Monocrystalline Superalloys Research Articles

Processing, Microstructures and Mechanical Properties of a Ni-Based Single Crystal Superalloy

A second-generation Ni-based superalloy has been directionally solidified by using a Bridgman method, and the key processing steps have been investigated with a focus on their effects on microstructure evolution and mechanical properties. The as-grown microstructure is of a typical dendrite structure with microscopic elemental segregation during solidification. Based on the microstructural evidence and the measured phase transformation temperatures, a step-wise solution treatment procedure is designed to effectively eliminate the compositional and microstructural inhomogeneities. Consequently, the homogenized microstructure consisting of γ/γ′ phases (size of γ′ cube is ~400 nm) have been successfully produced after a two-step (solid solution and aging) treatment. The mechanical properties of the resulting alloys with desirable microstructures at room and elevated temperatures are measured by tensile tests. The strength of the alloy is comparable to commercial monocrystalline superalloys, such as DD6 and CMSX-4. The fracture modes of the alloy at various temperatures have also been studied and the corresponding deformation mechanisms are discussed.

Microstructure evolution of monocrystalline superalloy with boron addition

The as-cast and heat treated structures of nickel-base monocrystalline superalloy DD90 were observed, using an optical microscope (OM) and a scanning electron microscope (SEM), and the effect of different contents of B on γ/γ′ eutectic structure, carbide and borides morphology and volume fraction were studied. Boron addition had no significant effect on dendrite morphology, the volume fraction of eutectic increased with the increasing of boron level. The morphology of carbides changed from layers, needle, block and the layer-typed to the smaller block. The morpgologies of carbides in as-cast state mainly were blocky, octahedral and Chinese script like. The carbides precipitated as dispersed blocky and octahedral after thermal treatment. With the increasing of boron level, the volume fraction of borides increased and the content of W + Mo + Cr + Re became much high. The morphology of borides mainly were slender rod and grain after thermal treatment.

Long-term strength determination for cooled blades made of monocrystalline superalloys

For the manufacture of blades for modern gas-turbine installations, monocrystalline alloys are used. Traditional methods for the calculation of stressed-deformed state and safety factors for these alloys developed and verified for polycrystalline materials need to be adjusted. This paper deals with methodological principles for an approach to the solving of the problem concerning a finite-element determination of the long-term static strength for cooled monocrystalline blades employed in gas-turbine installations based on the use of two different models (phenomenological and micromechanical) considering the inelastic deformation of monocrystalline superalloys. An analysis has been performed for the distribution of Schmid factors in the spherical triangle for primary and secondary octahedral and cubic slip systems. Calculations are performed using Larson–Miller’s parametric dependences taking into account the crystallographic orientation of the material. A determination procedure for the anisotropy coefficients of long-term strength is described based on data for different orientations. A comparative analysis of the results of finite-element calculations made using phenomenological and micromechanical (crystallographic) creep models for the long-term static strength of cooled monocrystalline blades used in a gas-turbine engine has been performed. It is shown that the location of the most loaded sections of such a blade coincide with the results of calculations according to these models. It has been found that the micromechanical deformation model results in the obtaining of the most conservative estimate for the long-term strength of turbine blades made of monocrystalline alloys. It is shown that the calculations using models for materials with isotropic properties can produce considerable errors in determining the durability of the blades. The possibility is considered for using 1D-, 2D-, and 3D-models for turbine monocrystalline blades in the determination of their durability parameters.

Microstructure analysis in the creep process of a gas turbine blade of the turbine engine

The technical condition of gas turbine blades of the turbine engine has a significant impact on the reliability and life span of the turbine and the entire engine. In order to assess the blades technical condition, a visual method, with the use of optoelectronic devices, is used. In order to verify this assessment, metallographic tests are conducted. The paper presents the results of microstructural tests of the turbine rotor blades made of nickel-based monocrystalline super alloys. The aim of these tests was to determine the consequences of impacting high exhaust gas temperature and stresses occurring during operation on the stability of the blade microstructure. The progress level of the blade microstructure changes in the post-operational stage for different sections towards the blade vertical axis and was compared with the blade microstructure on delivery made of the same super alloy. A varied degree of the microstructure degradation for different blade sections was shown. The changes typical of the high-temperature creep process – γ phase directional growth (rafting), were observed only for the thinnest walls of the upper section of the turbine blade leaf.

Influence of Different Etchants on the Representation of Microstructures in Nickel Alloys

Abstract This work presents a comparison of selected nickel alloys of the same condition which were treated by means of specifically chosen etching techniques. Microstructures on microscope images of wrought Alloy 617, a casting variant of Alloy 625, a polycrystalline casting alloy IN-738 LC, as well as of a monocrystalline superalloy CM 247 LC SX, respectively, are juxtaposed and evaluated. This approach allows for a comprehensive optical microscopy characterization of the characteristic microstructural features.

Fatigue Life Prediction of Single Crystals for Turbine Blade Applications

This paper describes a fatigue life prediction model accounting for high temperature applications of metallic materials. The formulation of the model is for general anisotropy and multiaxiality of loading. This phenomenological model distinguishes between an initiation phase and a propagation phase, and takes into account oxidation and creep effects on fatigue life. An application of the model is given for a coated single crystal superalloy for turbine blades. Model predictions are in good agreement with a large set of experimental data for mechanical loading including thermomechanical fatigue tests. A new experimental device, designed to produce a thermal gradient in the thickness of thin walled specimens, is also presented. This device has been used with polycrystalline and monocrystalline superalloys. Corresponding life predictions, performed by using recent anisotropic models, are presented for the single crystal superalloy.

Elastic properties of textured and directionally solidified nickel-based superalloys between 25 and 1200°C

Abstract The Young's modulus E and the shear modulus G of polycrystalline textured and directionally solidified nickel-based superalloys were measured in the temperature range from 25 to 1200°C by Forster's resonance method. The observed variation in elastic properties is explained by applying averaging concepts to the elastic properties of corresponding monocrystalline superalloys. Introducing the texture parameter a 4 of the orientation distribution function, a modified Voigt-Reuss-Hill approach leads to agreement with the measured upper and lower limits of E and G . The Poisson's ratios v of fibre-textured aggregates are calculated.

An Oxide Dispersion Strengthened Nickel-base Superalloy with Excellent High Temperature Strength

Oxide dispersion strengthened nickel-base superalloys with compositional variants were prepared by mechanical alloying, and their secondary recrystallization characteristics and creep rupture strength were examined. Their matrix compositions were determined by modifying cast superalloys containing a high volume fraction of γ' phase. One of the presently investigated alloys, Alloy 98, was shown to form directionally recrystallized grain structures by zone anneal and surpass monocrystalline superalloys in the creep rupture stress range below about 320MPa. Higher tantalum concentration in Alloy 98 than in other oxide dispersion strengthened alloys is considered to improve creep rupture strength. Aluminum addition, higher than about 6wt%, seems to degrade recrystallization properties and consequently decrease creep strength. M23C6 carbide precipitates besides γ' phase and oxide dispersoids were observed in Alloy 98 by means of transmission electron microscopy. Also observed was a formation of coalescent oxides and oxide denuded areas around them, and the higher creep strength will be expected by improved process to minimize these dispersion defects.

Elastic constants of a monocrystalline nickel-base superalloy

The interest in using a monocrystalline superalloy for turbine blades makes it imperative to understand the elastic response of the monocrystalline superalloy under external stress. While the elastic response in the direction of loading is well known for monocrystals, the transverse response under longitudinal loading is not well characterized. The present work presents an experimental measurement and an analytical treatment for characterizing the transverse elastic response under longitudinal loading. The work indicates that the transverse elastic response, in general, is sinusoidal. The in-plane transverse direction, therefore, affects the elastic deformation in the transverse direction under unit longitudinal load, and a proper choice of the in-plane crystallographic direction minimizes the in-plane transverse strain. By fitting the experimental data to an analytical equation, the elastic constants can be obtained from a cylindrical monocrystal of arbitrary axial orientation.