Rare-earth metals in nickel aluminide-based alloys: III. Structure and properties of multicomponent Ni<Subscript>3</Subscript>Al-based alloys

The possibility of increasing the life of heterophase cast light Ni₃Al-based superalloys at temperatures higher than 0.8T _m 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.     

High-temperature deformation of single-crystal Ni<Subscript>3</Subscript>Al-Based alloys

Comparative studies of the structure and strength properties of VKNA-1V and VKNA-4U 〈001〉 single-crystal alloys were performed using tensile tests at 1200 and 1250°C. Dynamic recovery is the principal relaxation mechanism. At 1250°C, the strength properties of the VKNA-4U alloy are higher than those of the VKNA-1V alloy.     

Direct laser deposition of a single-crystal Ni<Subscript>3</Subscript>Al-based IC221W alloy

Single-crystal (SX) nickel aluminide alloys have potential for structural applications where high-temperature strength and oxidation resistance are required. In this work, SX deposits of the Ni₃Al-based IC221W alloy were produced on a SX Ni-base superalloy substrate by means of the laser-engineered net shaping (LENS) process. The microstructure of the deposits was characterized. The effects of processing parameters on the SX solidification in the melt pool and on the fabricability by LENS were investigated. A simple relationship between the ratio of the temperature gradient to the growth velocity and the processing parameters was derived, which can be used qualitatively to guide the proper selection of processing conditions to maintain the columnar dendritic growth during the laser deposition. On the basis of analyses and experiments, the effects of processing parameters on the susceptibility to stray grain formation and solidification cracking are discussed.     

Rare-earth metals (REMs) in nickel aluminide-based alloys: II. Effect of a REM on the phase composition of a multicomponent Ni<Subscript>3</Subscript>Al-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.     

Influence of smelting on the structure and properties of an Ni<Subscript>3</Subscript>Al intermetallic alloy

Ni₃Al intermetallic alloys are produced by open inductive smelting with two different fluxes and subsequent centrifugal casting. The structure and phase composition of the alloys are investigated. Alloy samples undergo tensile tests at room temperature and elevated temperatures. The dependence of the structure and mechanical properties of the alloys on the smelting conditions is established.     

Structure and phase composition of alloyed intermetallic compound Ni<Subscript>3</Subscript>Al after annealing and high-temperature creep

The structure and phase composition of a VKNA-25 alloy based on the Ni₃Al intermetallic compound, produced by directional solidification, and alloyed with rare-earth metals are studied in the following two states: after annealing at 1100°C and after creep at 1100°C.     

Phase analysis of multicomponent alloys using an Ni<Subscript>50</Subscript>Co<Subscript>25</Subscript>Mo<Subscript>25</Subscript> alloy as an example

method for phase analysis of three-component alloys is proposed. It is based on a pair interaction model and an experimental determination of the sign of pair chemical interaction energy and includes an electron-microscopic investigation of microstructures above and below the ordering–separation phase transition temperature for each diffusion couple. This method is used to study an Ni₅₀Co₂₅Mo₂₅ alloy. The phases that precipitate in this alloy over the entire heating temperature range, including the liquid state, are detected.     

Effect of hydrogen charging on dislocation behavior in Ni-Cr and Ni<Subscript>2</Subscript>Cr alloys

The effects of hydrogen charging on the dislocation behaviour in Ni-Cr binary alloys have been investigated by means of transmission electron microscope (TEM) observations using single-crystalline specimens. The deformation mode of Ni-Cr alloys in the absence of hydrogen is characterized by planar dislocations. However, hydrogen charging changed the dislocation configurations to promote curved dislocations, such as dislocation loops and dipoles. The hydrogen-affected dislocation configurations are enhanced with increasing Ni content and reducing Cr content. Weak-beam images show that the Shockley partials of the hydrogen-affected dislocations frequently constrict to make kinks and cross-slip, as if the dislocations were generated by a thermally activated process. The effect of hydrogen charging on superdislocations of a Ni₂Cr superstructure has been also investigated using an aged 70Ni-30Cr alloy. While the deformation mode in the Ni₂Cr superlattice is classified as five variants of superdislocation triplets and one variant of ordinary dislocations, the hydrogen charging has preferred the ordinary dislocations to the superdislocation triplets. The results suggest that the charged hydrogen changes the local plasticity to affect the deformation dynamics in Ni-Cr alloys, where the influence of hydrogen on the plasticity is sensitive to the Ni/Cr concentration and the symmetry of atomic arrangement.     

Microstructure and properties of Ni<Subscript>3</Subscript>Si alloyed with Cr fabricated by self-propagating high-temperature synthesis casting route

Ni₃Si alloys with 20, 30, and 40 wt pct Cr were fabricated by self-propagating high-temperature synthesis casting at 543 K. Thermite reaction (Cr₂O₃+5CrO₃+12Al=7Cr+6Al₂O₃) was used in Cr alloying. The method is simple and economical when used to prepare Ni₃Si-based alloys. The process is described in detail. The alloys were analyzed with X-ray diffraction (XRD) and scanning electron microscopy (SEM) with X-ray energy dispersive spectroscopy (EDS). The results showed the alloys mainly consisted of Ni₃Si and Ni₅Si₂ with dissolved Cr and Cr phases. Phases and microstructures of the alloys varied with Cr content. Microhardness, bending and compressive strength, and wear rate of the alloys were measured. Microhardness of the alloys was higher than that of Ni₃Si without Cr and increased with Cr content. Bending and compressive strength of the alloys were better than those of the Ni₃Si without Cr, and those of the alloy with 30 wt pct Cr were the highest. The wear rate of the alloys was lower than that of the Ni₃Si without Cr and decreased with Cr content.     

Ordering transformations in Ni<Subscript>75</Subscript>Mo<Subscript>15</Subscript>Mn<Subscript>10</Subscript> alloy

Effect of partial substitution of Mo in Ni₃Mo by Mn on its ordering behaviour is studied here using Transmission Electron Microscopy (TEM). The change in alloy composition does not change the nature of the short range ordered (SRO) state, which shows intensity maxima at {1½ 0} and equivalent positions. However, prolonged aging at 973 K leads to transformation of the Ni₇₅Mo₁₅Mn₁₀ alloy, studied here, almost entirely into the ordered D0₂₂ phase unlike in the case of the binary Ni-Mo alloy. Curved antiphase boundaries (APBs), with wetting of the interdomain D0₂₂ regions by the disordered phase, characterize the microstructure of the aged alloy. A small volume fraction of a sigma-like topologically close packed (TCP) phase that appears in the as quenched alloy seems to remain unaffected by ageing.     

High-temperature synthesis of the Ni<Subscript>3</Subscript>Al intermetallic compound under pressure

We investigate by experimental and numerical methods the high-temperature synthesis of the Ni₃Al intermetallic compound in the thermal explosion mode under the pressure of a powder mixture of nickel and aluminum of a stoichiometric composition. The calculated and experimental thermograms of self-propagating high-temperature synthesis are analyzed depending on the temperature of preliminary heating of the powder mixture and the magnitude of external pressure.     

Kinetics of Ni<Subscript>3</Subscript>S<Subscript>2</Subscript> sulfide dissolution in solutions of sulfuric and hydrochloric acids

The kinetics of Ni₃S₂ sulfide (heazlewoodite) dissolution in solutions of hydrochloric and sulfuric acids is studied. The process under study in the temperature range of 30–90°C is found to occur in a kinetic regime and is controlled by the corresponding chemical reactions of the Ni₃S₂ decomposition by solutions of inorganic acids (E _a = 67–92 kJ/mol, or 16–22 kcal/mol). The only exception is the Ni₃S₂-HCl system at elevated temperatures (60–90°C). In this case, the apparent activation energy decreases sharply to 8.8 kJ/mol (2.1 kcal/mol), which is explained by the catalytic effect of gaseous chlorine formed under these conditions. The studies performed are related to the physicochemical substantiation of the hydrometallurgical processing of the copper-nickel converter mattes produced in the industrial cycle of the Norilsk Mining Company.     

Coarsening behavior of Ni<Subscript>3</Subscript>Ga precipitates in Ni-Ga alloys: Dependence of microstructure and kinetics on volume fraction

Coarsening of Ni₃Ga precipitates in binary Ni-Ga alloys containing 14.89 and 18.31 at. pct Ga, aged at 628 °C, was investigated using transmission electron microscopy and magnetic analysis. The equilibrium volume fractions, f _e , of Ni₃Ga were 0.053 and 0.486, respectively. The rate constant for the kinetics of average particle growth decreases anomalously as f _e increases. Ni₃Ga precipitates with concave interfaces are observed at small sizes (45 to 55 nm) in the 14.89 pct Ga alloy. Elastic interactions lead to rapid coalescence in the 18.31 pct Ga alloy, producing rod-shaped precipitates; this shape has not been observed before in Ni-base alloys aged under stress-free conditions. The small concave-cuboidal precipitates and rapid coalescence are attributed to the large elastic self- and interaction energies, respectively, associated with the large precipitate-matrix lattice mismatch in this alloy system. Analysis of the present and previous data yields a Ni₃Ga/matrix interfacial-free energy of 3.91±1.12 mJ/m², which is the smallest value in all the binary Ni-base systems investigated to date. A reasonable value of the chemical diffusion coefficient is also obtained from the data.     

Study of the Ni<Subscript>41.3</Subscript>Ti<Subscript>38.7</Subscript>Nb<Subscript>20</Subscript> wide transformation hysteresis shape-memory alloy

The shape-memory characteristics in the Ni_41.3Ti_38.7Nb₂₀ alloy have been investigated by means of cryogenic tensile tests and differential scanning calorimetry measurement. The martensite start temperature M _s could be adjusted to around the liquid nitrogen temperature by controlling the cooling condition. The reverse transformation start temperature A′ _s rose to about 70 °C after the specimens were deformed to 16 pct at different temperatures, where the initial states of the specimens were pure austenite phase, martensite phase, or duplex phase. The shape-memory effect and the reverse transformation temperatures were studied on the specimens deformed at (M _s +30 °C). It was found that once the specimens deformed to 16 pct, a transformation hysteresis width around 200 °C could be attained and the shape recovery ratio could remain at about 50 pct. The Ni_41.3Ti_38.7Nb₂₀ alloy is a promising candidate for the cryogenic engineering applications around the liquid nitrogen temperature. The experimental results also indicated that the transformation temperature interval of the stress-induced martensite is smaller by about one order of magnitude than that of the thermal-induced martensite.