Advanced Ultra-supercritical Power Plants Research Articles

Static and dynamic friction and wear of cobalt-based coatings at elevated temperatures

Advanced ultra-supercritical power plants, in which steam temperatures exceed 700 °C, demand the development of protective coatings, particularly for valves governing the steam flow where sealing surfaces are subjected to high static tribological solicitations. In this context, we systematically investigated the friction (static and dynamic) and wear of two cobalt-based coatings and one nickel-based substrate. Results indicate that oxidation kinetics exhibits a double-edged sword effect in terms of tribological properties at elevated temperatures. Fast oxidation kinetics promotes the sintering of oxides, generating a smoother oxide tribofilm and lowering the dynamic coefficient of friction (COF). However, this can also significantly increase the static COF over loading time due to welding/sintering of oxidized asperities, leading to increased torque requirements for valve operation.

Role of threshold stress in creep of IN740H, a γ′-lean Ni-based superalloy

IN740H, a Ni-based superalloy comprising a low-volume fraction of γ′, is a candidate material to be used over long periods at high temperatures and stresses, as conditions prevalent in advanced ultra-supercritical (AUSC) power plants. In this study, IN740H has been tested at temperatures above 700 °C to gain mechanistic insights into its high-temperature creep behavior. The material showed classic signatures of the presence of threshold stress, marked by observation of a high apparent creep stress exponent, n, (e.g., 10 at 750 °C) and a high apparent activation energy for creep, Qc (e.g., ∼545 kJ/mol), along with a rapid increase in n at lower stresses. Accounting for the threshold stress led to a decrease in the values of n and Qc to 4 and 280 kJ/mol, respectively. Furthermore, the transmission electron microscopy and atomic scale compositional analysis reveal the pinning of dislocations at γ-γ′ interface and segregation of Co, Cr and Mo atoms in the regions of γ-γ′ interface rich in dislocations. The above combination of n and Qc and the observation of dislocation pinning at the γ-γ′ interface indicate the dislocation climb over γ′ as the dominant creep mechanism in this γ′-lean Ni-based superalloy, with the detachment of dislocations from the γ-γ′ interface, augmented by the segregation of Co, Cr and Mo, as the mechanism responsible for the realization of the threshold stress. This work, thus, provides a new impetus to research on the long-term structural integrity of γ′-lean Ni-based superalloys exposed to extreme service conditions.

Analyzing dynamic strain aging and yield strength anomaly in IN740H, a nickel-based superalloy comprising low volume fraction of γ′

IN740H, a Ni-based superalloy comprising a low volume fraction of γ′, is a promising material for superheater and reheater components in advanced ultra-supercritical (AUSC) power plants. Here, its tensile deformation behavior across a temperature range of 25–800 °C and strain rates of 10−3 to 10−5 s−1 was investigated. Distinct stress-strain curve serrations, consistent with dynamic strain aging (DSA), were predominantly observed between 400 and 600 °C. At lower temperatures and strain rates, Type E serrations appeared, which transitioned to Type B and then to Type B + C serrations with increasing temperature. At 600 °C, Type B serrations emerged at high strain rates, gradually transforming into Type B + C serrations as rates decreased before disappearing at lower rates. Notably, the alloy displayed negative strain rate sensitivity in the DSA regime. Furthermore, the alloy also showed yield strength anomaly (YSA), wherein the yield strength decreased initially with temperature, followed by an increase above a specific temperature. Fractography reveals a transition from transgranular dimple fracture behavior to intergranular fracture behavior with an increase in temperature. Post-deformation electron backscatter diffraction analysis indicated predominant grain alignment along {100} and {111} planes, such that {111}-oriented grains exhibited a lower Schmidt factor and the formation of fewer sub-grain boundaries. This work comprehensively characterizes DSA and YSA in IN740H in the temperature range relevant to AUSC power plants, thereby providing much-needed information to assess its potential suitability for applications under harsh conditions.

Microstructure evolution of high-temperature deformation behavior of nickel based alloys for advanced ultra-supercritical applications

The microstructure evolution of nickel based alloy used in advanced ultra-supercritical power plants was studied in the strain range of 0.22–0.91 at 1000–1100 °C/0.01 s−1 by using the electron backscattered diffraction technique. The interaction of the dislocation with the Σ3 twin boundary causes the Σ3 twin boundary to deviate from its specific misorientation. The Σ3 twin boundary is stripped of its signature by the interaction between the dislocation and the Σ3 within the deformed grain. In the process of recrystallization, the Σ3 twin boundaries are affected by both recrystallization fraction and the size of recrystallization grain. Three recrystallization mechanisms, including DDRX, CDRX and TDRX, play their role in the deformation of the alloy. Leading to the recrystallized grains with high misorientation, CDRX tends to grow and promote twin nucleation, and the mechanism of CDRX becomes less significant with the increase of deformation temperature. Due to deformation, the orientation of the grain shifts to <101> direction. At 1000 °C and 1050 °C, the recrystallization texture inherits the deformation texture due to the effect of CDRX and the slow diffusion of the recrystallization process, which causes the recrystallization texture to show a sharp <101> fiber texture. At 1100 °C, the enhancement of the DDRX mechanism and the high diffusivity of the recrystallization process lead to the randomization of the texture, and the oriented growth of recrystallized grains at 1100 °C/0.91 enhances the <001> texture. In addition, both recrystallization and twinning are contributory to the randomization of texture.

Optimization of reheat-pressure ratios in Supercritical, Ultra Supercritical and advanced ultra-supercritical Thermal Power Plants with Single Reheating

This research paper presents the thermodynamic analysis of a modern supercritical (SC), ultra supercritical (USC) and advanced ultra supercritical (AUSC) steam power plants with a power generating capacity of 1200 MW, employing with single re-heat configuration. A MATLAB software code has been developed to accurately estimate the steam properties under varying conditions. The study focuses on the temperature and pressure at the turbine's inlet and outlet. The cycle's energy efficiency and exergy efficiency have been thoroughly examined, along with the impact of single reheat pressure on the cycle's performance. It has been observed that both efficiencies exhibit a more pronounced increase with rising temperatures compared to increasing pressures. The exergy losses incurred in each of the cycle's components have been meticulously studied and analyzed by systematically varying the pressure ratios of SC, USC and AUSC power plants.

Activated Tungsten Inert Gas Weld Characteristics of P91 Joint for Advanced Ultra Supercritical Power Plant Applications

Activated Tungsten Inert Gas Weld Characteristics of P91 Joint for Advanced Ultra Supercritical Power Plant Applications

Achieving High Strength and Creep Resistance in Inconel 740H Superalloy through Wire-Arc Additive Manufacturing and Thermodynamic-Guided Heat Treatment.

Inconel 740H superalloy is commonly used in advanced ultra-supercritical power plants since it possesses excellent strength and creep resistance. This study investigates the microstructure and mechanical properties of Inconel 740H superalloy fabricated using wire-arc additive manufacturing. The as-printed microstructure consisted of columnar γ grains with the Laves phase and (Nb, Ti)C carbides as secondary phases. The anisotropy in grain structure increased from the bottom to the top regions, while the hardness was highest in the middle portion of the build. To guide the post-heat treatment design, thermodynamic and kinetic simulations were employed to predict the temperature and time. Complete recrystallization with the Laves phase dissolution occurred throughout the build after homogenization at 1200 °C for 2 h. The peak hardness was achieved after aging at 760 °C for 12 h with the M23C6 carbides decorating the grain boundaries and γ' precipitates in the grain interior. The yield strength (655 MPa) and ductility (29.5%) in the post-heat treated condition exceeded the design targets (620 MPa, 20%). Stress rupture tests at 750 °C showed that the high-temperature performance was at par with the wrought counterparts. The fracture mode after rupture was identified to be intergranular with the presence of grain boundary cavities along with grain boundary sliding.

Microstructure and Pitting Corrosion Characteristics of Tig Welded Joints of Super 304HCu Austenitic Stainless Steel

Abstract Super 304HCu is an advanced ultra-super critical (A-USC) boiler grade austenitic stainless steel with the distinct addition of 3 wt.-% of Copper. A-USC power plants intended to operate in chloride rich environments (sea shore, feed water residues, etc.) are susceptible to chloride assisted corrosion failures. In this study, the pitting corrosion behaviour of the Super 304HCu parent material and tungsten inert gas weld joints was studied using a potentiodynamic cyclic polarization test in 3.5 % NaCl solution at three different pH levels (pH = 3, pH = 7, and pH = 11). The Epit values of the parent material is found to be much nobler than that of the weld joints. The micrographs of the pitted weld joints and the oxalic acid etched structure of Super 304HCu joints are presented. From the micrographs it is revealed that the heat affected zone is the most susceptible region to pitting corrosion.

Creep-fatigue life and damage evaluation under various strain waveforms for Ni-based Alloy 740H

This paper presents creep-fatigue life evaluation of a nickel-based Alloy 740H, for evaluating its performance as a material of advanced ultra-supercritical power plants. Strain controlled creep-fatigue tests were performed under six strain waveforms using solid bar specimens at 700 °C, and the effect of strain waveform on creep-fatigue life was discussed. Creep-fatigue lives were shorter than pure fatigue life depending on the strain waveform. The failure life in symmetrical waveform was reduced to about half as the strain rate decreased by one order of magnitude. The failure lives in the unsymmetrical waveforms were smaller than that in the symmetrical waveform and tensile slow straining was more detrimental to fatigue life than compressive slow straining. The creep-fatigue life decreased significantly due to strain hold, and tensile peak strain hold had more enhanced effect on fatigue life than compressive strain hold. Applicability of five existing approaches based on linear damage summation rule were discussed. In addition, a new approach based on modified ductility exhaustion model with several modifications was proposed in order to improve the life predictability of the classical approach. Time fraction rule with ASME failure envelope and new approach satisfactorily predicted the creep-fatigue lives almost within a scatter band of 2.

Three-step learning strategy for designing 15Cr ferritic steels with enhanced strength and plasticity at elevated temperature

15Cr ferrite steels are urgently required in advanced Ultra-supercritical power plants but meet design challenges in balancing excellent strength and plasticity at high temperatures. We developed a three-step learning strategy based on mutually driven machine learning and purposeful experiments to complete this multi-objective task. Compared with traditional adaptive learning and local-interpolation learning, this step-by-step modular manner provides good transparency and interpretability of the information flow, which is ensured by identifying essential factors from an exquisitely prepared composition-microstructure dataset, and learning valuable knowledge about the composition-property relationship. The requirement of only two groups of experiments indicates the low cost and high efficiency of the strategy. Performing the strategy, we found that Ti is another key element affecting the Laves phase besides Mo and W, and their effects on ultimate tensile strength (UTS) and elongation were also uncovered. Importantly, several low-cost steels free of Co were successfully designed, and the best steel exhibited 156%, 31%, and 62% higher UTS and elongation at 650 °C than the typical 9Cr, 15Cr, and 20Cr steels, respectively. Based on the advantages and success of the strategy in terms of alloy improvement, we believe the strategy suits other multi-objective design tasks in more materials systems.

Effect of microstructural evolution and mechanical properties of Haynes 230 superalloy during long-term aging at 700 °C

The effect of long-term isothermal aging of Haynes 230 super-alloy in an advanced ultra-supercritical (A-USC) power plant at 700 °C for up to 10,000 h on microstructural evolution and tensile behavior. And utilizing a combination of the scanning electron microscope (SEM), X-ray diffraction analysis (XRD), and transmission electron microscope (TEM). Although the grain size and grain boundary M23C6 carbides did not change significantly, the dissolution of intercrystallite primary M6C carbides and the precipitation of intracrystalline secondary lamellar M6C carbides were observed at 2000 h to 10,000 h. According to the SEM and TEM results, it is shown that the dissolution of primary M6C carbides makes the W, Mo, and Cr in the γ matrix increase, while the precipitation process of secondary M6C carbides is M+ 6 C→M6C. To reduce the interfacial energy of the γ matrix, secondary M6C carbides are precipitated in the crystal under long-term aging at 700 °C, with a gradual increase in time content. the best tensile and yield strengths at 750 °C occur after 4000 h of aging, with microscopic voids forming mainly around the granular M6C carbides in the crystal, but the solid solution strengthening effect of the matrix provides the strength. The worst strength and elongation occur after 10000 h aging, when the dissolution of granular M6C and secondary precipitation of M6C carbides are most abundant, interfacial cavity formation of granular M6C carbides and brittle fracture of M6C carbide particles, and the growth of non-co-grained secondary M6C carbides with the matrix allow more microscopic cavities to appear during high-temperature tension, reducing the strength of Haynes 230 and plasticity.

Investigations on wetting and structural behavior using CaF2-SiO2-CaO-22.5%TiO2 SMAW electrode coating for AUSC applications

This study investigates the structural behavior and high-temperature wettability of a CaF2-SiO2-CaO-22.5%TiO2 shielded metal arc welding electrode coating for advanced ultra-supercritical power plant applications. The wettability parameters were measured, including contact angle, spread area, adhesion energy, and surface tension. Extreme vertices mixture design approaches were used to develop 13 different electrode coatings. To determine the contact angle at solid/liquid interface, circular pellets were made from powder samples and heated inside the furnace at 1125°C at a constant heating rate. Young's and Boni's equations are used to calculate the surface tension value for 13 coated samples using the calculated contact angle. Adhesion energy for 13 coated samples is calculated using Dupre's equation. Phases and structural behavior of coating samples were measured by Fourier transform infrared spectroscopy and X-ray diffraction. The developed regression model estimates the impact of primary ingredients and their interaction on the calculated wettability characteristics. CaF2.SiO2, CaO.SiO2, and CaO.CaF2 binary mixture is the more favorable and substantially impacts contact angle, whereas ternary interactions of CaO.SiO2.CaF2 shows an opposite effect on contact angle. Coating numbers 2 and 10 give better wetting behavior and spread area due to the low value of contact angles. Individual ingredient shows an increasing effect on surface tension and adhesion energy. Ternary component of CaO.CaF2.SiO2 is favorable and shows an increasing impact on adhesion energy.

Tensile Behavior, Constitutive Model, and Deformation Mechanisms of MarBN Steel at Various Temperatures and Strain Rates.

To reduce harmful gas emission and improve the operational efficiency, advanced ultra-supercritical power plants put forward higher requirements on the high temperature mechanical properties of applied materials. In this paper, the tensile behavior and deformation mechanisms of MarBN steel are discussed at different strain rates (5 × 10-3 s-1, 5 × 10-4 s-1, and 5 × 10-5 s-1) under room temperature and 630 °C. The results show that the tensile behavior of the alloy is dependent on temperature and strain rate, which derived from the balance between the average dislocation velocity and dislocation density. Furthermore, observed dynamic recrystallized grains under severe deformation reveal the existence of dynamic recovery at 630 °C, which increases the elongation compared to room temperature. Finally, three typical constitutive equations are used to quantitatively describe the tensile deformation behavior of MarBN steel under different strain rates and temperatures. Meanwhile, the constitutive model of flow stress for MarBN steel is developed based on the hyperbolic sine law.

Microstructure, thermal stability and tensile properties of a Ni–Fe–Cr based superalloy with different Fe contents

The effects of Fe content on microstructure, thermal stability and tensile properties of a Ni–Fe–Cr based superalloy, mainly chosen as turbine rotor material in 700 °C advanced ultra-supercritical power plants, have been systematically investigated. The results show that the major precipitates of the heat-treated alloys with different Fe contents are all γ′ phase, MC and M23C6 carbides, whereas the precipitation of γ′ phase is suppressed in Fe-rich alloys. Fe content significantly improves the ductility of alloys, but slightly deteriorates the strength resulting from the decreased volume fraction of γ′ phase. After long-term thermal exposure, the morphology transformation of γ′ phase from spherical to cubic shape is observed in the alloy with 35% Fe content. The coarsening rate coefficient of γ′ phase decreases from 485.7 nm3/h to 280.8 nm3/h as increasing Fe content from 15% to 35%. The main reason is that there is a large concentration gradient of Fe element in the vicinity of γ/γ′ interface, which obviously restricts Fe atoms from diffusing away. The change of yield strength with particle size can be explained by the strongly coupled dislocation shearing and Orowan bowing models. When Fe content is up to 35%, the yield strength of the alloy reduces more rapidly than the theoretical value due to the precipitation of harmful Laves phase in the intermediate stage of thermal exposure.

Precipitation Behavior of Ni-Based Superalloy Alloy 625 for A-USC Power Plants

The age-hardening behavior of the wrought Ni-based superalloy Alloy 625 was investigated in the temperature range between 923 and 1173 K for the application to advanced ultra-supercritical (A-USC) power plants. The carbon content of the alloy was controlled as low as possible to minimize the precipitation of carbides during aging. A two-step increase of hardness was detected for the alloy at temperatures between 1000 and 1100 K; the first increase of hardness results from the precipitation of the metastable γ′′ phase, and the second increase corresponds to the precipitation of the orthorhombic δ phase. In contrast, a single-step increase of hardness was detected below 1000 K derived from the precipitation of γ′′ phase and above 1100 K derived from the precipitation of δ phase. The TTP (time–temperature–precipitation) diagram for the alloy was established on the basis of the results of hardness measurements and microstructure observations, where the nose temperatures of γ′′ and δ phases are determined as 1050 and 1123 K, respectively. The γ′′ particle coarsened along the Ostwald ripening. The activation energy for the γ′′ coarsening was evaluated as 202 kJ/mol, which is very close to that for the inter-diffusion of Nb in Ni.

Experimental investigation on SMAW electrode coatings developed using CaO–SiO2–CaF2–SrO based coating system

This research aims to provide a SMAW electrode coating developed using a CaO–SiO2–CaF2–SrO flux system for welding the components of an advanced ultra-supercritical power plant. The physicochemical and thermophysical characteristics of developed coatings were investigated. Twenty-one distinct electrode coating formulations were developed using the extreme vertices technique. Experiments for several thermophysical and physicochemical characteristics were carried out. The coating's qualities are critical for producing a strong weld that will last in the field. X-ray diffraction and Infrared spectroscopy were used to examine the structural behavior of the electrode coatings. Furthermore, the regression model was developed to explore the effect of coating compositions. The primary constituents and their interaction have an influential role in physicochemical and thermophysical characteristics. Individual and ternary mixtures significantly affect the density. CaO.SiO2.SrO and CaF2·SiO2·SrO is the most effective ternary mixture, which has an increasing effect on density. The weight loss of coating observed during thermogravimetric analysis is affected by individual constituents significantly. The ternary mixture CaO·CaF2·SiO2, CaO·CaF2·SrO, and CaF2·SiO2·SrO is the most favorable and has an increasing effect on weight loss. Individual constituents affect change in enthalpy significantly. The thermal properties of coating observed during hot disc are affected by binary and ternary mixture constituents. Binary mixture CaO·SiO2 is having an increasing effect on thermal conductivity. Binary mixture CaO·SrO and SrO containing ternary mixture have an increasing impact on thermal diffusivity. Ternary mixture constituents CaO·CaF2·SrO, CaF2·SiO2·SrO is the most effective and has an increasing effect on specific heat.

Some studies on dissimilar welds joint P92 steel and Inconel 617 alloy for AUSC power plant application

The Cr–Mo steel containing 9% Cr is generally preferred in producing high-temperature operating components (lower than 620 °C) like steam pipes and headers due to their outstanding combination of elevated temperature mechanical properties and creep strength. The components with an operating temperature of more than 620 °C (superheater and reheaters tubes) are mainly produced using stainless steel or Ni-based superalloy. Hence due to economic aspects of boilers, joining of P92 steel with Ni-based superalloy is inevitable in advanced ultra-supercritical (AUSC) plants. Thus the development of joining technology without diminishing the high-temperature mechanical properties of the weldments becomes essential. The present work introduces the microstructure evolution in gas tungsten arc welded (GTAW) dissimilar joint of P92 steel and Inconel 617 alloy produced by using the P92 filler. The microstructure study along weldments was examined using an optical microscope (OM) and field emission scanning electron microscope (FESEM). The mechanical properties of welded joint and joint integrity were determined by using the microhardness measurement, cross-weld tensile tests, and impact toughness test. The effect of the post-weld heat treatment (PWHT) on mechanical properties and microstructure evolution was also performed. The residual stresses were also examined using the blind hole drilling methods, and the effect of PWHT on the nature and magnitude of the residual stresses were also conducted. The microstructure observation showed the martensitic lath structure in the weld metal, which resulted in poor impact toughness (36 ± 5 J) and high hardness (458 ± 25 HV) of weld metal in the as-welded condition. A considerable increase in impact toughness (90 ± 4 J) and a decrease in hardness (371 ± 11 HV) of weld metal were observed after the PWHT. A wide region of the unmixed zone at the interface of weld metal and Inconel 617 was formed due to the difference in chemical composition and structure. The unmixed zone formation led to the poor tensile properties of the welded joint, and failure was observed both in P92 BM and at the interface of weld metal and Inconel 617 interface. The tensile tested sample failed from the interface region and showed the presence of Ti(C, N) particles, while secondary phase M23C6 particles were obtained from the tensile sample fractured from P92 BM. The impact test results also showed the variation in impact toughness along the weldments. The P92 BM was found the strongest zone, while weld metal was noticed as the weakest zone in terms of impact toughness. The variation in residual stresses was also measured along with the thickness of the plate, and the peak magnitude of the residual stresses was measured in the capping pass, which was compressive in nature.

Physicochemical and thermophysical properties of CaO–TiO2–SiO2–Na3AlF6 system based electrode coating for AUSC power plant

The aim of this study is to investigate the physicochemical and thermophysical properties of CaO–TiO2–SiO2–Na3AlF6 based electrode coating developed for the welding of advanced ultra-supercritical (AUSC) thermal power plant components. The extreme vertices approach has been used to create twenty-six electrode coating compositions. Various physicochemical and thermophysical properties were characterized using experiments. The coating properties are essential in ensuring sound weld with desirable in-service performance. The developed coating's structural behavior was analyzed using Fourier transformation infrared spectroscopy (FTIR) and X-ray diffraction (XRD). In addition, regression analysis was carried out to evaluate the influence of coating constituents. Individual constituents and their interaction were found to have a prominent role in influencing the physicochemical and thermophysical properties.

Study on hot cracking susceptibility of alloy 617M and an analysis on parameters used for its evaluation

Hot cracking study on nickel base superalloy, alloy 617M, a candidate material for Advanced Ultra Supercritical (AUSC) power plant applications, was carried out. The susceptibility of this alloy to solidification cracking was evaluated using varestraint (variable restraint) test set up and parameters like Brittleness Temperature Range (BTR) and Critical Strain rate per Temperature drop (CST) were estimated from the results. The BTR and CST for alloy 617M indicate higher susceptibility to solidification cracking than stabilized and un-stabilized austenitic stainless steels. However, mock-up welding of 400 mm diameter alloy 617M hollow forgings with weld thickness of 95 mm and alloy 617M boiler tubes of ~12 wall thickness made from this alloy did not reveal any significant cracking in the welds (except some isolated micro fissures) and hence, a detailed analysis of the results from varestraint tests was taken up. The study revealed the dual hot cracking behaviour of alloy 617M and the reasons behind such observation is elucidated. The microstructural characterisation of the hot crack tested specimens of alloy 617M revealed eutectics of austenite + carbides rich in Mo and Cr in the backfilled regions of the hot cracks in the fusion zone. The formation of significant fraction of eutectic phases along the interdendritic regions at the end of solidification is the primary reason for the high BTR exhibited by alloy 617M. Further, backfilling of the solidification cracks by this eutectic liquid is enabled under low augmented strain and the same is insufficient to aid backfilling in cracks at high restraints. The mechanism mentioned above was found to be the underlying reason for the dual behaviour revealed by alloy 617M. Hence, it was deduced that BTR and CST determined at saturated strain cannot be effectively used to predict the actual hot cracking behaviour of alloy 617M. A refined criterion-BTR at threshold strain is proposed to be an appropriate solidification cracking susceptibility index for this alloy.

Precipitation Behavior of Ni-Based Superalloy Alloy 625 for A-USC Power Plants

The age-hardening behavior of the wrought Ni-based superalloy Alloy 625 was investigated in the temperature range between 923 and 1173 K for the application to advanced ultra-supercritical (A-USC) power plants. The carbon content of the alloy was controlled as low as possible to minimize the precipitation of carbides during aging. A two-step increase of hardness was detected for the alloy at temperatures between 1000 and 1100 K; the first increase of hardness results from the precipitation of the metastable γ′′ phase, and the second increase corresponds to the precipitation of the orthorhombic δ phase. In contrast, a single-step increase of hardness was detected below 1000 K derived from the precipitation of γ′′ phase and above 1100 K derived from the precipitation of δ phase. The TTP (time–temperature–precipitation) diagram for the alloy was established on the basis of the results of hardness measurements and microstructure observations, where the nose temperatures of γ′′ and δ phases are determined as 1050 and 1123 K, respectively. The γ′′ particle coarsened along the Ostward ripening. The activation energy for the γ′′ coarsening was evaluated as 202 kJ/mol, which is very close to that for the inter-diffusion of Nb in Ni.