Subsolvus Solution Research Articles

Effect of the subsolvus and supersolvus solution treatments on the basket-weave microstructure, room and high temperature properties of TC21 alloy

In this study, two different microstructures were obtained after subsolvus solution treatment (910 °C + 550 °C) and supersolvus solution treatment (1010 °C + 550 °C) in TC21 alloy. The relationship of microstructure with room and high temperature properties was studied. The results showed that at room temperature, both samples after two treatments had high hardness. At room temperature, the subsolvus sample had high tensile strength of 1070 MPa and moderate plasticity and impact toughness, while the supersolvus sample was hard and brittle. The high strength and toughness of subsolvus sample was owing to its clear basket-weave structure within finer β grains which contributes to fine grain strengthening and the increase of crack tortuosity. Poor plasticity and toughness of supersolvus sample could be mainly attributed to its incomplete basket-weave structure, coarse β grains, Widmannstatten α and some coarse α-phase in the grain boundary. Both samples showed excellent high temperature tensile properties and stability at 400 °C, the tensile strength was still as high as 1000 MPa while plasticity was significantly improved. With the tensile temperature increasing to 600 °C, the alloys showed obvious softening as the tensile strength significantly decreased and elongation increased.

Evaluation on static mechanical stability of wrought Inconel 718 superalloy; effect of grain configuration and δ precipitation

Wrought nickel alloys are widely applied in hot static components for aero-engines, acquisition for the underlying mechanism of microstructural evolutions controlling mechanical degradation during elevated-temperature exposure is fundamentally vital to processing optimization as well as serving life prediction. In present work, the differential hot tensile characteristics during 500–2000 h exposure at 650 °C of wrought Inconel 718 alloy having specific microstructures designed via different processing routes were comparatively evaluated. The experimental results showed the static mechanical stability of Inconel 718 alloy was predominantly tailored by the complicated interaction of γ-matrix grain configuration and δ precipitation on grain boundaries (GBs). It mainly involved continuous GB-δ precipitation during the 500–2000 h exposure at 650 °C, which played opposite roles in homogeneous or heterogeneous grain configurations. GB-δ precipitation can improve hot ductility but deteriorate strength for homogeneous grain configuration, adequate GB-δ particles can improve the deformation compatibility between GBs and γ-matrix to achieve good balance between the strength and ductility as well as smaller mechanical fluctuations. The heterogeneous duplexing grain configuration exhibited enhanced mechanical sensitivity to sluggish GB-δ precipitation since the hot ductility was rapidly reduced by tiny δ particles with fracture mode transition. In comparison, appropriate sub-solvus solution prior to aging cycles can introduce GB-δ particles pre-existed in homogeneous grain configuration to benefit in superior long-term static mechanical stability for Inconel 718 alloy.

Creep Analysis and Microstructural Evaluation of a Novel Additively Manufactured Nickel-Base Superalloy (ABD®-900AM)

Abstract Nickel-base superalloys containing 30 to 50% gamma prime (γ') volume fraction are typically used in hot section components (e.g., guide vanes or blades) for power generating gas turbines, and suitable time-dependent properties are required for long-term elevated temperature operation. Additive manufacturing (AM) has recently been used to develop complex hot-section parts utilizing innovative designs with enhanced cooling features which improve efficiencies by reducing cooling air consumption. To further explore the opportunity to improve time-dependent AM superalloys, this paper focuses on a fundamental creep study and characterization of a novel nickel-base superalloy (ABD-900AM) that was manufactured using a laser-based powder bed fusion (LBPBF) AM process. The material was subjected to a subsolvus solution anneal and multistep aging heat treatment (HT) to produce a bi-modal distribution with ∼35% volume fraction of gamma prime without postprocessing hot isostatic pressing (HIP). Microstructural characterization was carried out for the as-built and fully heat-treated structures, and a creep-rupture test program was conducted to study the resultant creep properties. Activation energies and stress exponents in addition to rupture strength and deformation resistance were compared to traditionally cast IN939 and IN738 materials. After testing, specimens were evaluated using a variety of microscopy tools to determine location and features associated with creep damage. The optimized chemistry for ABD-900AM was printed crack free and fully dense in contrast to studies on similar alloys where significant process development and postbuild heat treatments were required. High-temperature mechanical properties in the heat-treated material showed some decrease in creep strength when compared to traditional casting. This strength and rupture life debit was dependent on build orientation, but a considerable increase in creep ductility was observed due to differences in the microstructure when compared with similar AM alloys. Analysis of creep data showed differences in creep mechanisms compared to traditional cast alloys. The relationship between microstructure and creep mechanisms is discussed, and ongoing work to further improve rupture strength through heat-treatment optimization will be highlighted.

Chemical redistribution and change in crystal lattice parameters during stress relaxation annealing of the AD730TM superalloy

The present study aims to understand the underlying mechanisms of macroscopic contraction that occurs in γ/γ′ nickel based superalloys submitted to isothermal annealing and resulting in stress relaxation retardation. Samples of the AD730TM alloy were characterized at different scales using dilatometry, X-Ray Diffraction (XRD) with in-situ heating, Electron Channeling Contrast Imaging (ECCI) in the Scanning Electron Microscope (SEM), conventional Transmission Electron Microscopy (TEM) and Atom Probe Tomography (APT). APT analyses revealed chemical composition changes in both the γ matrix and the γ′ hardening precipitates during annealing at 760°C after sub-solvus solution heat treatment. These chemical alterations lead to a decrease of the lattice parameters of both phases, which was captured by XRD analyses. The macroscopic volume contraction is shown to be mainly governed by that decrease in the lattice parameters. In addition, from APT analyses, the chemical alterations seem to be promoted by diffusion of solutes such as Cr, Co and Fe, along dislocations occurring under relaxation testing conditions.

Dissolution of the Primary γ′ Precipitates and Grain Growth during Solution Treatment of Three Nickel Base Superalloys

Second phase particles (SPP) play an essential role in controlling grain size and properties of polycrystalline nickel base superalloys. The understanding of the behavior of these precipitates is of prime importance in predicting microstructure evolutions. The dissolution kinetics of the primary γ′ precipitates during subsolvus solution treatments were investigated for three nickel base superalloys (René 65, AD730 and N19). A temperature-time codependency equation was established to describe the evolution of primary γ′ precipitates of each material using experimental data, the Thermo-Calc software and the Johnson–Mehl–Avrami–Kolmogorov (JMAK) model. The dissolution kinetics of precipitates was also simulated using the level-set (LS) method and the former phenomenological model. The precipitates are represented using an additional LS function and a numerical treatment around grain boundaries in the vicinity of the precipitates is applied to reproduce their pinning pressure correctly. Thus, considering the actual precipitate dissolution, these simulations aim to predict grain size evolution in the transient and stable states. Furthermore, it is illustrated how a population of Prior Particle Boundaries (PPB) particles can be considered in the numerical framework in order to reproduce the grain size evolution in the powder metallurgy N19 superalloy. The proposed full-field strategy is validated and the obtained results are in good agreement with experimental data regarding the precipitates and grain size.

Influence and Sensitivity of Temperature and Microstructure on the Fluctuation of Creep Properties in Ni-Base Superalloy.

In this work, the sensitivity zone of microstructure and temperature for precipitation-strengthened nickel-based superalloys, used for turbine applications in aero-engines, has been firstly established. Heat treatment experiments with different solution temperatures were carried out. The microstructure evolution and creep residual strain sensitivity, low cycle fatigue properties, and tensile properties are analyzed, and the essential reason for the fluctuation of the mechanical properties of nickel-based superalloys was revealed. The main results obtained are as follows: following subsolvus solution heat treatment with a temperature of 1020 °C, samples have a high primary γ′I phase content, which is beneficial to low creep residual strain. Above the supersolvus solution temperature of 1040 °C, the creep residual strain value and low cycle fatigue performance fluctuate significantly. The essential reason for the dramatic fluctuation of performance is the presence of γ′ phases in different sizes and quantities, especially following the solution heat treatment in the temperature-sensitive zone of the γ′I phase, which is likely to cause huge fluctuations in the microstructure of tertiary γ′III phases. A zone of particular sensitivity in terms of temperature and microstructure for the γ′I phase is proposed. The range of suitable solution temperatures are discussed. In order to maintain stable mechanical properties without large fluctuations, the influence of the sensitivity within this temperature and microstructure zone on the γ′ phase should be considered.

Influence of the γ́ Phase on the Creep Behavior of a New Polycrystalline Nickel-Based Superalloy (AD 730)

Abstract Demands for cost-efficiency and a flexible use of gas turbines are constantly increasing, creating a call for new alloys which allow achieving better efficiencies. Today's limit of use of common polycrystalline wrought alloys is 650 °C, whereas the new γ / γ́ wrought alloy AD 730 is designed for use at 700 °C. The presented work examines and discusses the impact of the microstructure on the alloy's creep behavior. Supersolvus and subsolvus solution annealing was performed to provide for a coarse-grained and fine-grained microstructure which was subsequently aged at different aging times.

Microstructure prediction of TMW-4M3 during heat treatment

The calculation model on the basis of multi-phase-field model, coupled with the calculation of phase diagram (CALPHAD) approach and classical nucleation theory (CNT) model, was studied for the precipitation behavior of γ′ precipitates in Ni-Co base superalloy TMW-4M3 (“TMW” is a trademark of National Institute for Materials Science registered in Japan) during the heat treatment process. This alloy is precipitation strengthened by γ′, and thus the control of size and distribution of γ′ precipitates is very important for practical applications. Precipitation behavior of γ′ in TMW-4M3 is strongly influenced by heat treatment conditions. Therefore, numerical simulation is useful for finding the most appropriate manufacturing condition prior to experimental trial and error approach. Firstly, we investigated the γ′ solvus temperature and improved several thermodynamic parameters of the conventional database to obtain reasonable phase boundaries between γ/η(Ni3Ti) and γ/γ′. Based on the new thermodynamic description, the nucleation rates related to temperature histories were calculated by using the CNT with composition balance equation under subsolvus solution treatment. Finally, the multi-phase-field method coupled with the CALPHAD approach and CNT model was applied to the microstructure evolution of intragranular secondary γ′ particles. The experimental and simulated microstructures show good agreement with each other. This method might be useful for microstructure evolution prediction of γ/γ′ dual phase during the heat-treatment.

High temperature creep properties of a linear friction welded newly developed wrought Ni-based superalloy

AD730™ Ni-based superalloy specimens in solution-treated conditions were linear friction welded. Then, post-weld heat treatment (PWHT), consisting of γ′ sub-solvus solution treatments followed by aging, was conducted on the linear friction welded samples. High temperature creep tests were performed on the as-welded and PWHTed joints at two different temperatures: 700°C under 600 and 750MPa stress levels, and 850°C under 100 and 200MPa stresses. The creep resistance of the PWHTed joints was higher than that of the as-welded samples. The PWHTed joints exhibited better ductility than that of the base material at 850°C, while they showed slightly lower creep life at 700°C in comparison to the base metal. Microstructure examination showed that cracks initiated at the interface of oxidized particles at 700°C. The decrease in creep resistance of the AD730™ Ni-based superalloy at 850°C was related to a combination of the formation of precipitate-free zones (PFZ) in the vicinity of the grain boundaries (GBs) and microcracking assisted by oxidation. The Larson-Miller Parameter (LMP) was used to correlate the creep strength, temperature and time to failure for the as-welded and PWHTed samples. LMP values varied between 21.5 × 103 and 24.5 × 103. It was found that in the investigated temperature range, the PWHTed AD730™ has similar creep characteristics as Udimet™720 Li and Inconel 738LC at low values of LMP and better creep properties than those of the Inconel 617 alloy at higher LMP values.

Impact of solution heat treatment on microstructure and creep behavior of a novel cast & wrought FGH4096 turbine disk alloy

By means of microstructural observation and creep tests, a study has been made into the effects of solution heat treatments on microstructure and creep resistance of a novel cast & wrought FGH4096 superalloy. The results show that grain size increases and the volume fraction of primary γ´ phase decreases with the increase of solution heat treatment temperature. The super-solvus solution heat treatment leads to an increase of the total volume fraction of secondary and tertiary γ´ phases, and superior creep property can be obtained. Various cooling rates from sub-solvus solution heat treatment are applied to gain cooling secondary γ´ phase with different sizes and morphologies. It is found that the solution cooling rate determines secondary γ´ morphology and particle distribution that develops. The alloy samples with fine γ´ particles obtained from the fast solution cooling rate possess narrow matrix channels, which can effectively enhance the Orowan resistance and hinder dislocation movement within the matrix and thus improve the creep resistance at 704°C/690MPa. The creep tests for three samples with different distribution of γ´ particles are interrupted when entering the third stage to investigate the corresponding dominant deformation mechanisms by transmission electron microscopy. It is indicated that partial dislocations shearing precipitates leaving abundant stacking faults behind is the dominant creep failure mechanisms for samples with coarser secondary γ´ particles and wide γ matrix channels. The main creep mechanism for the samples with fine secondary γ´ particles is microtwinning shearing both γ´ particles and matrix.

Dissolution kinetics and morphological changes ofγ′ in AD730TMsuperalloy

Alloy AD730TM is a recently developed Nickel base superalloy for application as turbine disk in modern gas turbines with improved thermal efficiency. Ingot casting followed by open die-forging and then heat treatments are the main manufacturing steps for the production of parts made of this alloy. Solution heat treatment operations are applied at different stages of the manufacturing in order to ease the deformation processing and/or prepare the microstructure for final heat treatment. In this research, the influence of various solution heat treatment schedules on morphology and distribution of the γ′ phase are investigated and documented. The obtained results will contribute to a better understanding of microstructure evolution of AD730TM during solution heat treatments. Differential Thermal Analysis (DTA) is used for the purposes of measurement of temperatures of phase transformations of the alloy. Based on DTA results, three solutionizing temperatures and three holding times were selected for performing and assessing the solution heat treatment process. Optical and electron microscopy were used to study the morphological evolution as well as the coarsening and dissolution of secondary phases at solvus and subsolvus temperatures. The results indicated that precipitate agglomeration and Ostwald ripening are the governing mechanisms during the initial stages and splitting and partial dissolution of γ′ precipitates takes place during subsolvus solution treatments.

Influence of sub-solvus solution heat treatment on γ′ morphological instability in a new Ni–Cr–Co-based powder metallurgy superalloy

Abstract The influence of the sub-solvus solution heat treatment on the microstructure, especially the γ′ morphology (γ′ fan-type structure), and microhardness of a new Ni–Cr–Co-based powder metallurgy superalloy was studied by means of field emission scanning electron microscopy (FESEM) and microhardness testing. The results show that sub-solvus solution heat treatment changes the microstructure of an as-forged alloy. It makes large primary γ′ phases at grain boundaries smaller and the distribution of secondary γ′ phases in the interior of the grains more homogeneous. Moreover, the grain boundaries widen because of the supplementary precipitate. The sub-solvus solution heat treatment before the super-solvus solution heat treatment does not change nucleation sites of the γ′ fan-type structures which precipitate during the super-solvus solution heat treatment. However, it influences the morphology of γ′ fan-type structures. Length distribution of the secondary γ′ dendrites in fan-type structures changes from a bimodal to a unimodal distribution, which means the lengths of the secondary γ′ dendrites become more uniform. Applying a sub-solvus solution heat treatment after the super-solvus solution heat treatment causes the secondary γ′ dendrites to be broken off in the fan-type structures and a refinement of the γ′ phases, and this improves stability of the γ′ phases.

Morphological evolution of γ′ precipitates in a nickel-based superalloy during various solution treatments

The morphological evolution of the γ′ phase in nickel-based superalloy жc6y during various solution heat treatments was investigated. The significant changes of the γ′ precipitates were observed in the solution-treated samples. The coarsening and dissolution of γ′ phase simultaneously occurred at intermediate temperatures. In some areas, the primary precipitates became blunt and the adjacent ones were interconnected with each other via a diffuse neck, indicating a coarsening process of the primary γ′ population. The coarsening was dominated by the precipitate agglomeration mechanism (PAM) rather than by the well-known Ostwald ripening mechanism. In other areas, the partial dissolution of the γ′ precipitates began to occur, spreading gradually from dendrite cores to interdendritic regions. In addition, a flower-like γ′ structure was developed during the subsolvus solution treatments. The observable long filaments composed of erraticly shaped precipitates were caused by the heterogeneous nucleation of the cooling precipitates during water quenching.

EFFECT OF SOLUTION TREATMENT ON THE INVERSE DYNAMIC STRAIN AGING OF A Ni-Co BASE SUPERALLOY

Serrated flow in a Ni-Co base superalloy was investigated after subsolvus and super- solvus solution treatments at the temperatures of 400 and 450 by tensile loading at different strain rates. The results suggested that the sizes of the grain and secondaryphase of the alloy after super- solvus solution were larger than that of the alloy after subsolvus solution. The serrated flow exhibited inverse DSA behavior after two solution treatments, which was caused by the interaction between sub- stitutional solutes and mobile dislocations. The variation of critical strain of serrated flow after two solution treatments may be related to different spacing between secondaryand different densities of mobile dislocations during plastic deformation. The different densities of mobile dislocations and the diffusion controlled by grain boundary were responsible for the variation of stress drop of serrated flow after two solution treatments.

Microstructure Characterization and Mechanical Properties of FGH 95 Turbine Blade Retainers

FGH 95 is a powder metallurgy (P/M) processed superalloy, which was developed in the 1980s in China. One of the applications of FGH 95 was high pressure turbine blade retainers. The manufacturing processes used to produce FGH 95 blade retainers consisted of atomization by plasma rotating electrode process (PREP), hot isostatic pressing (HIP) at super-solvus temperature and a sub-solvus solution heat treatment. The material had an equiaxed grain structure (ASTM 6, 5–7, 5). The γ' precipitates in as-HIP FGH 95 showed a tri-model distribution, Carbides in the alloy were MC type and precipitated at grain boundaries. The prior particle boundaries (PPB) in the material originated mainly from γ'z phase. Statistics of the mechanical properties data from batch production of the FGH 95 blade retainers were investigated. The as-HIP FGH 95 blade retainers showed high strength at room temperature and 650 °C, excellent creep resistance and outstanding stress rupture strength at 650 °C.

γ′ formation in superalloy U720LI

Nickel-base superalloys are a specialty class of heat treatable engineering materials that derive their mechanical properties from the precipitation of coherent or semi-coherent particles. The precipitation of {gamma}{prime} is one of the most important reactions in nickel-base superalloys, and is the focus of all industrial heat treatments. Superalloy U720LI has been previously studied with regard to current production subsolvus solution and aging heat treatment cycles. This work has shown that {gamma}{prime} develops a bi-modal distribution during precipitation from solution heat treatments cycles. The relationship between {gamma}{prime} size and mechanical properties has been reviewed and a new aging practice for subsolvus solution heat treated U720LI has been identified for optimum mechanical properties.

Thermal cycle simulation of a Ni-base superalloy

Thermal cycle simulation techniques have been investigated as a method of studying the effect of cooling rate, from sub-solvus solution heat treatment temperatures, on the microstructure and fatigue properties of Ni-based superalloy specimens. A Smitweld thermal cycle simulator (TCS) and a Gleeble 1500 TCS were used during the investigation. Grain growth was observed, predominantly in the sample centre, in a number of specimens simulated using the Smitweld TCS as a result of severe temperature gradients set up across the specimen. It is important both to quantify and minimise this temperature gradient, to allow accurate heat treatment at the desired temperature. The discrepancy between surface and core temperatures, under different simulation conditions, was determined using three different specimen geometries for internal thermocouple placement. The optimum conditions to minimise the temperature overheat were found to be a highly polished specimen surface tested under high vacuum using a small (0.15mm) diameter K-type thermocouple for temperature control. The radial temperature distribution in the simulated zone was further investigated by microstructural examination of the supersolvus grain growth region in a series of samples simulated to a range of peak temperatures.

Effects of cooling rate and γ′ morphology on creep and stress-rupture properties of a powder metallurgy superalloy

The 650 °C creep and stress-rupture (S/R) behavior of powder metallurgy (PM) RENE 95 alloy was investigated under various cooling rates from two subsolvus solution temperatures. The cooling conditions were chosen to alter the morphology of the cooling γ′ (γ′c), which constituted a high volume fraction (≃0.85) of all γ′ precipitates. The dispersion characteristics of γc′ changed from very fine at the highest cooling rate to progressively coarser with reduced cooling rate. At small γc′ (≃0.05 μm), a marked increase in creep and S/R resistance was observed, whereas at larger γc′ (≃0.1 μm), a marked decrease was observed. Also, for a givenγc′ size, both properties benefited from a higher solution temperature. The change in properties with the size of γc′ was associated with a change in deformation mechanism. The operative mechanism was essentially determined by the mean surface-to-surface spacing(ls) of the γc′ precipitates. Atls> 0.05 μm, the Orowan mechanism of dislocation expansion and looping became favorable, which led to higher primary creep strain and steady-state rate and also reduced S/R life. Atls< 0.05 μm, dislocation motion was significantly restricted, and the low dislocation density and inadequate dislocation sources were responsible for greatly suppressed primary creep strain and steady-state rate, and in some cases, for an incubation period in the creep curve. In addition, the S/R life increased significantly at lowls. Atls ≃ 0.05 μm, a mixed mode of deformation was observed due to inherent distribution of particle spacing in the structure, and this gave rise to intermediate creep and S/R resistance.