PROPERTIES OF NICKEL-BASED SUPERALLOYS OF EQUIAXIAL CRYSTALLIZATION

Glotka O. A. Ph.D., Associate professor, Associate professor of the Department of Physical material science of the National University “Zaporozhye Polytechnic”, Zaporizhzhia, Ukraine, e-mail: glotka-alexander@ukr.net Olshanetskii V. Yu. Doctor of Technical Sciences, Professor, Head of the Department of Physical material science of the National University “Zaporozhye Polytechnic”, Zaporizhzhia, Ukraine, e-mail: olshan@zp.edu.ua


Introduction
The development of new and optimization of existing alloys for cast parts, namely, the most heavily loaded, such as the working and nozzle blades of a gas turbine engine, is a material science, design and technological task that requires a comprehensive solution [1][2][3][4]. For modern thermally stressed gas turbine engines, the above-mentioned complex-profile parts are made from multicomponent heat-resistant alloys based on nickel, cobalt and iron by the methods of equiaxed, directional or monocrystalline casting [5][6][7][8][9].
Recent developments have focused on the study of blade materials with a low content of expensive elements for aircraft engine building. One of the problems of this type of materials is to increase their strength properties. To increase the high-temperature strength, the alloys are alloyed with a high chromium content. However, a high chromium content can cause the appearance of topologically close-packed phases of the μ, σ type in the casting structure during the development process, which will lead to phase-structural instability and embrittlement of parts [10][11][12][13][14].
Strengthening by the γ′-phase ensures long-term preservation of the high temperature performance of alloys of this class in a wide temperature range, up to 1150 °C. Consequently, the most important role in the resistance to high-temperature creep superalloys nickelbased equiaxial crystallization belongs to such structuralphase characteristics as the period of the crystal lattices of the γ-and γ′-phases and their dimensional mismatch δ or γ/γ′-mismatch [15][16][17][18][19][20].
The aim of the work is to obtain predictive regression models, with the help of which, it is possible to adequately calculate the mechanical properties of the nickel-based superalloys, without carrying out preliminary experiments.

Material and research technique
For experimental and theoretical studies of temperature performance, a working sample of alloys was formed, consisting of well-known industrial nickel-based superalloys for equiaxed casting of domestic and foreign production, the following brands: ZhS6U, ZhS6K, VZhL12U, VZhL12E, B1900, IN 100, MAR M200, MAR M246, TRW NASA 6A, WAZ16, U500, U700, ZhS3DK, ZhS3LS, VH4L, ChS88U, ChS104, RENE77, IN939, IN738LC, CM681, RENE220, NFP1916, ChS70S, CM939WELDABLE. The selection of alloys was made from the standpoint of a variety of chemical compositions (alloying systems), which, in terms of the content of the main elements, cover a wide range of alloying.
The obtained values were processed in the Microsoft Office software package in the EXCEL package using the least squares method to obtain correlation dependences of the "parameter-property" type with obtaining mathematical equations of regression models that optimally describe these dependences and plotting trend lines. The dependences have a sufficiently high coefficient of determination R2≥0.85 and are suitable for determining the temperature characteristics of the nickelbased superalloys.

Research results and discussion
Considering that the role in the high-temperature creep resistance of heat-resistant nickel alloys belongs to such a structural parameter as the dimensional mismatch δ (γ/γ′-mismatch), which depends on the alloying system, the urgent task is to obtain an optimal regression model for calculating this characteristic for based on the chemical composition of alloys of the class nickel-based superalloys equiaxed crystallization.
All components used for alloying nickel-based superalloys can be conditionally divided into three groups: dissolving mainly in the γ-solid solution (Co, Cr, Mo, W, Re), dissolving mainly in the-phase (Al, Ti, Ta, Hf ) and carbide-forming elements (Ti, Ta, Hf, Nb, V, W, Mo, Cr).
On the other hand, many elements can be included in the γ′-phase: Al, Ti, Nb, Cr, Co, Mo, W, V, etc. But their content in the γ′-phase and the effect on its amount in the structure are different. This effect is associated with the ability of the elements to form stable intermetallic phases of the Ni 3 Me type with nickel. Hence, it follows that the misfit and mechanical properties of alloys are influenced not only by the elements that belong to the γ′forming, but also those that are classified as γhard mortar hardeners [21 -24].
As a result of processing the experimental data and the above reasoning, for the first time a relation was proposed ∑ ∑ + + + + + (calibration factor 5 was determined empirically) for evaluating the mechanical properties, which takes into account the complex effect of the main components of the alloy. Since the dimensional mismatch of the lattice parameters is associated with the degree of concentration solid solution hardening of the γ-and γ′-phases, the efficiency of precipitation hardening of the alloy, the creep rate, and other properties, the Kγ′ ratio allows us to associate these properties with multicomponent systems [1][2][3][4][5][6][7][8][9][10].
It was found that the dimensional mismatch δ has parabolic dependences both at 20 and at 1000 (Fig. 1a, b) with the relations:  and has a tendency to a constant increase (Fig. 1c), since with an increase in the ratio, the number of elements forming the hardening phase increases. (c) -dependence of the short-term strength limit (σ В ) on the value of the ratio Кγ′ It is shown that at a test temperature of 1000 °C, the dependence of the limits of 100-and 1000-hour long-term strength on the value of the misfit (Fig. 2a, b) (Fig. 2b).
These dependences show that with an increase in the K γ′ coefficient, the long-term strength of the alloys increases in direct proportion, since the number of γ′forming elements increases, and, consequently, the volume of thephase in the alloy increases. It was found that the proposed ratio Kγ′ has a close correlation with the volume fraction of the γ′phase in equiaxed nickel-based superalloys (Fig. 3). All these dependences are linear with a positive slope and an error of no more than ±3.8 %. This behavior is explained by the fact that with an increase in Kγ′, the volumetric amount of the main strengthening elements increases, which form the γ′phase both at room temperature (Fig. 3a) and residual at elevated operating temperatures (Fig. 3a ), and consequently the limits of short-term (Fig. 1c) and longterm strength (Fig. 3) of alloys increase.
To eliminate the influence of volumetric diffusion processes at high temperatures, expensive heavy metals such as tungsten, molybdenum, rhenium and ruthenium are introduced into the composition of the nickel-based superalloys, which significantly increased the density of alloys, and, consequently, the weight of the finished product. It is known that the density ρ is closely correlated with the average atomic mass of the alloy Ac; therefore, the authors proposed a regression model obtained for multicomponent alloying systems of equiaxed nickelbased superalloys: with an error not exceeding ±1% (Fig. 4).
In fig. 4 shows the dependence of the specific density on the average atomic mass of alloys, which has a linear character, since an increase in the number of elements with a high atomic mass (refractory) will inevitably increase the density of equiaxed alloys. This tendency manifests itself as a consequence of the fact that elements with a high atomic mass belong to elements with a high melting point, which strengthen the γsolid solution and do not have a noticeable effect on the intermetallic hard-ening of alloys. The obtained regression models make it possible to predict the specific gravity, short-term and long-term strength limit, misfit according to the obtained ratio of Kγ′ of alloying elements in alloys and can be used both in the development of new equiaxed nickel-based superalloys and in the improvement of known industrial compositions within the graded composition.