Polycrystalline Ni-based Superalloys Research Articles

Microstructure-sensitive probabilistic modeling of HCF crack initiation and early crack growth in Ni-base superalloy IN100 notched components

Microstructure-sensitive simulation-based strategies for modeling fatigue life reduction in cyclically loaded notched components offer a means to augment costly experiments and to project performance of microstructures not yet processed. To advance design tools for notch fatigue resistance in aircraft gas turbine engine components, we present a formulation that links microstructure heterogeneity (grain size distribution) to size effects and fatigue scatter in notched polycrystalline Ni-base superalloy IN100 specimens. Simulated double-edged notched specimens with various notch radii are subjected to completely-reversed, quasistatic, isothermal (650 °C), strain-controlled loading at three different strain amplitudes. Polycrystal plasticity and experimentally-calibrated crack formation/growth laws are used to correlate cyclic plastic slip to the probability of forming and propagating a crack from grain scale to a transition crack length at which LEFM is applicable. Probabilistic strain-life and cumulative distribution function (CDF) plots show that larger notch sizes display a larger notch size effect and fatigue knock-down effect. The proposed CDF can be determined for any failure probability and number of cycles, which has implications for minimum fatigue life design of aircraft gas turbine engine components.

The Microstructure Changes in IN713LC during the Creep Exposure

Nickel-based creep resisting alloys (strengthened by γ´) are the basic materials for high-temperature constructional parts in aircraft engines and energy units. These parts are exposed to combined effects of mechanical stresses, high temperature and dioxide-corrosion conditions. The microstructure changes of cast polycrystalline Ni-based superalloy IN713LC after creep exposure were studied. Three specimens with three different diameters were used for creep tests. The degradation stage (damage parameter π) was determined for all parts of specimens. Individual parts of specimens were metallographic observed and analyzed by image analysis after rupture. The results were compared with model of stress distribution in the specimen with potential damage in the centre of the specimen.

Anomalous deformation behavior and twin formation of Ni-base superalloys at the intermediate temperatures

The tensile behaviors of polycrystalline Ni-base superalloys have been studied in the temperature range of 25–980 °C. Anomalous increase of yield strength was observed in precipitation hardened superalloys at intermediate temperature. The alloy with high γ′ volume fraction showed a remarkable increase of yield strength at intermediate temperature. A peak of yield strength was observed in the alloy with low γ′ volume fraction at intermediate temperature while solid solution strengthened alloys did not have such peak. Abrupt decrease of ductility in the intermediate temperature regime was observed not only in the γ′ strengthened superalloys but also in the solid solution strengthened superalloy. This result implies that γ′ precipitation is not a substantial cause for the occurrence of the ductility minimum in the superalloys. It was found that twinning was an important deformation mechanism of the superalloys at intermediate temperature where ductility was abnormally low. Deformation twins formed easily in the superalloys whose reduction of stacking fault energy was high regardless of strengthening mechanisms because alloys with low stacking fault energy was prone to extend stacking faults.

OS12F008 Assessment of Fatigue Crack Propagation Behavior near Crystallographic Orientation Boundaries in Polycrystalline Ni-base Superalloys

OS12F008 Assessment of Fatigue Crack Propagation Behavior near Crystallographic Orientation Boundaries in Polycrystalline Ni-base Superalloys

OS12-3-1 Assessment of Fatigue Crack Propagation Behavior near Crystallographic Orientation Boundaries in Polycrystalline Ni-base Superalloys

OS12-3-1 Assessment of Fatigue Crack Propagation Behavior near Crystallographic Orientation Boundaries in Polycrystalline Ni-base Superalloys

Behavior of Branch Cracking and the Microstructural Strengthening Mechanism of Polycrystalline Ni-Base Superalloy, IN100 under Creep Condition

Recently, the development of the high-efficiency technology for gas turbine and jet engine is required to minimize carbon dioxide and nitrogen oxide emission. It is effective way to increase the operational temperature to develop the high-efficiency technology for high temperature instruments. To increase the operating temperature, advanced Ni-base superalloys have been developed as a turbine blade material.Even though a Ni-base superalloy is used for a structural component, creep damages and creep cracks may be caused due to the external tensile load under high temperature conditions. Therefore, a predictive law of creep crack growth life is necessary to maintain operational safety.This study is aimed to clarify the branch cracking behavior due to the microstructural strengthening mechanism of polycrystalline Ni-base superalloy IN100 under the creep condition. The creep crack growth tests were conducted at a temperature of 900°C. The creep crack growth behavior and creep damage formulation were observed by in-situ observational system and FE-SEM/EBSD. Additionally, two-dimensional elastic-plastic creep finite element analyses were conducted for the model which describes the experimental results. The creep crack growth behavior and the creep damage progression were found to be affected by the distribution behaviors of grains and grain boundaries around the notch tip. By comparison of experimental results with mechanical analysis using FEM analyses, mechanisms of the creep crack growth and the creep damage formulation were clarified.

Nanoporous Membranes Produced from Polycrystalline Ni-Based Superalloys

Porous metal membranes are produced from a two phase system in which the discrete cubic gamma'-precipitates connect during self assembly. In the so called rafting process the cubic particles start to coarsen and finally create a network within the gamma-matrix. In a following electrochemical leaching process one of the phases can be removed leaving the nanoporous membrane. So far, single crystalline alloys have been used for producing thin nanoporous membranes. Now research is in progress to produce the nanoporous membranes from polycrystalline alloys in a creep process. A modification of the commercially available alloy Nimonic 115 was used for these membranes. The permeability of these membranes was proven in a gas-flow test.

Microstructure and creep strength of different γ/γ′-strengthened Co-base superalloy variants

The influence of W, Ta, Ti, Nb, V, Si, Mo, Ir and Cr on the high temperature properties of γ/γ′-strengthened Co–Al–W superalloys was investigated. All alloys exhibited a γ/γ′ microstructure with remarkably differing γ′ volume fractions. W, Ta, Ti, Nb and V increased the γ′ volume fraction and γ′ solvus temperature. An increased W content and alloying of additional elements, except Ir, decreased the liquidus temperature. First creep experiments revealed a creep strength comparable with polycrystalline Ni-base superalloys and the importance of grain boundary strengthening.

Localized cyclic deformation and corresponding dislocation arrangements of polycrystalline Ni-base superalloys and pure Nickel in the VHCF regime

Knowledge about the fatigue behaviour of metallic materials in the range of very high cycle fatigue (VHCF, N > 10 7) is a key factor to improve the safety against failure of component parts. Even in the conventional field of application of nickel-base alloys (e.g. turbine blades) superimposed high frequency loading requires a safe life at very high number of cycles. Thus, the fatigue behaviour of structural materials in the very high cycle regime has become an important and active subject of research. In this paper, polycrystalline nickel-base alloys (Nimonic 80A and Nimonic 75) and pure Nickel (wavy slip character) were investigated in the VHCF regime by varying the precipitation conditions (peak-aged, overaged and precipitation-free), dislocation slip behaviour and test frequency. In addition, mechanisms of fatigue failure in the VHCF regime were compared to conventional damage evolution (in the LCF and HCF region) on the basis of load-controlled low frequency tests. Surprisingly the overaged condition of Nimonic 80A shows a slightly higher fatigue strength in the VHCF regime as compared to the peak-aged condition. The results obtained document that fatigue failure can still occur beyond 10 7 cycles. Transmission electron microscopy (TEM) was carried out, in order to characterize the influence of microstructure, the resulting dislocation slip behaviour and the relevant dislocation particle interaction mechanism. Studies of the development of slip markings on surface grains were performed mainly using scanning electron microscopy (SEM).

Elemental partitioning of platinum group metal containing Ni-base superalloys using electron microprobe analysis and atom probe tomography

The partitioning of platinum group metal (PGM) additions in polycrystalline Ni-base superalloys has been investigated. Alloys with a baseline composition of 15Al–5Cr–1Re–2Ta–0.1Hf (at.%) with systematic variations in Pt, Ir, Ru and W were cast and heat-treated to produce bimodal two-phase γ– γ′ microstructures. Electron microprobe analysis was used to measure the composition of coarse γ′ particles. Selected alloys were studied using atom probe tomography. Pt and Ru weakly partition to the γ′ and γ phases, respectively, whereas Ir partitions equally between both phases. Pt and Ru do not change partitioning behavior appreciably with additional W and Re. Interestingly, Ir lowers γ′ volume fraction and reduces partitioning of W, Re, Ru, Al and Pt. Tungsten partitioning coefficients exhibit an unusually strong temperature dependence, with increased partitioning of W to the matrix at lower temperatures. The thermodynamics of these PGM-containing systems are analyzed and the implications for partitioning are discussed.

Microstructure-sensitive modeling of polycrystalline IN 100

A rate dependent, microstructure-sensitive crystal plasticity model is formulated for correlating the mechanical behavior of a polycrystalline Ni-base superalloy IN 100 at 650 °C. This model has the capability to capture first order effects on the stress–strain response due to (a) grain size, (b) γ′ precipitate size distribution, and (c) γ′ precipitate volume fraction. Experimental fatigue data with variable strain rates are used to calibrate the model for several distinct IN 100 microstructures (grain size, precipitate size distributions and volume fractions) obtained from thermomechanical processing. Physically based hardening laws are employed to evolve the dislocation densities for each slip system, taking into consideration the dislocation interaction mechanisms. The calibrated crystal plasticity model is used to inform microstructure dependent parameters of a macroscopic internal state variable (ISV) model, which is computationally feasible for use in component scale notch root analyses. A hierarchical methodology is outlined to embed this microstructure-dependence in the macroscale model.

Oxidation Behavior of a Cast Polycrystalline Ni-Base Superalloy in Air: At 900 and 1000 °C

The oxidation behavior of a cast polycrystalline Ni-base superalloy was studied at 900 and 1000 °C and analyzed by thermogravimetric analysis (TGA), X-ray diffraction (XRD) and SEM. The results indicate that the cast Ni-base superalloy exhibits subparabolic oxidation kinetics, which are controlled by the growth of the inner Al-rich layer. A mixed scale forms on the alloy after prolonged oxidation. The oxide scale formed on the γ matrix is composed of an outer layer of spinel and a uniform inner, compact Al-rich layer. The oxidation behavior on the region overlying MC carbide precipitates is slightly different from that in the γ matrix. The difference is explained by the big difference in composition between MC carbide and γ matrix.

MC Carbide Decomposition during Thermal Exposure of Polycrystalline Ni-Base Superalloys

MC decompositions during thermal exposure have been investigated in three conventional cast Ni-base superalloys, GTD111, IN738LC, and CM247LC. MC decomposition in GTD111 and IN738LC depends on exposure temperature. While MC decomposed into M23C6 at 982°C, η formed from MC after exposure at 927°C and 871°C. Ta and Ti separated from MC during thermal exposure made η phase to form instead of γ'. The decomposition of the MC in CM247LC depends on their morphology and position. Segregation during casting process affected the morphology and composition of MC type carbide in this alloy. Acicular M6C was found near scriptal MC in the dendrite. Blocky MC near γ-γ' eutectic decomposed into M23C6. Both M23C6 and M6C can be found on grain boundary after thermal exposure in CM247LC.

Microtwinning during intermediate temperature creep of polycrystalline Ni-based superalloys: mechanisms and modelling

Deformation mechanisms, operative during intermediate temperature creep of Ni-based polycrystalline superalloys, are poorly understood. The creep deformation substructure has been characterized in Renè 88DT following rapid cooling from the super-solvus temperature, yielding a fine γ′-precipitate microstructure. After creep to modest strain levels (up to 0.5% strain) at 650°C and an applied tensile stress of 838 MPa, microtwinning is found to be the predominant deformation mode. This surprising result has been confirmed using diffraction contrast and high-resolution transmission electron microscopy. Microtwinning occurs via the sequential movement of identical 1/6[11–2] Shockley partials on successive (111) planes. This mechanism necessitates reordering within the γ′ precipitates in the wake of the twinning partials, so that the L12 structure can be restored. A quantitative model for creep rate has been derived on the basis that the reordering process is rate-limiting. The model is in reasonable agreement with experimental data. The results are also discussed in relation to previous studies under similar deformation conditions.

Transversely isotropic viscoplasticity model for a directionally solidified Ni-base superalloy

A transversely isotropic continuum viscoplasticity model has been formulated to capture the fatigue and creep responses of a directionally solidified (DS) polycrystalline Ni-base superalloy used mainly in turbine blades. This model has been implemented as an ABAQUS User MATerial (UMAT) subroutine using a semi-implicit integration scheme. Isothermal uniaxial fatigue data from tests conducted with and without hold times and creep data are used to characterize the stress–strain response at temperatures ranging from 427 °C to 1038 °C. The scheme leads to reduction of the associated computational costs when compared to a crystal viscoplasticity model that explicitly considers 3-D grain structure. The macroscopic elastoviscoplastic model is shown to simulate the homogenized deformation response of the polycrystalline DS alloy for various isothermal histories. The predictive capability of this model is verified using both in-phase and out-of-phase TMF data, and is compared to the results of analysis of a single crystal in terms of stress concentration and stress distribution for a model problem of a plate with a central hole.

High-cycle fatigue of nickel-based superalloy ME3 at ambient and elevated temperatures: Role of grain-boundary engineering

High-cycle fatigue (HCF), involving the premature initiation and/or rapid propagation of cracks to failure due to high-frequency cyclic loading, remains a principal cause of failures in gas-turbine propulsion systems. In this work, we explore the feasibility of using “grain-boundary engineering” as a means to enhance the microstructural resistance to HCF. Specifically, sequential thermomechanical processing, involving alternate cycles of strain and annealing, was used to increase the fraction of “special” grain boundaries and to break up the interconnected network of “random” boundaries, in a commercial polycrystalline Ni-based superalloy (ME3). The effect of such grain-boundary engineering on the fatigue-crack-propagation behavior of large (∼8 to 20 mm), through-thickness cracks at 25 °C, 700 °C, and 800 °C was examined. Although there was little influence of an increased special boundary fraction at ambient temperatures, the resistance to near-threshold crack growth was definitively improved at elevated temperatures, with fatigue threshold stress intensities some 10 to 20 pct higher than at 25 °C, concomitant with a lower proportion (∼20 pct) of intergranular cracking.

Internal Nitridation during Creep Loading of Polycrystalline Ni-Base Superalloys

Internal Nitridation during Creep Loading of Polycrystalline Ni-Base Superalloys

Techniques for microstructural characterization of powder-processed nickel-based superalloys

Methods are described for sample preparation of polycrystalline Ni-based superalloys in order to perform a detailed microstructural characterization. Specific techniques for the precise definition of the various phases present are outlined and these are shown to be useful for the measurement of size, volume fractions and distribution. A number of optical, scanning and transmission microscopy techniques are used to provide the necessary information.

Measurement of the ductile-to-brittle transition temperature in a nickel aluminide coating by a miniaturised disc bending test technique

Nickel aluminide coatings are often employed to enhance the corrosion and oxidation resistance of nickel base gas turbine blades and vanes, as the high near-surface content of Al increases the ability to form an Al 2O 3 protective scale. The ductility of the coating depends on the type of aluminisation process and Ni-base material. In order to prevent coating degradation during service it is important to assess the ductile-to-brittle transition temperature (DBTT) in ductility of the coating. To determine the DBTT a miniaturised disc bending test (MDBT) technique is used, where a biaxial tensile stress is applied to a disc specimen. The DBTT of a NiAl coating, applied by a high-activity aluminium pack cementation process to a polycrystalline Ni-base superalloy (IN738 LC), was evaluated using the MDBT technique between room temperature (RT) and 860 °C. Test results gave a DBTT in biaxial ductility of the coating of approximately 760 °C. Above 760 °C, a significant increase in ductility was noted. Fractographic examination showed that the coating fractures in a mainly transgranular mode at RT but in a predominately intergranular mode at elevated temperatures, even at temperatures above DBTT.

The effect of grain-boundary-engineering-type processing on oxygen-induced cracking of IN718

Polycrystalline Ni-base superalloys are prone to time-dependent intergranular failure when the loading temperatures exceeds about 873 K. The fracture process is then controlled by oxygen grain-boundary diffusion followed by decohesion of the respective boundaries. It is shown that this kind of cracking for IN718 at 923 K in air can be reduced significantly by successive steps of deformation and annealing, which is known as grain-boundary-engineering processing. This illustrates the importance of the grain-boundary-character distribution with regard to this mode of failure, which is known as ‘dynamic embrittlement.’