N. Grudin, S.S. Grabchikov, L.B. Sosnovskaya, 2011, published in Izvestiya Rossiiskoi Akademii Nauk. Seriya Fizicheskaya, 2011, Vol. 75, No. 9, pp. 1274–1277.

Investigating the Structure of Electrolytically Deposited Alloys of the CoP–CoNiP System under Thermal Action E. B. Modina, O. V. Voitenkoa, A. P. Glukhova, A. V. Kirillova, E. V. Pustovalova, V. S. Plotnikova, B. N. Grudina, S. S. Grabchikovb, and L. B. Sosnovskayab a

b

Far Eastern State University, Vladivostok, 690950 Russia Joint Institute of Solid State Physics and Semiconductors, National Academy of Sciences of Belarus, Minsk, 220027 Belarus email: [email protected]

Abstract—The problems of structural relaxation and crystallization of electrolytically deposited amorphous metal alloys of the CoP–CoNiP system are investigated. The onset of the structural relaxation and crystalli zation is found to occur through the nucleation of nanocrystalline particles of an unrecognized CoxNiyP1–x–y phase. An increase in the concentration of Ni is shown to yield a substantial thickening of the amorphous structure and to increase the temperature of the onset of structural relaxation and crystallization by 50–70°C. DOI: 10.3103/S1062873811090188

INTRODUCTION The processes of structural relaxation and crystalli zation in amorphous metal alloys have been inten sively studied for several decades. At present, in con nection with the development of direct methods of investigation, it has become necessary to study the mechanisms of relaxation of the structure and the effect of external factors on it. The processes of struc tural relaxation are of particular scientific interest since they affect the physical properties of a material (mechanical, electrochemical, magnetic, etc.) [1]. In this work, the processes of structural relaxation and initial crystallization stage were investigated by direct observation through highresolution transmis sion electron microscopy and the microdiffraction of electrons. Alloys of the CoP–CoNiP system form a challenging class of materials with a unique combina tion of magnetic, electrophysical, and mechanical properties [2, 3]. These materials make it possible to study and contribute to the solving of both the funda mental problems of an amorphous state and the applied problems of producing functionally amor phous and nanocrystalline alloys with specified prop erties, plus the effect of external actions on these prop erties. The aim of this work was —to study the peculiarities of the alloy’s structure using highresolution transmission microscopy and electron tomography; —to analyze the effect of different thermal treat ment modes on the microstructure of amorphous and nanocrystalline alloys and the processes of their relax ation;

—to perform in situ investigations of the dynamics of microstructure variations during thermal action in the column of an electron microscope. EXPERIMENTAL Amorphous and nanocrystalline metal alloys of the (Co,Ni)P and (Co,Ni)W systems were analyzed. The specimens were obtained via electrolytic deposi tion from sulfuric acids with additions of sodium hypophosphite. The parameters of electrolytic deposi tion for the (Co,Ni)P system were as follow: acidity pH = 1.6; electrolyte temperature Т = 65°С; and cathode current density Dc = 15 mA/cm–2. Those for the (CoNi)W system were pH = 6.5; Т = 30–40°С; and Dc = 25 mA/cm–2. Deposition was accomplished in the stationary and nonstationary modes at frequen cies of 1 kHz to 10 kHz [4]. Polished 0.1 mmthick copper foil served as a substrate. The microstructure was studied using films with thicknesses of 400–800 Å placed on standard copper grids for transmission elec tron microscopy. The electron microscope studies of the structure were performed in a Zeiss Libra 200FE transmission electron microscope. The in situ experiments on heat ing and cooling of the specimens were carried out using Gatan 652 doubleaxis holders that allowed heating of up to 1100°С and a Gatan 636 holder that allowed cooling of down to –170°С. Heating was accomplished at a constant rate of 1–2°C up to the maximum temperature T = 300°C and stepwise with a 50°С step. Isothermal annealing was performed at T = 250°C for 30 min. The specimens were cooled accord

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The structure dynamics in the process of heating and cooling was observed in the diffraction and TEM modes. A control program allowing us to specify the required parameters (maximum temperature, heating and cooling rates, current, etc.) was developed to automate the experiment and record the process. Spe cial software using graphical processor units was devel oped to measure the diameter of the first diffusion ring on the electron diffraction images. RESULTS AND DISCUSSION

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Fig. 1. CoW specimen, initial state. Magnification, 155000×; TEM. Amorphous structure, fluctuations of density and chemical composition.

ing to two models: one with a maximum rate on the order of 70°C/min and one stepwise with a rate of 10°C/min.

According to the electron diffraction data, the specimens of the (Co,Ni)P system were in an amor phous state before the experiments, while the speci mens of the (Co,Ni)W system had a nanocrystalline structure. Figure 1 shows the image of the structure fragment in the initial state of the CoW alloy. The amorphous structure can be seen, but there are differ ent types of defects in the form of density and compo sition fluctuations. The processing and analysis of electron microscope images in the magnification range of 10 to 100000 revealed regions with reduced density in the structure of CoP–CoNiP alloys with sizes of 3 to 100 nm. As for the CoW–CoNiW system, analysis of the images revealed density fluctuations of 5 to 100 nm. Studies of the structure of alloys with different nickel concentrations show a change in the density fluctua

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Fig. 2. CoP specimen. Structural defects in the form of regions with reduced density can be seen.

Fig. 3. CoNiP specimen. Reduced density of defects upon an increase in Ni concentration.

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Fig. 4. CoP specimen. Heating to 250°С. Magnification, 400×; TEM. Ordered regions can be seen at the irregularity boundaries.

Fig. 5. CoNiP specimen. Heating to 150°C. Magnifica tion, 250×; TEM. A crystallized region d = 1.72 Å (section A) and an amorphous region (section B) can be seen.

tion parameters. Our statistical analysis of the distri butions of defect areas and sizes revealed the power dependence of the defect density on their effective diameter. Analysis of the images shows that as the Ni concentration increases, the electrons undergo stron ger scattering in the regions with an increased density, relative to the CoP alloy. Figure 2 shows a fragment of the CoP structure: defects in the form of regions with reduced density are clearly seen, while a smallscale grid structure with cell sizes of 5–10 nm is poorly pronounced. As the concentration of nickel increases, the density of the defects declines (Fig. 3). Based on the results from experiments on heating of the specimens in the micro scope column, we may conclude that the process of crystallization in the specimens made from CoP and CoNiP alloys began at a temperature of around 200°С. Figure 4 shows a fragment of the CoP film after heating. Polycrystalline rings on the diffraction image correspond in the best way to the phases of cubic Co (ICSD53805) and rhombic Co2P (d122 = 2.211 Å, a = 5.638, b = 3.507, c = 6.603, ICSD43687). Figure 5 demonstrates crystallized microregions (region “A” in Fig. 5) formed after specimen’s heating in the microscope column up to 150°С. No ordering is observed in the region “B”. Decoding of the images of the electron diffraction points to the formation of a hexagonal phase of NiP (ICSD43239). Note that as a

rule, the boundaries of irregularities in the film struc ture (e.g., density fluctuations and probably fluctua tions in the chemical composition of the film) and dif ferent structural defects (e.g., pores. cracks, and inho mogeneities in the film thickness) serve as regions of initial crystallization. The use of pulsed currents in the process of deposi tion (nonstationary electrolysis) makes it possible to substantially affect the microstructure of the films. The specimens obtained after pulsed electrolysis are characterized by a finer and more uniform structure than those obtained through stationary electrolysis. CONCLUSIONS Our investigations revealed defects at different lev els of scale in the amorphous structure of the CoP– CoNiP and CoW–CoNiW alloys. These structural defects can be from 3 to 100 nm in size and represent fluctuations in density and chemical composition. Morphological analysis of the film structure defects revealed that the density of object distribution takes the form of a Pareto distribution: P ( d ) = C 1v −1 . d The onset of structural relaxation and crystalliza tion of the alloys of the CoPCoNiP system was observed at a temperature of around 200°С. The effect of nickel concentrations on the alloy’s microstructure was studied. It was found that the amorphous structure

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thickens and the temperature of crystallization onset rises with an increase in the Ni concentration. ACKNOWLEDGMENTS The study was supported by the Program for Develop ing the Scientific Potential of Higher Education, project no. 2.1.1/955; the Russian Foundation for Basic Research, project no. 080290030; and the federal target program Cadres, state contract nos. 02.740.11.0549 of March 22, 2010 and 14.740.11.0471 of October 1, 2010.

REFERENCES 1. Grabchikov, S.S., Amorfnye elektroliticheski osazhden nye metallicheskie splavy (Amorphous Electrolytic Deposited Metallic Alloys), Minsk: Izd. tsentr BGU, 2006. 2. Krysova, S.K., Naberezhnykh, V.V., and Moshen skaya, I.N., Izv. Vyssh. Uchebn. Zaved., Ser. Chern. Met., 1984, no. 7, pp. 92–94. 3. Kullmann, U. and Dietz, G., J. Appl. Phys., 1981, vol. 24, no. 3, pp. 239–243. 4. Grabchikov, S.S. and Potuzhnaya, O.I., Poverkhn., 2009, no. 6, pp. 64–70.

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