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THERMAL STABILITY OF ALLOY STRUCTURE ON Ni 3 Al BASIS AND ITS APPLICATION IN ROTOR BLADES OF SMALL-SIZE TURBINE ENGINES (on material of the journal Metals 3, 2003)

 

2003. K.B.Povarova1, N.K.Kazanskaja1, V.P.Buntushkin2, V.G.Kostogryz3, V.G.Baharev3, V.I.Mironov3, O.A.Bazyleva2, A.A.Drozdov1, I.O.Bannyh1

 

1Institute of Metallurgy and Materials technologies named after A.A.Baykov of the Russian Academy of Science, Moscow

E-mail: povarova@ultra.imet.ac.ru

2Federal State Unitary Enterprise Russia Research Institute of Air Materials, Moscow

3Open Join-Stock Company Omsk Engine Design Bureau

 

Researches are executed at financial support of the Russian Federal Property Fund (grant 03-03-32463).

 

The comparative estimation of rotor blades structures stability from the VKNA-4U Mono experimental alloy and the GS6U serial alloy after operating time on the engine for service life of 480 cycles and 5000 h is carried out with the purpose of definition of their serviceability. Monocrystal rotor blades from the VKNA-4U Mono experimental alloy on the basis of y' - Ni3Al intermetallic save serviceability in conditions of a long-period operation with overtemperatures at least up to 1150. On a structure the experimental alloy is close to L-y'+y complex alloyed eutectic and can practically be considered as the natural composite self-organizing at crystallization.

The nozzle and rotor blades of state-of-the-art gas-turbine engines (GTD) are manufactured from complex alloyed heat resisting nickel super alloys representing solid solutions on the basis of nickel with GCK-structure, hardened by secondary extractions '-phase being a solid solution on the basis of the Ni3Al intermetallic with ordered crystal GCK-structure (L12). High-temperature strength both foreign, and domestic nickel super alloys such as GS (GS6, GS26, GS32, etc.) determined by a large volume fraction (up to 40 - 60 vol.%) and high dispersivity of secondary extractions of Ni3Al y'-phase, formed at an ageing of the supersaturated solid solution on nickel basis and having the cubic form, which period of a crystal lattice in comparison with a Ni-matrix differs on 0,3-0,5% [1]. The ceiling of operating temperatures of these alloys (1050-1100C) is determined by their loss of strength at the specified temperatures first of all because of decreasing of a volume fraction of secondary extractions of Ni3Al y'-phase owing to increase of its solubility in y-Ni-matrix, and also coarsing of corpuscles of y'-phase. One of the most perspective directions of activities with the purpose of increase of operating temperatures of "hot" details of a turbine engine, and first of all blades, is development of structural high-temperature alloys on the basis of Ni3Al y'-phase of series VKNA (such as VKNA-4U, VKNA-1V, etc.). Alloys have higher melting point, are easier and capable to work in oxidizing atmospheres without coatings due to the higher contents in them of aluminium in comparison with conventional nickel super alloys [2 - 6]. On basic performances of mechanical properties the new intermetallic casting (and capable to deformation) alloys meet the requirements, made in the industry to alloys for a casting of such details of a turbine engine, as nozzle guide vanes, adjustable nozzle flaps and elements of combustion chambers, fuel afterburnings, and also rotor blades of some turbine engines [6,7]. The present work is devoted to a comparative estimation of stability of rotor blades structures of the TVD-20 turbine, manufactured of VKNA-4U Mono experimental alloy and the GS6U serial alloy, after operating time on the engine for service life of 4800 cycles with the purpose of their service-ability estimation.

Materials and research methods. A structure of VKNA-4U Mono experimental alloy, mass percent: l 8,5 - 9,5; Cr 4,5 - 5,5; W <= 2,5; <= 5,0; Co 3,5 - 4,5; Ti 0,6 - 1,2; C <= 0,02. It has two-phase structure ~ 90 vol.% y '-Ni3Al+10 vol.% y-Ni [7]. Due to close values of the periods of crystal lattice of both phases at obtaining alloy by a method of an oriented crystallization there is no disturbance of crystallite increase and onset of "parasitic" equiaxial grains, i.e. in result it is the alloy with a monocrystalline structure of the VKNA-4U ono mark which plasticity at room temperature reaches 03V4 =30%, and strength - 1340 P. For endurance rig tests in the OEDB, the rotor blades of the TVD - 20 turbine have been manufactured from the VKNA-4U Mono alloy by method of high-gradient oriented crystallization on the UBNK installation on production process developed in VIAM. In the same turbines the blades from the GS6U serial alloy (casting, equiaxial grain, hardening of boundary lines of grains by carbides) have been tested.

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Figs. 1. A microstructure of the VKNA-4U Mono experimental alloy in initial state

For an estimation of serviceability of monocrystalline rotor blades from the VKNA-4U Mono experimental alloy on the basis of y'-Ni3Al intermetallic, the comparative research of microstructures of blade airfoil portion from the experimental and serial alloys which have worked in the TVD-20 turbine during 5000 h and for 4800 cycles (more than 5000 h) is carried out. Microstructures were investigated by method of the raster microscopy on "LEO-430" and "Hitachi S-2300" devices at increases up to 10000.

Results and their discussion. Microstructure of blades from the VKNA-4U Mono experimental alloy. In initial state the alloy is monocrystalline two-phase (y'-y) alloy, boundary lines of grains are absent (Figs. 1, a, b). The basis of this alloy is y'-Ni31 alloyed intermetallic (up to 90 vol. %). In y'-matrix the intermittent layers of tough plastic structural component - -phases (solid solution on Ni basis) - forming a grid are distributed in regular intervals. There are found rare and in regular intervals located on section segments of y'-phase, free from -layers. All this indicates that alloy on structure is close to structure of L=y'+y complex alloyed eutectic and can practically be observed as the natural composite which is self-organized at crystallization. Free from -actuations segments of y'-phase represent primary allocation of y'-Ni3Al. The single heat treatment, which this foundry material requires, is the shortterm high-temperature annealing for stress removals arising at crystallization.

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Figs. 2. Microstructure of trailing edge of hot section of rotor blade airfoil portion from the VKNA-4U Mono experimental alloy after operating time during 5000 h (I) and 4800 cycles (II).

 

After operating time of 5000 h (Figs. 2, I) and 4800 cycles (Figs. 2, II) it is possible to note following prominent features of a trailing edge structure of hot section of blade airfoil portion from experimental alloy: this alloy has kept uniform two-phase (+)-structure; volume fraction of y'-phase practically has not changed; new phases are not detected; in intermittent layers of -phase the disperse secondary grains of y'-phase were formed which were extracted from the supersaturated -solid solution, on nickel basis under effect of high temperatures and stress (the formed structure is similar to structure of conventional nickel high-temperature alloys); there was the integration of all structural components (Tab. 1) testifying about a volume, that the material of airfoil portion of a blade at operating time subjected to an overheating at least up to 1150.

Microstructure of blades from the GS6U serial alloy. In initial state the alloy has a coarse-grain structure (Figs. 3, a, ) and is multiphase: large corpuscles of irregular-shaped carbides and their accumulations are visible at the grain boundaries; dendrites are two-phase. The morphology of y'-phase in dendrite axes is cubic (volume fraction of phase ~60%), in interaxial segments the corpuscles of y'-phase are rounded and in 2-6 times larger.

After operating time during 5000 h and for 4800 cycles it is possible to note following prominent features of structure of a trailing edge of hot section of rotor blade airfoil portion from the serial alloy (Figs. 4, accordingly I and II): the alloy structure remains uniform geterophase (+'+carbides); for 4800 cycles the volume fraction of y'-phase (<20 vol.%) sharply decreases, its allocation are saved mainly near to boundary lines of grains stabilized by carbides and in the interdendrite spaces as intermittent layers in -matrix (solid solution on Ni basis) and practically disappear (1 - 7 vol.%) in dendrite axes.

Last structure change testifies, that during operating time the blades from the GS6U serial alloy were also exposed to an overheating at least up to 1150 that has called changes in structure of the serial alloy, conducting to significant or full loss of strength.

It is necessary to note, that in zone of root of the same blade (the root does not subject to an overheating) the changes of structure correspond to the usual changes inherent to the given alloy connected mainly with integration structural components (Figs. 5 a, ).

Thus, the lead metallographic research of hot zones of trailing edges of rotor blades of the TVD-20 engine, manufactured from the VKNA-4U Mono experimental alloy and the GSU serial alloy, in an initial state and after operating time on the engine during 5000 h and for service life of 4800 cycles allows to make the following conclu-sions about their serviceability.

1. Monocrystal rotor blades from the VKNA-4U Mono experimental alloy on the basis of y'-Ni3Al intermetallic preserve serviceability in conditions of a long-period operation with overtemperatures at least up to 1150, that is provided high stability of the alloy structure formed at crystallization in result of eutectic transformation L+y'+. High-temperature strength is caused by conservation of a high volume fraction of y'-phase and practically poorly varying ratio of shares of y'-and -phases. Some possible decrease of high-temperature strength because of coarsing of both structural components is compensated by layers hardening of -phase by disperse secondary extractions of y'-phase due to what in alloy in addition there are sections with the structure which is inherent to heat resisting nickel alloys.

2. Structure changes of blades from the GS6U serial alloy in the same conditions show, that its structure is less stable. At overtemperatures the corpuscles strengthening y'-phase dissolve, its volume fraction decreases that conducts to a loss of strength of the serial alloy, decline of its serviceability.

Observable changes of structure of rotor blades from both alloys after operating time for 5000 h and 4800 cycles completely correlate with data on mechanical properties of the VKNA-4U intermetallic alloy and the GS26 production nickel alloy at lengthy tests on more short-term bases of 100 and 500 h (Figs. 6, Tab. 2) [8 - 10]. It is visible, that for nickel alloy a thermal interval of stability of heat resisting properties is limited in temperatures below 1100 that is caused by surface oxidation or structural changes in metal volume. The VKNA-4U Mono intermetallic alloy is efficient at least up to 1150-1200, as it loses strength only owing to inevitable development of diffusive processes at temperature increase both in y'- and y-phases, and on interphase boundaries, and connected with it some coarsing of structural components. However in a certain extent it is compensated by hardening of -layers by secondary extractions of y'-phase in that temperature interval in which in alloys such as GS6 and GS26 there is a dissolution of strengthening extractions of y'-phase, caused by strong temperature dependence of its solubility in a -matrix.

The property complex of the VKNA-4U Mono certificate alloy received by high-gradient oriented crystallization is caused by unique high thermal stability of a structural - phase condition. It allows to recommend this alloy not only for cast nozzle guide vanes, but also for rotor blades of the air turbine engines working without protective coatings at temperatures up to 1200 in combustion products of aviation fuel.

 

Conclusions. Replacement of industrial production nickel alloys such as GS6U or GS26 by the VKNA-4U Mono intermetallic alloy provides with a capability of increase of permissible operation temperatures on rotor blades and nozzle guide vanes of perspective air turbine engines on 50-100, decrease of their mass on 7-10%, improvement of heat-resistance and, as consequence, increase of service life in 2-3 times. In conditions of more economic alloying it is possible to lower the consumption of such expensive deficient metals, as tungsten, cobalt, etc. One of the main advantages of intermetallic alloys such as VKNA-4U Mono is that alloys of this class do not require a strengthening heat treatment, and in this connection they are more simple and reliable, and also less labour-consuming in manufacturing.