The non-metallic inclusions (NMIs) population of two versions of AD730™ (the standard one and a high carbon-doped one) alloy were studied in terms of oxidation and low-cycle fatigue behavior at 450 and 700 °C. Cracking of NMIs was observed in the early stages of oxidation without any applied load, even at intermediate temperature. It is assisted by volume expansion and thermal mismatch between the NMIs and the surrounding γ/γ′ matrix. It was observed that the cracking took place in the Nb-rich part of the inclusions. However, the presence of pre-cracked inclusions does not have a detrimental effect on the fatigue lifetimes. The mechanical behaviors and mechanisms were investigated. Moreover, addition of carbon may lead to a debit in LCF life depending on the loading direction with respect to the NMIs cluster alignments. The observation of the fracture surfaces showed that no cracks initiated within inclusions in the high carbon content material despite a much higher density of NMIs. The inclusions and grain size distributions, the particles alignment orientations as well as the environment strongly contribute to the crack initiation mechanisms.

Transmission electron microscopy (TEM) experiments combining conventional and weak beam observations, energy dispersive X-ray and electron energy loss spectroscopies have been used to identify and quantify, in the polycrystalline AD730 (TM) Ni-Based superalloy, the effect of isothermal aging after solution treatment in the range 730-790 degrees C on the creep behavior at 700 degrees C - 500 MPa. The microstructure of different aged crept specimens has been first characterized as the creep behavior is directly influenced by the microstructural features (size of the gamma' precipitates and of the gamma-channel). The analyzes show similar results whatever the heat treatment. Then, the investigation of the dislocation configurations indicate that the deformation occurs by the propagation of straight perfect dislocations, which shear the gamma-matrix and the precipitates at 730 degrees C and 760 degrees C, whereas a combination of perfect and partial dislocations is observed after aging at 790 degrees C. The investigation of the local chemical composition has revealed some strong local variations in Cr content within the gamma-phase and smaller ones in Ti content within the gamma'-phase after aging at 790 degrees C. These elemental concentration quantifications have been used to evaluate the antiphase boundary and stacking fault energies. This allows to interpret the presence of partial dislocations after aging at 790 degrees C and thus the increase in the creep strain rate when the specimen is previously aged at 790 degrees C.

Hot ductility of the newly developed AD730 (TM) nickel-base superalloy was investigated in the temperature interval 1050-1240 degrees C. The nil strength and nil ductility temperatures were determined by hot tensile testing using the Gleeble (TM) 3800 weld thermal simulation method. The influence of heating rate, representing the weld thermal cycle, on hot ductility behavior of the alloy was also investigated. The microstructure and the fracture mode of samples were examined by optical and scanning electron microscopy. The influence of heating rate on the extent of grain boundary liquation and void formation was determined and it is shown that the significant ductility loss near the NDT point could be related to the reduction of surface tension at the grain boundary-matrix interface. In addition, the contribution of hard precipitates, such as grain boundary MC carbides, voids, and cavities as other damage mechanisms responsible for ductility loss at high temperature, are discussed.

Linear friction welding (LFW) is a near net shape solid state joining technology for aerospace applications. In this study LFW of a recently introduced Ni-based superalloy, AD730 (TM), with superior properties for use in the hot section of gas turbines was studied. In order to minimize the number of experiments for achieving sound welds, an analytical method was developed that allowed to determine the optimum process parameters with a limited number of experiments. The predictions of the method were validated by LFW experiments, and sound samples without defects were produced. Microstructure evolution of the as-welded samples from the weld center to the base metal was investigated using laser confocal and field emission scanning electron microscopy (SEM) including electron back scatter diffraction (EBSD), energy dispersive spectrometer (EDS) and backscattered imaging (BSE). Post-weld heat treatment (PWHT), consisting of a gamma' sub-solvus solutionizing followed by aging, was conducted on LFWed blocks. Then, microstructure evolution and mechanical testing were conducted on PWHTed samples and compared with the as welded blocks. The obtained results were correlated with microhardness as well as tensile testing at room temperature and 650 degrees C, and interpreted in terms of fundamental metallurgical processes. Macroscopic examination of the PWHTed joints revealed that the samples failed out of the weld zone, further demonstrating the appropriate selection of LFW processing parameters using the proposed analytical method. PWHTed samples exhibited better room and high temperature tensile properties compared to those of the as-welded samples. Microscopic examination of the fracture zones of samples showed that the higher levels of reprecipitation of gamma' particles in the thermomechanically-affected zone (TMAZ) of the PWHT samples were associated to their higher tensile properties compared to the as welded ones. (C) 2019 Published by Elsevier Ltd.

The high temperature creep properties of next generation cast and wrought AD730 superalloy have been investigated taking into consideration three microstructural parameters: the grain size, the presence of grain boundaries and the γ′ precipitates size and distribution. Definitive analysis of the influence of the grain boundaries and γ′ precipitates size distribution has been enabled by the study of single crystalline versions of the polycrystalline alloys studied. At high temperature (equal to or in excess of 850°C), the grain size controls creep properties. Comparisons between polycrystalline and single crystalline specimens indicate that the grain boundaries provide a strengthening effect, especially in the small strain regime. At intermediate temperature (700°C), the γ′ precipitates size is the main creep-rate controlling parameter. In this temperature domain, creep strength seems to be mainly controlled by dislocation motion. A very striking grain boundary strengthening mechanism is observed at small creep strain and intermediate temperature.

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.

AD730 (TM), which is a new nickel base superalloy developed by Aubert & Duval, was designed to provide a better combination of higher temperature strength and cost than the currently available wrought superalloys. In comparison with U720Li, which can be considered as one of the most efficient cast & wrought (C&W) superalloy, previous studies have shown that the mechanical properties of AD730 (TM) are equivalent but its formability for the C&W route is much better. This alloy has also been considered for high temperature fastener applications thanks to its high mechanical properties up to 750 degrees C and its good workability. In this study bars with diameters in the 15-27mm diameter range were successfully hot rolled to enable the evaluation of AD730 (TM) for fastener applications.Preliminary tests show that AD730 (TM), heat-treated in standard conditions with an average grain size equal to ASTM 10, was notch sensitive above 700 degrees C in the presence of stress concentration factor higher than 4. Heat-treatment of AD730 (TM) was investigated and optimized for fasteners applications against notch sensitivity and balanced tensile, creep and fatigue properties. Results show that creep notch sensitivity can be mitigated by increasing the aging temperature, decreasing the cooling rate after solution heat-treatment or decreasing the solution heat-treatment temperature. The best notch fatigue properties were obtained with the largest gamma prime precipitates which promote stress relaxation at the notch. The implication of these fmdings is that the material properties of AD730 (TM) can be customized via specific heat treatment to suit different requirements in fastener applications.

This paper summarizes five years of joint efforts by Aubert & Duval, Ecole de Technologie Superieure, and ISAE-ENSMA/Institut Pprime in developing microstructure graded turbine disks. It is mainly focused on the characterization of the mechanical properties of cast & wrought hybrid disks made of UDIMET (TM) 720Li and the newly developed AD730 (TM) alloys. In such disks, a coarse grain structure (grain size greater than 100 mu m) has been introduced in the rim sections, where time dependent damage processes (creep, dwell-fatigue crack growth) are the main life limiting factors, while bore sections keep a fine grain structure (grain size between 5 to 15 mu m). Smooth disks with a various diameters and height were processed and investigated. Tensile properties at room temperature, 550 degrees C, and 700 degrees C, creep properties at 700 degrees C/750 MPa, 770 degrees C/540 MPa, and 850 degrees C/300 MPa together with low cycle fatigue properties at 550 degrees C were investigated as a function of radial position. Moreover, the effect of an aging heat treatment after the dual microstructure heat treatment (DMHT) has also been investigated.From this study, it is shown that tensile and low cycle fatigue (LCF) properties, as well as creep properties at 770 degrees C/540 MPa and 850 degrees C/300 MPa, are mainly controlled by the grain size, whatever the gamma' precipitation state. Moreover, a gamma'-subsolvus solution heat treatment is recommended after the DMHT to homogenize the intragranular microstructure through the disks and optimize tensile and LCF properties in the bore sections. Finally, creep and tensile properties in the grain size transition areas are shown to be highly dependent to the fraction of coarse grains.

The reprecipitation mechanisms and kinetics of γ′ particles during cooling from supersolvus and subsolvus temperatures were studied in AD730 TM Ni-based superalloy using Differential Thermal Analysis (DTA). The evolution in the morphology and distribution of reprecipitated γ′ particles was investigated using Field Emission Gun Scanning Electron Microscopy (FEG-SEM). Depending on the cooling rate, γ′ particles showed multi or monomodal distribution. The irregularity growth characteristics observed at lower cooling rates were analyzed in the context of Mullins and Sekerka theory, and allowed the determination of a critical size of γ′ particles above which morphological instability appears. Precipitation kinetics parameters were determined using a non-isothermal JMA model and DTA data. The Avrami exponent was determined to be in the 1.5–2.3 range, suggesting spherical or irregular growth. A methodology was developed to take into account the temperature dependence of the rate coefficient k ( T ) in the non-isothermal JMA equation. In that regard, a function for k ( T ) was developed. Based on the results obtained, reprecipitation kinetics models for low and high cooling rates are proposed to quantify and predict the volume fraction of reprecipitated γ′ particles during the cooling process.

High temperature creep and dwell-fatigue properties of the new nickel-based superalloy AD730™ have been investigated. Three microstructures have been studied in creep (850 °C and 700 °C) and dwell-fatigue (700 °C stress control with trapezoidal signals, and dwell times ranging from 1 s to 3600 s): a coarse grains microstructure, a fine grains one, and single crystalline samples. The aim of this study is to assess the influence of the grain size on creep and creep-fatigue properties. It is demonstrated that fine and coarse grains microstructures perform similarly in creep at 700 °C, showing that the creep properties at this temperature are controlled by the intragranular precipitation. Moreover, both the coarse grains and the fine grains microstructures show changes in creep deformation mechanisms depending on the applied stress in creep at 700 °C. At higher creep temperatures, the coarse grains microstructure performs better and almost no effect is observed by suppressing grain boundaries. During dwell-fatigue tests at 700 °C, a clear effect of the mechanical cycling has been evidenced on the time to failure on both the coarse and the fine grains microstructures. At high applied stresses, a beneficial effect of the cyclic unloading to the lifetime has been observed whereas at lower applied stresses, mechanical cycling is detrimental compared to the pure creep lifetime due to the development of a fatigue damage. Complex creep-fatigue interactions are hence clearly evidenced and they depend on the pure creep behavior reference.

The enhancement of efficiency in gas turbine engines requires the development of new superalloys capable of withstanding higher temperatures. The development of new industrial cast and wrought (C&W) disk alloys with required combination of strength, creep and fatigue properties at 700 degrees C is highly desired due to expensive cost of powder metallurgy. The development of U720Li in the nineties provided an intermediate solution in term of cost / properties combination between 718 and powder metallurgy superalloys. However, U720Li is known to be difficult to be manufactured due to its high gamma ' content (45%) and can be considered as the limit between C&W route and powder metallurgy route. Because of this and its intrinsic raw material content, U720Li is significantly more expensive than alloy IN718. To remain competitive on worldwide aeronautical market, engine designers are more and more challenged to optimize cost/properties combination of each component. In this context, Aubert&Duval has developed a new C&W superalloy, AD730 (TM) which was designed to offer a better combination between high temperature properties at 700 degrees C and cost compared to U720Li. The lower content of Ti and the lower gamma ' amount in AD730 (TM) compared to U720Li is expected to reduce the segregation tendency. Contrary to U720Li, the better ability of AD730 (TM) for C&W route enables the vacuum arc remelting of larger ingots up to 635 mm diameter, which is expected to have a beneficial impact on billet cost. This paper describes the work performed within the European Cleansky project CESAME (Cost Effective Superalloy for Advanced Modern Engine) which evaluated the feasibility of larger AD730 (TM) ingots (635 mm diameter ingots) in order to improve AD730 (TM) manufacturing cost. Comparisons between AD730 (TM) ingot of 635 mm diameter (called in the following text AD730 (TM) large ingot), AD730 (TM) ingot of 508 mm diameter (called in the following text AD730 (TM) small ingot) and U720Li ingot of 508 mm diameter (called in the following text U720Li ingot) were performed after conversion to billets of 254 mm but also on close-die forged disks. Results on billets showed that increasing ingot size for AD730 (TM) alloy had a beneficial impact on billet microstructure and ultrasonic testing results in comparison with AD730 (TM) and U720Li smaller ingots (508 mm diameter). Increasing AD730 (TM) ingot diameter up to 635 mm was not harmful on chemical homogeneity but led as expected to an increase of carbonitrides size. Tensile, creep and fatigue tests on close-die forged disks showed no detrimental influence of AD730 (TM) larger ingot size (635 mm diameter) compared to AD730 (TM) smaller ingot (508 mm diameter). However, homogeneous microstructure and better UT detectability than those on U720Li disks were obtained on AD730 (TM) disks. Mechanical properties of AD730 (TM) and U720Li are very similar with advantages on tensile strength at high temperature and creep elongation for AD730 (TM). Finally, fatigue tests on close-die forged disks showed that the largest carbonitrides observed on disk from AD730 (TM) large ingot did not lead to detrimental effect on fatigue properties. This work was a reliable demonstration of AD730 (TM) larger ingot feasibility to enhance the combination between cost and properties.

The enhancement of efficiency in power generation gas turbine requires the development of new superalloys capable of withstanding higher temperatures. The development of AD730TM superalloy was achieved to provide to this new cast & wrought (C&W) superalloy a higher combination between mechanical properties, microstructural stability and cost than that of other C&W superalloys with a temperature capability up to 750 ∘C. Supersolvus heat-treatment of AD730TM was studied to improve the creep properties of fine grain AD730TM superalloy which were not high enough to reach the foreseen conditions of future power generation gas turbine disks. Firstly, the grain growth was studied to select the supersolvus temperature 1120 ∘C and to obtain a homogeneous coarse grain microstructure. Then, various supersolvus heat-treatments with different cycles were tested and applied on a forged pancake with a section representative of power generation gas turbine disk. The average grain size was evaluated to be close to 200 μm for all heat-treatments. Tensile, creep, fatigue and fatigue crack growth tests were performed to compare the various heat-treatments. FEG-SEM examinations were also realized to discuss the relationships between heat-treatment, intragranular gamma prime precipitation and mechanical properties. Finally, a comparison made with other supersolvus heat treated C&W superalloys shows that AD730TM properties obtained with coarse grain microstructure are at the expected level and enable applications for power generation gas turbine discs.

AD730 (TM) was designed to be a cost effective superalloy for high temperature turbine disks. This superalloy, strengthened by gamma prime precipitates, presents a good ability for cast and wrought process route and a high combination between high temperature mechanical properties and cost. A study was performed to optimize the aging sequence considering tensile, creep and dwell-fatigue crack growth rate properties. Isothermal aging in the 730-790 degrees C range with various durations in the 4-16h range were performed after a 1080 degrees C/4h solution heat treatment followed by air cooling on coupons to investigate the effects of temperature and time on gamma prime precipitation and on mechanical properties. The effect of a second additional aging step (700 degrees C/8h) was also investigated and discussed. Results show that the strengthening peak during aging is close to 730 degrees C. Above this aging temperature, tensile strength decreases due to a slight increase of secondary gamma prime diameter. Gamma prime precipitates remain however very fine for all aging sequences with a diameter in the 20-50nm range. Creep properties at 700 degrees C were strongly decreased by aging temperatures above 760 degrees C. A special attention was also paid to dwell-fatigue crack growth rate behavior. Relaxation tests at 650 degrees C were performed to compare the different microstructures assuming that best behavior during crack propagation would be obtained with microstructures that promote the highest stress relaxation magnitude. Finally, mechanical properties were assessed on a forged disk heat-treated in industrial conditions with the selected aging 730 degrees C/8h/Air and were at the expected level.

To reduce CO2 emissions on coal-fired power plant, A-ultra supercritical (A-USC) power plant whose steam conditions exceed 700 °C are being developed. At these elevated temperatures, the use of Ni-base superalloys becomes necessary. In this context and within the European project NextGenPower, focus is made on commercial Nimonic C-263 as a candidate material for turbine rotors. Nimonic C-263 is known to have low sensitivity to segregation, high workability and high weldability which are major properties for the manufacture of large shafts. Long-term creep strength is also required for this application and unfortunately Nimonic C-263 shows η-phase precipitation after long-time exposure between 700 °C–900 °C which is detrimental for long-term creep properties. The composition of Nimonic C-263 was thus optimised to overcome the formation of η-phase. Trial tests were made in order to study the effect of hardening contribution elements on microstructural and mechanical properties. Then, a 500 mm diameter forged rotor was made from optimised 263 alloy and shows promising properties.

The enhancement of efficiency in gas turbine engines requires the development of new superalloys capable of withstanding higher temperatures. The development of new industrial cast and wrought (C&W) disk alloys with required combination of strength, creep and fatigue properties at 700 degrees C is highly desired due to the expensive cost of powder metallurgy. AD730 (TM), which is the newly nickel base superalloys developed by Aubert & Duval, was therefore designed to offer a better combination between high temperature properties at 700 degrees C and cost compared to other C&W superalloys. This paper describes the alloy design based upon the chemistries of the previous experimental alloys Ni30 and Ni33 [1-2]. The control of expensive elements contents and the presence of iron in AD730 (TM) alloy confer to this alloy an attractive cost compared to other C&W superalloys for disk applications. Gamma prime solvus was decreased compared to Ni33 in order to improve hot workability and (Ti+Nb)/Al ratio was decreased compared to Ni30 and Ni33 in order to avoid any risk of Eta-phase precipitation. It was actually observed that the precipitation of the needle-shape Eta-phase predicted by the thermodynamic databases was not in agreement with experimental results obtained on various alloys of the AD730 (TM) chemical system. Industrial ingots with a diameter equal to 500mm were produced (Vacuum melting and remelting) and converted to evaluate the mechanical properties and the ability for the conventional C&W route. A special attention is made in this paper to the AD730 (TM) workability which was highly evaluated with various industrial forging process routes (close-die forging, ring-rolling. etc). Heat treatment optimization was then performed on this alloy in regard to tensile and creep properties. The effect of solution heat-treatment temperature and cooling rate after solution heat treatment were investigated on AD730 (TM). Solution heat treatment temperature has a slight effect on the tensile strength if the temperature is lower than the gamma prime solvus. Yield strength remains stable and close to 1100MPa at 700 degrees C. Solution heat-treatment was therefore optimized in regard of grain size in order to increase creep properties. As most of superalloys strengthened by gamma prime phase, cooling rate after solution heat-treatment has to be as fast as possible to get the highest tensile and creep properties.Oil quenching can be easily performed on AD730 (TM) without any issues due to the moderate gamma prime content in the alloy (35-40%) and the fine grain size. Tensile, creep, long-term aging performed on a forged disk heat-treated in optimized conditions, are presented and discussed in this paper. A comparison with Udimet720 (TM) properties and 718Plus (TM) ones show that AD730 (TM) alloy presents a higher combination between cost and mechanical properties at 700 degrees C than current C&W superalloys.

The enhancement of efficiency in gas turbine engines requires the development of new superalloys capable of withstanding higher temperatures. The development of novel industrial cast and wrought (C&W) disk alloys with required combination of strength, creep and fatigue resistances at 700°C is particularly desired due to the expensive cost of powder metallurgy. In this context, new C&W disk alloys were recently developed to fulfill these requirements. TMW4 shows higher properties than the current C&W disk alloy despite an expensive cost due to its high cobalt content, where as 718Plus presents a moderate cost with restricted creep properties at 700°C compared to the current U720Li disk alloy. The new nickel base superalloys developed by Aubert & Duval were therefore designed to offer a better compromise between high temperature properties at 700°C and cost. This paper describes the alloy metallurgical features and is especially focused on the alloy design which is extensively based on phase diagram modeling. The study was firstly carried out on small ingots of 6 kg to optimize the chemistry before forging 200 kg ingots by industrial processes. The ability to be processed by the conventional cast & wrought route and the control of the highly expensive elements contents confer to the alloys an attractive cost comparable to that of 718Plus alloy. The high amount of ’ and the molybdenum-tungsten levels insure higher creep and tensile properties than those obtained with 718Plus.

The tensile properties of a forged UDIMET 720Li alloy have been investigated at room temperature.The aim of this study was to increase both yield and ultimate tensile stresses using adequate thermal treatments.Classical three steps heat treatments after hot forging were applied: a solution treatment followed by a quench and a two steps aging treatment.Several combinations were investigated: four hours solution treatments, either sub-solvus (1080 C -1120 C) or super-solvus (1160 C); two different cooling rates (10 C/min or 3600 C/min); four different two-steps aging treatments: 650 C/24h/Air Quench (AQ) + 760 C/16h/AQ, 760 C/16h/AQ + 650 C/24h/AQ, 700 C/24h/AQ + 815 C/16h/AQ, 815 C/16h/AQ + 700 C/24h/AQ.The tensile properties appeared to be maximized with the following combination of heat treatments: sub-solvus solutioning (1120 C or 1080 C), fast cooling rate (oil quench, 3600 C/min), and the 760 C/16h/AQ + 650 C/24h/AQ aging.Both EBSD measurements and systematic stereological analyses were performed to characterize for each condition, grain size and γ' distribution (primary γ', secondary γ', tertiary γ'), respectively.The increase of the 0.2% yield stress, is managed by a competition between keeping a small grain size (i.e. by using a low temperature sub-solvus solution treatment) and increasing the intragranular γ' content (i.e. by increasing the solutioning temperature).In order to evaluate the relative contributions to the deformation mechanisms of grain size and of γ' particles, especially intergranular one (primary γ'), Scanning Electron Microscope (SEM) insitu tensile tests have been performed at room temperature on both sub-and super-solvus samples.It is finally demonstrated that the main controlling parameter to reach a very high yield stress is grain size.

Gamma double prime (γ′′), precipitation was studied in Alloy 718 using isothermal and isochronal aging heat treatments applied between 943 and 1003K. It is shown, that the coarsening behavior of γ′′ precipitates follows the coarsening kinetic predictions of the Lifshitz–Slyozov–Wagner (LSW) theory. The activation energy for γ′′ growth has been determined as equal to 272kJmol−1 and seems to be controlled by volume diffusion of niobium in the matrix. The energy of the γ′′/matrix interface, Γ, has been found to be 95±17mJm−2 by assuming that the γ′′ precipitates adopt a disk shape which minimizes the total energy. This energy includes a volume distortion term calculated from the Eshelby inclusion theory and a surface component which is assumed to be isotropic. This interfacial energy is discussed and compared with the energy of γ′/matrix and γ′′/matrix interfaces in other superalloys. The constant K′′ of the LSW law time dependence has been calculated using the value of interfacial energy and the activation energy of γ′′ precipitates coarsening and is found to be in good agreement with our experimental values.