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The paper examines how the structure and phase composition of Ti-Al-B alloys evolve at various consolidation stages in the composite powder mechanochemical synthesis and subsequent sintering under pressure. Two powder alloys with different boron content are studied. The amount of aluminum in both initial powder mixtures is the same and corresponds to TiAl. The content of boron is selected so as to form an aluminide matrix with 10 and 25 vol.% borides. It is established that phases form in the mechanochemical synthesis in the following sequence: Ti + Al → Ti (Al) → TiAl 3 and Ti 3 Al → TiAl. Titanium borides are formed simultaneously with TiO 2 and TiAl or after them, which confirms that these processes are interrelated. The mechanochemical synthesis for hours in a planetary-ball mill results in the formation of micron particles that have agglomerated or conglomerated (sometimes layered) composite structures. X-ray analysis is used to study the phase evolution of Ti-Al-and Ti-Al-B alloys. It is shown that the presence of boron in mechanical alloying reduces the degree of amorphization and promotes the formation of fine crystalline structure. In addition, the presence of boron prevents the formation of metal oxides and a number of intermetallides. It is established that the sintered dispersion-hardened Ti-Al-B material consists of an aluminide matrix with micron and submicron inclusions of borides. The increase in boron content leads to a higher amount of boride inclusions. As a result, the distance between them decreases and thus microhardness increases.

We have studied the structure and mechanical properties of eutectic alloys β-NiAl + γ-Re of the ternary system Ni − Al − Re. We have established that the best combination of mechanical characteristics, determined by local loading with a rigid indentor, is exhibited by the alloy containing 2.5 at.% Re, the structure of which consists of the eutectic β-NiAl + γ-Re. Rhenium inclusions can inhibit movement of cracks in the material, and also can play the role of traps for cracks. Brittle intercrystallite fracture is characteristic of the alloy consisting of one-phase intermetallic NiAl. Mixed fracture is typical of the eutectic alloy β-NiAl + γ-Re, with transcrystallite cleavage predominating. We have shown that plastic interlayers of a rhenium phase within the microstructure increase the crack resistance of a detonation coating made from eutectic alloys β-NiAl + γ-Re.Nickel aluminide NiAl, having (along with high scaling resistance (up to 1400°C)) a density 1.35 times lower and a thermal conductivity 4 times higher than nickel superalloys, is a promising compound to use as a basis for developing new structural materials and protective coatings for gas turbine engines for aviation [1][2][3]. Major factor holding back widespread use of materials based on nickel aluminide is lack of plasticity at room temperature and low crack resistance.Progress in improvement of the class of materials under consideration is mainly connected with development of the optimal composite structure by introducing alloying components into their composition: Cr, Mo, Nb, V, Ti, Ta, etc.[4-6]. One such promising component is rhenium, which as we know [9-11] has an unusual combination of unique characteristics: high modulus of elasticity, high recrystallization temperature, high fatigue strength. From experience using rhenium in superalloys, we know [3,4] that introducing rhenium into a material may simultaneously increase both the strength and the plasticity. Moreover, rhenium makes it possible to form eutectic structures that often have more fortunate combination of characteristics, compared with other structural composites. A slight amount of rhenium is contained in an NiAl solid solution (0.2 at.%) [9].Our previous investigations [11] allowed us to determine the limits of the region of existence for eutectic alloys β-NiAl + γ-Re that are two-phase in crystallization. From this region, we have selected alloys containing from 0.2 to 3.0 at.% rhenium in order to assess the mechanical properties of β-NiAl + γ-Re eutectic alloys and coatings sprayed from powders, obtained from the ingots by mechanical crushing. EXPERIMENTAL SECTIONThe alloys were melted in an electric arc furnace with a nonconsumable tungsten electrode on a water-cooled copper hearth in a purified argon atmosphere. The elemental distribution in the structure was determined using a Camebax SX-50 electron probe microanalyzer (France). In order to assess the mechanical properties of the materials, we used the method of local loading by a rigid indentor [9], including plotting of the...

621.793We have studied phase formation in detonation coatings sprayed from Ti − 50 at.% Al powders. The powders of the alloy were obtained by various methods: crushing an ingot and mechanical alloying of Ti and Al. Using polyphase nanostructural materials activated by mechanical alloying makes the process of phase formation in the gas-thermal sprayed coatings based on them more general-purpose and controlled due to the more active and more subtle reaction of the material with the gaseous atmosphere.We have shown that from mechanically alloyed Ti − 50 at.% Al powder, using the detonation-gas spraying method we can consolidate a coating based on Al 2 TiO 5 by oxidizing action of the working gas on the powder and also a coating based on titanium aluminides with TiN inclusions by nitriding action.The phase composition of the cast microstructural γ-TiAl powder is inherited by the coating.

The paper examines and compares the properties of Al 2 O 3 coatings sprayed using two methods: arc plasma spraying (APS) of micron powders (average particle size is 45 µm) and suspension plasma spraying (SPS) (average particle size is 2.9 µm). A system for feeding suspension into plasma spray is developed and fabricated. It is established that SPS coatings contain finer structural components than APS. This improves their mechanical characteristics such as microhardness and indentation fracture toughness.The use of submicron and nanosized powder materials to produce coatings with unique properties is one of the promising areas of materials science and gas spraying engineering [1][2][3][4][5][6]. The research in this area was initiated in 1996-97 when the first nanostructured gas-sprayed coatings were obtained from preliminary conglomerated nanosized powders [2]. The improved mechanical and tribological properties of gas-sprayed coatings from nanostructured ceramics are reported in [3][4][5][6].The spraying of fine powders using conventional gas-spraying methods is associated with process drawbacks: low bulk density and yield, clumping, and high hygroscopicity. There are methods to deal with these drawbacks to some extent. They involve calcination and preliminary drying of powders immediately before spraying, use of heated, screw, and vortex feeders, use of flexible cords of fine powders, conglomeration of powders, etc. However, all these methods require additional operations or complex, expensive equipment or decrease the effectiveness of the process.The use of conglomerated powders is one of the most efficient ways to deposit a coating based on fine powders. However, the production of such powders is a complex process commonly including many operations: prepare a charge from a fine starting powder and binder; dry and simultaneously mix the charge; grind, classify, and separate the required grain size; dispose waste or reconglomerate it. This affects the cost of powders and restricts the application of the process. In addition, binders pollute the initial material and sometimes substantially deteriorate the characteristics of coatings. The major drawback of conglomerated submicron and nanosized powders is associated with their behavior when sprayed. Conglomerated powder completely or partially dissolves in a hightemperature plasma jet and its fine components coagulate and form a drop of tens of micrometers. Hence, the unique properties peculiar to the initial material deteriorate.

The influence of the magnetic field on the dislocation structure of intermetallic NiAl-Re alloy is studied and is experimentally confirmed for the first time. Active displacement of dislocations is established. Rhenium inclusions perform stop functions in these conditions. In addition, Ni atoms move to the surface of the alloy and an Al-rich internal layer forms under the magnetic gradient. These results indicate that the new atomic configuration forms by diffusion due to jumps of Ni atoms to adjacent Ni vacancies in intermetallic NiAl compound.

A phenomenological model is proposed to show the dependence of the heat conductivity of a porous coating on the pore size using experimental data on ZrO 2 + 7% Y 2 O 3 coatings. To obtain plasmasprayed coatings with different structures, powders with different properties are used. The concept of critical pore size at a given porosity is proposed. It is the maximum pore size at which the heat conductivity depends not only on the total porosity but also on the pore size, which is especially important for materials with submicron and nanosized structural components. Since the free path of phonons scattered by pores depends on the distance between them, the concept of critical distance between pores is introduced. It is the maximum distance beyond which the heat conductivity depends on the integral porosity alone.Ceramics based on stabilized zirconia are used as a thermal-barrier coating (TBC) to protect parts of gas turbines [1-3]. One of the most efficient methods to increase the performance of TBCs is to decrease their heat conductivity. There are many studies on the porosity dependence of heat conductivity of coatings, but they generally take into account only the volume fraction of pores, disregarding their size, shape, and branching. The morphology of pores and cracks occurring during deposition also has a strong effect [4].A real coating deposited by air plasma spraying is inhomogeneous. It generally consists of several layers successively formed of a powder whose particles have different sizes and, thus, have different energy statestemperature and velocity. Thus, the coating forms when its portions are in different thermal states and differently interact with the medium in which coatings are deposited and cooled down. As a result, the coating contains microvoids (pores, cracks) and many interfaces. These factors affect the heat conductivity of the coating. Moreover, its heat conductivity is also affected by the gas in microvoids. Certainly, each factor influences the heat conductivity to a different degree, which is primarily dependent on the type of material and the dimensions and shape of structural elements (grain boundaries and pores). In real production of porous materials (coatings in our case), these factors can be controlled by varying the size and morphology of particles of the starting powder [5].The heat conductivity λ of a dielectric material is given bywhere c is heat capacity; ν is the velocity of phonons; l is the free path of phonons; and ρ is the density of the material.