Single-crystals of Co8.4Al9.4W1.9Ta alloy with macrosegregation of Al and W were directionally solidified with a flat front. Mini-specimens of different composition were cut at different heights of the single-crystals and tested for compression and oxidation at 900°C. It was found that W increases yield strength while Al retards oxidation.

The method of hot gas extrusion differs from other methods of metal pressure treatment in that the processed material is subjected to intense plastic deformation by extrusion under conditions of high isostatic pressure of inert gas and heating in the area of plastic deformation. The method makes it possible to process powder materials, while inevitably there is a need to control mechanical properties by controlling porosity and pressure inside the pores. The paper presents a method for calculating the pressure inside the pores from the minimum size of the closed pore. The pore pressure was evaluated on materials obtained by the consolidation of nickel nanopowders by hot gas extrusion. Quantitative processing of the material cross-section view images obtained by scanning electron microscopy showed that the minimum pore size is 220 nm. The formula for calculating the pore pressure is derived from the formula for calculating the critical pore radius. It takes into account the external impact pressure (the gas pressure during the extrusion process was equal to 400 MPa), the surface tension coefficient and the yield strength of nickel (at the extrusion start temperature of 910 °С), as well as the minimum pore radius. The proposed method for calculating the pressure inside a closed pore will allow us to evaluate the mechanical properties of materials obtained by various methods of powder metallurgy.

Negative creep of single crystals of two nickel-based superalloys has been studied. This phenomenon was observed for both alloys at temperatures of 980–1000°C and low or zero loading stresses. It is assumed that the main reason for negative creep is the formation of short-range order of atoms in the highly alloyed lattice of the matrix γ-phase. Additional factors influencing the magnitude and anisotropy of negative creep deformation can be the relaxation of residual stresses: at the microscopic level - misfit stresses between the γ-matrix and the strengthening γ′-precipitates, and at the mesoscopic level - dendritic stresses between the dendritic axes and interdendritic regions.

High-alloyed nickel-based superalloys used for casting of single crystal blades of aircraft gas-turbine engines have to be subjected to a long-term high-temperature homogenization annealing for dissolution of nonequilibrium γ′-eutectics and smoothing of dendritic segregation of alloying elements. Single crystal nickel-based superalloy of the 3-rd generation CMSX-10 developed by Cannon-Muskegon corporation for casting highly loaded uncooled single crystal blades of the first stage of a high pressure turbine of an aviation gas turbine engine Trent 800 belongs to such alloys. In the present work, the processes occurring during high-temperature homogenization annealing of this alloy have been investigated. It is shown that the kinetics of dissolution of nonequilibrium γ′-eutectics strongly depends on temperature: in the range of 1340 – 1360 °C the dissolution rate increases by about an order of magnitude with increasing temperature by 10 °C. Two types of homogenization annealing with a stepwise increase of temperature proposed for CMSX-10 by Cannon-Muskegon have been investigated: a long 45-hour annealing with 10 steps of temperature increase and a shortened 20-hour annealing with 6 temperature steps. The change in the solidus temperature of the alloy during the homogenization was studied by the method of measurement of the specific electrical resistance. It is shown, that during homogenization annealing of the both types the nonequilibrium eutectics is completely dissolved, and a longer 45-hour annealing provides a deeper chemical homogenization of the alloy. This is especially important for such alloying element as rhenium, which has a high segregation coefficient and very slow diffusion mobility in nickel. Deeper homogenization provides better thermal stability of the alloy microstructure and consequently higher long-term creep strength. A negative effect of high-temperature homogenization is increased porosity resulting from the dissolution of γ′-eutectics. Homogenization porosity can be reduced by optimizing the alloy composition and reducing the fraction of nonequilibrium γ′-eutectics.

Microporosity is a dangerous defect observed in single-crystal gas turbine blades cast from nickel-based superalloys (NBSs). The volume fraction of porosity in single-crystal alloys does not exceed several tenths of a percent; however, it can result in shortening of the lifetime of the material of gas turbine blades under fatigue loading by many times. This work presents the results of determination of the volume fraction of porosity in single-crystal NBSs. Single crystals of a CMSX-4 NBS obtained according to the industrial technology of manufacturing of single-crystal blades are used as a test object. It is found that the methods applied, except for optical microscopy, have accuracy sufficient for measuring the volume fraction of microporosity of about 0.2 vol

Based on the results of performed thermophysical measurements and available experimental data for adiabatic elastic stiffnesses, isothermal elastic characteristics of the single crystal nickel-based superalloy CMSX-4 have been calculated for a wide temperature range, from room temperature to 1300°C. According to the results obtained, the adiabatic and isothermal values of such elastic characteristics as elastic stiffnesses c_11 , c_12 and bulk modulus of elasticity B differ significantly at high temperatures. The reasons for this are significant changes in the thermophysical properties of the alloy with temperature and a temperature increase of its Poisson’s ratio approaching the limiting value for cubic crystals, equal to 0.5. It is shown that the use of adiabatic elastic constants instead of isothermal ones in engineering calculations affects the relationship between the volumetric strain and hydrostatic stress, and this effect is similar to introducing a field of thermal dilatation into the analyzed object. At low temperatures, this effect is small, but at high temperatures, typical for the service conditions of the blade material of aircraft gas turbine engines, it increases many times.

Samples cut from a disk blank of Russian granulated nickel-based superalloy VZH178P were tested for tensile strength at room temperature and long-term creep at temperature of 750 °C. Transmission electron microscopy showed that, in both cases, stacking faults and microtwins formed during plastic deformation of the alloy. During long-term creep at 750 °C, the alloying elements Cr, Co, Mo, and W segregate on the stacking faults, leading first to the formation of Suzuki atmospheres and then to the nucleation and growth of TCP particles with stoichiometry (Co,Cr)3(Mo,W).

An analytical solution for residual stresses and their energy in an elastically anisotropic two-component plate structure, where the components have an identical type of elastic anisotropy, identical or proportional elastic constants and coinciding principal axes of elastic anisotropy is obtained. The obtained solution has been applied to analyze the anisotropy of the elastic energy of such crystalline structures as the raft structure γ/γ' of single-crystal nickel-base superalloys, multilayer erosion-resistant nanocoatings ZrN/CrN and single-layer coatings of various types. It has been shown that the factor of minimizing the elastic energy of residual stresses has a significant effect on the crystallographic orientation of the interface in multilayer structures and the direction of axis of the growth texture axis of coatings.

Abstract—Single crystals of superalloys CMSX-4 and CMSX-10 were tested for creep at 750-1100°C and low-cycle fatigue at 700°C. It is shown that the type of creep curves depends on the test temperature. It is found that during creep and fatigue tests the specimens are fractured from the near-surface casting pores. The pores significantly decrease the fatigue life, but do not have a significant effect on the long-term creep strength.

Using the method of directional solidification, single crystals of experimental nickel-based superalloys with negative, zero, and positive γ/γ' misfits are obtained. The γ' solvus, solidus, and liquidus temperatures of the alloys are determined, and the microstructures of the alloys after directional solidification, heat treatment, and creep tests are investigated. Creep tests are performed at temperatures of 800 and 1000°C. It is found that single crystals of the alloy with a negative γ/γ' misfit have the highest creep resistance and lifetime (the crystal lattice period of the γ' phase is smaller than that of the γ matrix).

Specimens cut from a powder-metallurgy manufactured disk of the Russian nickel-base superalloy VZh178P have been tested for tensile strength at room temperature and creep strength at 750°С. Transmission electron microscopy has shown that stacking faults and microtwins form in both cases during plastic deformation of the alloy. Creep testing at 750°C causes segregation of alloying elements Cr, Co, Mo, and W at stacking faults, resulting first in a Suzuki atmosphere and then in the nucleation and growth of TCP particles with (Co, Cr)3(Mo, W) stoichiometry.

A model for the porosity formation which occurs in single-crystal nickel-based superalloys when the non-equilibrium eutectic melts during high-temperature heat treatment is proposed. It is assumed that the porosity formation results from plastic deformation of the γ-solid solution of nickel caused by dilatation of the melting eutectic. This mechanism of porosity formation is described analytically using the elasto-plastic properties of superalloy experimentally measured at near solidus temperatures. The proposed model was used to predict the pore formation in the 3rd generation single-crystal nickel-based superalloy CMSX-10. Comparison of the obtained theoretical and experimental results showed a good agreement.

The dependence of the elastic properties of monatomic (simple substances) and diatomic (AB type compounds) crystals of the cubic syngony on the types of their crystal structure and interatomic bond is analyzed. It is shown that the elastic properties of these crystals follow one of two trends, covalent or ionic. The auxetics (materials with a negative Poisson’s ratio) are identified among monatomic crystals with the A1 structure (fcc), such as to alkaline earth, transition and post-transition metals, actinides, and lanthanides, as well as among monatomic crystals with the A2 structure (bcc), such as to alkali metals. Among binary diatomic compounds, the largest number of auxetics is found among diamond-like crystals with the B3 structure (zinc blende) and stoichiometry A N B 8–N .

The morphology of pores partially shrunk during a half-hour HIP at temperature of 1288 °C and pressure of 103 MPa has been investigated in nickel-based superalloy CMSX-4. The investigation resulted in the following findings: surrounding the shrinking pores by a γ ′-shell (Ni 3 Al), faceting of the pores surface by {023} and {011} planes, and formation the submicroscopic satellite pores connected by channels with the neighboring larger pores. It is assumed that the formation of the γ ′-shell around the pores and the faceting of the pore surface is due to diffusion processes occurring during pore shrinkage, and therefore these findings can be considered as arguments supporting the vacancy model of pore annihilation. The submicroscopic satellite pores are expected to be the result of dividing the casting pores of a complex initial shape during their shrinking. The connecting channels are probably required for the gas to escape from the rapidly shrinking small satellite pores into the slowly shrinking large pore. Thus, it is reasonable to assume that the casting pores may contain some amount of gas.

An analytical model for forecasting the temperature dependence of γ/γ' misfit in heat-resistant nickel alloys is proposed. The model accounts for the concentration dependences of the periods of crystalline lattices of the γ and γ' phases (Vegard law), thermal expansion of the γ and γ' lattices, and dissolution of the γ' phase at high temperatures. Adequacy of calculations of misfit is confirmed by comparison with the results of measurements using methods of X-ray and neutron diffraction. The model is applied for development of a nickel alloy with positive misfit.

The temperature dependences of the periods of the crystal lattices of the γ and γ' phases, their dimensional mismatch (misfit), and volume fraction of the γ' phase of an experimental single-crystal high-temperature nickel-based alloy have been determined by X-ray diffraction analysis in the temperature range of 18–1150°C. The temperature ranges in which intense changes in the structural and phase characteristics of the alloy under study take place have been determined.

A nickel-based eutectic alloy with a γ/γ '-NbC structure was directionally solidified with a planar front. The specimens were tested for creep under sawtooth thermal cycling in the temperature range from 600 to 1100°C. It has been established that the lifetime under the conditions of thermal cycling is about five times shorter than it is predicted by the linear damage accumulation rule on the basis of results of isothermal creep tests. Faster creep under thermal cycling is caused by the rapid coarsening of the γ/γ ' microstructure due to the periodic partial dissolution and reprecipitation of the γ ' phase in heating and cooling half-cycles.

The formation of pores in CMSX-4 nickel based superalloys is detrimental to the service life of the material. A way to avoid the problem is to treat the superalloys under Hot Isostatic Pressing (HIP), which enables a large volume fraction of pores to be annihilated. This paper aims to understand the contribution of plastic activity related to the gliding of dislocations on the pore annihilation. Simulations based on a phase-field model of dislocation are performed and make it possible to consider the strong anisotropy of the CMSX-4 under HIP conditions in conjunction to the strong elastic heterogeneity introduced by the pore. For pores with a radius of few micrometers, it is shown that edge parts of dislocation lines that present an extra half atomic plane oriented towards the pore are stacked above and under it in the direction which is perpendicular to their slip-planes, causing an increase of the number of dislocation along the four octahedral directions of the FCC single crystal which intersect the pore center. Results are streamlined within the isotropic elastic theory of dislocations. Effects of elastic anisotropy and dislocation reactions are also investigated in order to specify what would be the dislocation configuration around a pore in CMSX-4 under HIP conditions. Notably, the elastic anisotropy is shown to significantly modify the arrangement of dislocations close to the pore equator. Simulations also allow for the characterization of pore/dislocation interactions when dislocations are involved in Low Angle Boundaries as experimentally observed.

Comparative long-term creep tests of [001] single crystals of third-generation rhenium-containing nickel-based superalloy VZhM1 and fourth generation rhenium- and ruthenium-containing nickel-based superalloy VZhM4 were performed at temperatures of 850, 1000, 1100, and 1150°C for durations up to ~3000 h. It has been established that the alloys are equally strong at a temperature of 850°C. At higher temperatures of 1000, 1100, and 1150°C the curves of long-term creep strength $$\sigma = f\left( \tau \right)$$ of the alloys intersect each other: at shorter times the creep strength of the VZhM1 alloy is higher than that of the VZhM4 alloy, while at longer times we have the inversion of creep strength. The time at which the long-term strength curves intersect each other shifts towards the lower values as the temperature increases and corresponds to the duration of incubation period of precipitation of topologically close-packed (TCP) phases in the structure of the VZhM1 alloy. Under similar test conditions, the volume fraction of TCP phases in the structure of the VZhM1 alloy is considerably larger than that in the VZhM4 alloy.

An improved diffusion model of pore annihilation during hot isostatic pressing of single crystals of nickel-base superalloys is proposed. The model considers dissolution of pores by emission of vacancies and their diffusion sink to low-angle boundaries. The calculation, which takes into account pore size distribution, predicts the kinetics of pore annihilation similar to experimental one.