Structure relaxation and crystallization of the CoW-CoNiW-NiW electrodeposited alloys
© Pustovalov et al.; licensee Springer. 2014
Received: 30 October 2013
Accepted: 31 January 2014
Published: 10 February 2014
The structure of electrolytically deposited nanocrystalline alloys of the CoW-CoNiW-NiW systems under low-temperature heating was investigated by means of high-resolution transmission electron microscopy (HRTEM), high-angle annular dark-field scanning transmission electron microscopy (HAADF STEM), and analytical methods such as energy dispersive x-ray spectroscopy (EDS) and electron energy loss spectroscopy (EELS). Structural relaxation and crystallization were investigated at temperatures of 200°C to 300°C. The structural and compositional inhomogeneities were found in the CoW-CoNiW-NiW alloys, while the local changes in composition were found to reach 18 at.%. Nanocrystals in the alloys grew most intensely in the presence of a free surface, and we found their nuclei density to range from 2 × 1023 /m3 to 3 × 1023 /m3. It was determined that the local diffusion coefficient ranged from 0.9 to 1.7 10−18 m2/s, which could be explained by the prevalence of surface diffusion. The data gathered in these investigations can be used to predict the thermal stability of CoW-CoNiW-NiW alloys.
Tungsten-based alloys with iron group metals (Ni and Co), particularly CoW and CoNiW, possess better functional properties and in our case alloys were formed by electrochemical deposition. These alloys can be used as thermo-resistant and hard-wearing materials [1, 2] and as alternatives to chromium coatings . Tungsten-based alloys can be found in hydrogen power engineering, sewage sterilization, and toxic waste putrefaction . Thin magnetic films based on CoNiW alloys are promising as materials for perpendicular or near-perpendicular magnetic recording because of their columnar structure with perpendicular magnetic anisotropy [5–7]. Researchers are interested in these films because of their wide range of magnetic properties that are dependent on deposition conditions and chemical composition [4–6, 8–10]. It is well known that the alloy structure of CoW-CoNiW-NiW may be nanocrystalline or amorphous depending on the composition and preparation conditions [7–14]. At the same time, the degree of order of the structure significantly changes depending on the processing history of the alloy. One simple treatment, low-temperature annealing, is interesting from a practical perspective. While the structure changes of these alloys are well-studied at higher temperatures, they are not well-studied between 200°C and 300°C. However, the initial stages of atomic structure relaxation and crystallization are extremely important in order to understand further changes in the macrostructure and physical properties.
Deposition was performed in stationary- and pulsed-current conditions at frequencies of 1 to 10 kHz. A 0.1-mm-thick polished copper foil was used as the substrate. Studies of the microstructure were performed on films 40- to 80-nm thick, placed on standard copper grids for transmission electron microscopy (TEM). In situ heating experiments were used according to various schemes. In one case, heat was applied at a constant rate of 1 to 2°С/min to a maximum temperature of 300°C. In another, it was applied stepwise in increments of 50°С. Isothermal annealing was performed at 200°C, 250°C, and 300°C. Three electron microscopes were used: FEI Titan™ 80–300 (FEI Company, Hillsboro, OR, USA), JEOL ARM™ 200 (JEOL Ltd., Tokyo, Japan) equipped with aberration correctors of the objective lens, and Carl Zeiss Libra® 200FE (Carl Zeiss AG, Oberkochen, Germany) equipped with an omega filter. Local chemical analysis was completed using both energy dispersive x-ray spectroscopy (EDS) and electron energy loss spectroscopy (EELS). The accelerating voltages were 80 and 300 kV for the Titan, and 200 kV for the ARM200 and Libra 200FE. In situ experiments were carried out using the FEI Titan 80–300 and Zeiss Libra 200 FE with a specialized Gatan dual-axis heating holder (Gatan, Pleasanton, CA, USA). Comparable in situ heating experiments were carried out with the Libra and Titan, both with and without electron beam irradiation. It was found that electron beam irradiation can lead to a temperature difference in the specimen of up to 300°C, depending on the current density of the electron beam.
Results and discussion
The CoW-CoNiW-NiW alloys have a quasi-network structure, with nanocrystals in the cells separated by a ‘skeleton’ amorphous structure [11, 12]. The high scattering capability of the tungsten atoms allows the ordered structure to be visualized by aberration-free high-resolution transmission electron microscopy (HRTEM) with sufficient contrast down to an area on the order of 1 nm, which is a few unit cells of the crystalline phases of tungsten as well as the crystalline phases and solid solutions of NiW and CoW.
It is well known that a NiW alloy structure changes due to the concentration of tungsten . Below 19.6 at.% W, the structure is crystalline, whereas above 23.5 at.% it is amorphous. If the composition is between these two values, the structure is in a transition zone between crystalline and amorphous. Chen at al.  investigated the transition range under low-temperature annealing and found that at 19.6 at.%, W, the as-prepared alloy's structure, was completely crystalline. In that case, the NiW alloy film was prepared by magnetron deposition and was about 1-μm thick. A metastable crystalline phase can form under those conditions. Our NiW alloy film was prepared by electrochemical deposition at a thickness of about 40 to 80 nm. The temperature difference of the surface atoms as well as the tungsten concentration (32 at.% in our case) explain the initial structural differences.
It is well known that the local atomic structure can be modified by an electron beam and is visible in TEM as radiation damage, nanoparticle coagulation, or other changes [18–21]. The density of such areas and the level of structure damage depend on the current density and the incident beam energy. In our investigations, the current density did not exceed 10 to 20 A/cm2 at beam energy of 80 to 300 kV. This allowed us to choose the conditions under which local structure modification was negligible and not visible under electron beam irradiation.
One method proposed for estimating diffusion coefficients of amorphous alloys is by direct measurement of the crystals' size changes under heat using the electron microscope . We estimated the diffusion coefficient by direct observation of atoms moving in the specimens by using TEM with high-pass diffusion  at the beginning of structure relaxation and at crystallization at elevated temperatures. The most visible changes in the alloy structure occurred at the vacuum-crystal interface. In these areas, the local diffusion coefficient was much higher, up to 10−18 cm2/s. This does not contradict prior findings that the mean value of the diffusion coefficient ranges from 10−25 to 10−24 cm2/s for Co/Ni in W and W in Co/Ni [24, 25] at 200°C. Our primary goal was to estimate the diffusion coefficient through direct local observation of the beginning of atomic structure relaxation and crystallization at low-temperature annealing.
Ni and W content of NiW alloy at the points of interest using EDS analysis
Atomic percentage of Ni
Atomic percentage of W
Co and W content of the CoW alloy at the points of interest using EDS analysis
Atomic percentage of Co
Atomic percentage of W
Investigations showed the presence of structural and compositional inhomogeneities in the CoW-CoNiW-NiW alloys. Atomic electron microscopy allowed us to determine the preferential areas of the structural relaxation and crystallization processes. The most intensive nanocrystal growth occurs on free surfaces. Based on direct observation of the atoms' movements, it was determined that the diffusion coefficient is in the range of 0.9 to 1.7 × 10–18 m2/s, which was significantly higher than the volume diffusion coefficient for similar alloys. This can be explained by the prevalence of surface diffusion, which can exceed volume diffusion by three to five orders of magnitude [26–28]. It was found that local changes in the composition can reach 18 at.% for the CoW alloy and 4 at.% for the NiW alloy. In addition, tungsten is more homogeneously distributed than nickel or cobalt. This is associated with the higher mobility of nickel and cobalt atoms in the electrolyte. Thicker areas of the alloys are enriched by nickel, whereas the thinner ones have increased tungsten percentages. This data can be used to predict the thermal stability of the CoW-CoNiW-NiW alloys.
EVP is an associate professor of computer systems department in School of Natural Sciences in Far Eastern Federal University. He has a Ph.D. in Physics and great experience in electron microscopy. His scientific interests are electron microscopy, physics of condensed matter, image processing, and high-performance computations on GPU. EBM is currently a Ph.D. student of School of Natural Sciences in Far Eastern Federal University. His Ph.D. project focuses on electron microscopy of amorphous and nanocrystalline metallic alloys and their structure changes under external impact. OVV is a Ph.D. student of School of Natural Sciences in Far Eastern Federal University. His Ph.D. project focuses on electron microscopy and electron tomography of structure inhomogeneities in amorphous metallic alloys.ANF holds a BS degree in Information Systems from Far Eastern Federal University. He is currently working toward a master's degree in Information Systems and Technologies at Far Eastern Federal University. He has interests and experience in image processing, computer simulation and electron microscopy. AVD holds a BS degree in Information Systems from Far Eastern Federal University. He is currently working toward a master's degree in Information Systems and Technologies at Far Eastern Federal University. He has interests and experience in multiscale modeling and development high-performance solutions. BNG is a full professor of Computer Systems Department in School of Natural Sciences in Far Eastern Federal University. He has many years of experience in electron microscopy image processing and modeling. VSP is a full professor of Computer Systems Department in School of Natural Sciences in Far Eastern Federal University and head of electron microscopy and image processing laboratory. His research activities started in 1970s and were focused on electron microscopy and physics of condensed matter. SSG is chief researcher of Scientific and Practical Centre of Material Science, Belarus National Academy. His scientific interests are microstructure studies, magnetic and mechanical properties of electrolytically deposited amorphous metal alloys. He has great experience in electrochemistry and experienced in obtaining alloys with specified functional characteristics.
The authors thank Professor Ute Kaiser and Dr. J. Biskupek (Ulm University, Germany) for their help with the experiments and productive discussions. The work was supported by the Russian Fund of Basic Research (RFBR) and the Far Eastern Federal University (FEFU) Scientific Fund.
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