Base Single Crystal Superalloy Research Articles

Ni基単結晶超合金のラフト組織

Ni基単結晶超合金のラフト組織

Progress of phase-field investigations of γ′ rafting in nickel-base single-crystal superalloys

With the development of nucleation theory, the calculation of phase diagrams(CALPHAD) method and microscopic elasto-plasticity mechanics, it is possible to apply the phase-field method to simulate the γ′ rafting behavior of nickel-base single-crystal superalloys under the conditions of heat treatment, fatigue and creep testing. Based on the experimental progress in γ′ rafting behavior and in combination with the microstructural simulation of the precipitation and rafting of γ′ phase, this paper reviews the phase-field investigation of γ′ rafting in nickel–base single-crystal superalloys.

Effect of climb on dislocation mechanisms and creep rates in γ′-strengthened Ni base superalloy single crystals: A discrete dislocation dynamics study

Creep of single-crystal superalloys is governed by dislocation glide, climb, reactions and annihilation. Discrete three-dimensional (3D) dislocation dynamics (DDD) simulations are used to study the evolution of the dislocation substructure in a γ/γ′ microstructure of a single-crystal superalloy for different climb rates and loading conditions. A hybrid mobility law for glide and climb is used to map the interactions of dislocations with γ′ cubes. The focus is on the early stages of creep, where dislocation plasticity is confined to narrow γ channels. With enhancing climb mobility, the creep strain increases, even if the applied resolved shear stress is below the critical stress required for squeezing dislocations into the γ channels. The simulated creep microstructure consists of long dislocations and a network near the corners of the γ′ precipitate in the low-stress regime. In the high-stress regime, dislocations squeeze into the γ channels, where they deposit dislocation segments at the γ/γ′ interfaces. These observations are in good agreement with experimentally observed dislocation structures that form during high-temperature and low-stress creep.

Mechanism of the oxidation and degradation of the aluminide coating on the nickel–base single-crystal superalloy DD32M

[Liu, C. T.; Ma, J.] Qingdao Univ Sci & Technol, Coll Mat Sci & Engn, Qingdao 266061, Peoples R China. [Sun, X. F.] Chinese Acad Sci, Inst Met Res, State Key Lab Corros & Protect, Shenyang 110016, Peoples R China. [Zhao, P. C.] Qingdao Univ Sci & Technol, Coll Electromech Engn, Qingdao 266061, Peoples R China.;Liu, CT (reprint author), Qingdao Univ Sci & Technol, Coll Mat Sci & Engn, Qingdao 266061, Peoples R China;liuchunting76@yahoo.com.cn

Novel experimentation techniques to study damage accumulation and microstructural instability during creep of super alloy single crystals at high temperatures (>1000°C)

The objective of the present work is to demonstrate that it is well worth the effort to develop novel experimentation techniques to study creep in Ni base super alloy single crystals above 1000°C. A miniature tensile creep specimen and a double shear creep specimen have been developed. These are useful in high temperature technology because they can be directly taken out from cast turbine blades (miniature tensile creep specimens) and provide multiaxial creep data (double shear creep specimens). Most importantly, these new techniques can be used to increase the level of microstructural understanding of creep. Thus miniature tensile creep experiments can be used to demonstrate that <100> directions parallel to, and perpendicular to, the solidification direction differ in creep rupture behaviour. This is related to the presence of cast micropores which align along primary dendrites. The double shear creep specimens, on the other hand, help in understanding the influence of stress multiaxiality on rafting, the directional coarsening of the γ′ phase. In a macroscopic crystallographic shear system of type <110> {110}, rafting occurs at 45° to the shear direction and is governed by the maximum principal stress.

Effect of Re on deformation and slip systems of a Ni base single-crystal superalloy

Using transmission electron microscopy (TEM) and scanning electron microscopy (SEM), the influence of Re on deformation and slip system of Ni base single-crystal superalloy was studied. Stress rupture tests were performed at 280 MPa and 1000 °C, where the effect of Re clusters was prominent. The γ′ precipitates in the DD32 stress rupture samples were rafted to P–N type directional coarsening. The carbides in the DD32 samples deformed along stress axis direction. The finer dislocation network in DD32 could act as a very effective barrier to dislocation cutting into γ′ platelet. The slip of DD32 samples could occur on the two slip systems.

Joining of nickel base superalloy single crystals

The paper reviews scientific and technological issues related to repair weldability of nickel base single crystal superalloys. The role of weld pool geometry and crystallographic orientation of the single crystals on the solidification grain structure was discussed. Results of asymmetric cracking during welding of thin crystals due to stray grain formation were introduced. The role of high resolution characterisation tools including electron microscopy and atom probe field ion microscopy in understangding non-equilibrium microstructure evolution was demonstrated. Computational methodologies to describe the dynamic thermal, stress and microstrutural evolutions to describe weldability as well as the performance of these welds were discussed.

Role of secondary γ′ precipitation and of hydrogen in the first stage of the plastic deformation of the γ matrix of a Ni base superalloy single crystal

A detailed analysis of the tensile properties at room temperature of fully γ AM3 nickel base superalloy single crystals was performed to understand the mechanisms of plastic deformation of the γ matrix of the AM3 industrial γ–γ′ superalloy. The nucleation of the small γ′ precipitates after aging at 750°C of the fully γ alloy strongly increases the critical resolved shear stress of [001] oriented single crystals and contributes to the observed coplanar dislocation motion. Hydrogenated samples exhibit a different behavior depending on the presence of γ′ microprecipitates: internal hydrogen increases the critical resolved shear stress of fully γ specimens whereas the main effect of hydrogen on the first stage of the plastic deformation of the aged samples is an enhancement of the localized plasticity.

Sans analysis of γ′ precipitation in the γ matrix of Ni base superalloy single crystals

Sans analysis of γ′ precipitation in the γ matrix of Ni base superalloy single crystals

Hydrogen transport by mobile dislocations in nickel base superalloy single crystals

Hydrogen deformation interactions have been shown to strongly control the mechanisms of hydrogen embrittlement (H.E.) and stress corrosion cracking (SCC) of numerous metals and alloys. Among the different reported interactions an accelerated transport of hydrogen by mobile dislocations was first suggested by Bastien and Azou. This paper reports the first results of an experimental work aimed at the quantitative characterization of the hydrogen transport by mobile dislocations in a nickel base alloy. The procedure was chosen with the goal of correlating the tritium desorption from a tritiated sample undergoing a tensile straining, with the stress/strain curve.

Retardation of creep in 〈001〉-oriented superalloy CMSX-4 single crystals by superimposed cyclic stress

Experimentally it was shown that the cyclic stress superimposed on static load in the case of CMSX-4 single crystals at 800 °C slows down the rate of the creep process. This effect was explained with the help of a recently developed model of creep in 〈001〉-oriented nickel base superalloy single crystals taking into account the interplay of creep in y channels by dislocation slip, dynamic recovery of the dislocation structure and migration of the γ/ γ′ interfaces. The simulated creep curves are in reasonable agreement with the experimentally measured ones both for loading with and without the cyclic stress component provided the jog density for creep/cyclic loading is taken to be lower than that for pure creep loading. Thus the retardation of creep by superimposed cyclic loading is attributed to the more difficult formation and/or easier annihilation of jogs on vibrating dislocations.

Activation energy of creep in 〈001〉-oriented superalloy CMSX-4 single crystals

The available experimental data on the creep behaviour of the two-phase nickel-base superalloy CMSX-4 single crystals show that the activation energy of the steady-state creep is much higher than the activation energies of self-diffusion in both the phases. A model of creep in 〈001〉-oriented nickel base superalloy single crystals taking into account dislocation slip, dynamic recovery, migration of the γ/γ′ interfaces and coarsening of the rafted structure has been developed. The simulated creep curves and the predicted values of the activation energy are in reasonable agreement with the experimental findings of Schneider and Mughrabi [1].

Influence of cycling frequency on cyclic creep characteristics of nickel base single-crystal superalloy

The static and the cyclic creep load-controlled tests of the nickel base single-crystal superalloy, CMSX 3 have been studied. In this study, two different testing procedures of cyclic creep were carried out under specific loading schedules defined by the stress amplitude, the stress range, stress ratio, and cycling frequency. The different cycling frequencies for two cycling hold periods were investigated to influence the creep/fatigue behaviour of the alloy. The influence of these variables was evaluated through the deformation behaviour and the fracture life data. It is concluded that the additional fatigue stress component superimposed onto creep stress component is of great significance influencing the deformation process and fracture life and accelerating the creep strain rate for chosen types of testing.

Welding of nickel base superalloy single crystals

Welding of nickel base superalloy single crystals

Welding of nickel base superalloy single crystals

The welding and weldability characteristics of single crystal nickel base superalloy PWA 1480 were investigated over a range of welding conditions and orientations using electron beam and laser welding processes. Using differential thermal analysis, the freezing range for this alloy was established and the formation of the γ/γ′ eutectic during the last stages of solidification was identified. Both electron beam and laser welds showed extensive fusion zone cracking, and the cracks were identified as solidification cracks, i.e. hot cracks. Crack free welds could be made over a very narrow range of welding conditions with preheat. Most of the microstructural features observed in the weld are similar to those previously observed in Fe–Ni–Cr alloy single crystal welds. The welds contain misoriented stray grains which playa critical role in the promotion of hot cracks in the welds. The origin of these grains is explained in terms of constitutional supercooling and the growth conditions. Detailed atom probe field ion microscopy analysis was carried out to determine the γ/γ′ compositions and the segregation at the γ/γ′ interface. The results revealed no solutal segregation at this interface, and the composition of the γ/γ′ was close to that of the equilibrium composition.

Change of phase morpholigies during creep of CMSX-4 at 1253K

Morphology changes of {gamma}{prime} precipitates during uniaxial stress of nickel base superalloy single crystals containing high volume fractions of {gamma}{prime} phase are well known. For negative misfit material and tensile stress in [001] direction, the initially cuboidal {gamma}{prime} particles transform into rafts oriented perpendicular to the load axis. In [011] orientation {gamma}{prime} rods, aligned perpendicular to the tensile external load direction, are generated. For the [111] load axis no shape changes were observed neither for tensile nor for compressive stresses. During investigations of creep anisotropy of CMSX-4 and microstructural evolution connected with creep in different orientations, the changes in matrix plate thickness occurring during creep at elevated temperatures are of interest. In [001] as well as in [110] orientation, bowing of dislocation dipoles in matrix channels is a frequently observed process which is controlled by the local resolved shear stress and the thickness of matrix plates. The rhenium containing alloy CMSX-4 has a volume fraction of about 70% of ordered {gamma}{prime} phase after standard heat treatment (solutioning: 1h/1,553 K, 2h/1,563 K, 6h/1,573 K and annealing 6h/1,413 K, 16h/1,143 K).

Creep resistance of CMSX-3 nickel base superalloy single crystals

Creep deformation in 〈001〉 oriented nickel base superalloy single crystals has been studied in an effort to assess the factors which contribute to the overall creep resistance of superalloys with high volume fractions of γ′ phase. Detailed observations of three dimensional dislocation arrangements produced by creep have been made with the use of stereo electron microscopy. In the temperature range of 800–900°C at stresses of 552 MPa or lower, the dislocation-free γ′ precipitates are resistant to shearing by dislocations. As a result, creep deformation occurs by forced bowing of dislocations through the narrow γ matrix channels on {111} planes. At moderate levels of temperature and stress there are incubation periods in virgin crystals prior to the onset of primary creep. The incubations arise because of the difficult process of filling the initially dislocation starved material with creep dislocations from widely spaced sources. When the newly generated dislocations percolate through the cross section, incubation comes to an end and primary creep begins. In primary creep neither work hardening nor any type of recovery plays an important role. The creep rate decelerates because the favorable initial thermal misfit stresses between γ and γ′ phases are relieved by creep flow. Continued creep leads to a build-up of a three-dimensional nodal network of dislocations. This three-dimensional network fills the γ matrix channels during steady state creep and achieves a quasi-stationary structure in time. In situ annealing experiments show that static recovery is ineffective at causing rearrangements in the three-dimensional network at temperatures of 850°C or lower. The kinematical dislocation replacement processes which maintain the quasi-stationary dislocation network structures during apparent steady state creep are not understood and require further study. Because of the impenetrability of the γ′ precipitates, dislocations move through the γ matrix by forced Orowan bowing, and this accounts for a major component of the creep resistance. In addition, the frictional constraint of the coherent or semi-coherent precipitates leads to the build-up of pressure gradients in the microstructure, and this provides load carrying capacity. There is also a smaller component of solid solution strengthening. Work hardening is comparatively unimportant. Finite element analysis shows that the non-deforming precipitates are increasingly stressed as creep deformation accumulates in the matrix. In the later stages of steady state creep and during tertiary creep the stresses in the precipitates rise to high enough levels to cause shearing of the γ′ particles by dislocations entering from the γ matrix. The recovery resistance of the material is in part due to a very low effective diffusion constant and in another part due to the fact that the three-dimensional dislocation networks formed in the γ matrix serve to neutralize the misfit between the γ and γ′ phases.

The maximal dissipation rate criterion—II: Analysis of fatigue crack propagation in FCC single crystals

The following hypothesis has recently been offered for consideration; Crack propagation occurs such that a cracked body 's global dissipation rate is maximized. The present work summarizes results of two recent experimental investigations into the fatigue crack propagation behavior of SRR-99 nickel base superalloy single crystals. Several observations made during the course of these studies are rationalized using a special case of the maximal dissipation rate criterion. It is recalled that the thermodynamic energy balance used to define the dissipation rate also yields a general fatigue crack propagation rate equation. Implications of this equation for the modeling of fatigue crack propagation are briefly discussed.

Ｎｉ基超合金の単結晶化

Ｎｉ基超合金の単結晶化

Apparent and effective creep parameters in single crystals of a nickel base superalloy—II. Secondary creep

At intermediate and high temperatures (700–1000°C) primary creep of nickel base superalloy single crystals appears as an incubation period (part I) with a very low creep rate ( ge < 10 −5/ h). Following a transition period where ge increases rapidly, secondary creep sets in with a constant creep rate of the order ge ∼- 10 −4/ h. Secondary creep is analized in terms of effective stress σ e = σ a − σ i ( σ a , T) and effective creep parameters Q e and n e are estimated Experimental determinations of the apparent creep parameters yield n a = 11 and Q a ∼- 135 kcal/mole in this stage. For creep rates of the order of 10 −4/h, internal stress measurements, as estimated from stress dip tests, point to values of σ i in the vicinity of 0.85−0.95 σ a . When performed under various temperature and applied stress conditions, they lead to the determination of σ isu∗ = ( ∂σ i ∂T) σ a equal to −0.5 MPa/°C and of σ' i = ( ∂σ i ∂σ a ) T equal to 0.2. An effective activation energy of about 60 kcal/mole and an effective stress exponent of the order of 1 are derived from these results. Direct observation of dislocation and precipitate structures in relation with effective creep parameters during secondary creep suggests a high temperature creep model for nickel base superalloys in the range of temperature and applied stress where shearing of the γ' phase does not control the straining process. During secondary creep, strain is mainly the result of climb of dense edge dislocation networks in the γ-γ' interfaces: the dislocation density (− 10 9/cm 2) is kept constant by the glide of screw dislocations between γ' precipitates. Effective creep parameters are typical of a diffusion controlled process: edge dislocations are climbing in the γ-γ' interfaces. This mechanism is responsible for the mass transport leading to the oriented coarsening of the hardening phase taking place during high temperature deformation and also for the typically unstable plastic flow usually observed in this type of high performance material.