Properties Of Single Crystal Superalloys Research Articles

Crystal Plasticity Finite Element Method for Cyclic Behavior of Single Crystal Nickel-Based Superalloy

This paper reports the modeling and simulation of cyclic behavior of single crystal nickel-based superalloy by using the crystal plasticity finite element method. Material constitutive model based on the crystal plasticity theory is developed and is implemented in a parallel way as user subroutine modules embedded in the commercial Abaqus[Formula: see text] software. For simplicity in calibration and without loss of generality, the crystal plasticity constitutive relationship used in this work takes the form that only contains a few parameters. The parameters are optimized by using the Powell algorithm. We employ the calibrated constitutive model with the finite element solver on a cuboid and a blade to simulate cyclic and anisotropic properties of single crystal superalloy. Results show that the predicted stress–strain curves are in good agreement with the experimental measurements, and anisotropic results are presented in both elastic and plastic regions.

Co-alloying effects on the site preference and the elastic properties of γ′-Ni3Al from first-principles calculations

The influences of co-alloying atom pairs Ta–X (X: Ta, Mo, W, Cr, Co, and Re) on the site preference and elastic properties of γ′-Ni3Al precipitate phase were studied using first-principles calculations, and the underlying mechanism for the strengthening effect is determined. It is found that the site preferences of Ta–X co-alloying atom pairs are in the sequence of Al–Al, Al–Ni, Ni–Al, and Ni–Ni sites except for Ta–Co. The atom pair Ta–Co does not display a strong site preference tendency. The elastic properties for the Al–Al configuration were examined, and the results were compared with those of single-alloying to investigate the co-alloying effect. It is shown that the elastic modulus and hardness are obviously improved by co-alloying. Moreover, the effect of co-alloying is stronger than that of single-alloying. The further analyses by the charge density difference and the electronic density of states reveal that the strong effect of the co-alloying is attributed to the enrichment of the charge density between X and its nearest-neighbor Ni, as well as the orbital hybridization between Ta/X-d and Ni-d which acts as covalentlike bonds. Our results can provide an in-depth insight into the physics in the strengthening effect of co-alloying elements on the mechanical properties of single crystal superalloys.

Microstructure evolution and tensile property of a first-generation single crystal superalloy fabricated by laser melting deposition

Additive manufacturing (AM) of single crystal superalloys has got some progress in recent researches, but there are few reports on eliminating recrystallization or mechanical properties of single crystal superalloys by AM. In this work, single-crystal samples of SRR99 were fabricated at high temperature by laser melting deposition (LMD). Owing to the high temperature of substrate in the deposition process, recrystallization in the following heat treatment process was eliminated. The contrast sample of SRR99 was prepared by directional solidification, and the microstructure evolution during heat treatment and tensile property of deposited samples were analyzed. The results showed that the shapes of the γ′ phase became irregular after solution treatment in deposited samples. After a solid solution and aging treatment, the γ′ phase size is larger and the γ′ volume fraction is slightly lower in deposited samples than in the contrast sample. As a result, the yield and tensile strength of deposited samples are slightly lower than that of contrast samples, but the plasticity of deposited samples is better.

Interactions between interfacial dislocations and γ/γ′ interface during high temperature – low stress creep in nickel-based single crystal superalloy

ABSTRACTThe interaction between interfacial dislocations and γ/γ′ interface is critical to the high temperature creep properties of single crystal superalloys. However, only a few studies have paid attention to the detailed structure such as local interfacial morphologies and the elemental distribution around interfacial dislocations. In this paper, the interfacial protrusions and related dislocations in a single crystal superalloy after creep at high temperature – low stress have been investigated in detail. It is found that the morphology and size of the interfacial protrusions remain almost the same during the early and middle stages of high temperature creep, which indicates a local equilibrium at the interfacial protrusions. Steps at different height are formed at the γ/γ′ interface at the initial stage of high temperature creep since dislocations could move along the γ/γ′ interface, which indicates that dislocation motion at different creep stage may affect the morphology of γ/γ′ interface.

Improved 1200 °C stress rupture property of single crystal superalloys by γ′-forming elements addition

The 1200°C/80MPa stress rupture properties of single crystal superalloys are pronouncedly improved by adding γ′-forming elements Al and Ta. The fraction of γ′-precipitate at high temperature enhances with Al and Ta added. This fraction change yields the increase in concentrations of γ-strengthening elements Mo and Re in γ-channel. It produces the elevated solid solution hardening of γ-matrix and the more negative two-phase lattice misfit. The property improvement is ascribed to the increased γ′ fraction and the elevated γ solid solution level, both induced by Al and Ta addition.

Effect of thermal stability of γ′ phase on the recrystallization behaviors of Ni-based single crystal superalloys

The recrystallization may deteriorate the mechanical properties of single crystal superalloys greatly, which concerns the engineers and scientists in the aviation industry. Recrystallization behaviors after grit-blasting and annealing at different temperatures of a γ′-richen experimental single crystal alloy IC32 (the as-cast and heat-treated) were investigated. Samples of the as-cast ones are less prone to recrystallization than the heat-treated ones. In the as-cast state, the interdendritic regions show a stronger resistance to recrystallization than the dendritic regions. According to the results, we can conclude that the size characteristics and thermal stabilities of the γ′-phases play important roles in deciding the recrystallization resistances of the Ni-based single crystal superalloys.

Effect of Nb content on microstructure stability and stress rupture properties of single crystal superalloy containing Re and Ru

Three experimental single crystal superalloys with 0%Nb, 0.5%Nb, 1.0%Nb were cast in the directionally solidified furnace, while other alloying element contents were basically kept unchanged. The effect of Nb on the microstructure, stability at 1100 °C and stress rupture properties at 1070 °C and 160 MPa of the single crystal superalloy were investigated. The experiment results show that the primary dendrite arm spacing decreases and the volume fraction of γ/γ′ eutectic of the alloy increases with the increase of Nb content in the as-cast microstructures. The size of γ′ phase particles becomes small and uniform and the cubic shape does not obviously change with the increase of Nb content. The precipitating rate and volume fraction of TCP phases increase significantly with the increase of Nb content in the process of long term aging at 1100 °C. The stress rupture lives increase and elongation decreases with increasing Nb content at 1070 °C/160 MPa. At last, the relationship between the microstructures stability, stress rupture properties of the alloy and Nb content is discussed based on JMatPro software and the lastest relevant database for single crystal superalloy.

Effects of Cr content on microstructure and mechanical properties of single crystal superalloy

Two experimental single crystal superalloys with 2% Cr and 4% Cr (mass fraction) were cast in a directionally solidified furnace, while other alloying element contents were kept unchanged. The effects of Cr content on the microstructure, phase stability, tensile properties at 1100 °C and stress rupture properties at 1070 °C and 160 MPa of the single crystal superalloy were investigated. The results show that the size of γ' phase particles become small and uniform, and the cubic shape turns a little regular with the increase of Cr content. The γ' directional coarsening and rafting were observed in the 2% Cr and 4% Cr alloys after long term aging (LTA) at 1100 °C. The rafting rate of γ' phase increased with increasing Cr content. Needle-shaped topologically close packed (TCP) phases precipitated and grew along fixed direction in both alloys. The precipitating rate and volume fraction of TCP phases significantly increased with the increase of Cr content. The tensile property of the alloy increased and the stress rupture properties of the alloy decreased with the increase of Cr content at high temperature. The increase of Cr content increased the partition ratio of TCP forming elements, Re, W, and Mo, and the saturation degrees of these elements in γ phases increased. Therefore, the high temperature phase stability of the alloy decreased with the increase of Cr content.

Microsegregation Behavior of Single Crystal Superalloy

The major advancements in some mechanical properties of single crystal superalloys can be attributed to the carbon addition. The present study investigated the effect of carbon addition levels on the microsegregation behavior of single crystal superalloys. Quantitatative partitioning results indicated that typical single crystal superalloy segregation behavior for some elements such as W, Ni, Co, are all towards the dendrite core regions, while for elements of Al, Mo, Ta, Ti, Cr, partition towards the interdendrite regions. For all the alloys studied, the baseline alloy showed the most severe segregation degree, while the no carbon alloy the slightest. Furthermore, the segregation behavior of Ti and Ta were affected by the carbon additions, and that of W was greatly affected by the carbon levels.

High-Temperature Ultrasonic Fatigue Testing at 1000°C

A high-temperature ultrasonic fatigue testing system was developed to evaluate the gigacycle fatigue properties of single-crystal superalloys used in aircraft engine turbine blades. In this development, a commercial ultrasonic fatigue testing machine was considerably modified to achieve high-temperature fatigue testing. The developed system took account of temperature dependency of Youngs modulus, and also had a function to evaluate the Youngs modulus. In order to protect the testing system from the heat of a specimen, straight and round rods were inserted between the testing system and the specimen. Other modifications achieved accurate control of temperature, edge displacement and resonance frequency, which were necessary for accurate control of stress amplitude. The testing system was first applied to a heat-resistant steel at 650 °C to check its accuracy, and next to SC superalloy samples at 1000 °C. In the conventional fatigue tests on the heat-resistant steel, the results were coincident in a frequency range from 1 Hz to 800 Hz, suggesting that comparable results would be obtained in ultrasonic fatigue testing at 20 kHz. In case of the SC superalloy samples, conventional fatigue tests were conducted at only 10 Hz, so the frequency effects were not clarified. In both cases, ultrasonic fatigue testing showed good agreement with conventional fatigue testing. The accuracy of the developed system is therefore high, even at 1000 °C. In these results, the SC superalloys showed no fatigue limit, indicating gigacycle fatigue tests to be necessary.

Influence of Cast Dimension on Rotary Bending High Cycle Fatigue Properties of Single Crystal Superalloy

Samples of DD3 single crystal superalloy with different dimensions were cast in the directionally solidified furnace with high temperature gradient. The effect of cast dimension on the rotary bending high cycle fatigue (HCF) properties of the alloy was investigated at 800 °C in ambient atmosphere. SEM was used to examine the fracture surface and fracture mechanism of the alloy. The results show that the rotary bending HCF properties of the alloy decreases with increasing cast dimension. The cast dimension has little effect on the HCF fracture mechanism of the alloy. The HCF fracture mechanism of the alloy with different cast dimensions is all quasi-cleavage fracture. The fatigue cracks initiated on the surface or near the surface of the specimens. The crack would propagate along {111} octahedral slip planes. Typical fatigue arc and striation formed on fatigue crack steady propagation. The degeneration of HCF properties is due to the increase of dendrite arm spacing and size of γ′ phase particles and maximal microporosity.

NUMERICAL SIMULATION OF RECRYSTALLIZATIONPROCESS OF NICKEL BASE SINGLECRYSTAL SUPERALLOY

The formation of recrystallization can cause great damage on the performance of nickel base single crystal superalloy turbine blade. The stress, strain and stored energy distribution of single crystal superalloy were modeled and simulated under the condition of inhomogeneous deformation. The critical stored energy was established which takes the anisotropic mechanical properties of single crystal superalloy and the recrystallization grain boundary migration process into account. Based on the calculation of the critical stored energy, the recrystallization microstructure in the local deformation zone after heat treatment was simulated by using cellular automaton (CA) method. DD6 single crystal test bars were treated by the cold deformation process and then annealed at the temperature of 1280 for 30, 60 and 240 min. The experimental results showed that the recrystallization occurred on the surface near the deformation area. The recrystallization area and the grain size increased gradually with time. The simulated microstructure results * J vO A g ki 2011CB706801, J !PR v+i 51171089, %|J R f ii 2012ZX04012–011 2011ZX04014–052 qJ4Y4 : 2013–01–21, q +4Y4 : 2013–03–10 )j ( : PY{, , 1987 #p, p DOI: 10.3724/SP.J.1037.2013.00036

Microstructure evolution and creep behavior of a [111] oriented single crystal nickel-based superalloy during tensile creep

By means of creep curve measurements and microstructure observation, the microstructure evolution and creep properties of [001] and [111] oriented single crystal nickel-based superalloys are investigated. Results show that, after full heat treatment, the microstructure of both the oriented single crystal nickel-based superalloys consist of the cuboidal γ′ phase embedded coherently in the γ matrix phase, and arranged regularly along the <100> direction. During tensile creep, the cuboidal γ′ phase in the [001] oriented superalloy is transformed into the rafted structure along the direction vertical to the stress axis, while rafting orientation of the γ′ phase in the [111] oriented alloy is at an angle of about 40°–60° relative to the stress axis. During steady-state creep, the deformation mechanism of the [001] oriented alloy is mainly dislocations being activated in γ matrix channels and climbing over the rafted γ′ phase, while that of the [111] oriented alloy is dislocations slipping in γ matrix channels and shearing into the rafted γ′ phase. As creep goes on, dislocations concentrate to form the subgrain structure in the [111] oriented alloy. From the study of this paper, the creep anisotropy, especially the microstructure-evolution anisotropy and its effect on creep properties of single crystal superalloys, are further specified, which are expected to provide useful data for the design and production of single crystal superalloys.

Effect of Ru on stress rupture properties of nickel-based single crystal superalloy at high temperature

Two experimental single crystal superalloys, Ru-free alloy and Ru-containing alloy with [001] orientation, other alloying element contents being basically kept same, were cast in the directionally solidified furnace. The effect of Ru on the stress rupture properties of the single crystal superalloy was investigated at (980°C, 250 MPa), (1100°C, 140 MPa) and (1120°C, 140 MPa). The results show that Ru can enhance high temperature stress rupture properties of single crystal superalloy. The improvement effect of Ru addition on stress rupture properties decreases with increasing test temperature. The γ′ coarsening and rafting directionally are observed in Ru-free alloy and Ru-containing alloy after stress rupture test. Needle shaped TCP phases precipitated in both of alloys after stress rupture test at (1100°C, 140 MPa) and (1120°C, 140 MPa) and no TCP phase was observed in both of alloys after stress rupture test (980°C, 250 MPa). The precipitate volume fraction of TCP phases is significantly decreased by the addition of Ru. At last, the relationship between the microstructure change with Ru addition and improvement of stress rupture properties was discussed.

Predicting size effects in nickel-base single crystal superalloys with the Discrete-Continuous Model

The Discrete-Continuous Model, a coupling between dislocation dynamics and finite elements simulations, is used for modelling size effects in the mechanical properties of single-crystal superalloys. Both formation and evolution of the dislocation microstructures are analysed, and the crucial role of the storage of signed dislocations at the interfaces is discussed. The onset of plasticity is found to scale as the inverse of the channel width, and polarised dislocation networks at the interfaces significantly increase the flow stress with respect to a bulk crystal.

Role of Rhenium in Single Crystal Ni-Based Superalloys

The effect of rhenium on the microstructure and mechanical properties of single crystal superalloys with a nominal composition of Ni-3Cr-12Co-1Mo-6W-6Al-8Ta-0.1Hf-(0, 2, 4) Re has been studied. With the rise of rhenium content, the size of as-cast  particles becomes smaller. Rhenium addition elevates the incipient melting temperature and slows down the solid solution process. Even after 2000h prolonged aging both at 950°C and 1050°C, no topologically close-packed phase precipitation is found in the three alloys with Re content up to 4%. Re retards the  coarsening and is beneficial to improving the stress rupture life. The deformation mechanisms together with the dislocation configuration have been studied and discussed.

Predicting size effects in nickel-base single crystal superalloys with the Discrete-Continuous Model

The Discrete-Continuous Model, a coupling between dislocation dynamics and finite elements simulations, is used for modelling size effects in the mechanical properties of single-crystal superalloys. Both formation and evolution of the dislocation microstructures are analysed, and the crucial role of the storage of signed dislocations at the interfaces is discussed. The onset of plasticity is found to scale as the inverse of the channel width, and polarised dislocation networks at the interfaces significantly increase the flow stress with respect to a bulk crystal.

Anisotropy of stress rupture properties of a Ni base single crystal superalloy at two temperatures

The stress rupture properties of a single crystal superalloy in three orientations were tested under 871 °C/552 MPa and 1010 °C/248 MPa. The [0 0 1] orientation shows the longest life and good elongation at 871 °C, the [1 1 1] orientation exhibits the best elongation, while the life and elongation of [0 1 1] orientation are both poor at this test condition. At 1010 °C, the anisotropy between [0 0 1] and [0 1 1] orientation is evidently reduced, however the [1 1 1] orientation shows the longest life at the same condition. In [0 0 1] orientation, the N type rafts are formed under two test conditions. In [1 1 1] orientation, the γ′ keep cuboidal shape under 871 °C/552 MPa, while rafts with direction varying with local position are formed under 1010 °C/248 MPa. In [0 1 1] orientation, lots of stacking faults appears in γ′ under 871 °C/552 MPa, while many twins are formed under 1010 °C/248 MPa, which are both related to the activation of {1 1 1} 〈1 1 2〉 slip system. The influence of orientation and test condition on the deformation mechanisms and stress rupture properties is discussed.

High-Temperature Fatigue Properties of Single Crystal Superalloys in Air and Hydrogen

Hot section components in high-performance aircraft and rocket engines are increasingly being made of single crystal nickel superalloys such as PWA1480, PWA1484, CMSX-4, and Rene N-4 as these materials provide superior creep, stress rupture, melt resistance, and thermomechanical fatigue capabilities over their polycrystalline counterparts. Fatigue failures in PWA1480 single crystal nickel-base superalloy turbine blades used in the space shuttle main engine fuel turbopump are discussed. During testing many turbine blades experienced stage II noncrystallographic fatigue cracks with multiple origins at the core leading edge radius and extending down the airfoil span along the core surface. The longer cracks transitioned from stage II fatigue to crystallographic stage I fatigue propagation, on octahedral planes. An investigation of crack depths on the population of blades as a function of secondary crystallographic orientation (β) revealed that for β=45+/−15 deg tip cracks arrested after some growth or did not initiate at all. Finite element analysis of stress response at the blade tip, as a function of primary and secondary crystal orientation, revealed that there are preferential β orientations for which crack growth is minimized at the blade tip. To assess blade fatigue life and durability extensive testing of uniaxial single crystal specimens with different orientations has been tested over a wide temperature range in air and hydrogen. A detailed analysis of the experimentally determined low cycle fatigue properties for PWA1480 and SC 7-14-6 single crystal materials as a function of specimen crystallographic orientation is presented at high temperature (75°F–1800°F) in high-pressure hydrogen and air. Fatigue failure parameters are investigated for low cycle fatigue data of single crystal material based on the shear stress amplitudes on the 24 octahedral and 6 cube slip systems for FCC single crystals. The max shear stress amplitude [Δτmax] on the slip planes reduces the scatter in the low cycle fatigue data and is found to be a good fatigue damage parameter, especially at elevated temperatures. The parameter Δτmax did not characterize the room temperature low cycle fatigue data in high-pressure hydrogen well because of the noncrystallographic eutectic failure mechanism activated by hydrogen at room temperature. Fatigue life equations are developed for various temperature ranges and environmental conditions based on power-law curve fits of the failure parameter with low cycle fatigue test data. These curve fits can be used for assessing blade fatigue life.

MULTIAXIAL LIFETIME PREDICTIONS OF SINGLE-CRYSTAL SUPERALLOYS: USE OF REFERENCE STRESSES

The high temperature microstructural stability and good thermal-mechanical properties of single crystal superalloys has played a crucial role in the development of these materials for gas turbine blade application. A form of Continuum Damage Mechanics has been successfully developed over a decade or so, to model the anisotropic creep deformation of single crystal superalloys, where in a general formulation the creep rate can be obtained by the evolution of several state variables, each related to changes in the alloy microstructure during its lifetime. The model assumes that creep strain arises from the accumulation of shear strain on at least two families of active slip systems: the octahedral system G 1=111}⟨11¯0⟩ and cube system G 2=001}⟨110⟩. Primary creep is viewed to result from a stress transfer process between the γ-matrix and the γ′-particles, and is described by the normalized variable Sk . The tertiary creep behavior in these materials is dominated by strain softening process, associated with an increasing mobile dislocation density in the γ-channels. It is assumed that the rate of damage, ωk , is proportional to the creep rate. The resulting set of constitutive relations has been incorporated into a finite element routine, in order to carry out component calculations that may be under complex states of multiaxial stress. However, the CPU times require to carry out component calculations can be considerable for complex structures such as a turbine blade. In this paper, an alternative procedure is investigated for the predictions of lifetimes under a multiaxial loading using the skeletal point concept. Lifetime calculations of the Bridgman notch specimens were done using equation (1) under the assumption that the multiaxial stress state in the notch be represented by a uniform multiaxial stress, identical to that at the skeletal point.