Abstract—The results of the density functional theory calculation of the segregation energy of refractory metals at grain boundaries (GBs) in the alloys based on nickel (fcc solid solutions), chromium (bcc), and titanium (hcp) are analyzed. The influence of segregation on the cohesive strength of GBs is estimated using the Rice–Wong model, in which the mechanical strength of GBs is characterized by the boundary splitting energy. Refractory metals are shown to contain “useful” elements, which enrich GBs and increase their cohesive strength in alloys of all types. The calculated data are used to design experimental high-temperature alloys (HTAs), the alloying system of which contains useful elements in the form of a package of low-alloy additives in HTAs, namely, nickel (Zr, Nb, Hf, Ta), chromium-based (Nb, Ta), and titanium (W, Ta, Re) alloys. The results of testing the mechanical properties of these experimental alloys are presented. The low-alloy additives are shown to increase the creep resistance of all alloys at elevated temperatures.

We use ab initio calculations to analyze the influence of 4d and 5d transition metal alloying elements on cohesive properties of the bulk and a representative grain boundary in Cr within the framework of the Rice–Thomson–Wang approach. The results obtained for Cr are combined with the analogous results for Ni to select Ta and Nb as promising alloying additions to dual-phase (α/γ) Cr-Ni-base high-temperature alloys. Ta and Nb are added to the alloying system of an existing alloy I (Cr-Ni-W-V-Ti) in an attempt to design a chemical composition of a new alloy II (Cr-Ni-W-V-Ti) + (Ta-Nb). Investigation of the microstructure of the Ta-bearing Cr-Ni-alloy reveals a Ta enrichment of large γ-areas near GBs in α-matrix that we consider as potency to increase the cohesive strength of GBs and the cohesive energy of the bulk in γ-phase. Mechanical testing of alloys I and II demonstrates that the alloy II has improved tensile strength and creep resistance at high temperatures.

We used the results of ab initio calculations to improve the high temperature mechanical properties of a Cr-Ni-base alloy (Cr-33Ni-2W-0,3Ti-0,3V, wt.%) (alloy I) with two-phase α - γ microstructure. It was established that γ – phase in Cr-Ni-base alloy (I) plays a key role in the processes of plastic deformation. By analogy with Ni-base superalloys the bulk and grain boundaries cohesion in γ – phase of the Cr-Ni-base alloy (I) were strengthened by adding a package of the “low alloying” elements (Zr, Hf, Nb, Ta) (alloy II) chosen in accordance with our theoretical predictions. We further investigated an influence of a sum (Ta, Nb, Hf, Zr) like the low alloying additions on the mechanical properties of Cr-Ni-base alloy (I). The results of mechanical testing revealed a significant strengthening of the alloy (II) in comparison with (I) at the temperature 1080 oC in accordance with our predictions. We also investigated the microstructure’s peculiarities of the alloys (I) and (II).

This paper presents a detailed study of the formation of chromium-based alloys, Cr–Ta–W + plasticizing additives (Nb and Zr) and Cr–Ta–Si, during milling of powder mixtures in a Fritsch (P-7) planetary mill under an Ar atmosphere. It is shown that, after milling for 18 h, all the components of the starting mixtures convert into a Cr-based BCC solid solution. The powders of chromium alloys obtained in this study are readily compacted by hot isostatic pressing (HIP) under conditions typical of the processing of powders of high-temperature nickel alloys. Heating of the powders and compacts leads to the decomposition of the supersaturated solid solution and the formation of two forms of the Cr2M Laves phase with cubic crystal lattices. The formation of a mixed-phase fine microstructure in the chromium alloys after HIP suggests that the materials studied here are potentially attractive as a base of next-generation chromium-based high-temperature alloys.

First-principles-aided design of a new Ni-base superalloy : Influence of transition metal alloying elements on grain boundary and bulk cohesion (vol 82, pg 369, 2015) ERRATUM

Effect of comprehensive alloying system (W, Ta, Nb, Mo, V, Hf, Ti, Zr, Ni) and impurity N on cohesive properties of the bulk and the special high-angle grain boundary (GB) Σ5 (210)[100] in Cr-base alloys, as well as segregation behavior of impurities at the GB and the (210) free surface are studied by first principles calculations. The analysis of the data obtained allows us to single out W, Ta as the best and nitrogen as the worst interatomic bond strengthening elements for both the bulk and GB in Cr-base alloys. To verify these theoretical results by means of experiment, we investigated an influence of W (up to 10 wt. %), and (Ta, Nb, Hf and Zr) on mechanical properties of Cr-base alloy. We observed an increasing of the strength properties due to W addition and increasing of the plasticity as a result of (Ta, Nb, Hf, Zr) adding.

A new approach to the design of Ni-base polycrystalline superalloys is proposed. In this approach, we assume that the creep–rupture characteristics of a superalloy are mostly determined by the strength of interatomic bonding at grain boundaries (GBs) and in the bulk of γ matrix. The ideal work of separation, Wsep, of a GB is used as a fundamental thermodynamic quantity that controls the mechanical strength of an interface, whereas the partial cohesive energy, χ, of an alloy component serves to characterize its contribution into the strength of the bulk. Using the Σ5 (210)[100] symmetric tilt GB as a representative high-angle GB in Ni, we calculate Wsep,χ, and GB segregation energies, Eseg, for the complete set of 4d and 5d transition metal impurities, to which we add B (a typical microalloying addition), S and Bi (notoriously known as harmful impurities in Ni-base superalloys). The purpose of the analysis is to identify the elements that demonstrate a high tendency to segregate to GBs, have positive (preferably high) partial cohesive energies in the bulk, and have positive impact on Wsep of GBs. We refer to these elements as low-alloying additions. Our study reveals Zr, Hf, Nb, Ta and B as the most promising low-alloying additions. Our next step is to introduce the elements found in the first step into a new powder metallurgy (P/M) Ni-base superalloy. The results of the subsequent testing confirm that the newly created P/M superalloy indeed demonstrates superior mechanical properties at high temperatures compared to the existing Russian P/M alloy EP741NP.

The segregation energies of B, Si, P, Cr, Ni, Zr, and Mg on the special grain boundary (GB) Σ5 (210)[100] and on the open (210) surface of aluminum have been determined and the GB splitting energy has been calculated by the density functional theory methods. It has been shown that all elements listed above enrich the GB; for B, Si, P, Cr, Ni and Zr, Mg, interstitial and substitutional sites are preferred, respectively. The effect of alloying elements on the GB binding has been estimated using the parameter η equal to the change in the fracture work of the aluminum GB when adding alloying element atoms. From the viewpoint of strengthening the GB binding forces, Zr, Cr, Ni, and Mg are efficient, Si and B are neutral and phosphorus weakens GBs.

A new approach to the design of Ni-based polycrystalline superalloys is proposed. It is based on a concept that under given structural conditions, the performance of superalloys is determined by the strength of interatomic bonding both in the bulk and at grain boundaries of material. We characterize the former by the cohesive energy of the bulk alloy, whereas for the latter we employ the work of separation of a representative high angle grain boundary. On the basis of our first principle calculations we suggest Hf and Zr as “minor alloying additions” to Ni-based alloys. Re, on the other hand, appears to be of little importance in polycrystalline alloys.

The effect of γ’ particle size upon the mechanical properties of Ni base superalloy EP741NP obtained by powder metallurgy was investigated. The particle size of γ’ phase in γ-γ’ microstructure was varied by changing the cooling rate V from the temperature of the solid solution treatment at 1200 °C (V = 80, 200 and 400 °C \ min.). After solid solution treatment billets were subjected to aging in the standard mode. It was established that as V increases from 80 to 200 °C \ min., the average particle’s size of γ’ phase decreases from 0.54 microns to 0.22 microns in the aged state. This improves the characteristics of creep and low cycle fatigue at 650°C: time to rupture under load 1000 MPa increased from 132 hours to 416 hours and low cycle fatigue increased from 42,215 to 82,016 cycles.

The effect of B, Si, P, Cr, Ni, Zr and Mg on cohesive properties of Al and the special grain boundary (GB) Σ5 (210)[100], as well as their segregation behavior at the GB and the (210) surface are studied by first principles method. The analysis of these parameters allows us to single out Ni as the best and phosphorus as the worst interatomic bond strengthening alloying elements.

Typical defects of the cast engine blades from Ni base superalloys in the as-cast state and after long operation were investigated: the cast porosity, internal and external microcracs. The initial cast porosity and voids, which have appeared during long operation, reduce the fatigue and other service characteristics of the blades from Ni base superalloys. Hot isostatic pressing (HIP) reduces the size and number of micropores in the cast blades from Ni base superalloys with a polycrystalline (alloy GS6U) and single crystal (alloy GS32) structures. As a result, HIP improves the mechanical properties of the blades. Application of the HIP, together with the standard heat treatment in the renovation of the blades after long operation can provide a virtually complete restoration of their structure and efficiency.

A new approach to the design of Ni-based single crystal superalloys is proposed. It is based on a concept that under given structural conditions, the creep-rupture characteristics of superalloys are mainly determined by interatomic bonding given by the cohesive energy. In order to characterize the individual contribution of each alloying element to the strength properties at high temperature, we introduce a parameter, χ, which is the partial molar cohesive energy of an alloy component. This parameter is then obtained in the total energy first-principles calculations for a usual set of alloying elements. We demonstrate that creep-rupture characteristics of alloys indeed correlate with the total gain partial molar cohesive energy due to alloying and find that W, Ta, and Re have the highest values of χ, and should therefore play the major role in providing high-temperature strength of superalloys. Based on this finding, we design three new superalloys with a high content of W and show that they have superior creep-rupture properties compared not only with their counterparts with the lower content of W, but also with the best Ru-bearing Ni-based superalloys.

The density of alloy ZhS6U in cast state is studied after testing for long-term strength, hot isostatic pressing, thermal vacuum treatment, and long-term operation. The density of the alloy is determined using the method of hydrostatic weighing of test specimens and specimens cut from turbine blades.