A. E. Morozov

Physicochemical laws of the interaction of nickel aluminides with alloying elements: I. Formation of nickel aluminide-based solid solutions

Physicochemical interactions in Ni-Al-M, where M is an alloying element (metal or metalloid), have been systematically analyzed. In most cases, the order of the solubilities of alloying elements (AEs) in the nickel aluminides β-NiAl and γ′-Ni3Al (with an ordered crystal lattice) and the character of substitution of AEs for the positions of Ni (Group VIIIA electronegative transition metal with the valence-electron configuration d8s2 and atomic radius rat = 0.124 nm) and/or Al (Group IIIA electropositive nontransition s2p1 metal with atomic radius rat = 0.143 nm) in these aluminides can be satisfactorily explained using the positions of these elements in the periodic system, the atomic radii of the isolated elements, and their outer electron configurations. The study of β-NiAl-based solid solutions demonstrates that the transition of the sp electrons of Al to the d band of transition metals changes the effective atomic radii of both Al and the transition metals. As a result, the ratio of their atomic radii can change. A correlation has been established between the ability of AE atoms to substitute for Ni and Al atoms in the crystal lattice of the intermetallic compound β-NiAl or γ′-Ni3Al and the effective atomic radii of the components of solid solutions. The use of the effective atomic radii of the solution components to estimate the atomic-size misfit makes it possible to take into account the atomic radii of pure components and their changes as a result of chemical interaction in a solid solution caused by electron redistribution. These changes depend on the position of an AE in the periodic table. This approach explains some contradictions that appear when the shape of a homogeneity area and the solubility of the AE in β-NiAl or γ′-Ni3Al are related to the atomic radii of pure components.

Rare-earth metals (REMs) in nickel aluminide-based alloys: I. Physicochemical laws of interaction in the Ni-Al-REM and NixAly-REM-AE (alloying element) systems

The data on the Ni-Al-R (R = REM Sc, Y, La, lanthanides) binary and ternary systems and the interactions of three rare-earth metals (yttrium, lanthanum, cerium) with the main alloying elements (Ti (Zr, Hf), Cr (Mo, W) that are introduced into Ni3Al-based VKNA alloys are analyzed. The binary aluminides of REMs in the Ni-Al-R ternary systems are shown to be in equilibrium with neither NiAl nor Ni3Al. The solid solution of aluminum in RNi5, which penetrates deep into these ternary systems, is the most stable phase in equilibrium with Ni3Al. In the NiAl (Ni3Al)-AE-R systems, REM precipitation (segregation) on various defects and interfaces in nickel aluminides is likely to be the most probable, and REMs are thought to interact with the most active impurities in real alloys (C, O, N), since REMs have a large atomic radius and, thus, are virtually undissolved in nickel, aluminum, and nickel aluminides.

NiAl Powder Alloys: II. Compacting of NiAl Powders Produced by Various Methods

The technological properties of granulated NiAl powders produced by gas spraying of melts and NiAl powders produced by calcium hydride reduction (CHR) of mixtures of nickel and aluminum oxides are compared. The possibilities of production of compact workpieces from these powders using hydrostatic pressing, hot pressing, hot isostatic pressing, and hot extrusion are estimated. To improve compressibility, preliminary milling and/or mechanical activation of the powders are proposed. The strength properties of NiAl rods with a diameter of 20 mm extruded from a temperature of 1100°C and made from the granulated powders are slightly higher than those made from the CHR powders. At temperatures higher than 800°C the properties becomes similar. Transition point td.b from the ductile to brittle state of samples made from powders sprayed in nitrogen and argon is 100–150°C higher than those made from the CHR powders. The difference in the mechanical properties is caused by the structural and chemical microheterogeneity of granules (microingots), which is inherited in the rods after hot deformation and annealing at 1200–1400°C and is (0.67–0.88)Tm NiAl (Tm is the melting point, K).

Physicochemical laws of the interaction of nickel aluminides with alloying elements: II. Interaction of nickel aluminides with alloying elements and/or interstitial phases

The Ni-Al-X (X is an interstitial element or phase) phase diagrams are analyzed to reveal systems that can be used as the basis for designing promising alloys and natural composites based on nickel aluminides reinforced by interstitial phases (natural composites I). The most thermally stable materials are shown to be heterophase alloys and composite materials (CMs) located in the eutectic-type (including degenerate eutectic) pseudobinary sections of ternary or multicomponent phase diagrams. They exhibit insignificant (or zero) dissolution of interstitial phases at operating temperatures and the absence of an intense interaction between CM components (natural composites II). Natural composites I based on the NiAl-or Ni3Al-interstitial phase alloys produced upon cooling from a melt can be reinforced by the refractory thermally stable rigid interstitial phases, namely, borides and carbides, that are present in pseudobinary sections in equilibrium with these nickel aluminides, since the elements forming these phases dissolve completely in matrix melts and the mutual solubility of these phases in the solid state is low. Such borides are TiB2 and HfB2 in equilibrium with β-NiAl, and such carbides are, e.g., TiC and HfC in equilibrium with β-NiAl and La2C3, NbC, and TaC in equilibrium with γ′-Ni3Al. Natural composites II should be produced using solid-phase methods (NiAl with AlN, Y2O3, Al2O3) or a combination of methods, where a refractory interstitial phase of the Al2O3 or Y2O3 type is solid and the intermetallic NiAl or Ni3Al matrix is liquid. NiAl-TiB2 (HfB2), NiAl-Al2O3 (Y2O3), and Ni3Al-La2C3 (NbC, TaC) composites are considered as examples.

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.

NiAl powder alloys: I. Production of NiAl powders

The influence of five methods of production of Ni50Al50 powder alloys on the processes occurring during reactive alloy formation of nickel monoaluminide during heating is considered. It is shown that, when powder mixtures obtained by agitation in ball mills and cladded composite powders with a low level of internal stresses are used, it is possible to produce a material with a nearly equilibrium phase composition in the course of reactive sintering due to an exothermic effect with the participation of a liquid phase (aluminum melt) in the reaction. The sintered material is porous and has an island structure. Mechanical alloying in a high-energy ball mill (attritor) results in the formation of layered Ni/Al granules with a developed interface and a high level of internal stresses and defects, which makes it possible to decrease the temperatures of initiation of reactive interaction by ∼300°C. This interaction develops in the solid phase according to a slow diffusive mechanism leading to the formation of intermediate nickel aluminides and hindering the achievement of equilibrium phase composition. The microingot granules (∼80 wt % particles 100–400 μm in size) produced by melt spraying by gases (N, Ar) has the composition of the melt, but grain boundaries are depleted of aluminum in comparison with the volume. The NiAl powders (∼90 wt % particles <40 μm in size) produced by combined hydride-calcium reduction are characterized by a highly homogeneous nickel and aluminum distribution, and their composition is close to equilibrium. These two types of powders are selected as the initial material for investigating the compacting and production of NiAl-based alloys.

Structure and hot hardness of RuAl-based alloys produced by reactive sintering using hot isostatic pressing

The structure and hot hardness (at temperatures up to 1100°C) of RuAl-based powder alloys with 1–3 at % Ni, Mo, Re, or Ru are studied. The alloys are produced by the reactive sintering of cold-compacted bars and subsequent threefold isostatic pressing with intermediate annealing at 1500°C performed after the first hot isostatic pressing. The samples have a residual pore content of 1–2.5 vol % and are characterized by a micrononuniform distribution of base and alloying elements. The alloys with refractory metals, such as Re, Mo, or Ru, are found to have the maximum hardness at all temperatures under study. At low temperatures, the effect is more substantial; the hardness of the Re-containing alloys exceeds that of the other alloys by a factor of 1.3–3.6. The increase in the hardness related to solid-solution alloying becomes more substantial owing to the microinhomogeneity of the sintered powder alloys and weakens because of microporosity. Recommendations that allow the uniformity of the distribution of the base and alloying elements to be increased are given.

Effect of the Preparation Methods of Me+Al Powder Blends on the Compaction Behavior, Structure, Phase Transformations, and Properties of the Compacted Aluminides of Transition Metals

The effect of the methods for preparing powder blend by conventional milling (Me+Al particles), attriting (Me/Al/ Me/Al composite particles), and plating of Me by Al (Me/Al composite particles) on the structure, internal stress level, and compactability of the powder blends as well as the structure and phase composition of the MeAl compacts was investigated. The Me+Al→MeAl exothermic reaction of these powders occurs at T≥650°C. The reaction sintering (RS) or hightemperature self-propagation synthesis (HTSPS) occurs through the formation of Al melt (liquidphase reaction) and lower-melting MeAl3, Me2Al3, Me3Al aluminides. An increase in the level of internal stresses (IS) upon attritting activates RS at lower temperatures and decreases the value of high-temperature exoeffect. This suppresses the HTSPS development. A large high-temperature exoeffect ensures the intensity and completeness of the reaction interaction, and the application of pressure upon RS or HTSPS provides a high, near-theoretical density of the compacted material.