Gamma Prime Particles Research Articles

Development of morphology and lattice misfit in modified Ni-base superalloy with Al, Co and Ni additions

Abstract This research work studied the effect of aluminium, cobalt and nickel additions in superalloys grade GTD-111 and IN-738 on morphology of gamma prime particles and their lattice misfits after subjected to heat treatment and long-term cyclic exposure by using SEM and XRD. It was found that the particles became more cuboidal in shape after heat treatment in modified GTD-111 and IN-738 alloys. When adding 1 wt% aluminium and 6 wt% nickel, larger negative values of lattice misfit were found at −0.57 % and −0.28 %, respectively. However, 6 wt% cobalt addition reduced the negative value of the lattice misfit to −0.20 % in modified GTD-111 and small positive lattice misfit (+0.08 %) was found in modified IN-738. Also, modified GTD-111 alloy shows more spherical shape of the gamma prime precipitates than those in modified IN-738. After long-term cyclic exposure at 1173 K for 1440 ks, it was found that the particles become more spherical shape comparing to those of heat treated ones. The shape of particles in both modified alloys with 1 wt% aluminium and 6 wt% nickel additions maintained more cuboidal compared to others. Furthermore, the effect of 6 wt% cobalt addition is more pronounced for modified IN-738 than that of modified GTD-111.

Effects of Re and Co additions on lattice parameters and lattice misfit in cast Ni-based superalloys

Abstract Morphological evolution of gamma prime precipitates was governed by an elemental partitioning ratio between the gamma and gamma prime phases. In the present study, Ni-based MGA 1400 alloy was modified by adding 4 wt% of Re and 6.4 wt% of Co by vacuum arc melting. The effects of Re and Co additions on lattice parameters of gamma and gamma prime phases were studied by TEM and XRD techniques. Calculated lattice misfit values obtained from XRD and TEM techniques were used to identify the shape and morphology of gamma prime precipitated particles after long-term exposure at 1173 K for 1800 ks? After long-term exposure, gamma prime particles became much coarser. The addition of Re retarded the coarsening rate of the gamma prime precipitates, resulting in smaller gamma prime particles at the same annealing times. Even though, both Re and Co atoms are strongly partitioned in the gamma matrix, the effect of Re and Co additions on lattice misfits was contradictory. The addition of Re produced rounded corner cuboidal morphology of gamma prime precipitates due to higher magnitude with negative lattice misfit (−0.3857%), while Co addition provided a more rounded shape of gamma prime particles due to the lattice misfit value approach to zero (+0.0936%).

Effect of Re and Ru additions on morphology and long-term stability of gamma prime particles in new modified superalloys prepared by a vacuum arc melting process

Abstract This research investigated the effects of adding rhenium and ruthenium with various contents to new modified nickel-based superalloys, which were prepared and cast using a vacuum arc melting process. Heat treatment processes were then carried out. The size, circularity and the area fraction of the gamma prime phase were characterized via scanning electron microscopy after solution treatment, precipitation aging and long-term heating processes. The addition of ruthenium seems to provide a stronger effect than adding rhenium by increasing the area fraction of the gamma prime phase after precipitation aging. Furthermore, rhenium and/or ruthenium additions maintain the precipitation of the gamma prime particles in a cubic shape. For phase stability analyses, the alloys were subsequently heated at 1173 K in intervals up to 1080 ks. The study showed that the coarsening rate of γ′ particles can be significantly retarded with the addition of rhenium and/or ruthenium, and topologically close-packed phase can occur with excessive rhenium addition. In addition, the highest addition of ruthenium content in this study strongly exhibited high gamma prime phase stability. Thus, increasing the content of ruthenium and decreasing the content of rhenium can inhibit the formation of the topologically close-packed phase.

Compositions of Gamma and Gamma Prime Phases in an As-Cast Nickel-Based Single Crystal Superalloy Turbine Blade

The core and the interdendritic regions of an as-cast nickel based single crystal turbine blade were observed by electron microscopy to understand the microstructural development during an investment casting process. The dendrite core region shows an irregular morphology of gamma prime in gamma due to a relatively short casting time, which prevented the development of gamma prime expected in a solution heat-treated microstructure. By comparison, the interdendritic region comprises three different regions composed of: several elongated gamma prime particles, relatively tiny and irregular gamma prime, and gamma prime with relatively regular morphology. The chemical analysis of these phases showed that, regardless of the analysis point in the core or the interdendritic region, almost the same compositions were acquired in the regular type of gamma and gamma prime phases. This result suggests that if the gamma prime forms in the gamma matrix, the composition of gamma prime is almost uniform regardless of the region and prevailing general chemical composition. In contrast, the composition of the elongated gamma prime in the interdendritic region was slightly different depending on the analysis point even within the same elongated particle.

Long term heating effects at 1173 K and 1273 K on microstructural rejuvenation in various modified alloys based on GTD-111

Abstract This research work studied and evaluated the effects of reheat treatment conditions, which consisted of solution treatment at a temperature of 1448 K for 14.4 ks, followed by air cooling and precipitate aging at a temperature of 1118 K for 86.4 ks, on the microstructural rejuvenation or refurbishment of various modified alloys based on the cast nickel base superalloy, GTD-111 with aluminum, nickel and/or cobalt additions after long term heating at temperatures of 1173 K and 1273 K for 1440 ks. From the results obtained, it was found that the reheat treatment conditions applied are more suitable for microstructures after long term heating at a temperature of 1173 K. However, such reheat treatment conditions could not fully return reheat treated microstructures to microstructures similar to those of previous research work. It seems that the selected solutioning temperatures and/or times were not sufficient to completely dissolve all coarse gamma prime particles after long term heating for all samples with alloying additions. Typical size and area fractions of the gamma prime particles of the reheat treated microstructures are very similar to those of the original alloyed ones but with lower values, especially those related to the size of the gamma prime particles.

Effect of cooling rates of solution treatment on rejuvenation heat-treated microstructures of a cast nickel-based superalloy

Abstract IN-738 turbine blade samples, deteriorated after long term service at high temperatures, were solution heat-treated at two temperatures, 1398 K and 1473 K, for 7.2 ks. Subsequently, the samples were cooled down in different atmospheres, in air and in furnace, for the purpose of studying the effects of different cooling media (rates) on the restored microstructures. Following this, the samples were aged at 1118 K for 43.2 ks and 86.4 ks in order to determine the characteristic of re-precipitated gamma prime particles. A scanning electron microscope (SEM) and ImageJ analysis software were used. The results show that the cooling in air provided gamma prime particles re-precipitating in spherical shape while the cooling in a furnace resulted in coarse gamma prime particles re-precipitating in irregular shape. The samples solutionized at 1398 K for 7.2 ks cooled down in air and then aging at 1118 K provided bimodal microstructure, while the sample solutionized at 1473 K for 7.2 ks, followed by air cooling and aging at 1118 K generated unimodal γ’ precipitation in spherical shape. Cooling in a furnace provides coarse γ’ recipitated particles in more irregular shape for the both solutionizing temperatures studied here. Cooling in a furnace provides coarse γ’ precipitated particles in more irregular shape for the both solutionizing temperatures studied here.

New alloy development from modified cast Ni-base superalloy GTD-111 with additions of Al, Ni and/or Co prepared by vacuum arc melting process

Abstract The aim of this research is to study and evaluate the effects of cobalt and nickel additions on heat-treated and thermal-exposed microstructures in cast nickel-base superalloy grades based on GTD-111 with cobalt and nickel modifications. In the study, 1 wt.-% aluminium, as well as various cobalt and nickel contents, was added to all the modified alloy. After standard heat treatment, it was found that increasing the 1 wt.-% aluminium content affected the morphology of gamma prime particles, causing them to become more cuboidal in shape and larger in size. On the other hand, the size of gamma prime in all modified alloys decreased slightly with increasing cobalt contents. However, cobalt and nickel additions did not strongly affect the shape of the gamma prime particles. After long-term exposure at temperatures of 1173 K and 1273 K for 1440 ks, these gamma prime particles were much larger in size than those of heat-treated ones. The higher temperature caused extreme increases in the average size of thermal-exposed gamma prime particles. Also, the gamma prime particles became nearly spherical in shape and had lower coarsening rate with higher cobalt and lower nickel contents. Moreover, the addition of cobalt reduced the coalescence of gamma prime particles. Samples with cobalt content higher than 13.1 wt.-% were able to maintain the cubic shape of the particles even after long-term exposure at a temperature of 1273 K for 1440 ks. No topologically close-packed phase was found in any analyzed specimens.

Effects of adding Ni and Co on microstructure and gamma prime phase stability in Ni superalloy Inconel 738 with 1 wt.-% Al

Abstract The effects of adding nickel and cobalt to Inconel 738 along with adding 1 wt.-% of aluminum were investigated in this study. The alloys, with various amounts of nickel and cobalt, were prepared using a vacuum arc melting process. The alloys were heat treated using a standard heat treatment procedure which consisted of solution treatment at 1448 K for 14.4 ks and precipitation aging at 1118 K for 86.4 ks. Increased nickel content was shown to enhance the stability of gamma prime particles and the gamma matrix as well as assist the formation of gamma prime particles. In the case of the addition of cobalt, the coarsening rate of gamma prime particles decreased with increasing cobalt content. Furthermore, a further addition of aluminum to Inconel 738 resulted in an increase in the area fraction of the gamma prime phase and more rapid growth of gamma prime particles during long-term heating compared to standard Inconel 738.

Effect of Cooling Rate on Microstructure of Rejuvenated Fe-Ni Based Superalloys

Fe-Ni based superalloys have been widely used in land-base gas turbine application. The turbine blade was in service for 50,000 h at high temperature and stresses. When subjected to long-term exposure at high temperature, the microstructure lost its best mechanical properties due to the microstructural instability. The aim of this research is to understand the effect of cooling rate on gamma (γ) grain size and gamma prime (γ’) particle size, morphology, and its volume fraction in rejuvenated Fe-Ni based superalloys. The alloys were solutionized above the γ’ solvus temperature at 1125 °C for 2 h for homogenization and cooling to room temperature at different cooling rates. The alloys were experienced with furnace cooling, air cooling, oil quenching, and water quenching. Microstructural analyses were investigated. Grain size, morphology, volume fraction of γ’ precipitates were investigated. Preliminary mechanical properties such as microhardness was conducted.

New alloy development from modified cast Ni-base superalloy GTD-111 with additions of Al, Ni and/or Co prepared by vacuum arc melting process

Abstract The aim of this research is to study and evaluate the effects of cobalt and nickel additions on heat-treated and thermal-exposed microstructures in cast nickel-base superalloy grades based on GTD-111 with cobalt and nickel modifications. In the study, 1 wt.-% aluminium, as well as various cobalt and nickel contents, was added to all the modified alloy. After standard heat treatment, it was found that increasing the 1 wt.-% aluminium content affected the morphology of gamma prime particles, causing them to become more cuboidal in shape and larger in size. On the other hand, the size of gamma prime in all modified alloys decreased slightly with increasing cobalt contents. However, cobalt and nickel additions did not strongly affect the shape of the gamma prime particles. After long-term exposure at temperatures of 1173 K and 1273 K for 1440 ks, these gamma prime particles were much larger in size than those of heat-treated ones. The higher temperature caused extreme increases in the average size of thermal-exposed gamma prime particles. Also, the gamma prime particles became nearly spherical in shape and had lower coarsening rate with higher cobalt and lower nickel contents. Moreover, the addition of cobalt reduced the coalescence of gamma prime particles. Samples with cobalt content higher than 13.1 wt.-% were able to maintain the cubic shape of the particles even after long-term exposure at a temperature of 1273 K for 1440 ks. No topologically close-packed phase was found in any analyzed specimens.

The Mechanism of Grain Boundary Serration and Fan-Type Structure Formation in Ni-Based Superalloys

A model of discontinuous precipitation of γ′ phase at grain boundaries (GBs) in polycrystalline Ni-based superalloys during continuous cooling from supersolvus temperature was developed. The model calculates the size and γ′ phase fraction in GB precipitates as a function of cooling rate and GB mobility. The model is based on the classical mechanism of diffusion-controlled precipitation. It considers fast diffusion of γ′-forming elements along the GB and the motion of the GBs under driving force induced by alloy decomposition. The model predicts either discrete GB gamma-prime particles and GB serration or the growth of fan-type structures depending on the cooling rate and GB mobility. We conclude from the model predictions that the GB mobility is the key factor controlling the type and morphology of GB precipitates. High GB mobility results in formation of fan-type structures, while the lower GB mobility leads to GB serration. The predicted dependence of the size of GB precipitates on the cooling rate is in good agreement with the experimental observations.

Effects of adding Ni and Co on microstructure and gamma prime phase stability in Ni superalloy Inconel 738 with 1 wt.-% Al

Abstract The effects of adding nickel and cobalt to Inconel 738 along with adding 1 wt.-% of aluminum were investigated in this study. The alloys, with various amounts of nickel and cobalt, were prepared using a vacuum arc melting process. The alloys were heat treated using a standard heat treatment procedure which consisted of solution treatment at 1448 K for 14.4 ks and precipitation aging at 1118 K for 86.4 ks. Increased nickel content was shown to enhance the stability of gamma prime particles and the gamma matrix as well as assist the formation of gamma prime particles. In the case of the addition of cobalt, the coarsening rate of gamma prime particles decreased with increasing cobalt content. Furthermore, a further addition of aluminum to Inconel 738 resulted in an increase in the area fraction of the gamma prime phase and more rapid growth of gamma prime particles during long-term heating compared to standard Inconel 738.

Effects of pre- and post-weld heat treatment conditions on microstructures of cast nickel based superalloys GTD-111 in the laser welding process

Abstract This research work aims to study the effects of pre-weld and post-weld heat treatment conditions in the laser welding process on the microstructure of cast nickel based superalloy grade, GTD-111. Pre-weld heat treatment conditions consist of a solution treatment at a temperature of 1473 K for 7.2 ks and three over-aging processes at temperatures of 1393, 1413, 1433 K for 7.2 ks. All as-received and pre-weld heat treated specimens received the same post-weld heat treatment, which consisted of a solution treatment at a temperature of 1473 K for 7.2 ks and precipitation aging at a temperature of 1118 K for 86.4 ks. From the results, it can be seen that the higher temperature of the solution treatment with respect to the pre-weld heat treatment provided coarser gamma prime particles precipitating in a gamma matrix with a more uniform microstructure and very fine gamma prime precipitated particles. Furthermore, when followed by the post-weld heat treatment or standard heat treatment of the alloy, the final microstructures of all as-received and pre-weld heat treated specimens were very similar with regard to uniform gamma prime particles. No significant effect of pre-weld heat treatment conditions on the microstructure was found after post-weld heat treatment.

Effect of rhenium and cobalt additions on the microstructure and gamma prime phase stability of vacuum arc melted modified nickel-based superalloys grade MGA 1400

Abstract The effect of rhenium and cobalt additions on the microstructure and gamma prime phase stability of modified nickel-based superalloy, grade MGA1400 by means of a vacuum arc melting method with different contents of rhenium (Re) and cobalt (Co) was investigated. After being subjected to heat treatment, the size of the gamma prime particles was slightly reduced with increasing rhenium content. Then, the alloys were exposed at a temperature of 1173 K for 1800 ks. The results showed that increasing the rhenium addition leads to a slower coarsening rate of the gamma prime particles. Furthermore, the shape of the gamma prime particles changed to a more cubic shape. However, an excessive rhenium content (>1.21 wt.-%) promotes the formation of a topologically close-packed phase after a 360 ks exposure at 1173 K. It can be concluded that an increase in rhenium content can result in a lower coarsening rate of the gamma prime particles. The cobalt addition is beneficial for impeding or slowing the precipitation of topologically close-packed phases at an elevated temperature.

Effect of Temperature Dropping Conditions during Solution Treatments on Final Microstructures of Cast Nickel Base Superalloy, MGA-1400

Abstract This research has an aim to evaluate the effect of temperature dropping conditions during solution treatment of reheat treatment process on final restored microstructure of cast nickel base superalloy, grade MGA-1400, a gas turbine blades materials, which had been degraded under long-term service at high temperatures and pressure. Generally, reheat treatment process to restore the degraded microstructure of the alloy consists of solution treatment of 1100-1125°C for 4 hours and precipitation aging of 845-865°C for 24 hours. However, the temperature dropping could be practically occurred due to the machine error of high temperature vacuum furnace. Therefore, the research had simulated various conditions of temperature dropping during solution treatment at temperature of 1100°C and 1125°C with decreasing temperature to 845°C after a) solutioning for 1 hours b) solutioning for 2 hours and c) solutioning for 3 hours then temperature was increased again to the required 1100°C and 1125°C. Then all various solution treated specimens were performed with precipitation aging at temperature of 845°C and/or 865°C for 24 hours. From all obtained results, it was found that the effect of temperature dropping provided the different in received reheat treated microstructures when considering in term of precipitation gamma prime particle morphology such as shape, size and phase density.

Influence of Cr Content on Cellular Precipitation Behaviour of Ni-38Cr-3.8Al Alloy with Lamellar Structure

Ni-Cr binary alloys containing high amount of Cr demonstrate gamma/alpha-Cr lamellar structure by discontinuous precipitation (DP) reaction from grain boundary. The mechanism of DP reaction is caused by supersaturated Cr in the gamma phase. Supersaturated Cr concentration influences the driving force for the DP reaction and the lamellar spacing. Moreover, the Ni-based alloys with high Cr, containing Al, significantly increase the hardness and strength due to the very narrow lamellar structure. Al addition brings on Ni consumption in the matrix by precipitation of the gamma prime phase. Therefore, Cr supersaturates dramatically in the matrix. The wrought Ni-Cr-Al alloy, Ni-38Cr-3.8Al (mass%) , reaches extremely high tensile strength, which is over 2 GPa, after annealing treatment. Even though chemical composition of Ni-38Cr-3.8Al is simple, the microstructure is complex because it consists of the gamma/alpha Cr lamellar structure with the fine gamma prime particles. Therefore, in this study, we investigated the influence of Cr concentration on the cellular precipitation behaviour. In order to understand the influence of Cr concentration, Ni-34, 36 and 38Cr-3.8Al alloys were prepared. Forged bars were subjected to solution treatment in the gamma single phase region. Subsequently, the alloys were aged from 873K to 1073 K for various times. The cellular precipitation reaction is suppressed by a decrease in Cr concentration, particularly at low temperature annealing treatment condition. The hardness is low in lower Cr concentration alloys in all range of annealing treatment temperature. These results indicate that Cr concentration remarkably affects the driving force for the DP reaction.

Effect of solution treatment and precipitation aging conditions on microstructural refurbishment in cast nickel base superalloy, grade MGA 1400

The aim of this project is to study and evaluate the effect of reheat treatment conditions on microstructural refurbishment in cast nickel base superalloy, MGA 1400. This cast nickel base superalloy is used as a turbine blade material in Japanese gas turbine engines. The evaluated reheat treatment process consists of a solution treatment at 1125°C, 1150°C and 1175°C for 4 hours and followed by 1) final precipitation aging at 845°C for 25 hours or 2) primary aging at 925°C for 2 hours and secondary aging at 845°C for 25 hours. All reheat treated microstructures were investigated and analyzed by SEM and image analysis program. From all obtained results, it was found that at temperature of 1125°C for 4 hours followed by single precipitation aging at 845°C for 25 hours (without 2-steps aging) provides the best microstructural characteristics, which consists of the most uniform distribution of very dense gamma prime particles precipitating with its proper size and shape in the gamma matrix.

Effect of Temperature Dropping during Solution Treatment in Rejuvenation Heat Treatment and its Long-Term Heating Simulation on Microstructures of Nickel Base Alloy, Udimet 520

Udimet 520 is a low precipitation strengthened nickel-based superalloy, which was designed and developed to be gas turbine blades at elevated temperatures. However, after long-term service under high stresses and temperatures, the microstructure of the turbine blades could be continually degraded. Therefore, the mechanical properties could be worse than the new ones. The rejuvenation heat treatment of degraded turbine blades, which were made of cast Udimet 520, was following by solution treatment at 1,121oC / 4 hours and then double aging processes including primary aging at 843 oC / 24 hours and secondary aging at 760oC / 16 hours, respectively. However, in practical reheat treatment processes, the temperature during solution treatment could be dropped by error or malfunction of high temperature heating furnace because the furnace has to be operated continually at very high temperature for very long time resulting in final reheat treated microstructures in many nickel base superalloys. To simulate this effect, the droppings of temperature during solution treatment are chosen and performed for 3 levels; 840oC, 800oC and 760oC, which could happen in practical working then heated up again immediately to solution temperature level. The maximum number of temperature dropping during the single solution treatment is up to 3 times. Received results show that the effect of temperature dropping during solution treatment has influenced on the final rejuvenated microstructures slightly due to the low precipitation behavior of the alloy. The long term heating at 800oC and 900oC / 1000 hours provided much effect in gamma prime particle coarsening.

Effect of Precipitation Aging Temperatures on Reheat Treated Microstructures and its Phase Stability after Long-Term Exposure in Cast Nickel Base Superalloy, Grade Inconel 738

This research work has an aim to investigate the effect of precipitation aging temperatures of 845°C, 865°C, 885°C and 905°C for 24 hours after solutioning treatment at temperature of 1145°C for 4 hours on final microstructure of cast nickel base superalloy, grade Inconel 738, which is used as a material for turbine blades in land base gas turbine engines to generate electricity in power plants. Further interesting is also extended to study and evaluate the phase stability of precipitated gamma prime particles after long-term heating at tempeatures of 900°C and 1000°C for 200 hours of all received final microstructures after various reheat treatment conditions. From all obtained results, it was found that the higher precipitation aging temperatures provided the more coarsening size of both coarse and fine gamma prime particles. Furthermore, after long-term exposure at high temperatures, this resulted in an increasing of both area density and size of gamma prime particles.

Metallography Evaluation of Cast and Wrought Ni-Base Superalloys

. The Ni-base superalloys have an interesting history and evolution since they start to be used in aero jet engines. Microstructures of superalloys have dramatically changed through the years as modern technology of its casting or forging becomes more sophisticated. The first superalloys have polyedric microstructure consist of gamma solid solution, some fraction of gamma prime and of course grain boundaries. As demands on higher performance of aero jet engine increases, the changes in superalloys microstructure become more significant. Further step in microstructure evolution was directionally solidified alloys with columnar gamma prime particles. The latest microstructures are mostly monocrystalline, oriented in [001] direction of FCC gamma matrix. What does not changed through the years is elementary FCC structure of matrix and fundamental group of alloying elements. All microstructure changes bring necessity of proper preparation and evaluation of microstructure. Except already mentioned structures have gamma double prime and various carbides form appear. These structural parameters have mainly positive influence on important mechanical properties of superalloys. However, some detrimental phases as Laves, σ-phase appears as well and have negative influence on heat resistance of superalloys. Paper deals with such microstructural evaluation of both groups of alloys – cast and wrought as well. Microstructure evaluation helps to describe mechanism at various loading and failure of progressive superalloys. Such example where microstructure evaluation is employed is fractography of failure surfaces after fatigue tests, which are as example of metallography evaluation described in this paper as secondary objective. Fatigue test done in this article were at high frequency with push-pull loading, so called high frequency fatigue loading (HFFL) and at low frequency three point flexure, so called low frequency fatigue loading (LFFL).