High Strength Nickel-base Superalloy Research Articles

HOT TENSILE PROPERTIES OF INCONEL 706 ALLOY

INCONEL 706 is a high-strength nickel-based super alloy that is commonly used in high-temperature applications such as gas turbines and nuclear reactors. In this study, the hot tensile properties of Inconel 706 were investigated to better understand the material's behavior under high-temperature and high-stress conditions. Tensile tests were conducted at temperatures ranging from 740°, 860°, 940°, 1020°1080°and strain rates ranging from 0.001/s to 1/s. The results showed that the ultimate tensile strength and yield strength of Inconel 706 decreased with increasing temperature, while the elongation increased. The strain rate had a negligible effect on the mechanical properties. Microstructural analysis revealed that the deformation mechanism changed from dislocation slip to dynamic recrystallization as the temperature increased. The activation energy for deformation was calculated to be 421 kJ/mol, indicating that diffusion-controlled mechanisms play an important role in the deformation process. Overall, the hot tensile properties of Inconel 706 were found to be highly dependent on temperature and microstructure, which should be considered in the design and operation of high-temperature components made of this alloy. The output from the tensile testing machine is used for the data in analysis software like MATLAB, Ansys. The aim of investigations conducted in this project is to study the change of the mechanical properties, especially the high temperature mechanical properties of HTPI, at elevated temperatures.

Failure analysis of a heavy gauge fastener of the ITER toroidal field gravity support system

The Gravity Supports (GS) of the ITER Toroidal Field (TF) coils are positioned at the bottom of the machine and are situated under the 18 toroidal field coils. Each support includes 26 high strength bolts with M60 and M85 gauge. The TFGS will sustain a total load of about 11,000 tonnes of dead weight of the magnet system. Moreover, they will be exposed in operation to large electromagnetic forces and possible seismic dynamic loads. The bolts are manufactured from forged rods of double aged UNS N07718 (also known as Inconel® 718), a high strength nickel base superalloy. Stringent material specification and quality controls requirements apply to the threaded bolts and the other components of the GS. A complete break of one M85 bolt was discovered, that occurred about 40 days after its installation and preloading, prior to the application of any operating stress. Half of the part blew out from the bolthole and was found lying horizontally on the GS top area. Following the incident, a comprehensive failure analysis was carried out, based on a combination of non-destructive and destructive examinations including advanced techniques such as computed microtomography, immersion ultrasonic testing (UT) and fracture mechanics. The delayed rupture was understood as due to a combination of cavities not closed by the forging operations associated to a continuous network of brittle secondary phases at the grain boundaries, resulting in a local lack of ductility, poor impact toughness and mechanical properties locally lower than specified. As a consequence of the incident, UT procedures were developed to confirm that the remaining bolts, including the installed ones, are free of cavities using sufficiently conservative criteria and fit for purpose prior to operation. In particular, in-situ axial inspections allowed installed bolts that could contain relevant imperfections to be identified and replaced.

Prediction and optimization of work-piece temperature during 2.5-D milling of Inconel 625 using regression and Genetic Algorithm

The estimation of heat distribution during metal cutting is essential, as it contributes in workpiece deflection and quality of the machining. Inconel 625 is a high strength nickel-based superalloy...

On the Influence of Nb/Ti Ratio on Environmentally-Assisted Crack Growth in High-Strength Nickel-Based Superalloys

The effect of Nb/Ti ratio on environmentally-assisted crack growth of three prototype Ni-based superalloys is studied. For these alloys, the yield strength is unaltered with increasing Nb/Ti ratio due to an increase in grain size. This situation has allowed the rationalization of the factors influencing damage tolerance at 700 °C. Primary intergranular cracks have been investigated using energy-dispersive X-ray spectroscopy in a scanning transmission electron microscope and the analysis of electron back-scatter diffraction patterns. Any possible detrimental effect of Nb on the observed crack tip damage due to Nb-rich oxide formation is not observed. Instead, evidence is presented to indicate that the tertiary γ′-precipitates are dissolving ahead of the crack consistent with the formation of oxides such as alumina and rutile. Our results have implications for alloy design efforts; at any given strength level, both more and less damage-tolerant variants of these alloys can be designed.

Characterization of Phase Chemistry and Partitioning in a Family of High-Strength Nickel-Based Superalloys

A family of novel polycrystalline Ni-based superalloys with varying Ti:Nb ratios has been created using computational alloy design techniques, and subsequently characterized using atom probe tomography and electron microscopy. Phase chemistry, elemental partitioning, and γ′ character have been analyzed and compared with thermodynamic predictions created using Thermo-Calc. Phase compositions and γ′ volume fraction were found to compare favorably with the thermodynamically predicted values, while predicted partitioning behavior for Ti, Nb, Cr, and Co tended to overestimate γ′ preference over the γ matrix, often with opposing trends vs Nb concentration.

Time-resolved synchrotron diffractometry of phase transformations in high strength nickel-based superalloys

Phase transformations in prototype high strength polycrystalline nickel-based superalloys of varying Ti/Nb ratio are studied using time-resolved, high resolution X-ray synchrotron diffractometry. The dissolution kinetics of the ordered phase Ni3(Al, Ti, Nb, Ta) upon heating to the solutioning temperature of ∼1200°C and its reprecipitation on cooling are deduced; effects of varying Nb and Ti alloy composition on the reaction kinetics are identified. Heating to 800°C does not alter substantially the fraction of the strengthening phase Ni3(Al, Ti, Nb, Ta) but further heating causes its rapid dissolution. At higher temperatures, evidence is provided for the formation of further ordered phases; Ni3(Ti, Ta) is proposed and possibly Ni0.45Ta0.55; cooling causes their dissolution and reprecipitation of Ni3(Al, Ti, Nb, Ta), so that it seems probable that the reactions are coupled. The unforeseen high temperature precipitation of further ordering by phases other than Ni3(Al, Ti, Nb, Ta) implies the possibility of a contribution by them to the high temperature mechanical behaviour of these materials, which until now has been thought to be solely due to Ni3(Al, Ti, Nb, Ta). The MC carbide, probably TiC, is stable even at the solution heat treatment temperature; no evidence of reactions involving other carbides such as M23C6 is found.

Modelling of the influence of alloy composition on flow stress in high-strength nickel-based superalloys

A model is proposed for the variation of the yield strength of nickel-based superalloys as a function of chemical composition. Consistent with hardening theory, alloy strength is assumed to be proportional to the product of the anti-phase boundary (APB) energy and the square root of the fraction of the strengthening γ′ phase. A relationship is established between the APB energy estimated using a CALPHAD database and predictions from density functional theory. Quantitative estimates of the role played by Ti, Ta, Nb, Cr, W and Mo suggest that these elements have a profound effect on APB energy. A procedure is proposed to enable the strength to be estimated from an initial input of the chemical composition alone. Predictions are made for new multicomponent alloys. Insight is provided into how composition may be isolated for optimal strengthening. However, the size and spacing of the γ′ precipitates is not explicitly predicted or considered; future work must address this.

열 노출에 의한 IN738LC의 기계적 특성 및 미세조직 변화

니켈기 초내열합금은 높은 강도, 피로 및 산화 저항성이 우수하여 비행기 엔진, 선박 엔진 및 발전용 가스터빈 고온 부품 등을 만드는 소재로 오래전부터 사용되어 왔다. 본 연구에서는 가스터빈 블레이드 소재인 니켈기 초내열합금 IN738LC에 대하여 실제 운전환경과 유사한 조건을 설정하여 다양한 변형률 범위와 온도에서 인장시험을 수행하였다. IN738LC 소재를 871℃ 및 982℃ 에서 각각 1,000 ~ 10,000시간 동안 열 노출한 시편을 준비하여 기계적 특성 및 미세조직 변화를 관찰하였다. 기계적 특성 변화는 열 노출 시간에 따른 '의 변화와 관련된 것을 확인하였다.<BR>

A Comparison of Residual Stress Development in Inertia Friction Welded Fine Grain and Coarse Grain Nickel-Base Superalloy

The effect of the base material microstructure on the development of residual stresses across the weld line in inertia friction welds (IFWs) of high-strength nickel-base superalloy RR1000 was studied using neutron diffraction. A comparison was carried out between tubular IFW specimens generated from RR1000 heat treated below (fine grain (FG) structure) and above (coarse grain (CG) structure) the γ′-solvus. Residual stresses were mapped in the as-welded (AW) condition and, after a postweld heat treatment (PWHT), optimized for maximum alloy strength. The highest tensile stresses were generally found in the hoop direction at the weld line near the inner diameter of the tubular-shaped specimens. A comparison between the residual stresses generated in FG and CG RR1000 suggests that the starting microstructure has little influence on the maximum residual stresses generated in the weld even though different levels of energy must be input to achieve a successful weld in each case. The residual stresses in the postweld heat treated samples were about 35 pct less than for the AW condition. Despite the fact that the high-temperature properties of the two parent microstructures are different, no significant differences in terms of stress relief were found between the FG and CG RR1000 IFWs. Since the actual weld microstructures of FG and CG RR1000 inertia welds are very similar, the results suggest that it is the weld microstructure and its associated high-temperature properties rather than the parent material that affects the overall weld stress distribution and its subsequent stress relief.

Effect of secondary orientation on notch-tip plasticity in superalloy single crystals

Numerical and experimental evolutions of slip fields in notched Ni-Base Single Crystal superalloy tensile specimens are presented as a function of secondary crystallographic orientation. The numerical predictions based on three-dimensional anisotropic elasticity and crystal plasticity are compared with experimental observations. The results illustrate the strong dependence of the slip patterns and the plastic zone size and shape on the secondary orientation of notches, which can have important consequences on crack initiation. Specific orientations or non-symmetric notch geometries lead to non-symmetric patterns on both sides of the sample. The computations show that strongly different plastic zones are expected in the core of the sample and at free surfaces. The ability of the anisotropic elastic model to anticipate the plastic domains, based on identifying dominant slip systems, is confirmed by the crystal plasticity computations, at low load levels. An important observation is that kink shear banding is a real deformation mode operating at crack tips and notches in high strength nickel-based single crystal superalloys for specific orientations.

Low Cycle Fatigue Behavior of Ni-Base Superalloy IN738LC at Elevated Temperature

High strength nickel-base super alloys have been used in turbine blades for many years because of their superior performance at high temperature. The prediction of fatigue life for superalloys is important for improving the efficiency. In this study, low cycle fatigue tests are performed the variables of total strain range, and room and elevated temperature. The relations between plastic and total strain energy densities and number of cycles to failure are examined in order to predict the low cycle fatigue life of IN738LC super alloy. The fatigue life is evaluated by the Coffin-Manson equation, also the predicted lives by plastic and total strain energy density are compared with experimental results.

니켈기 초내열합금 IN738LC의 고온 저주기피로 거동

니켈기 초내열합금은 고온 강도를 지속적으로 증가시키며 현재 비행기 엔진, 선박 엔진 및 발전용 가스터빈 고온 부품 등을 만드는 가장 중요한 소재로 오래전부터 사용되어져 왔다. 이러한 부품의 수명을 연장하기 위해서는 사용 환경과 유사한 조건에서의 피로수명 예측이 매우 중요하다. 따라서 본 연구에서는 가스터빈 블레이드 소재인 니켈기 초내열합금 IN738LC에 대하여 실제운전환경과 유사한 조건을 설정하여 다양한 변형률 범위와 온도에서 시험을 수행하였다. 저주기 피로수명을 예측하기 위하여 변형률 에너지 밀도와 파단 사이클과의 관계를 사용하였다. 수명의 예측은 시험결과를 토대로 변형률 에너지법과 Coffin-Manson법에 의하여 예측을 하였다.

Application of X-ray and neutron diffraction to determine lattice parameters and precipitate volume fractions in low misfit nickel base superalloys

Synchrotron X-ray diffraction and neutron diffraction techniques are employed to characterise the lattice parameters and volume fraction of γ and γ′ phases in a high strength nickel base superalloy. Samples of Rene88DT were solutionised at 1150°C and cooled under three different rates to provide fine and coarse γ′ size distributions. Samples were aged at 760°C to precipitate tertiary γ′ and coarsen secondary precipitates. Lattice parameter misfit and coefficient of thermal expansion parameters were also determined. Results indicate significant microstructure changes within the samples during the initial 25 h of aging.

Residual stresses in face finish turning of high strength nickel-based superalloy

This paper investigates the residual stresses distributions introduced in a new generation nickel-based superalloy RR1000 by surface finish turning. The residual stresses introduced as a function of depth have been analysed for a series of machining trials with round and rhombic inserts, coated and uncoated inserts, new and worn tools, and chipped tool. The residual stress depth profiles obtained by X-ray diffraction, and layer removal show that the tool type, tool coating, tool wear and tool breakage influence the residual stress. The extent of plastic deformation for different cutting conditions has been inferred from the different peak width. Overall, residual stresses tend to have a tensile character at all depths in the hoop direction, but exhibit a significant compressively stressed zone in the radial direction.

Characterisation of Residual Stresses in Machined Surfaces of a High Strength Nickel-Base Superalloy

The near surface residual stresses introduced by turning on a nickel-base superalloy RR1000 are studied. X-ray diffraction residual stress measurements have been made in both radial and circumferential directions as a function of depth. These show systematic changes in stress magnitude and depth profile. The extent of the plastic deformation has been determined by the variation of the diffraction peak width in the XRD measurement. Both the residual stresses and the plastic work have been found to vary sharply over the first 100 mu m. The variation in unstressed lattice spacing with depth points to a heat affected zone of 40 mu m in thickness.

Microstructure Modeling as a Tool to Optimize Forging of Critical Aircraft Parts

Forging of high strength nickel base superalloys 720 and 718 for aircraft parts requires the usage of finite element simulations to ensure a proper thermo-mechanical treatment. Because of the strong mechanical requirements and narrow specifications of such parts not only a correct, defect free final geometry is necessary, but also a defined microstructure. The crucial point is therefore, to control all process parameters in a way to achieve the demanded properties. The typical forging processes like hydraulic, screw press and hammer forging imply a broad spectrum of strain rates. The influence of this different strain rates as well as forging temperature and strain on dynamic and post-dynamic recrystallization have been examined experimentally. Annealing tests at various temperatures and time periods have been performed, to investigate the grain growth behavior and dissolution processes in this before mentioned materials during heating periods. The obtained data was used to build phenomenological models, which were implemented into finite element code of a commercial special purpose finite element program. 2D and 3D Simulations of multiple step thermo-mechanical processes are compared with microstructure examinations of forged parts to show the usability and accuracy of such models as a tool to optimize complex forging processes of critical aircraft parts. In combination with systematic process data collection during production a stable processes and satisfactory mechanical product properties are guaranteed.

Effect of the microstructure on the creep behavior of PM Udimet 720 superalloy—experiments and modeling

Powder metallurgy processed Udimet 720 is a high creep strength nickel-based superalloy considered for high temperature turbine disks for nuclear gas cooled reactors working under 700 °C. Both fine-grained and coarse-grained microstructures have been obtained by applying respectively a subsolvus or a supersolvus solution treatments, followed by ageing treatments. In both microstructures, the distribution of the strengthening γ′ precipitates has been characterized by transmission electron microscopy (TEM). The creep curves of the coarse-grained microstructure show the three usual creep stages. On the contrary, the creep curves of the fine-grained microstructure show a transition directly from primary to apparent tertiary creep without any obvious steady state. According to TEM analyses, Orowan loops surround Udimet 720 CR γ′ and U720 HS γ′ at high stress whereas U720 HS γ′ are sheared at low stress. To describe the behavior of the superalloy Udimet 720, a specific creep model is developed on the basis of McLean and Dyson models including physical damage parameters.

The determination of the residual strains and stresses in a tungsten inert gas welded sheet of IN718 superalloy using neutron diffraction

The residual stress state in a tungsten inert gas (TIG) welded sheet of IN718, a high-strength nickel-based superalloy, has been characterized using neutron diffraction. The measurements were performed using a time-of-flight diffractometer, which allowed lattice strains from the γ-γ′ {111} and γ-γ′ {311} composite peaks to be compared with the Reitveld-refined spectra. Residual stresses were estimated using plane-specific values of Young's modulus and Poisson's ratio for the {311} and {111} peaks, and the macroscopic material response for the Reitveld-refined data. These values were obtained from a theoretical analysis of existing data after Kröner. The weld considered was an autogenous TIG weld 180mm long placed centrally on to a 2mm × 100mm × 200mm solution heat-treated sheet of IN718. The strain was mapped over the central 140mm of the plate; within this region, the measured strains were almost constant along the length, with the peak bulk longitudinal strain of 1700 × 10-6 detected 4mm from the weld centre, in the heat-affected zone. The corresponding peak longitudinal stress was 270 MPa, and the tensile region 11 mm wide, with the longitudinal residual stresses typically up to 2.5 times greater than those in the transverse direction. It was also found that, while in-weld stresses derived from analysis of single peaks may be incorrect due to textural and compositional effects, those derived from Reitveld refinement of the entire spectrum showed reasonable agreement with those obtained from incremental hole drilling measurements made with the Matthar-Soete method.

Comparison of three different techniques for measuring the residual stresses in an electron beam-welded plate of WASPALOY

The longitudinal, transverse, and through-thickness (short-transverse) residual stresses in an electron beam-welded plate of WASPALOY, a high-strength nickel-based superalloy, have been characterized using neutron diffraction, X-ray diffraction, and a hole-drilling method. Where possible, the results from the different techniques, and the associated uncertainties, have been compared. For the neutron measurements, the γ/γ′ {111} peak was used for the determination of lattice strains. The X-ray measurements were carried out using Fe Kα radiation, the sin2ψ technique, and the {311} γ/γ′ composite peak. The Matthar-Soete method was used for the incremental hole-drilling measurements. Unfortunately, due to texture effects, it was not possible to detect the residual stresses within the weld metal by the diffraction-based methods. For the estimation of residual stresses, plane-specific values of the Young’s modulus and Poisson’s ratio were determined from tensile testpieces using in situ neutron diffractometry. When these data are used, it is found that the neutron, X-ray, and hole-drilling residual stress data are mutually consistent, although the absolute certainties vary with the method employed. The results indicate that, next to the weld, the longitudinal residual stresses approach 1000 MPa and are typically far greater (up to 5 times) than those in the transverse and through-thickness directions. Measurements of the longitudinal strain with distance along the welding direction indicate that the stress state reaches a steady state over the central portion of the plate; for this reason, the majority of the diffraction measurements have been made in the plane perpendicular to the weld at the center of the plate. A simple analysis of the thermal cycles and the extent of plastic deformation induced in the specimen is presented. The plastic “upset zone” has a size which is at least 3 times greater than the cross-sectional area of the weld metal; this suggests that, for accurate analysis of weld-induced distortion, attention should be paid to the evolution of residual stresses in the heat-affected zone as well as the fusion zone.

Characterization and modeling of the precipitation of the sigma phase in UDIMET 720 and UDIMET 720Li

The kinetics of the formation of the sigma phase in the high strength nickel-based superalloys UDIMET 720 (U720) and UDIMET 720Li (U720Li) have been characterized using a combination of electrolytic extraction and quantitative X-ray diffraction (XRD) involving the Rietveld method. In order to perform the analysis, a database of crystallographic parameters is required and details of this are presented. It is shown that the numerical data generated using this technique are consistent with observations made using conventional optical, scanning, and transmission electron microscopies. The results are presented in the form of temperature-time-transformation (TTT) diagrams. It appears that U720Li is very much less prone to sigma precipitation than U720. The presence of a tensile stress accelerates the reaction, but the kinetics are very slow at temperatures lower than 700 °C. Thermodynamic calculations have been used in order to infer that the formation of sigma is associated predominantly with the dissolution of the γ matrix. Theoretical modeling of sigma formation is carried out using a coupled thermodynamic/kinetic analysis assuming multicomponent diffusion-controlled growth. The numerical results are broadly consistent with the experimental data; in the U720Li alloy, there appears to be an incubation period associated with the onset of reaction, but this is negligible in U720. In principle, the method could be adapted for the quantification of microstructural instabilities in other superalloys. If a maximum tolerable amount of microstructural degradation is defined, the method provides a rational basis for the estimation of the limiting combinations of temperature, time, and stress that any given alloy can withstand.