K. B. Povarova

Preparation, Structure, and Properties of Ni3Al and NiAl Light Powder Alloys for Aerospace

New light super-heat-resistant powder Ni3Al and NiAl-based alloys (of the Ni-Al-Mo-B, Ni-Al-Fe-La, and Ni-Al-Y2O3 systems), as well as a new technology for preparing and processing them have been developed. The density of the alloys was 7.3-7.5 and ~6 g/cm 3, respectively. The Ni3Al sheets were used to prepare shields for combustion chambers in gas-turbine engines by roomtemperature deformation; the shields are intended for the long-term operation at 1100-1200°C and for the short-term use at 1300°C. The activated NiAl powders alloyed with Fe+La were used to produce sintered complex-shape articles, such as combustion stabilizers in a jet unit of combustion chamber of the gas-turbine installation, heat sources, etc. capable of operating at t≤1500°C under low mechanical stresses. At 1100, 1300, and 1500°C, the 100-h strength of the heat-resistant NiAl- (2-7.5) vol. % Y2O3 alloys subjected to directional recrystallization is 70, 35 and ≥10 MPa, respectively. The vanes, in which the length of recrystallized grain is smaller than the vane length by a factor of 1.5-2, were manufactured from these alloys.

Rare-earth metals (REMs) in nickel aluminide-based alloys: II. Effect of a REM on the phase composition of a multicomponent Ni3Al-based alloy

The hardening mechanisms are studied in the cast high-temperature next-generation materials that are based on the intermetallic compound Ni3Al and are low alloyed with refractory (W, Re, Mo, Cr) and reaction- and surface-active (REM, Ti, etc.) metals. The interaction of the main impurities (C, O, Si, S) with three characteristic representatives of the REM group (namely, Y, La, Ce), which can be used for alloying, is analyzed. The reported data on the behavior of some REMs in the alloys based on nickel monoaluminide NiAl are considered. The effect of the REMs on the phase compositions of real multicomponent semicommercial Ni3Al-based VKNA alloys produced by directional solidification is investigated, and the excess phases precipitating upon alloying are revealed. Alloying with refractory metals and REMs is shown to lead to the formation of nanophases that stabilize the dendritic or single-crystal structure of VKNA-type cast alloys and strengthen the interface boundaries in them.

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 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.

Structure and properties of W-Ni-Fe-Co heavy alloys compacted from nanopowders

Heavy tungsten alloy (HTA) W-Ni-Fe-Co nanopowders synthesized by a chemical-metallurgical method are used to produce a compacted material with a theoretical density and a grain size of 2.9–4.6 μm. Upon solid-phase sintering (SPS) at 1350–1450°C, the binder composition of the produced material coincides with the binder composition of the alloy fabricated by liquid-phase sintering (LPS) according to a traditional technology. The hardness of the material is 4400–3400 MPa (as compared to 1750 ± 50 and 2950 ± 50 MPa after LPS followed by a hardening treatment for a standard HTA), and its ultimate tensile strength after SPS is 950–1050 MPa (as in the case of the standard alloy after LPS). The melting temperature of the binder is 25–30°C lower than that of the traditional alloy.

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.

Rare-earth metals in nickel aluminide-based alloys: III. Structure and properties of multicomponent Ni3Al-based alloys

The possibility of increasing the life of heterophase cast light Ni3Al-based superalloys at temperatures higher than 0.8Tm of Ni3Al is studied when their directional structure is additionally stabilized by nanoprecipitates, which form upon additional alloying of these alloys by refractory and active metals, and using special methods for preparing and melting of an alloy charge. The effect of the method of introducing the main components and refractory reaction-active and surface-active alloying elements into Ni3Al-based cast superalloys, which are thermally stable natural composite materials of the eutectic type, on the structure-phase state and the life of these alloys is studied. When these alloys are melted, it is necessary to perform a set of measures to form particles of refractory oxide cores covered with the β-NiAl phase and, then, γ′prim-Ni3Al phase precipitates during solidification. The latter phase forms the outer shell of grain nuclei, which provides high thermal stability and hot strength of an intermetallic compound-based alloy. As a result, a modified structure that is stabilized by the nanoprecipitates of nickel and aluminum lanthanides and the nanoprecipitates of phases containing refractory metals is formed. This structure enhances the life of the alloy at 1000 °C by a factor of 1.8–2.5.