Behavior Of Single Crystal Superalloys Research Articles

Initial oxidation behavior of a single crystal superalloy during stress at 1150\u2009\xb0C

Revealing the initial oxidation behavior of single crystal superalloys is significant for a better understanding of the oxidation mechanism of turbine blades during service condition. The purpose of current research was to observe the initial oxidation of a single crystal superalloy. In-situ oxidation experiment during only thermal exposure and thermal-stress pattern were carried out. The mechanism of nucleation and growth of oxide scale was discussed. Results showed that the oxide on the interface of γ/γ′ phase was constituted of Al2O3 precipitates and formed by external diffusion of Al atoms or ions. Loading stress enhanced the diffusion of Al atom causing high oxidation rate. A logarithmic model was proposed and fitted well with the oxidation process.

Effect of orientation on low-cycle fatigue behaviour of single crystal superalloys at 900°C

ABSTRACTLow-cycle fatigue tests on [001], [011] and Ni-based single crystal superalloys were conducted at 900°C under a constant plastic strain control. Cracks will more readily propagate between the {111} planes than migrate between the {111} and {001} planes, as shown by the longer crack propagation period and fatigue life for each [001] specimen (all crystal orientations). The oxidation could proceed via two different diffusion paths: short-range diffusion and long-range diffusion. Fatigue cracks prefer to nucleate at one oxide and then propagate along the oxidised slip bands until they link to the next oxide, which could be predicted from fatigue crack growth data, including the fractographic slip band density and oxide depth.

Effects of Gas Film Hole on Mechanical Property and Oxidative Damage of Turbine Blades

With the development of aero-engine with high specific flux, high thrus-weight ratio, and high turbine inlet temperature, it is an effective way to improve the cooling efficiency by designing dense gas film holes on the blade surface of the turbine blades, especially at the intake side. However, the introduction of gas film holes destroys the geometrical integrity of the blade structure, resulting in a multiaxial high stress state of the blade material around the small holes, which seriously affects the strength and life of the cooling blades. In addition, the geometry design of the gas film holes also directly affects the temperature and the oxidation rate of the turbine rotor blades. In this paper, the researches on the mechanical properties and oxidative damage of the gas film holes at home and abroad in recent years have been systematically introduced. The effects of gas film holes on the durability and fatigue properties, the high temperature oxidation behavior of single crystal superalloys, and the thin-walled effect of oxidative damage of gas film holes have been systematically described.

Friction and Wear Characteristics of Single Crystal Ni-Based Superalloys at Elevated Temperatures

The purpose of this study was to investigate the friction and wear behavior of single crystal superalloys at elevated temperatures. Pin-on-plate experiments were conducted using a custom-built high-temperature fretting/wear apparatus. Measurements were performed on two single crystal Ni-based alloys and Waspaloy® (used as a baseline material). The coefficient of friction for the single crystal materials (i.e., during running-in and steady state) was lower compared to the Waspaloy®. In addition, the experiments showed that the friction coefficient of the single crystal is dependent on the crystallographic plane; the friction coefficient was lower for the tests on the {100} plane compared to the {111} plane. The wear behavior was aligned with the friction behavior, where the single crystal Ni-based alloys showed slightly higher wear resistance compared to the Waspaloy®. Ex situ analysis by means of FIB/SEM and XPS analysis revealed the formation of Co-base metal oxide layer on the surface of the single crystal alloy. Similarly, a Co-base oxide layer is observed on the counterface providing a self-mated oxide-on-oxide contact and thus lower friction and wear compared to the Waspaloy®.

A tensorial thermodynamic framework to account for the [formula omitted] rafting in nickel-based single crystal superalloys

A new model is developed in order to account for the effect of microstructural degradation (i.e. precipitate-directional-coarsening) on the viscoplastic behavior of single crystal superalloys under high temperature exposure. Microstructural changes are modeled by a tensorial description of γ channel width evolution and coupled to the Kelvin modes based viscoplasticity thanks to the Orowan stress. Such a coupling is performed within a thermodynamics framework. Isotropic and directional coarsening of the γ' hardening phase are modeled as well as its dissolution with temperature changes. Results are presented for isothermal creep of CMSX-4 alloy at 1050∘ C along <001> and <111> crystal directions but the formulation allows to account for anisothermal loadings, isotropization of the creep response at high temperature and misaligned loadings. This newly developed tensorial framework for rafting can also apply to single crystal superalloys having a positive misfit.

Effect of Alloy Chemistry and Growth Rate on the Unidirectional Solidification behaviour of Single Crystal Superalloys

The addition of Re to Ni-based cast superalloys significantly changes the solidification behaviour and thereby improves the creep strength and resistance to high temperature corrosion and oxidation in both columnar grained and single crystal blades. This is because the partitioning behaviour of Re is different from that of other alloying additions in nickel based superalloys. Re containing alloys in the present study solidifies as dendrites of γ-phase together with inter dendritic eutectic of γ–γ′. It has been observed that the growth rate, apart from affecting the solidification structure influences the solid-state transformation of γ–γ′ and thus changes the γ–γ′ distribution and morphology.

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.

Experimental characterization and mechanical modeling of creep induced rafting in superalloys

A constitutive model has been developed for the high temperature mechanical behavior of single crystal superalloys, including rafting and its consequences. The flow stress depends on the γ channel width via the Orowan stress. An evolution equation for channel widening during high temperature straining has been derived and calibrated with measurements. Therein, rafting is assumed to be driven by the relaxation of internal stresses. The model is able to represent the mechanical softening at high stresses consecutive to rafting. The model has been applied to simulate rafting during uniaxial creep in several crystal orientations, in notched specimens as well as in cyclically loaded specimens.

Constitutive modeling of the creep behavior of single crystal superalloys under non-isothermal conditions inducing phase transformations

The prediction of the viscoplastic behavior of Ni-based single crystal superalloy is still a challenging issue due to the non-isothermal loadings which can be encountered by aeronautical engines components such as high pressure turbine blades. Under particular in-service conditions, these materials may experience temperature cycles which promote the dissolution of the strengthening γ′ phase of the material on (over)heating, and subsequent precipitation on cooling, leading to a transient viscoplastic behavior. New internal variables representing the microstructural changes under those specific thermal loadings have been introduced in the framework of crystal plasticity using a macroscopic approach (no representation of the γ/γ′ microstructure of the alloy) to account for the transient creep behavior induced by microstructure changes. This modeling approach captures first order effects on the creep behavior due to (a) γ′ precipitates volume fraction evolution of each kind of particles of a bimodal distribution of precipitates (which evolves according to thermal history), (b) recovery of the dislocation density and, (c) material orientation. In addition, a damage law keeping in memory all the thermal history and recovery processes has been introduced to account for the unconventional post-overheating creep life. This model is calibrated on non-isothermal creep experiments on [0 0 1] oriented single crystals made of MC2 alloy. It is able to predict creep strain (primary, secondary, tertiary), whatever the temperature history of the material. In addition, it can be used to quantify the effect of slight variation of the as-received γ′ volume fraction on the creep behavior.

Low cycle fatigue behavior of single crystal superalloy with temperature gradient

In present study, low cycle fatigue model based on rate dependent constitutive formulation with kinematic hardening and fatigue damage rule is introduced to investigate the fatigue behavior of single crystal superalloys with temperature gradient. Low cycle fatigue tests with uniform temperature and with temperature gradient are carried out to investigate the influence of temperature gradient. The microstructure reveals that the slip deformation is a principal mechanism of low cycle fatigue with temperature gradient. Complex stress experiments of low cycle fatigue are carried out to verify the life prediction rule. Bauschinger effect , ratcheting effect and fatigue damage of single crystal superalloy at different temperatures are studied. Special attention is placed on the simulation of low cycle fatigue behavior with temperature gradient. The simulation results show that the ratcheting effect at high temperature is more remarkable than that at low temperature due to the influence of temperature gradient, which results in the difference of damage evolution between high temperature zone and low temperature zone. Comparison between experiments and simulations with temperature gradient proves that the predicted model based on the damage of the second cycle is reasonable.

Environmental effects on tensile and low cycle fatigue behavior of single crystal nickel base superalloys

The effect of γ ′ rafting on the tensile and cyclic behavior of the single crystal superalloys PWA1483 and N5 was investigated at room temperature. Both alloys with rafted microstructures exhibited susceptibility to environment i.e. moisture. This environmental effect was manifested by a reduction in ductility and fatigue life accompanied by {1 0 0} cleavage.

Anisotropic damage model under continuum slip crystal plasticity theory for single crystals

Modern concepts in the safe and economical design of components and structures need modern material models, which describe the material behavior more correctly. In high temperature design, the study of creep damage accumulation and its influence on the deformation behavior is very important. For ductile materials large deformation takes place at the level of damage appearance. Damage is anisotropic in nature. In this paper an anisotropic damage mechanics model based on a continuum damage mechanics (CDM) has been developed to model creep behavior of single crystal superalloys. Using the theory of CDM, the slip system model coupling with anisotropic damage model is developed. The model is formulated in the context of irreversible thermodynamics and the internal state variable theory. The distinguishing characteristic of the proposed constitutive model is that, by construction, the corresponding incremental stress–strain relations including damage evolution equation derive from a thermodynamical framework. The finite element program ABAQUS has been used and the slip system model is written using a user material subroutine. The numerical simulations show that the developed damage crystal model can reflect the microstructure such as the lattice orientation, self- and latent-hardening and rate sensitivity has great influence on the creep and damage development.

Modelling and prediction of the stress rupture behaviour of single crystal superalloys

This paper is concerned with modelling and prediction of the stress rupture properties of several single crystal superalloys. The stress rupture theory is based on a modified form of the damage mechanics equations of Kachanov and Rabotnov. The ultimate tensile strength (UTS) is included and acts as an upper bound in these equations, which corrects for the fact that failure should occur when the effective stress reaches the UTS, rather than infinity, and allows the curvature in stress rupture data to be modelled effectively. The interpretation of damage on a microscopic scale is discussed. The time–temperature behaviour of these alloys is shown to obey a stress modified Sherby–Dorn parameter. A consideration is made for the changing strength of the material with temperature by comparing rupture data at constant fractions of the UTS rather than at constant stress. The activation energy used in the Sherby–Dorn parameter is similar to that for self-diffusion of nickel. By basing the rupture model on physical properties (namely the UTS and activation energy for diffusion), predictions of rupture behaviour of new alloys can be made when these corresponding physical properties are determined. However, such predictions of behaviour can only be made accurately between alloys of relatively similar composition. A major distinction is shown to exist between non-rhenium containing first generation alloys, and alloys containing rhenium, due to the increased rupture strength and different rafting kinetics of the latter.

Microstructure dependent modelling for specimens made of single crystal superalloys loaded under torsion

Microscopic dislocation mechanisms were investigated by means of transmission electron microscopy in specimens made of CMSX-4 loaded at 1000 °C. Torsional tests were carried out on tubular specimens. In order to model the anisotropic behaviour of single crystal superalloys, two cases are distinguished: At low temperatures, the γ′-particles remain cubic and the initial cubic symmetry of the continuum is preserved. The overall cubic symmetries of the component disappear during rafting of the γ′ phase, which occurs at low stresses and high temperatures. Creep potentials adapted to the evolution of the microstructure are presented. Predictions of the anisotropic creep behaviour are discussed.

Modelling of anisotropic creep deformation and damage in single crystal superalloys

Previous studies on the creep behavior of single crystal nickel base superalloys reveal that both creep deformation behavior and stress rupture properties of single crystal alloys are strongly dependent on crystal orientation and temperature. A physical model of creep deformation in single crystal superalloys has been proposed by Ghosh et al. This model accounts for the anisotropy and asymmetry of creep by viscous glide on specific crystallographic slip systems and for the tertiary creep behavior through a damage parameter that is related to the accumulation of mobile dislocations in each slip system. The model has been applied to analyze the creep curves of the single crystal nickel base superalloy SRR99, with particular reference to [001] and [111] orientations. The model also attempts to predict the change in the anisotropy of creep behavior with stress and temperature, and the changes in the orientation and specimen cross-section with creep strain. The creep model proposed by Ghosh et al has been modified and extended to describe the anisotropic creep behavior of single crystal superalloys for multiaxial stress states. The equations must be used with a general transformation procedure to relate the crystallographic deformation to the global loading directions. Three creep rupture criteria basedmore » on material damage in single crystal superalloys have been suggested. The creep damage criterion that takes into account of the different effects of damage on octahedral and cubic slip system and includes interaction effects between the two types of slip provided the best predictions of creep rupture life for the single crystal nickel base superalloy SRR99.« less

Asymmetric creep deformation of a single crystal superalloy

The anisotropic creep behaviour of single crystal superalloys leads to changes in the orientation and shape of specimens of arbitrary initial orientations during high temperature deformation. A model of anisotropic creep has been applied to a creep database for the single crystal superalloy SRR99 tested in uniaxial tension on 〈011〉 and 〈111〉 orientations in order to evaluate the model parameters. These have been used to predict the changes in specimen cross-sectional shape resulting from creep deformation of more complex orientations. Experimental measurements have been made of the cross-sectional dimensions of initially cylindrical specimens as a function of local creep strain. The agreement between experimental measurement and model prediction is normally very good. The biggest disagreement, for the 〈211〉 specimen, indicates that macroscopic cube slip is dominant rather than mixed cube and octahedral deformation as indicated by the model.