Material Origins of Accelerated Operational Wear of RD-33 Engine Blades

: The structural and strength analysis of the material used to construct such an important engine element as the turbine is of great significance, both at the design stage as well as during tests and expertises related to emergency situations. Bearing in mind the conditions above mentioned, the paper presents the results of research on the chemical composition, morphology and phased structure of the metallic construction material used to produce the blades of the high and low pressure turbine of the RD-33 jet engine, which is the propulsion unit of the MiG-29 aircraft. The data obtained as a result of the material tests of the blades allowed, on the basis of the analysis of chemical composition and phased structure, to determine the grade of the alloy used to construct the tested elements of the jet engine turbine. The structural stability of the material was found to be lower in comparison with engine operating conditions, which manifested itself as a clear decrease in the resistance properties of the blade material. The results obtained can be used as a basis for analyzing the life span of an object or a selection of material replacements, which enable to produce the analyzed engine element.


Introduction
The introduction should briefly place the study in a broad context and highlight why it is important. It should define the purpose of the work and its significance. The current state of the research field should be reviewed carefully and key publications cited. Please highlight controversial and diverging hypotheses when necessary. Finally, briefly mention the main aim of the work and highlight the principal conclusions. As far as possible, please keep the introduction comprehensible to scientists outside your particular field of research. References should be numbered in order of appearance and indicated by a numeral or numerals in square brackets, e.g., [1] or [2,3], or [4][5][6]. See the end of the document for further details on references. The basic parameter characterizing the ember-resistance of nickel-based super alloys intended for the construction of turbine blades of jet engines is their creep resistance. The increase of this parameter depending on the phased structure of the alloy, and in particular the share of γ/γ' phases, reinforcement carbides, Laves phases, etc., is obtained by modifying the chemical composition and morphology of the crystal structure of the blade matrix material. The same also happens in case of Russian superalloys. An increase in creep resistance of materials used to produce turbine blade was obtained by using additives Re and Ta, which, together with directional crystallization (ZhS-26) or monocrystalline crystallization (ZhS-32) allowed, in comparison with alloy ZhS-6U, to significantly increase this parameter (Table 1). Increase of the creep resistance of ZhS-32 alloy according to the authors [2.] allowed to increase the temperature of gases on the blades of the first stage of the turbine from 1263°C (for ZhS-6U-VI alloy [3.]) up to 1400°C. However, due to the acceptable period of use in service of RD-33 engine, prolonged temperature of the blade material exceeding 1100°C is not recommended.
The preservation of these exploitation regimes allowed to extend the acceptable period of use in service of the RD-33 engine blades made of superalloy ZhS-32 from 300 hours (ZhS-6) up to 1000 hours of exploitation. Nevertheless, one must keep in mind that prolonged exposure to hot exhaust gases can lead, especially if the protective coating is damaged, to structural changes within the blade material, thus leading to its destruction. Detailed analysis of the RD-33 engine damage showed that over 42% of the failures were caused by damage to the blades, of which the damages to blades of the first stage of the turbine accounted for over 60% [3.]. The main causes of damages are: gaseous corrosion at the leading edge, cracks at the leading and trailing edges and also thermo-mechanical damages ( Figure 2). The scope of application of alloys used to construct blades of the RD-33 engine, and in particular the creep resistance depending on temperature and duration of its long-term impact on the material, is best represented by the Larson-Miller parametric coordinates defined by the formula (1)   Higher permissible exploitation stress of the ZhS-32 alloy compared to ZhS-6U should be explained by a higher volume content of the phase γ' in the material structure, 67% and 57% respectively, as well as its higher thermal stability, caused by Re and Ta additives, which increase the temperature of the solidus from 1240°C (ZhS-6U) up to 1310°C (ZhS-32) [4.]. It is obvious that the long-drawn process of exploitation, linked with the high-temperature influence of exhaust gases, causes the grain coarsening and decrease of the phase γ' content in the blade material structure (   In connection with the above mentioned conditionalities / exploitation and strength rationale, for the purpose of research on the construction of the aircraft engine assemblies in the area of structural analysis, a thorough analysis of material degradation shall be carried out. It must take into account the tests of the grain structure and the chemical composition that determines the phased structure, which directly affects the operational properties, including the mechanical properties that determine the performance of such a complex structure as the modern turbine system. Incorrect use of a given[?] construction material, or incorrect exploitation or other factors causing the limits (resulting from strength calculations) of the applied materials to be exceeded, may lead to a reduction in strength. This, in turn, may lead to damage to the turbine assembly as well as the engine.
The above assumptions are the basis for the research undertaken to determine the causes of premature damages to the blades of the turbine of the RD-33 engine of the MiG-29 airplane, resulting in the necessity to dismount the engines and perform engine overhauls.

Sample preparation and test methodology
The mass loads, and in particular the heat loads of the turbine blades caused by the exhaust gas stream coming from the combustion chamber (1560 K) require cooling of this engine component. In the RD-33 motor under analysis, blades of both LPT and HPT are cooled by means of internal channels (Figure 6.), and blades of HPT, due to the higher gas temperature, have also channels for cooling the trailing edge.  This blade design poses serious problems in selecting a place to take the strength test specimens to properly perform the static tensile test. Therefore, taking into account the nature of the mechanical loads carried by the material of the component tested, it was assumed that the specimens for the static tensile tests will be cut along the blade feather of the LPT, i.e. in the direction of centrifugal forces, and in case of HPT from the lock material, i.e. the component subjected to the highest mass loads (  Nevertheless, the three-dimensional shape with variable cross-sections and curvatures made it impossible to clamp and thus to make the strength specimens with classical machines for cavity machining (edging method machining). Therefore, in order to preserve the assumed shape, dimensions and geometric quality of the surface of the specimens, as well as to minimize the impact of the preparation process on the structural changes of the blade material, the electro-discharge machining was used as the cutting method. All the samples were made by using the electro-discharge wire rod numerically controlled machine and assumed geometric dimensions have been respected.
In order to make microscopic observations to reveal the grain structure of the blade material, metallographic samples were prepared (Figure 8.) by mounting them in a conductive resin and then grinding them with abrasive papers of varying granulation in the range 1001000, and by final polishing with a diamond slurry of polishing powder, using the Struers polishing machine. Microscopic observations at magnifications of 10010000 times were made by using the Quanta 3D FEG (SEM/FIB) high resolution scanning microscope, which, in addition to grain structure morphology analysis, also enables, with the use of EDS/WDS/EBSD attachments, complex studies of chemical composition in microareas, as well as analysis of crystallographic orientation.
Since the microanalysis of chemical composition indirectly allows only for identification of the phased structure of the material under investigation, it was necessary to perform X-ray phase analysis (XRD). Identifying the type of crystal lattice and measuring the parameters of the elementary crystal cell allows, in a precise and unambiguous way, for identification of the phased structure of the material under investigation, on which functional properties, including strength properties depend. The phase X-ray analysis was carried out with ULTIMA IV Rigaku diffractometer by using a parallel beam. The measurements were carried out using a cobalt lamp with radiation length CoKα1=1.78892 Å with power 1600W, scanning step 0.02º and scanning speed 2º/min in the angular range 2Θ=20º-140º. The identification of the obtained reflections and the phased analysis were carried out on the basis of the crystallographic database PDF-4.
The evaluation of mechanical properties of the material of the tested blades was based on hardness, microhardness and the static tensile test. Due to the ultrafine grain structure of the tested material, in order to unequivocally confirm the correctness of the hardness measurements, the testing of this strength parameter was carried out using three measurement methods (HBW2.5/187.5, HV10 -Wolpert Wilson Testor and HRC -Rockwell PW 106 hardness meter), and then (using appropriate nomograms enabling comparison of hardness values measured by different methods), the results obtained were evaluated taking into account statistical analysis for the correctness of the data obtained. In order to further assess the homogeneity of the matrix structure, Vickers microhardness tests were performed, in various areas of the blades, using the SHIMADZU-DORERNST M hardness tester with a load of 25G.
A static tensile test, considered a destructive test, was carried out with the Instron 8501 Plus universal testing machine in accordance with PN-EN ISO 6892-1:2010 standard. Due to the nonstandardized, small dimensions of the strength specimens obtained (Figure 7.), in order to execute the static tensile test it was necessary to design and manufacture special holders enabling proper performance of the test ( Figure 9.).

The results of tests
The microscopic observations carried out at magnifications of 100-10000x ( Figure 10) showed that the TWC blade material is characterized by a multiphase structure typical for nickel-based heatresistant alloys, after the supersaturation and aging process, consisting of the original grains of the solid solution γ (Figure 10. a) with cuboidal ultrafine grain precipitates of the γ 'intermetallic phase ( Figure 10.b), as well as reinforcing carbides with the morphology of "Chinese script" (Figure 10.c). The available data shows that the blades of the turbine of the RD-33 engine are currently made of two types of alloys: monocrystalline ZhS-32 or directionally crystallized Zhs-26. Nevertheless, the microanalysis of the chemical composition of the multi-phase blade structure allowed to identify significant discrepancies, mainly in terms of the content of chromium and titanium, between the material tested and the alloys ZhS-32 and ZhS-26. The chemical composition of the analyzed material of the blades suggests that they are made of an alloy with a chemical composition corresponding to the heat-resistant nickel-based superalloy MAR-M 200 [9], which also corresponds to the chemical composition of the Russian ZhS-6U alloy [10] (Figure 11.). Since the maximum exploitation temperature of the alloy MAR-M 200 is (according to the CES Edu Pack database) in the range 815-983°C and the temperature of the exhaust gases before the rotor wreath of a high pressure turbine (HPT) of the RD-33 engine reaches the upper level of the temperature range of the material used for the blades of HPT, therefore, in order to protect the material of the blade core it was covered with a barrier coating (Figure 12.) created by using the diffusional aluminizing technology. This process has led to the constitution on the surface of the blade material of a three-layer heatresistant coating of NiAl intermetallic phase protecting the blade material from overheating.
Nevertheless, in spite of the material of the core of the HPT blade being protected against the temperature influence of exhaust gases, the barrier coating was damaged at the leading edge, which, due to temperature increase, led to changes in the grain structure of the blade at the overheating point. In addition to a small but noticeable growth of grains of the γ matrix from 80 up to 95μm, an anomalous selective growth and coagulation of precipitations of the γ' phase and an increase in the content of carbides in the structure are also observed, which may lead to a decrease of the creep resistance of the blade material. Although the thermally less loaded material of the LPT blade coated with a diffusion barrier layer made of aluminides is also constructed of a multi-phased structure γ + γ' + MC, no abnormal growth of the superstructure γ' and carbides was observed in material of the LPT blade. A microanalysis of the chemical composition suggests that a similar phased composition to that of material of the HPT blade is the result of a similar chemical composition, suggesting that the same alloy was used for material of the LPT blade as in case of the HPT blade (Figure 14). The conducted microscopic observations made it possible to measure, by means of microscopic image analysis, the volume content of individual phases in the structures of the analyzed blades, as well as to assess the average grain size of the identified structural components. From the collected data presented in Table 2, it can be concluded that the structure of the material of the blades is stable as long as it is protected by a barrier coating. The damage to the protective layer, as already indicated, leads to a noticeable growth of the matrix grains and tripling increase of the carbide volume content.
Microscopic observations have been confirmed by the XRD analysis. Based on the X-ray phase analysis measurements, by analyzing the positions of the obtained reflections, it was possible to unequivocally confirm the multi-phase structure of the blade material observed during microscopic observations ( Figure 15). The distribution of alloying elements suggested the occurrence of the three-phased structure of the material based on the solid solution of aluminum in nickel γ and of the areas of intermetallic phase γ' on the matrix of the Ni3Al superstructure and carbide precipitates, mainly Cr23C6, which was fully confirmed by the comparison analysis of obtained reflections[results?] with the data contained in PDF-4 database.
Additional evidence confirming the comparable structural-phase composition of the analyzed blades are microhardness measurements [7] carried out with the materials of the tested blades. These tests were carried out with the semi-automatic microhardness tester SHIMADZU-DORERNST M and the obtained results were subjected to statistical analysis which confirmed the structural homogeneity of the analyzed material areas, characterized by an average value of microhardness at a statistically uniform level within the range 410-430HV0.025 (Figure 16.). The observed structural changes were reflected in the strength parameters of the alloy used for the construction of the tested blades. As can be seen in Figure 17, where the hardness values obtained were marked with the standard deviation determined, the intersection points (marked with green color) of the hardnesses obtained are located on the curves representing the dependency between Brinell and Rockwell hardness as the function of Vickers hardness (curves HB=f(HV) and HRB=f(HV)), which clearly proves the correctness of the measurements carried out. It also proves despite the multiphase nature of the tested material, its high homogeneity caused by the refinement of the grain structure. However, the average value of hardness of the analyzed material after its exploitation has decreased to the level of 350HV10 compared to 450HV10, which is characteristic in case of the correct structure of the ZhS-6U alloy.
It should also be noted that the values of hardness (Figure 17.) and microhardness (Figure 16.) measured by the same Vickers method differ significantly, reaching 70 HV hardness units in extreme cases. However, the observed phenomenon, indentation size effect (ISE), is in accordance with the variable hardness law, which describes the influence of elastic deformation on the value of obtained results, i.e. the smaller the load, the greater the influence of elastic deformation and thus the greater the value of hardness. At this point it should be recalled that the load for microhardness measurements was 25G and for hardness measurements was 10kG.
The decrease in strength properties is confirmed by the results of the static tensile test ( Figure  18).   The decrease in the share of the γ 'phase in the material structure and there is growth, caused, in accordance with the Hall-Petch relationship, a decrease in the yield point from the original level of 770MPa for the ZhS-6U alloy to 678MPa of the material of the tested blade (Table 3).

Summary
The data obtained as a result of material tests of blades of rotors of the low and high pressure turbine of the RD-33 engine, taking into account the chemical composition and the phase structure corresponds to the Russian alloy, the type Zhs-6U, used in the early versions of the RD-33 engine. The deployment of blades made of the ZhS-26 and ZhS-32 alloys allowed to increase the temperature of exhaust gases before the turbine, thus increasing the engine performance. However, the conscious or unconscious use, in this exploitation conditions, of blades made of ZhS-6U alloy with noticeably lower structural stability, combined with local damage to the barrier protective coating, resulted in a distinctive, more than 10 per cent reduction of strength properties, primarily plasticity limit, which could ultimately lead to engine failure and even damage to the engine. The values of the determined strength parameters will be used in further work related to numerical analyses in the field of strength issues concerning the life and failure of engines manufactured by Russia, including the RD-33 engine. The importance and need for such works [3.], [11.], [12.] may be proved by the number of RD-33 engines dismounted from the airplanes in one of the air bases due to the damages in the high pressure turbine, which is shown in the diagram in Fig.  19. In addition, the analysis of structural changes caused by exploitation conditions can be helpful in the search for and selection of material substitutes, used to construct the engine element under analysis, which will meet increasingly higher, mainly temperature-related design requirements.