Elastic instability condition of the raft structure during creep deformation in nickel-base superalloys

A condition for destabilizing the raft structure has been deduced from elastic energy calculations with the concept of “effective eigenstrain”, where the effect of creep deformation is included in addition to the lattice mismatch. The calculations indicate that the 0 0 1 raft structure is stabilized by a small amount of creep deformation but becomes unstable when the creep strain in the γ phase exceeds the magnitude required to fully relax the lattice mismatch. The excess creep strain is required to produce an internal elastic field that suppresses further creep deformation, and has to be introduced in the primary creep stage. Via the instability of the 0 0 1 raft structure, the raft structure gradually turns into a wavy one in the second creep stage before its collapse.     

Lattice strain evolution during creep in single-crystal superalloys

In situ neutron diffraction studies are carried out to characterize the micromechanical deformation occurring during tensile creep of a typical single-crystal nickel-based superalloy, CMSX-4. The loading responses of the matrix γ phase and the precipitate γ′ are distinct. Moreover, the behaviour in the tertiary creep regime (in which the γ′ phase remains intact) is qualitatively different from that in the primary creep regime (when γ′ is sheared). In tertiary creep, initial deformation of the matrix leads to a release of misfit between the phases in the (1 0 0), resulting in elastic compression of the γ in the loading direction. The load state then remains fairly constant during creep. During the initial stages of primary creep, elastic compression of the γ phase is observed until at around 2–4% creep strain this compression stabilizes as the (1 0 0) misfit is released. This is the point at which γ′ shearing is thought to begin. Subsequently, the load in the γ increases by around 200 MPa until a maximum is reached at around 8% creep strain. This load is then suddenly released, which may be due to the release of back-stress.     

Evolution of orientation distributions of γ and γ′ phases during creep deformation of Ni-base single crystal superalloys

The evolution of orientation distributions of γ and γ′ phases in crept Ni-base single crystal superalloys have been investigated by theoretical calculations with elastic–plastic models and by experiments. As creep deformation proceeds, the crystallographic orientation distributions for both phases are broadened as a result of the waving of the raft structure, which occurs to reduce the total mechanical energy. The broadening of the orientation distribution occurs in such a way that the 0 0 1 pole broadens isotropically while the h k 0 poles broaden preferentially along the 〈0 0 1〉 directions. Since the extent of the broadening increases almost linearly with the number of creep deformation, the measurement of the broadening by X-ray diffraction can be utilized in non-destructive methods to predict the lifetime of Ni-base superalloys.     

A model for the creep deformation behaviour of nickel-based single crystal superalloys

A physical model for the creep deformation of single crystal superalloys is presented that is sensitive to chemical composition and microstructure. The rate-controlling step is assumed to be climb of dislocations at the matrix/particle interfaces and their rate of escape from trapped configurations; a strong dependence on alloy composition then arises. By testing the predictions of the model against the considerable body of published experimental data, the dependence of the kinetics of creep deformation on alloy chemistry is rationalized. The effects of microstructural scale – precipitate size, geometry and spacing – are also studied. The climb processes assumed at the matrix/precipitate interfaces give rise to the vacancy flux required for the mass transport needed for rafting. For creep deformation at higher temperatures, a modification to the basic theory is proposed to account for a rafting-induced strengthening effect. A first-order estimate for the rate of creep deformation emerges from the model, which is useful for the purposes of alloy design.     

High-temperature and low-stress creep anisotropy of single-crystal superalloys

The high-temperature and low-stress creep (1293 K, 160 MPa) of the single-crystal Ni-based superalloy LEK 94 is investigated, comparing the tensile creep behavior of miniature creep specimens in [0 0 1] and [1 1 0] directions. In the early stages of creep, the [0 0 1]-direction loading shows higher minimum creep rates, because a greater number of microscopic crystallographic slip systems are activated, the dislocation networks at γ/γ′ interfaces accommodate lattice misfit better, and γ channels are wider. After the creep rate minimum, creep rates increase more strongly as a function of strain for [1 1 0] tensile loading. This may be related to the nature of rafting during [1 1 0] tensile creep, which results in a more open topology of the γ channels. It may also be related to more frequent γ′ cutting events compared with [1 0 0] tensile creep.     

Development of γ phase stacking faults during high temperature creep of Ru-containing single crystal superalloys

An unusual deformation mode involving the formation of intrinsic stacking faults in the γ matrix of experimental Ru-containing γ–γ′ superalloys with high Co and Re contents during high temperature creep at 950 °C/290 MPa has been observed. The morphology, distribution and dependence of these stacking faults on alloy chemistry has been investigated along with their formation mechanism. Additions of Re and Co substantially decrease the stacking fault energy of the γ matrix. The observed stacking faults in the γ matrix form by the dissociation of a/2〈1 1 0〉 matrix dislocations with Burgers vectors perpendicular to the loading direction in the early stages of creep. The dependence of creep properties on elemental additions that influence stacking fault energy is discussed.     

Deformation structure during creep deformation in soft orientation PST crystals

Deformation microstructure of soft polysynthetically twinned (PST) crystals Ti–48A1 was investigated. The soft orientation with the lamellar plates oriented 35° to compression axis was deformed at 1150 K under applied stresses of 100–251 MPa. Macroscopically anisotropic deformation was observed after creep deformation. Deformation took place in the maximum shear stress direction in the soft orientation. Dislocation structures in γ domains of six different variants were examined by transmission electron microscopy. Operative slip and twinning systems were also analysed. Macroscopic plastic strain and strain compatibility at domain and lamellar boundaries were discussed in terms of the slip systems in each domain. Refinement of lamellar occurred by mechanical twinning during creep deformation.     

Modeling thermally activated deformation with a variety of obstacles,and its application to creep transients

A material is modeled as an array of a variety of coupled elements of varied strength, each of which is characterized by a slip probability that is a function of local stress and temperature. A stochastic cellular automaton is used to run simulations of nominally constant structure creep where simple rules are used to ensure rough compliance with mechanical equilibrium and compatibility. Three cases are studied that incorporate distinctly different statistical and spatial strength distributions. For all three simulation conditions, a general form of creep curve is obtained. The general form, when plotted as log(strain) vs. log(time), has a slope near unity at short and long times which are connected by a region of minimum slope. The slope of the central region increases systematically with increasing temperature. These features are consistent with several experimental observations. The same simulation can also provide reasonable predictions of anelastic backflow. This analysis can be of value in interpreting experimental observations in both forward and reverse creep transients.     

Effect of Ti content on creep properties of Ni-base single crystal superalloys

The effect of Ti content on the creep properties and microstructures of experimental Ni-base single crystal superalloys has been investigated. The experimental alloys were designed to provide better high temperature properties than the commercial single crystal alloy CMSX-4. The creep properties of the experimental alloys, Alloy 2 and Alloy 3, were superior to those of CMSX-4. Alloy 3 showed a longer creep life than Alloy 2 at 900 °C and 950 °C, while it has similar creep life with Alloy 2 at 982 °C. Transmission electron microscopy micrographs of the experimental alloys after the creep test showed distinct deformation features as a function of temperature and Ti content. The dissociation of dislocations into partial dislocations with stacking faults in Alloy 3 was found to improve resistance to creep deformation at 950 °C. The effect of Ti on the creep deformation mechanism was not evident at 982 °C, which resulted in similar creep properties in both experimental alloys. The transition of the γ′ cutting mechanism from dislocations coupled with stacking faults to anti-phase boundary coupled pairs occurred both in Alloy 2 and Alloy 3. However, the transition temperature was higher in Alloy 3 than in Alloy 2 because of the difference in Ti contents.     

A physically based methodology for predicting anisotropic creep properties of Ni-based superalloys

This paper is focused on developing suitable methodology for predicting creep characteristics(i.e., the minimum creep strain rate, stress rupture life and time to a specified creep strain) of typical Ni-based directionally solidified(DS) and single-crystal(SC) superalloys. A modern method with high accuracy on simulating wide ranging creep properties was fully validated by a sufficient amount of experimental data, which was then developed to model anisotropic creep characteristics by introducing a simple orientation factor defined by the ultimate tensile strength(UTS). Physical confidence on this methodology is provided by the well-predicted transitions of creep deformation mechanisms. Meanwhile, this method was further adopted to innovatively evaluate the creep properties of different materials from a relative perspective.     

Parallel twinning during creep deformation in soft orientation PST crystal of TiAl alloy

Compression creep tests were conducted in the soft orientation PST TiAl crystals under the constant stress condition with applied stress of 100–316 MPa at 1150 K to investigate the effect of parallel twinning on creep deformation behavior. The effect of applied stress and strain on parallel twinning during creep deformation is quantitatively investigated. The generation of parallel twins reduced the average lamellar spacing during creep deformation. The critical resolved shear stress for nucleation of parallel twins was measured as 50 MPa. The activity of parallel twinning increased with increasing applied stress. The nucleation of parallel twinning finished at a strain less than 0.02. The lamellar refinement by parallel twinning did not change the creep rate of the PST crystal under the creep conditions studied. The effect of parallel twinning on creep behavior is discussed on the basis of microstructural investigation.     

Influence of microstructure stability on creep properties of single crystal nickel base superalloys

The influence of microstructure stability on the creep properties of single crystal nickel-based superalloys was investigated by means of the measurement of the creep curves and microstructure observation. Results show that the superalloy with 4%（mass fraction）W in Ni-AI-Cr-Ta-Co-5.5%Mo-x%W systems displays a better microstructure stability, but the β phase is precipitated in the superalloy with 6% W during aging. The strip-like μ phase is precipitated to be parallel or perpendicular to each other along the 〈110〉 orientation, and grown into the slice-like morphology along the { 111 } planes. The superalloy with 4%W displays a better creep rupture lifetime under the applied stress of 200 MPa at 982 ℃, but the creep lifetime of alloy is obviously decreased with the increase of the element W content up to 6%. The fact that the μ phase is precipitated in the superalloy with 6% W during applied stress and unstress aging results in the appearance of the poor regions for the refractory elements. This is one of the main reasons for reducing the creep rupture lifetime of the superalloy.     

High temperature deformation mechanism and evolution of dislocation structure during creep of Ni-Base superalloy 713LC

Creep deformation of cast nickel base superalloy 713LC has been investigated in a temperature range of 723 to 982°C. The values of the stress exponent and activation energy for creep of the alloy vary with a combination of temperature and stress. Introduction of threshold stress for creep of the alloy provided an explanation of the high values of the stress exponent and the apparent activation energy. Microstructural evolution of the alloy with creep deformation has also been studied. The analysis of the creep mechanism has been supplemented by microstructural observations after deformation under various test conditions. The dislocation structure of the alloy at high temperature and low stress was different from that at low temperature and high stress. Shearing of γ′ particles by dislocation pairs was the dominant creep mechanism at low temperature and high stress whereas dislocation climb over γ′ particles was the rate controlling process of creep at high temperature and low stress.     

Oxidation of a Mn-Cr austenitic steel during creep testing and its interaction with the fracture

The interaction between oxidation and creep rupture was studied in a 17 Mn-10 Cr austenitic steel, of interest as structural material for the internal components of fusion reactors. The observation of the creep specimens tested in air at temperatures ranging from 773 to 973 K revealed the presence of an adherent oxide scale and of a ferritic phase underneath, which forms as a consequence of the Mn depletion of the austenitic matrix. The microstructure of the two layers was investigated by optical microscopy, SEM, X-ray diffraction, EDS and magnetic permeability measurements. The scale has a complex structure, being composed mainly of manganese oxide. The ferritic layer is completely recrystallized and does not present grain boundary precipitates as the austenitic phase does. The effect of the surface modification on the creep rupture process is discussed in the light of a recent model of deformation-oxidation interaction.     

The intermediate temperature deformation of Ni-based superalloys: Importance of reordering

A number of planar deformation mechanisms, such as microtwinning, a[112] dislocation ribbon, and superlattice intrinsic and superlattice extrinsic stacking fault formation, can operate during the intermediate temperature deformation of nickle-based superalloys. The fundamental, rate-limiting processes controlling these deformation mechanisms are not fully understood. It has been recently postulated that reordering of atoms in the wake of the gliding partial dislocations as they shear the γ′precipitates within the γ/γ′microstructure is the limiting process. Experimental evidence that substantiates the validity of the reordering model for the microtwinning mechanism is provided. A conceptual approach to study reordering at the atomic scale using ab-initio calculation methods is also presented. The results of this approach provide a clear conceptualization of the energetics and kinetics of the reordering process, which may be generically important for the aforementioned planar deformation modes.