Model forecasting of phase composition of electrolytic alloys Co-Ni-Mn (part 1)

With the help of four criteria for phase formation, a model forecasting of the phase composition of electrolytic alloy Co-Ni-Mn was carried out; the expected phases were calculated. The boundaries of the chemical content of the metal-solvent (Co) in these phases are determined, depending on the ratio of metal ions in the electrolyte of deposition. Model forecasting of the phase composition of Co-Ni-Mn alloys makes it possible to predict the type and number of Co phases (hexagonal close-packed - HCP-α-Co, face-centered cubic - FCC-β-Co) depending on the mole fraction of the solvent metal (Co). In the first approximation, the forecast allows one to determine the phase and chemical composition of the coating, which corresponds to the specified operational properties.


Introduction
Modern production places high demands on structural materials, such as hardness, cold resistance, catalytic and magnetic properties, etc. In turn, the latter directly depends on the structure of the material (phase and chemical composition, nanostructure, surface morphology).
An important role among the promising materials is played by galvanic coatings. Most often, binary alloys are used in practice, but selectivity and quality of high-alloy steels is higher.
Ternary alloys are preferable, but their introduction is inhibited due to the complex phase and chemical composition, the difficulties of experimental research, structural features and a number of other factors.
The solution of this most important task becomes possible with the help of preliminary theoretical modeling of the phase composition and the chemical content of the coating, taking into account the specified operational properties.
The aim of the work is to forecast the phase and chemical composition of ternary electrochemical alloys using the example of Co-Ni-Mn alloy with some given physicochemical properties.
Research objectives: 1. Forecasting the phase composition and chemical content of metals in the Co-Ni-Mn alloy using four phase-formation criteria, depending on the ratio of the component ions in the deposition solution.
2. Forecasting the ratio of the Co +2 :Ni +2 :Mn +2 content in the electrolyte corresponding to the desired phase composition for the specified operational properties.

Materials and methods
Properties of electrochemical alloys of the same chemical composition often do not coincide with the properties of pyrolytic alloys. First of all, this is due to the phase composition of galvanic sediments, To control the phase composition, it is necessary to have certain criteria. For binary mixed crystals, Hume-Rosery [1] suggested that metals could form mixed crystals if their atomic radii differ by no more than 15%. When estimating the distortion of the structure of the components crystal lattice, it is necessary to take into account the differences in the dimensions of the atoms, so Hume-Rosery [1] introduced size factor n d =d 1 /d 2 (d 1 , d 2 -the diameter of the first and second components) for solid solutions.
Later, Gordy [3] introduced energy factor n ε to take into account the electronic interaction. The phase formation criteria for binary electrochemical alloys were proposed by A.I. Zhikharev and I.G. Zhikhareva [4], and for ternary ones, I.G. Zhikhareva and V.V. Schmidt [5][6][7] forecasted the phases of solid solutions using four criteria: n s -entropic, n d -dimensional, n ε -energy factor, n о = n d + n ε -general factor.
Unlike the criteria proposed in [1][2] for cast alloys, the authors of [5] took into account the chemical content of the components in the electrolyte and alloy in calculating n s and n d , and in n s , the correction for the mixing entropy was additionally introduced. Thus, the calculation of n s in the research was carried out using formulas [8]: where U 1 , U al -the relative ionization potential of a metal and an alloy win relation to the ionization potential of hydrogen [8].
n ε characterizes the possibility of a redistribution of the electrons of the outer shells and a change in the configuration of the electron shells [8].
General factor ε n n n d о + = (4), where n о characterizes the difference in the geometric dimensions of atoms and the density of compound (n d ) and the distortion of the electron shells (n ε ).
The main task in forecasting the Co-Ni-Mn phase composition was to obtain a coating with a maximum α-Со phase content and a minimum α-Mn phase content. For this, the ratio of the metal ions in the electrolyte was forecasted at first, at which the maximum extent of the α-Co phase boundaries is expected. The widest range of the existence of the HCP phase of Co is expected at a ratio of Co +2 :Ni +2 :Mn +2 = 0.6:0.3:0. 1 The phase composition of electrodeposited Co-Ni-Mn alloys was determined using equations (1) -(4). The calculation takes into account that alloys based on cobalt have a unique feature since Co is able to be in the alloy in the form of a hexagonal close-packed phase (hcp) α-Co, face-centered cubic (fcc) -β-Co) or a mixture of α-Co + β-Co [9-10], i.e. metal Co can be considered as a pseudoalloy. Theoretically, the type of phase can only be determined by comparing the work of nucleation (twodimensional or three-dimensional) phase. According to theoretical calculations, the α-Co phase is the main phase; the FCC-phase of Co is metastable.
The formation of solid solutions is indicated by values n s = 0.992 ÷ 1.001; and the appearance of three-phase coatings is indicated by higher negative values of n d and n о ( Table 1).
Comparison of the calculated dependences of the phase formation criteria on the molar fraction of Co in the Co-Ni-Mn alloy with a polynomial dependence (Fig. 1) showed their satisfactory agreement. The boundaries of the phase homogeneity of a solid solution can be judged from the kinks in the dependences n i -y. Both the calculation data (Table 1) and the approximating curves indicate that in the Co-Ni-Mn alloys (when y Co = 0.654-0.704), the formation of two phases α-Co, α-Mn is possible.
As is seen from Fig. 1, phase-formation criteria n s , n d and n о give practically identical boundary conditions for the existence of α-Co and α-Mn phases. This indicates the prevailing influence of the chemical bond, geometric dimensions and density of matter on the limits of existence of these phases in comparison with redistribution of electronic shells and distortion of their configuration (n ε ). Thus, a model forecast of the Co-Ni-Mn ternary alloy was carried out at a certain ratio of metal ions in the electrolyte according to four phase-formation criteria. The boundaries of the homogeneity of the calculated phases were determined.
The second task of the theoretical research was to determine the phase and chemical composition of the Co-Ni-Mn alloy, which corresponds to specified operational properties.
It is known that Co-Ni-Mn alloy, on the one hand, has magnetic properties, and on the other hand, it is a heterogeneous catalyst for the Fischer-Tropsch reaction.
To ensure high magnetic properties, the alloy should possess a hexagonal close-packed lattice and contain a small amount of α-Mn (2 ÷ 2.4 mass %). The alloys ( Table 1, №3 and №8) correspond to these conditions.

Conclusion
The theoretical calculation of the phase composition of the Co-Ni-Mn alloy allows us to forecast not only the phase composition depending on the chemical content of metals in the alloy and the ratio of their ions in the electrolyte, but also forecasts, in the first approximation, the chemical and phase composition corresponding to the specified operational properties.
To estimate the accuracy of the forecasted modeling of the phase composition, it is required to The relationship between the predicted structural characteristics and the given magnetic properties is described in the publication "Structure and magnetic properties of Co-Ni-Mn alloy coatings (part 2)" by Shmidt V.V., et al.