Behavior Of Superalloy Research Articles

Elevated temperature erosive wear behavior of superalloy coatings deposited using cold spray technology

The boiler tube components in thermal power plants and steam generating systems suffer from erosion and corrosion problems and are even today posturing a grave threat to industries, resulting in forced outages. Unrelenting efforts to decrease the components’ maintenance costs have been made by protecting with coatings. Among the various coating methodologies are thermal spray, chemical vapour deposition (CVD), physical vapour deposition (PVD), etc., in vogue. However, recently, the additive manufacturing-based cold spray technique is gaining interest among material scientists, incorporating high-velocity impacts associated with the low-temperature regime. Nickel-based superalloy Inconel 625 has been chosen and cold sprayed on The American Society of Mechanical Engineers (ASME) T11 and T22 boiler steels. The coated surface is characterized by scanning electron microscopy and energy dispersive X-ray analysis (EDAX), X-ray diffraction, micro-hardness, and elemental X-ray mapping techniques. The solid particle erosion studies have been carried out both at room and an elevated temperature of 700 °C as per American Society for Testing and Materials (ASTM) standard for 30°C and 90 °C impact angles. An optical profilometer has been used to evaluate the erosion volume loss. The results showed that the coated samples exhibit superior erosion resistance than the uncoated samples at both room and elevated temperatures. The erosion data obtained have been substantiated using scanning electron microscopy by analyzing the damage features and correlating with the erosion data.

Frequency Analysis of Transients in Electrochemical Noise of Superalloys Waspaloy and Ultimet

Nickel or Cobalt-based superalloys represent an important class of engineering materials, finding widespread application in critical components within the gas turbine engines used for jet propulsion and electricity generation. This research aimed at the frequency analysis of transients in electrochemical noise of Waspaloy and Ultimet superalloys, immersed in 3.5 wt.% in H2SO4 and NaCl solutions at two different temperatures, 25 and 60 °C. Localized corrosion behavior of superalloys was assessed using the electrochemical noise technique (EN) according to ASTM-G199 standard. Three different statistical methods filtered the EN signal, and the polynomial method was employed to obtain the noise resistance (Rn), the localization index (LI), skew and kurtosis, and the power spectral density analysis (PSD). Results indicate that the current and potential noise transients have a better behavior with better clarity when a polynomial is used to show a localized corrosion kurtosis for both superalloys.

Physical-Based Constitutive Modeling of Hot Deformation in a Hot-Extruded Powder Metallurgy Nickel-Based Superalloy

For simulating the hot deformation behavior of a powder metallurgy nickel-base superalloy during isothermal forging, the hot compression tests were performed in the temperatures of 1020-1110 °C with the strain rates from 10−3 to 1 s−1. The thermoplastic deformation and dynamic recrystallization (DRX) behavior of the superalloy during thermocompression were systematically studied. It can be shown that the flow stress of the superalloy exhibits apparent characteristics of DRX. A relationship indicating the dependency of flow stress on deforming temperature and strain rates is built through introducing the Z parameter. The work-hardening rate (θ) curves of the superalloy at various deformation conditions are obtained, and the values of θ are found to decrease with decreasing Z parameter, i.e., decreasing strain rates and increasing deforming temperature. There exists a power exponential relationship between critical strain ec and Z parameter. Combining the experimental data, a two-stage constitutive model on the basis of the Estrin–Mecking constitutive model and the DRX kinetic model is set up to model the flow stress of the superalloy. The flow stresses obtained from the established model are well consistence with the measured values.

Effect of Hot Deformation Parameters on Grain Refinement of an As-Cast Ni-Based Superalloy

The grain refinement behavior in superalloys has been identified for a long time, while its exact underlying mechanism remains to be explored. In this study, transmission electron microscope and electron backscatter diffraction technique were employed to investigate the fundamental mechanism governing the process of grain refinement for as-cast superalloys GH4720LI. The results show that grain refinement highly depends on the distributions of γ′ precipitates. In γ + γ′ two-phase region, γ′ precipitates could promote the dislocation multiplication, hinder the dislocation motion and lead to the formation of high-density dislocation substructure. Hence, the sub-boundaries and high-angle grain boundaries (HABs) are formed resulting from the gradual transformation of the dislocation substructures. In this process, the new grains are developed by quasi-continuous dynamic recrystallization (quasi-CDRX) and discontinuous dynamic recrystallization (DDRX). In addition, strain rate has little influence on grain size and DRX behavior at high strain rate and low temperature. While in γ single-phase region, the dislocation within grains is partly consumed through continuous original boundary migration (COBM) at 0.01 s−1. Moreover, in some deformed grains which are expected to possess high dislocation density, new grains can be formed by DDRX. With the strain rate increasing, the process of COBM was suppressed while DDRX was promoted. Therefore, the main dynamic softening mechanism of alloy is DDRX under the high temperature and high strain rate.

A cohesive zone model for thermomechanical fatigue

In this study, a temperature dependent elastic-viscoplastic cohesive zone model is developed to predict the thermomechanical fatigue (TMF) behaviour of superalloys. The theory is based on rheological models enhanced with fatigue and creep damage variables which are micro-mechanically motivated. This cohesive law can capture monotonous, cyclic, thermomechanical, and rate dependent loads.The material model is used to simulate the deformation behaviour and to determine the lifetime for isothermal, in-phase- (IP) and out-of-phase- (OP) TMF loading. The capability of the model to capture phase difference between mechanical and thermal loading is demonstrated, and the effects are analysed.

Experimental and Numerical Studies on Recrystallization Behavior of Single-Crystal Ni-Base Superalloy.

The recrystallization (RX) behavior of superalloy during standard solution heat treatment (SSHT) varies significantly with deformation temperature. Single-crystal (SX) samples of Ni-base superalloy were compressed to 5% plastic deformation at room temperature (RT) and 980 °C, and the deformed samples were then subjected to SSHT process which consists of 1290 °C/1 h, 1300 °C/2 h, and 1315 °C/4 h, air cooling. RT-deformed samples showed almost no RX grains until the annealing temperature was elevated to 1315 °C, while 980 °C-deformed samples showed a large number of RX grains in the initial stage of SSHT. It is inferred that the strengthening effect of γ’ phases and the stacking faults in them increase the driving force of RX for 980 °C-deformed samples. The RX grains nucleate and grow in dendritic arms preferentially when the microstructural inhomogeneity is not completely eliminated by SSHT. A model coupling crystal plasticity finite element method (CPFEM) and cellular automaton (CA) method was proposed to simulate the RX evolution during SSHT. One ({111} <110>) and three ({111} <110>, {100} <110>, {111} <112>) slip modes were assumed to be activated at RT and 980 °C in CPFEM calculations, respectively. The simulation takes the inhomogeneous as-cast dendritic microstructure into consideration. The simulated RX morphology and density conform well to experimental results.

Relationship between unusual high-temperature fatigue crack growth threshold behavior in superalloys and sudden failure mode transitions

An investigation of high temperature cyclic fatigue crack growth (FCG) threshold behavior of two advanced nickel disk alloys was conducted. The focus of the study was the unusual crossover effect in the near-threshold region of these type of alloys where conditions which produce higher crack growth rates in the Paris regime, produce higher resistance to crack growth in the near threshold regime. It was shown that this crossover effect is associated with a sudden change in the fatigue failure mode from a predominant transgranular mode in the Paris regime to fully intergranular mode in the threshold fatigue crack growth region. This type of a sudden change in the fracture mechanisms has not been previously reported and is surprising considering that intergranular failure is typically associated with faster crack growth rates and not the slow FCG rates of the near-threshold regime. By characterizing this behavior as a function of test temperature, environment and cyclic frequency, it was determined that both the crossover effect and the onset of intergranular failure are caused by environmentally driven mechanisms which have not as yet been fully identified. A plausible explanation for the observed behavior is proposed.

Hot Corrosion Behaviour of Superalloys and Plasma Coatings as a Protective Technique – A Review

Hot Corrosion Behaviour of Superalloys and Plasma Coatings as a Protective Technique – A Review

High-Temperature Oxidation Behavior of Two Nickel-Based Superalloys Produced by Metal Injection Molding for Aero Engine Applications

For different high-temperature applications like aero engines or turbochargers, metal injection molding (MIM) of superalloys is an interesting processing alternative. For operation at high temperatures, oxidation behavior of superalloys produced by MIM needs to match the standard of cast or forged material. The oxidation behavior of nickel-based superalloys Inconel 713 and MAR-M247 in the temperature interval from 1073 K to 1373 K (800 °C to 1100 °C) is investigated and compared to cast material. Weight gain is measured discontinuously at different oxidation temperatures and times. Analysis of oxidized samples is done via SEM and EDX-measurements. MIM samples exhibit homogeneous oxide layers with a thickness up to 4 µm. After processing by MIM, Inconel 713 exhibits lower weight gain and thinner oxide layers than MAR-M247.

Microstructural Assessment of IN718: A Closed-Loop Fatigue Approach

The goal of relating a local fatigue life approach with different microstructures requires the consideration of the main forging process dependent influence factors and their effect on grain size, grain shape, grain contiguity and others. The presented methodology shows the generation and use of a microstructural based evaluation method to link the grain-shape based texture and morphology to the low-cycle-fatigue behavior of superalloy 718. The developed microstructural based energy approach supports an alternative description of the microstructure and grain shape texture. Both, the morphology and the statistical distribution, although covering as-large-as grains, are assessed by two independent numerical parameters. A parametric link to local fatigue parameters was established using this alternative microstructural characterization technique. A prediction of the microstructural evolution during the forging process is already available at design stage for hot-forging of this nickel-base superalloy. In this regard the value of the first parameter e correlates with the mean grain size; the value of the second parameter b is affected by the local forging process history. This enables the lifetime assessment of local forging process at design stage using advanced forging simulation tools. This holistic approach of establishment an experimental based methodology from specimen tests, extensive companying metallographic evaluations, linking them with local forging parameters, implementing the supported microstructural parameters to forging simulation codes and calculating the local component lifetime closes the simulation chain for superalloy 718.

Diffraction Profile, Strain Distribution and Dislocation Densities during Stage II Creep of a Superalloy

One of the major ingredients of modelling the mechanical behaviour of superalloys is the knowledge of dislocation densities and strain distribution. Both can be measured using post mortem BF TEM and CBED, but such methods do not allow following their variations during a test. The aim of the present work is to investigate the usefulness of in situ X-Ray Three Crystal Diffractometry (TCD) to measure the density and distribution of dislocations within a rafted superalloy, i.e. during stage II of high temperature creep. As the instrument contribution is very low, the two-peaked experimental profiles are representative of the lattice parameter distribution within the material. The profiles were measured within bulk specimens at the BW5 high energy beamline Hasylab (DESY), during high temperature (1050°C to 1180°C) tests under loads between 0 MPa and 300 MPa. The peak shapes were observed to change with varying experimental conditions. The peak width follows different patterns under low and high stress, i.e. with low and high strain rates. The distribution of elastic strains was calculated by assuming two main contributions: dislocation segments trapped at the γ/γ’ interfaces in a more or less regular network, and dislocations moving within the γ’ rafts. A comparison between experimental and simulated peaks shows that several features of their behaviour can be explained: the absolute magnitude of the peak width, the observed decrease of the peak width under low loads with increasing interfacial dislocation densities. The larger increase in the width of the γ’ peak under high load (and strain rate) may be attributed to a dislocation density within the 1013 m-2 range within the rafts. The present results are presently being cross-checked by post mortem TEM observations.

&lt;i&gt;In Situ&lt;/i&gt; Measurement of Internal Stresses and Strain Rates by High Energy X-Ray Diffraction during High Temperature Mechanical Testing

The combination of high temperature (1050°C -1150°C) testing and in situ high energy X-Ray diffraction measurements using synchrotron Three Crystal Diffractometry may give various insights into the mechanical behaviour of superalloys: measurement of the lattice mismatch, order within the ' phase, elastic constants, and dynamic response to changes in the experimental conditions. Several examples are given on the rafted AM1 superalloy, resulting from experiments at the ID15A (ESRF) and BW5 (DESY) high energy beamlines.

Use of instrumented micro-indentation to study the mesoscopic elasto-plastic behavior of GH4145/SQ superalloy during high-temperature cyclic straining

In this paper the instrumented micro-indentation testing (IIT) was used to study the mesoscopic elasto-plastic behavior of nickel-base superalloy GH4145/SQ subjected to cyclic straining at an elevated-temperature. For this purpose, a series of experiments including high-temperature low-cycle fatigue tests, instrumented micro-indentation measurements and OM (TEM) examinations were carried out. The characteristic parameters of indentation (C, hmax, S, hr, We and WP) during high-temperature fatigue failure process were determined from the experimental indentation load–depth (P–h) curves. The fatigue mesoscopic elasto-plastic properties such as E, σy and n were estimated using Dao et al.’s analysis algorithm [19], combined with the indentation characteristic parameters, and their distribution patterns were verified in a statistical sense. Microstructural examinations using both OM and TEM were performed to provide the micromechanisms for the fatigue mesoscopic elasto-plastic behavior of the superalloy during high-temperature low-cycle fatigue.

The Behaviour of Superalloys in Marine Gas Turbine Engine Conditions

This paper presents hot corrosion results carried out systematically on the selected nickel based superalloys such as IN 738 LC, GTM-SU-718 and GTM-SU-263 for marine gas turbine engines both at high and low temperatures that represent type I and type II hot corrosion respectively. The results were compared with advanced superalloy under similar conditions in order to understand the characteristics of the selected superalloys. It is observed that the selected superalloys are relatively more resistant to type I and type II hot corrosion when compared to advanced superalloy. In fact, the advanced superalloy is extremely vulnerable to both types of hot corrosion. Subsequently, the relevant reaction mechanisms that are responsible for slow and faster degradation of various superalloys under varied hot corrosion conditions were discussed. Based on the results obtained with different techniques, a degradation mechanism for all the selected superalloys as well as advanced superalloy under both types of hot corrosion conditions was explained. Finally, the necessity as well as developmental efforts with regard to smart corrosion resistant coatings for their effective protection under high temperature conditions was stressed for their enhanced efficiency.

Tribocorrosion of Stellite 706 and Tribaloy 400 superalloys

Co based superalloys such as Stellites and Tribaloys have been extensively used in many engineering applications owing to their inherent superior corrosion and wear resistance and hot hardness property at elevated temperatures. The combined interaction among wear and corrosion which is known as tribocorrosion often results in a significant increase in material loss especially in aqueous environments. In this study tribocorrosion performance of Stellite 706 and Tribaloy T400 was investigated by electrochemical techniques such as potentiodynamic (anodic polarisation) and open circuit potential (OCP) measurement with three electrode electrochemical cell set-up under sliding contact in 3·5% NaCl solution. The effect of friction on anodic polarisation behaviour of superalloys was studied at different loads of 20 and 80 N. During OCP measurements the frequency was increased from 0·5 to 1·5 Hz by an increment of 0·5 Hz. Thus the effect of frequency on tribocorrosion behaviour of superalloys was determined. Tribaloy 400 exhibited greater performance in potentiodynamic polarisation test. On the other hand, Tribaloy 400 was more affected by corrosive wear in OCP measurements as compared to Stellite 706.

Effect of Original Microstructure on the Hot Compression Behavior of Superalloy 718

Hot compression tests at different temperatures and strain rates were performed on superalloy 718 annealed at 1100 °C for 30 min as well as 950 °C for 30 min in order to investigate the effect of the original microstructure on the hot deformation behavior. The results show that the δ phase precipitated on the grain boundaries of superalloy 718 prior to hot deformation leads to a decrease of the flow stress, which can be attributed to the drop of the matrix strength. The activation energy (Q) for hot deformation of the fine-grained material with δ phase on the grain boundaries is determined to be 381 kJ/mol, which is much lower than that of the coarse-grained material without δ phase (472 kJ/mol). It is suggested that the initial δ phase precipitated on the grain boundaries of the annealed material affect the thermal activation process of hot deformation for superalloy 718, resulting in the decrease of activation energy.

An Investigation on Electromagnetic Shaping and Solidification Behavior of Superalloys under Vacuum Environment

An Investigation on Electromagnetic Shaping and Solidification Behavior of Superalloys under Vacuum Environment

High-temperature erosion resistance of coatings for use in turbomachinery

Industrial gas turbines, aircraft gas turbines, steam turbines and coal-utilization turbines, when operating under particulate flows, are exposed to erosion and performance deterioration. This paper describes an experimental method used to find the erosion behavior of superalloys and coatings subjected to gas-particle flows at high temperatures. The erosion behaviors are investigated experimentally at high temperatures using a specially designed erosion wind tunnel. The erosion results show the effects of velocity, temperature and impact angle on the erosion rate.

Interfacial dislocation based criterion for the prediction of rafting behavior in superalloys

The effect that the morphological changes in {gamma}/{gamma}{prime} superalloys have on the mechanical properties of these materials is still an unresolved issue of the greatest interest. Experimental studies on [001] oriented single crystals have shown, for instance, that when a microstructure consisting of evenly spaced cuboids of {gamma}{prime} embedded in {gamma} was subjected to externally applied stresses at high temperature, then the coalescence occurred in an orientation-dependent way, and also that these rafting phenomenon can have a significant influence on the creep or fatigue life in load-bearing applications. In some alloys, the coalescence produced long rods or needles of {gamma}{prime} oriented parallel to the axis of stress, and others produced flat plates or rafts in an orientation perpendicular to the stress axis. In most alloys this orientation dependence was found to invert when a stress of opposite sign was applied. Recent attempts to predict the rafting behavior of these materials have suggested using a combined Monte Carlo/finite element approach which relies on the elastic strain energy and an isotropic interfacial energy term. While this approach succeeded in reproducing the map developed by Pineau and based on the elastic energy of inclusions, a large amount of computation was needed even formore » small lattices in simulations of the microstructural evolution. The present study, which is part of a wider research program on the mechanical performance of superalloys, is aimed at presenting a novel criterion providing an improved rafting prediction map and also a simpler way to model the energy anisotropy, so as to allow simulations of larger systems or for longer microstructural evolutions.« less

Oxidation Behaviour of Superalloys Protected by Plasma Sprayed M Cr AlYTa Coatings

Oxidation Behaviour of Superalloys Protected by Plasma Sprayed M Cr AlYTa Coatings