Cast Nickel-base Superalloy Research Articles

Effects of sub-solvus ageing on the tensile and creep properties of a new cast nickel-based superalloy

For nickel-based superalloys with medium volume-fraction γʹ phase (20 %–40 %), dual or multi-stage aging treatments are usually conducted to generate a microstructure containing the multimodal distribution of γʹ for a balance of strength and plasticity. In the present study, the microstructure and high-temperature properties of a novel cast nickel-based superalloy K4800 were investigated after being subjected to three heat treatments (HT) procedures, namely HT1: 1180 °C/4 h + 1090 °C/2 h + 800 °C/16 h, HT2: 1180 °C/4 h + 1060 °C/2 h + 800 °C/16 h and HT3: 1180 °C/4 h + 800 °C/16 h. It was found that the sub-solvus aging treatments at 1090 and 1060 °C precipitated sub-micron-sized (∼300 nm) primary γʹ phase which enhanced the ductility during 800 °C tensile (the total elongation of T1, T2, and T3 samples were 6.75 %, 7.3 %, and 3.25 %, respectively) without evidently impairing the strength. After careful microstructure observation and deformation mechanism analysis, the enhancement of elongation was rationalized that the precipitation of the sub-micron-sized primary γʹ phase decreased the volume-fraction and size of the nanometer-sized γʹ phase which was precipitated at 800 °C, and simultaneously, promoted the dislocation movement by suppressing the non-planar slip. However, an excessive amount of the sub-micron-sized primary γʹ phase led to a faster ripening process of the nanometer-sized γʹ during creep, which decreased the creep life at 800 °C/430 MPa (T1: 125 h, T2: 199 h, and T3: 198 h). Based on this, we monitored the number density of nanometer-sized γʹ phase coexisting with different amounts of large γʹ during creep. An area fraction less than 7 % of the sub-micron-sized γʹ phase was considered to have little detrimental effect on the creep life of K4800 alloy, which corresponded to a sub-solvus temperature range about 1080–1090 °C.

Thermomechanical and isothermal fatigue properties of MAR-M247 superalloy

Thermomechanical low-cycle fatigue (TMF) tests were performed on polycrystalline cast nickel-based superalloy MAR-M247 under in-phase (IP) and out-of phase (OP) loading in the temperature range of 500–900 °C. A constant heating and cooling rate of 5 °C.s−1 was selected. Furthermore, isothermal low-cycle fatigue (ILCF) tests with the same cycle period as TMF cycle were carried out at the maximal temperature of TMF cycle (900 °C). All tests ran under strain control and fully reversed cycle on solid cylindrical specimens in laboratory air. High resolution scanning electron microscopy and transmission electron microscopy were utilized to characterize the damage mechanism occurring during TMF and ILCF loading. The cyclic deformation and lifetime behaviour as well as damage mechanisms dependent on loading regime were examined. Results show that the TMF loading has a detrimental effect on the lifetime of MAR-M247. Lifetime reduction is most significant under IP loading. The fracture surfaces are characterized by typical striation fields, where the largest spacing between striations was observed for IP loading with prevailing intercrystalline cracking followed by OP and isothermal ILCF loading with transcrystalline crack propagation. To improve discussion of obtained results, experimental data were compared with damage and life prediction models.

Microalloying effects of Y on performance of cast nickel-based superalloy IN713C

Microalloying is a cost-effective strategy to develop high performance nickel-based superalloys. Here, the mechanisms of rare earth Y in IN713C on purification, mechanical property improvement, and microstructure modification were systematically investigated. Via melt purification of Y, the O, N, and S elements are reduced to 5 ppm in the IN713C alloy. By adding Y to 74 ppm, the yield strength, tensile strength, and elongation of the IN713C alloy at 650 °C are increased by 12.2%, 14.5%, and 66%, respectively. Microstructure analysis indicated that the strength of the alloy is improved by refining the size and increasing the volume fraction of the γ’ phase. The plasticity of the alloy is improved by modifying the morphology and distribution of MC carbides. The modification of carbide morphology can be attributed to the enrichment of Y at the surface of carbides and the change of segregation of Nb, which is the main carbide-formation element.

High-Temperature Low Cycle Fatigue of Nickel-Based Superalloy IN738LC

Polycrystalline cast nickel-based superalloy IN738LC is employed for critical parts of gas turbine components in the power industry, aircraft engines, and the marine sector. These components undergo severe degradation by low cycle fatigue caused by thermal gradients, particularly during start-up and shut-down periods. The present work reports the cyclic deformation behaviour and fatigue damage of IN738LC during high-temperature isothermal fatigue. Cylindrical specimens were cyclically deformed under strain control with constant total strain amplitude in symmetrical cycling at 800 °C and 950 °C in air. The microstructure is typical of coarse dendritic grains with carbides, eutectic, and shrinkage pores. SEM imaging revealed a γ matrix with coherent L12 γ′ precipitates with bimodal morphology. Cyclic hardening/softening curves, cyclic stress-strain response, and fatigue life diagrams were determined. An increase in testing temperature is associated with a significant decrease in stress amplitude and an increase in plastic strain amplitude. The fatigue life gradually decreases with increasing temperature. The fracture surfaces and polished sections parallel to the specimen axis were examined to study damage mechanisms in cyclic loading at high temperatures.

IMPROVING THE PROPERTIES OF NICKEL-BASED SUPERALLOYS OF DIRECTED CRYSTALLIZATION BY OPTIMIZING THE CHEMICAL COMPOSITION

Purpose. It consists in establishing the influence of the chemical composition on the type, chemical composition and morphology of the primary carbides of the nickel-based superalloy, which allows to increase the operational properties of the parts through the structure of the carbide component.
 Research methods. Process modeling was carried out using the CALPHAD method. The initial data were the chemical compositions of model alloys with different concentrations of carbide-forming elements. The result of the calculation was the chemical composition of the carbides released in the corresponding systems. The experimental values were processed by statistical methods to obtain correlation dependencies of the “parameter-property” type and establish mathematical equations of regression models that optimally describe these dependencies.
 Results. The regularities of the effect of metal chemical composition on the morphology of carbides of MC type have been established. It is shown that depending on chemical elements introduced in the system, the basis of carbides can change, which causes a change in their shape and an increase in the crack resistance of the material. It is shown that the obtained dependences are closely correlated with metallographic studies of alloys of this class.
 Scientific novelty. The dependences of the influence of the chemical composition variation of the multicomponent system Ni-Cr-Co-Al-W-Re-Ta-Mo-Nb-C on the chemical composition and morphology of carbides have been established. This makes it possible to change the basis of the carbides, their composition and morphology, thereby improving the mechanical properties of the material, especially fatigue and heat-resistant characteristics.
 Practical value. An effective solution for establishing the structural-phase state of nickel-based superalloys by optimizing their chemical composition is proposed, which made it possible to improve the operational properties of the material. The established dependencies can be used in the optimization of the composition of industrial cast nickel-based superalloys and in the development of new compositions.

Effects of Heat Treatment on the Microstructure and Properties of a Cast Nickel-Based High-Cr Superalloy

The effect of solution treatment and intermediate heat treatment on the microstructure and properties of a new cast nickel-based high-Cr superalloy was investigated in this paper. The results indicate that the tensile strength and elongation at 900 °C increase when the solution temperature increases from 1160 °C to 1180 °C and then decrease when the solution temperature changes from 1180 °C to 1200 °C and 1220 °C. The stress rupture test results of the high-Cr superalloy under conditions of 900 °C/275 MPa shows that the rupture time, elongation, and reduction of area initially increased and then decreased with the increase in solution treatment temperatures. The results of stress rupture tests for the alloy after intermediate heat treatment followed by furnace-cooling, air-cooling, and water-cooling show that the morphology and distribution of γ’ phase have a great influence on the tensile test results at 900 °C of the alloy but no obvious influence on the test at 900 °C/275 MPa. The microstructure analysis of the superalloy after heat treatment shows that: when the solution treatment temperatures are at 1200 °C and 1220 °C, the incipient melting appears in the interdendritic region, which can severely deteriorate mechanical properties; the morphology of γ′ phase changes gradually from cube to spherical; and a large number of fine γ’ phase precipitates in the γ channel are found with increasing cooling rate after intermediate heat treatment.

Influence of pre- and post-weld heat treats on microstructures of laser welded GTD-111 with IN-718 as filler metal

Abstract The present research work studied and evaluated conditions of pre-weld and post-weld heat treatments for the laser welding process, aiming to avoid weld cracking, which usually occurs after tungsten inert gas and/or laser welding processes in cast nickel-based superalloys with high aluminum and titanium contents. The pre-weld heat treatment conditions used in the experiments were (1) 1473 K for 7.2 ks, (2) 1433 K for 7.2 ks + 1298 K for 56.7 ks, (3) 1413 K for 7.2 ks + 1298 K for 56.7 ks, (4) 1393 K for 7.2 ks + 1298 K for 56.7 ks. A laser welding process on cast nickel-based superalloys, grade GTD-111, with metal filler of Inconel 718, a nickel-iron based alloy was conducted. From all obtained results, it was found that the pre-weld heat treatment conditions significantly influence microstructures before laser welding. After laser welding and post-weld heat treatment at a temperature of 1473 K for 7.2 ks followed by precipitation aging at a temperature of 1118 K for 86.4 ks (standard heat treatment), all welded specimens exhibited similar microstructures and hardness values. Furthermore, no weld cracking was observed in all welded specimens.

Dissimilar Welding of Cast Alloy 706 with Different Prior Heat Treatment Conditions and Austenitic Stainless Steel 321

Abstract The welding of dissimilar joints is most common in the aerospace applications, especially, cast nickel-based superalloys are welded with austenitic stainless steel for high-temperature service conditions. It is well known that heat-affected zone (HAZ) cracking is a common phenomenon while welding age-hardened alloy 706. In this study, an alloy equivalent to cast alloy 706 subjected to three different heat treatment cycles and was welded with annealed austenitic stainless steel 321. The HAZ on the alloy 706 was investigated, and it was found that the HAZ of alloy 706 that underwent stabilization heat treatment was free from microfissures. The fusion zone microstructures were also investigated. The mechanical properties were evaluated, and it was found that the alloy 706 welded in stabilized and age-hardened condition furnished better mechanical properties than in both room temperature and elevated temperature.

On the creep resistance in cast Ni-base superalloys

Abstract In this paper, we attempt to apply the modified hardening theory to estimate the creep resistance, which is considered as a key factor for controlling the creep deformation mechanism in cast nickel-base superalloys. It is suggested that, when the applied stress is high enough for the dislocations to cut into the γ′ particles, the creep resistance is almost a constant and independent of applied stress. At low applied stress, creep deformation is mainly controlled by dislocations climb, where the creep resistance has two components of a threshold stress and a friction stress. The model is verified for two cast nickel-base superalloys DZ17G and IN738. The results of detailed calculations are in good agreement with the experimental data.

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.

High Temperature Creep Behaviour of Cast Nickel-Based Superalloys INC 713 LC, B1914 and MAR-M247

Cast nickel-based superalloys INC713 LC, B1914 and MAR-M247 are widely used for high temperature components in the aerospace, automotive and power industries due to their good castability, high level of strength properties at high temperature and hot corrosion resistance. The present study is focused on the mutual comparison of the creep properties of the above-mentioned superalloys, their creep and fracture behaviour and the identification of creep deformation mechanism(s). Standard constant load uniaxial creep tests were carried out up to the rupture at applied stress ranging from 150 to 700 MPa and temperatures of 800–1000 °C. The experimentally determined values of the stress exponent of the minimum creep rate, n, were rationalized by considering the existence of the threshold stress, σ0. The corrected values of the stress exponent correspond to the power-law creep regime and suggest dislocation climb and glide as dominating creep deformation mechanisms. Fractographic observations clearly indicate that the creep fracture is a brittle mostly mixed transgranular and intergranular mode, resulting in relatively low values of fracture strain. Determined main creep parameters show that the superalloy MAR-M247 exhibits the best creep properties, followed by B1914 and then the superalloy INC713 LC. However, that each of the investigated superalloys can be successfully used for high temperature components fulfils the required service loading conditions.

Research Progress of the Effects of Trace S Element on the Microstructure of Cast Nickel-base Superalloys

In the process of alloy melting and vacuum pouring, the impurity S element will inevitably be introduced into the nickel-base superalloys, which will cause adverse effects on the microstructure and properties of the cast nickel-base superalloys. In this paper, the research progress of S element in cast nickel-base superalloys is summarized. The effects of S element on the microstructure are described in detail from experimental researches and first-principles. The reasons for the grain boundary embrittlement caused by S atom are discussed, and the authors wish to provide some references for the better development of cast nickel-base superalloys.

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.

Three-dimensional analysis of dendrites via automated serial sectioning using a Robo-Met.3D

The dendrite morphologies of the cast nickel-based superalloy CMSX-4® (CMSX-4® is registered trademarks of the Cannon-Muskegon Corporation) and the austenitic stainless steel HP microalloy have been obtained via an automated serial-sectioning process which allows three-dimensional (3D) microstructural characterization. The dendrite arm spacing, volume fraction of segregation, and fraction of porosity have been determined. This technique not only increases the depth, scope, and level of detailed microstructural characterization but also delivers microstructural data for modeling and simulation.

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.

Effect of heat treatment on microstructural evolution and creep behaviors of a conventionally cast nickel-based superalloy

The evolution of microstructures caused by heat treatment on Ni-based superalloy CM 247 LC was examined, and the creep behaviors were analyzed at various temperature and stress conditions. Secondary γ′ hardly precipitated in aged specimens subjected to solution treatment, but secondary γ′ precipitated in specimen subjected to two-step aging at 1080 °C and 871 °C without solution treatment, because the size and volume fraction of precipitated γ′ particles and the width of the γ channel depend on the heat-treatment path. The creep properties of the specimen subjected to two-step aging without solution heat treatment under low temperature and high stress was the best, owing to the uniform deformation and damage-relieving effect of the fine secondary γ′ particles in the γ channel and the eutectic γ/γ′ phase. At and above 871 °C, the effect of γ′ rafting on the creep properties was very important, and the creep lifetimes of the solution-treated specimens with better rafting were longer than those of the aged specimens.

On the influence of Ta/W ratio on the morphological evolution of γ+γ’ eutectics and solidification path in cast nickel-base superalloys

ABSTRACTThe relation between Ta/W ratio and mechanism of γ/γ’ eutectics morphological changes and solidification path was investigated for experimental nickel-base superalloys. Thermal analysis and thermodynamic modelling was used to determine the solidification path. The results indicated that an increase in Ta content alters chemical composition of the dominant carbides from HfC to (Hf,Ta)C and then to TaC. Meanwhile, at low Ta/W ratios, uniformly distributed fine eutectic pools of γ+γ’ form and large phases of γ’ are rarely observed beside these pools, while at high Ta/W ratios, relatively coarse eutectic pools of γ+γ’ are formed with a rather non-uniform distribution grown epitaxially during the latest solidification step. According to the proposed heat treating guide map, the alloys with Ta/W ratio lower than 0.5 or greater than 2 with a rather narrow or smaller than 10°C heat treatment window are located in the region with a high risk of incipient melting.

Electrochemical deep grinding of cast nickel-base superalloys

Nickel-based superalloys are considered as typical difficult-to-cut materials for traditional machining. Compared with wrought nickel-based alloys, numerous of disordered micro-particles are involved in the matrix of cast nickel-based superalloys, further worsen their machinability. In this paper, a hybrid process of electrochemical deep grinding (ECDG) combined by electrochemical machining (ECM) and deep grinding was employed to process cast nickel-base superalloy. By means of high-speed dissolution of anode materials by ECM and timely scraping of electrolytic products and micro-particles by mechanical grinding to realize the expectation of high-performance machining of cast nickel-based superalloy materials. Machining parameters, such as abrasive particle size, cathode rotational speed, etc., were studied in detail through experiments. The results show that better machining performance can be obtained when the cathode rotational speed is 1000 rpm and the abrasive particle size is 325 #. Furthermore, specimens of cast nickel-base superalloys were machined, and the machining performances of ECDG was compared with that of ECG and ECM. The results show that there are significant differences between ECDG and ECG, and the proportion of material removed by mechanical grinding in ECDG process is significantly reduced. Besides, the better machining efficiency and surface quality of cast nickel-based superalloys machined by ECDG is better than that by ECM.

Lifespan of super-alloy Waspaloy cutting tools

An airplane has about 6-million components and parts, mainly the engine, undercarriage, constructions and so on. Among them, nickel base materials are widely used, including engine, cartridge receiver, compressor drum and other industries application, such as energy, petrochemical industry, mould, etc. Nickel base alloy, with anti-corrosion and thermo stability, has good mechanical properties under high temperature. Since 1950s, due to the development of precision casting technology, a series of cast nickel base super-alloy with high strength has been developed, namely the super-alloy complying with the above conditions. In recent years, with the changing of military and civil aerospace, the usage amount of nickel base super-alloy also increases along with the date. This research mainly used Waspaloy of nickel base material to do cutting research and uses regression analysis to find significant factor of cutting tool's life, and performs the optimization experiment. The cutting research mainly uses TiAlN to coat cutting tools. In the conclusion, it discusses main factors that influence the tool life, such as the cutting speed, depth, feed rate and so on. Finally it establishes tool life formula with regression analysis method, in this case when cutting speed V = 30.77 m/min and cutting depth dp = 0.0367mm, the minimum wear prediction can be obtained.