A. V. Antonova

Physicochemical approaches to designing NiAl-based alloys for high-temperature operation

The structure and properties of new-type materials based on light refractory nickel monoaluminide NiAl as a structural material are analytically reviewed. The choice of various alloying systems and structural-phase states of NiAl-based structural materials, including structural materials, is analyzed, and the choice of the processes of production of the materials is grounded, as applied to their composition.

Influence of rare-earth metals on the high-temperature strength of Ni3Al-based alloys

The influence of the content of reaction- and surface-active alloying elements (rare-earth metals (REMs)) and the method of their introduction into cast high-temperature γ′-Ni3Al-based intermetallic alloys, which are thermally stable natural eutectic composites, on their structure-phase state and the mechanical properties is studied. The life of low-alloy heterophase γ′ + γ cast high-temperature light Ni3Al-based alloys is shown can be increased at temperatures exceeding 0.8Tm (Tm is the melting temperature of Ni3Al) due to additional stabilization of the single-crystal structure of these alloys with submicron and nanometer-sized particles of the phases formed by refractory and active REMs. It is also shown that stage-by-stage fractional introduction of all components into alloys during vacuum induction melting with allowance for their reaction activities (most refractory metals are introduced in the form of low-melting-point master alloys at the first stage of vacuum induction melting, and lanthanum is introduced with a master alloy in the optimal contents of 0.1–2 wt % into the charge of VKNA-1V and VKNA-25 alloys at the final stage) leads to the formation of a modified structure stabilized by nanoprecipitates of nickel and aluminum lanthanides and the phases formed by refractory metals. This method increases the life of VKNV-1V-type alloys (0.5 wt % Re) at 1000–1200°C by a factor of ∼1.7 and that of VKNA-25-type alloys (1.2 wt % Re and Co) by a factor of ∼3.

Effect of the method of producing Ni3Al-based alloy single crystals on the macro- and microhomogeneity of component distribution, structure, and properties

The mechanisms of hardening heterophase Ni3Al-based cast alloys, which are thermally stable natural eutectic composites, are studied in the operating temperature range. The distribution of basic and alloying elements and impurities in macrovolumes along the height of a charge billet prepared in a vacuum induction furnace is analyzed. The effect of the deviation of the axis of intermetallic alloy single crystals from the 〈111〉 orientation on their mechanical properties is considered. It is shown that the deviation from this orientation within 2.5°–5.4° does not affect the short-term strength characteristics and substantially affects the ductility characteristics of the single crystals. The effect of the method of introducing basic components and refractory reaction- and surface-active alloying elements in the alloys on the structure-phase state of Ni3Al-based alloys and their service life is investigated.

Method for the production of TiAl-based layered composite billets containing a ductile constituent

Various schemes are studied to fabricate layered composite billets consisting of a Ti-48 at % Al alloy (phase composition γ-TiAl + α2-Ti3Al) and plates of a ductile metallic constituent made of a titanium alloy or niobium. Hot isostatic pressing followed by hot isothermal pressing are used to fabricate high-quality defectless multilayered billets of layered composite materials intended for high-reduction hot rolling to produce sheets.

Composite materials with an intermetallic matrix based on nickel and titanium monoaluminides hardened by oxide particles or fibers

Hardening phase/intermetallic matrix pairs are chosen for composite materials (CMs) intended for long-term high-temperature operation. These materials must have high and stable mechanical properties during a long time at high temperatures and loads. The compatibility of the physicochemical and mechanical properties of CM components is estimated to choose hardening phase/intermetallic matrix pairs in which the matrix is represented by an alloy based on NiAl or TiAl monoaluminide and the hardening phase is a refractory thermodynamically stable oxide of a Group III transition metal M2O3. The following two schemes are used to perform hardening of a CM with a matrix consisting of a TiAl or NiAl alloy by the most thermodynamically stable interstitial phases, i.e., refractory oxides, at temperatures higher than the operating temperature (Top) of the IMM. The first scheme consists in creating Al2O3/TiAl CMs hardened by continuous single-crystal sapphire fibers using the impregnation of a bundle of single-crystal fibers with a matrix melt followed by directional solidification. The TiAl-based matrix in these CMs serves as a binder connecting oxide phase fibers and preventing them from fracture due to high adhesion forces between oxide fibers and the matrix and a high fiber/matrix interface strength. In the second scheme, Y2O3/NiAl CMs are produced by powder metallurgy methods, which include severe deformation by extrusion accompanied by the formation of deformation texture and subsequent recrystallization annealing. In these CMs, disperse refractory oxide particles stabilize grain boundaries in a recrystallized matrix material and lead to the formation of directional structures with coarse elongated grains and a low fraction of transverse boundaries. Al2O3/TiAl CMs containing 20–25 vol % hardening single-crystal sapphire Al2O3 fibers can operate at temperatures of 1000–1050°C (∼0.7Tm of matrix), which is 250–300°C higher than the maximum values of Top of a TiAl-based matrix and 400-450°C higher than the maximum values of Top of a Ti-based matrix. An Y2O3/NiAl composite with a directionally recrystallized structure of a NiAl-based matrix hardened by 2.5 vol % Y{ia2}O3 particles can be recommended for operation at temperatures of 1400–1500°C ((0.8–0.9)Tm of matrix), which are higher by 100–400°C than not only Top but also Tm of Ni superalloys.

Effect of the method of production of TiAl-based layered composite billets containing a ductile constituent on the structure of rolled sheets

The effect of the method of production of TiAl-based layered composite billets intended for hot rolling on the structure of rolled sheets is studied. The billets consist of (γ-TiAl + α2-Ti3Al)/ductile metallic constituent (DMC) materials, where DMC is a titanium alloy (VT1-00 or VT-9) or niobium (Nb1). The most promising methods for the upsetting of the billets after compacting by hot isostatic pressing have been chosen, and a pilot technology for the production of high-quality defectless 11-layer 3-mm-thick sheets using hot rolling to a total reduction of ∼90% has been developed. Structural studies of the sheets indicate that the most promising materials for the DMC are a high-strength VT-9 alloy and, especially, high-melting-temperature niobium (Nb1). The strain resistance of Nb1 is comparable with that of a γ-TiAl-based superlight alloy, and diffusion processes between the layers of the related composite material are significantly slower than those in the layered composite materials with a DMC made of VT-9 and, especially, VT1-00.

Dendrite segregation in Ni3Al-based intermetallic single crystals alloyed with Cr, Mo, W, Ti, Co, and Re

The character of dendrite segregation in Ni3Al-based intermetallic VKNA-type alloy single crystals with a dendritic–cellular structure is studied. Distribution coefficient kd of an alloying element (AE) in the alloy during solidification kd = cd.a.I/c0 (c0 is the AE content in the alloy (liquid phase composition), cd.a.I is the AE content in primary dendrite arms of the alloy (in the solid phase)) and segregation coefficient ks = cd.a.I/ci.d (ci.d is the AE content in the interdendritic space) have been found. A comparative study of the dendrite segregation parameters in VKNA-nype Ni3Al-based intermetallic alloys and the well-known ZhS36-type nickel superalloy shows that the intermetallic alloys satisfy to the rule deduced for two- and three-component nickel-based superalloys: if an introduced AE increases the melting temperature of the basic metal, we have kd > 1 (Co, W, Re); if it decreases the melting temperature, we have kd < 1 (Al, Ti, Cr, Mo). Dendrite segregation coefficients ks are dependent on the proportion of the AE contents in the alloys. In nickel superalloys, the dendrite segregation of aluminum, tungsten, and rhenium is higher than that in the intermetallic alloys. The dendrite segregation coefficients of tungsten and rhenium is higher by a factor of 1.5–2 than that in the VKNA-type intermetallic alloys with a low content of refractory metals. This can be due to the retardation of diffusion of refractory metals in the solid phase of a nickel superalloy highly alloyed with these elements.

Effect of directional solidification on the structure and properties of Ni3Al-based alloy single crystals alloyed with W, Mo, Cr, and REM

The effect of the solidification gradient (G = 60 and 150°C/cm) at a solidification rate R = 10 mm/min on the structural parameters and the short- and long-term strength characteristics of blade-type single-crystal workpieces made of a heterophase γ′ + γ VKNA-1V-type γ′(Ni3Al)-based alloy with low contents of refractory metals is studied. The single crystals have a cellular-dendritic structure: dendrites are heterophase and consist of thin discontinuous nickel-based γ solid solution layers between γ′(Ni3Al)-matrix regions. Primary γ′-phase precipitates are located in the interdendritic space. An increase in solidification gradient G from 60 to 150°C/cm (by a factor of 2.5) at a solidification rate R = 10 mm/min leads to a decrease in the dendrite arm spacing by ∼1.5 times, the size of primary γ′-phase precipitates by 2.5–3 times, and the refinement of γ′ regions between γ layers in dendrite arms and at the periphery of dendrites by 2–3 times. The strength characteristics of the single crystals grown at G = 150°C/cm are higher than those of the single crystals grown at G = 60°C/cm by 10%. An increase in gradient G weakly affects the long-term strength of the single crystals. During long-term high-temperature tests under loading, secondary disperse γsec′ particles precipitate in the discontinuous γ solid solution layers forming inclusions in two-phase γ′ + γ dendrites, and the morphology of the γ layers changes (they become thicker and shorter). The 〈111〉 VKNA-1V alloy single crystals grown at G = 150°C/cm and R = 10 mm/min have a set of the required properties, namely, a high high-temperature strength over the entire temperature range, moderate high-temperature plasticity, and the absence of the plasticity drop at 800°C (which is characteristic of single crystals with other crystallographic orientations). These properties make 〈111〉 VKNA-1V alloy single crystals promising for working and nozzle gas turbine engine blades, including the blades in “blisk” assembly units.

Structure of a bimetallic strip produced by plasma spraying of a TiAl powder on a niobium sheet

Ti-48 at % Al alloy granules produced by centrifugal spraying are milled into a powder with a particle size of 40–100 μm, and are applied onto a niobium foil using plasma spraying in an argon atmosphere. The fabricated TiAl/Nb bimetallic strip consists of a 100-μm-thick niobium layer and a porous 300-to 400-μm-thick TiAl layer formed by flattened particles. Directly after the preparation of the bimetallic strip, the surface of the TiAl porous layer is rough. Vacuum annealing at 1000, 1100, and 1200°C for 0.5–1.5 h leads to intense pore healing. After deposition and annealing, the interlayer adhesion is strong. The preparation of TiAl granules and spraying of the powder is accompanied by aluminum depletion of the Ti-48 at % Al alloy to 42–45 at % and an increase in the fraction of the α2-Ti3Al phase in the deposited layer. The prepared material has a duplex structure. An intermediate diffuse layer characterized by a variable composition and thickness is formed at the interface. This layer consists of two solid solutions; one of them, which is formed at the TiAl layer, is an α2-Ti3Al-based solid solution of niobium and the other, which is formed at the niobium foil, is a niobium-based solid solution of titanium and aluminum.

Structural heat-resistant β-NiAl + γ′-Ni 3 Al alloys of the Ni–Al–Co system: I. Solidification and structure

When analyzing the ternary Ni–Al–M phase diagrams, where M is a group VI–VIII transition metal, we chose the Ni–Al–Co system, where the γ′ and γ phases are in equilibrium with the β phase, as a base for designing alloys with the following physicochemical properties: a moderate density (≤7.2 g/cm3) and satisfactory heat resistance at temperatures up to 1300°C. The structure formation in heterophase β + γ′ alloys during directional solidification is studied. It is found that, in contrast to cobalt-free β + γ′ alloys (where the γ′-Ni3Al aluminide forms according to the peritectic reaction L + β ⇄ γ′), the alloys with 8–10 at % Co studied in this work during directional solidification at 1370°C contain the degenerate eutectic L ⇄ β + γ. The transition from the β + γ field to the β + γ′ + γ field occurs in the temperature range 1323–1334°C, and the γ′ phase then forms according to the reaction β + γ ⇄ γ′.