Single Crystal Superalloy CMSX-4 Research Articles

Ni基単結晶超合金の表面溶融部の組織学的検討

Crystal growth behavior in the surface-melted region of Ni-base single crystal superalloy CMSX-4 was investigated. The surface melting was conducted using 2kW diode laser or gas tungsten arc. Laser power and scanning speed in laser surface melting were varied while defocusing distance and shielding gas (Ar) flow rate were fixed. For arc surface melting, arc current and welding speed were varied. Specimen surfaces were arranged parallel to the (001) of the base metal. The microstructure and crystal orientation in the melted region were analyzed by using optical microscopy, scanning electron microscopy(SEM) and electron backscattered pattern analysis(EBSP). The microstructure and crystal orientation in laser surface-melted region were markedly influenced by heat input. The laser surface-melted region under low heat input conditions was found to be solidified as the single-crystalline state with unidirectional dendrites which grew along the [001] directions. The laser surface-melted region under intermediate heat input conditions was also single-crystalline with dendrites which grew along the [001], [010] and [100] directions. In contrast, in the case of high heat input conditions, the laser surface-melted region was consisted of polycrystal with stray crystals. Such tendencies were also observed in the melted region with gas tungsten arc. These results demonstrate that heat input significantly affects single-crystallinity regardless of the type of heat source, such as laser and arc. Furthermore, stray crystal formation in surface-melted region was investigated in detail by using SEM and EBSP. From the results of these experiments, it is revealed that stray crystal formation was affected by constitutional supercooling and dendrite growth direction.

Ni基単結晶超合金の表面溶融部の再結晶挙動と機械的特性

Recrystallization behavior in postheat treatment of the surface-melted regions of Ni-base single crystal superalloy CMSX-4 have been investigated. In, addition, mechanical properties of surface-melted regions after postheat treatment were evaluated. The surface melting was conducted using 2kW diode laser. The microstructure and crystal orientation in the melted regions were analyzed by using optical microscopy, scanning electron microscopy (SEM) and electron backscattered pattern analysis (EBSP). The effect of postheat treatment condition on microstructure was investigated using the specimens whose surface-melted regions were single-crystallized. The surface-melted regions were recrystallized when the temperature of postheat treatment was higher than 1513K. On the basis of this result, the partial solution treatment condition was proposed as 1493K×3.6ks to maintain single crystal structure without recrystallization in the surface-melted regions. Furthermore, the mechanical properties of surface-melted regions after partial solution treatment and subsequent aging were investigated by the creep rupture test. The creep rupture strength of surface-melted specimens was comparable to those of the base metal. It could be concluded that the crystalline controlled repairing technique would be applicable to the repair of single crystal superalloy turbine blades.

Ni基単結晶超合金のレーザー肉盛溶接部の結晶成長挙動

Crystal growth behavior in the overlay welded regions of Ni-base single crystal superalloy CMSX-4 has been investigated. The Ni-base superalloy René142 was used as the filler metal for overlay welding. The surface of the specimens was prepared as to be coincident with the (001) plane. The overlay welding was conducted using a 2kW diode laser along the [100] direction. Microstructural observation revealed that the overlay weld metal grew epitaxially on the substrate and stray crystal tended to form in the conditions of higher power and (Vf /Vw) ratio (Vf : wire feeding speed, Vw : laser scanning speed). Furthermore, the dendritic growth direction and the single-crystallized condition were analyzed by the theoretical model with combining the geometric model, CET (columnar to equiaxed transition) model and CFD (computational fluid dynamics) analysis. The predicted results of the dendritic growth direction and the stray crystal formation in the overlaid regions corresponded approximately with the experimental ones, which indicated that the current theoretical calculation can be used to predict the conditions to suppress the formation of stray crystals and to promote single-crystal production in the overlay welded region with epitaxial dendritic growth of [001] orientation and/or the dedritic growth of [100] and [010] orientations. On the basis of the condition by the theoretical prediction, a single crystalline layer of 3 mm thickness on a single crystal substrate could be achieved by laser overlay welding with ten passes.

High temperature nanoindentation of a Re-bearing single crystal Ni-base superalloy

Nanoindentation studies of single crystal Ni-base superalloy CMSX-4 oriented in the 〈0 0 1〉 and 〈1 1 0〉 directions were conducted over a range of temperatures from 303 to 673 K. The creep compliance at peak load was measured at each of the test temperatures and incorporated into the contact mechanics analyses to calculate the elastic modulus of CMSX-4 as a function of temperature. The measured variations in hardness and modulus as a function of temperature determined via nanoindentation are consistent with previously reported values.

Crystalline controlled build-up repairing utilizing unidirectional solidification

Ni-base single crystal superalloy CMSX-4 has been widely used for turbine blades and vanes. However, since they are used under severe environments, the blades sometime suffer from erosion. Therfore, it is of great interest to develop repairing techniques that allow the life of single crystal turbine blades and vanes to be extended. So, in this study, the applicability of repairing techniques of the eroded blades by directional solidification treatment was investigated. Ni-Cr-B and Ni-Cr-Si filler metals were used as built-up repairing for the eroded region of the blade. The built-up repairing treatment was developed by utilizing unidirectionally solidification of a repaierd region under temperature gradient using a vertical type furnace with a movable heater. Microstructure and crystal orientaion in the repaired regions under various heat treatment conditions were examined by SEM and EBSP. The microstructure of the repaired region could be classified into several zones; eutectic zone, normally solidified zone, isothermally solidified zone and substrate. The solidification morphology of the normally solidified zone changes from dendoritic to planar with decreasing the moving speed of the heater. The single-crystallization could be achieved in the isothermally solidified zone and the normally solidified zone when its solidification morphology was planar. The thickness of the built-up region of about 2590μm can be obtained when the moving speed of heater is 0.28μm/s using Ni-Cr-Si filler metal.

Kinetics of rafting in a single crystal superalloy: effects of residual microsegregation

The kinetics of directional γ′ coarsening, known as rafting, are examined in the nickel based single crystal superalloy CMSX-4. Electron microscopy and image analysis are used to characterise heat treated specimens, prestrained in creep to a plastic strain beyond the threshold required to initiate rafting. The kinetics of rafting are found to be sensitive to the presence of microsegregation from casting, incompletely removed by the heat treatment process; thus the dendritic regions raft appreciably faster than the interdendritic ones – this despite rhenium enrichment at the dendrite cores consistent with its preferred partitioning during solidification. Estimates of the activation energy required for rafting in the two regions are established and Avrami-type equations developed. Since the important γ′ forming elements aluminium and tantalum are known to partition interdendritically causing an enhanced γ′ fraction, it is suggested that the correspondingly smaller γ channel width so caused is responsible for the greater resistance to rafting displayed by the interdendritic regions.

Femtosecond Laser Ablation of a Single Crystal Superalloy CMSX-4 in Different Environments

The femtosecond laser ablation has been investigated on second generation single crystal superalloy CMSX-4 using a commercial titanium:sapphire laser system (λ = 780nm, τ = 120 fs). The ablation thresholds of bulk alloy CMSX-4 have been determined as a function of the pulse number (1, 10, 100, 1000) in air, argon gas and vacuum. The results indicate that the multiple-pulse threshold of this material decreased with increasing the pulse number in all the cases. For the same pulse number, the threshold changed in the descending order: air, vacuum and argon gas. The incubation coefficient in air has been determined: ξ = 0.86 ± 0.03. The preliminary results showed the better quality of femtosecond laser machining of CMSX-4 in vacuum and argon gas than in air.

Experiences in the determination of TMF, LCF and creep life of CMSX-4 in four-point bending experiments

The thermo-mechanical fatigue (TMF), low cycle fatigue (LCF) and creep life of rectangular flat specimen were evaluated by means of four-point bending tests. Experiments were performed on the single crystal superalloy CMSX-4 mainly in the orientation 〈0 0 1〉, but also orientations 〈0 1 1〉 and 〈1 1 1〉. While the TMF-Tests were performed in the temperature ranges from 250 to 750 °C, 450–950 °C and 550–1050 °C with a heating rate of 25 °C/s and a cooling rate of 12.5 °C/s, the LCF and creep tests were performed at 750, 950 and 1050 °C. Two different bending devices were designed and machined for the creep test and the LCF and TMF test respectively. In this work the experiences with the design and performance of the experimental test setup of the LCF and TMF experiments, especially with temperature and strain control are reported. Results and theoretical interpretation are given in [Xu J, Reuter S, Rothkegel W. Tensile and bending thermo-mechanical fatigue testing on cylindrical and flat specimens of CMSX-4 for design of future high-pressure turbine blades, TMF-Workshop, 22–23 September, Berlin; 2005.].

Crack growth in a coated gas turbine superalloy under thermo-mechanical fatigue

The out-of-phase thermo-mechanical fatigue (TMF) behaviour of the single crystal superalloy CMSX-4 was studied in the uncoated and in the low-pressure plasma sprayed (LPPS) coated condition. In addition, growth measurements of in-depth coating cracks were performed. Cyclic hardening of the substrate and superimposed multiple crack initiation/propagation within the coating leads to cyclic softening that consumes a significant portion of TMF lifetime. A three-step crack propagation mode of the coated material is described. Special attention is paid to the role of the inter-diffusion zone, since coating cracks are decelerated in the inter-diffusion zone before penetrating into the substrate.

Influence of phase shift and strain path on the thermomechanical fatigue behavior of CMSX-4 specimens

In this paper, the fatigue and lifetime effects resulting from thermomechanical loadings with different phase shifts between temperature and mechanical strain and with different strain paths are investigated for the single crystal nickel-based superalloy CMSX-4. In-phase tests, experiments with a phase shift of −20° between mechanical strain and temperature as well as tests with a non-proportional ratio of mechanical strain and temperature were carried out. It could be shown that the induced stress alone does not allow a lifetime estimation. For all the tests carried out at the same mechanical strain range but different phase relations the induced stresses were approximately the same but the resulting lifetime varied. It turned out that the time interval in which high temperature and high mechanically induced tensile stresses are acting simultaneously is a major aspect for the damage development. For cracks connected to the surface the crack propagation is supported by an oxide induced γ′-dissolving, whereas mechanically induced γ′-dissolving may arise in the region between approaching internal cracks.

High temperature oxidation behaviour of single crystal superalloys RR3000 and CMSX-4

This paper describes the oxidation behaviour of two technologically important nickel based superalloys, RR3000 and CMSX-4, used extensively in gas turbine blade applications. Oxidation tests were conducted in air at 1100°C for equal and extended periods of time, up to 100 h to compare the superalloys characteristic properties. The morphology of oxide growth in the test samples was investigated using metallography, SEM, EDX, X-ray diffraction (XRD) and Cr3+ photoluminescence spectroscopy. The results indicated that irrespective of the duration of exposure, alloy RR3000 develops a much thicker oxide scale than alloy CMSX-4. Examination of cross-sections revealed that the scale grown on the alloy RR3000 is porous and has a three layered structure consisting of an outer layer of (Ni, Co)O; a sublayer of CrTaO4 rich oxide with different spinels; and an inner layer of α-Al2O3. By contrast, the scale that forms on alloy CMSX-4 is compact and consists of two layers. The upper layer is primarily composed of spinels (Ni,Co)Al2O4 and (Ni,Cr)2O4, and the inner layer of α-Al2O3. Residual stress measurements using luminescence spectroscopy point to a weak adhesion of the oxide scale to the substrate for alloy RR3000 and a strong adhesion for alloy CMSX-4.

Damage accumulation during creep deformation of a single crystal superalloy at 1150 °C

Microstructural degradation in the CMSX-4 single crystal superalloy during creep deformation at 1150 °C and 100 MPa is studied. Tensile deformation in the 〈0 0 1〉 direction is considered due to its technological importance. Under these conditions, rafting of the γ′ structure is completed quickly and within the first 10 h. It is demonstrated that the creep rupture event is highly localised, the instability being associated with a critical and well-defined strain being reached with failure occurring within a further few tens of hours. It is shown that the high strain rates and shear stresses associated with the rupture process are sufficient to cause realignment of the rafted γ′ structure with respect to the γ matrix. Creep cavitation damage near to the rupture surface is prevalent, at microporosity inherited from the casting process but more significantly, at topologically close-packed (TCP) phases and associated pores and voids formed in their vicinity which have formed via vacancy condensation. Hot isostatic pressing (HIPing) prior to creep testing reduces the incidence of casting microporosity, but the creep rupture life is not improved significantly. It is suggested that it is the formation of TCP phases which limits creep rupture life.

等温凝固による結晶制御充填補修に関する検討

Ni-base single crystal superalloy CMSX-4 has been widely used for turbine blades and vanes because of its superior heat-resistance. However, since they are usually used under severe environments, cracks sometime occur. Since this material is so expensive, there exists strong demand for repairing technique which enables to form singlecrystal in the repaired region to prevent deterioration of fatigue resistance. Therefore, in this study, the applicability of repairing technique utilizing iso-thermal solidification treatment with using Ni-Cr-B and Ni-Cr-Si alloys as filler metals to cracks occurred in single crystal superalloy CMSX-4 was investigated. The microstructure and crystal orientaion in the repaired regions under various heat treatment conditions were examined by SEM and EBSP. SEM and EBSP analyses have revealed that isothermal solidification and single-crystallization in the repaired region was achieved by repairing with using Ni-4Cr-2.5B filler metal under the condition of 1448K×176.4ks. The same result could be obtained also by using Ni-4Cr-10Si filler metal under the condition of 1473K×90ks. Creep rupture test for the repaired regions with Ni-4Cr-2.5B and Ni-4Cr-10Si filler metals has shown that the creep rupture strentgh of repaired region with Ni-4Cr-10Si filler metal reached about 2/3 of that of the base metal.

Rafting of Single Crystal Superalloy by High Thermal Stress

Rafting of the nickel-base single crystal superalloy CMSX-2 was investigated under the condition of high thermal stress using a newly constructed experimental equipment. The test piece with the diameter of 25 mm was tested at the maximum material temperature of 1 000 degree Celsius for 80 hours. The results showed that the raft structure occurred only under the influence of thermal stress. The raft structure developed parallel to the direction of the compressive thermal stress because of negative lattice misfit. The thickness of gamma phase and gamma prime phase in rafting increase with the increase in the temperature and the thermal stress.

一方向性凝固を利用した結晶制御肉盛補修に関する検討

Ni-base single crystal superalloy CMSX-4 has been widely used for turbine blades and vanes. However, since they are used under severe environments, the blades sometime suffer from erosion. Therfore, it is of great interest to develop repairing techniques that allow the life of single crystal turbine blades and vanes to be extended. So, in this study, the applicability of repairing techniques of the eroded blades by directional solidification treatment was investigated. Ni-Cr-B and Ni-Cr-Si filler metals were used as built-up repairing for the eroded region of the blade. The built-up repairing treatment was developed by utilizing unidirectionally solidification of a repaierd region under temperature gradient using a vertical type furnace with a movable heater. Microstructure and crystal orientaion in the repaired regions under various heat treatment conditions were examined by SEM and EBSP. The microstructure of the repaired region could be classified into several zones; eutectic zone, normally solidified zone, isothermally solidified zone and substrate. The solidification morphology of the normally solidified zone changes from dendoritic to planar with decreasing the moving speed of the heater. The single-crystallization could be achieved in the isothermally solidified zone and the normally solidified zone when its solidification morphology was planar. The thickness of the built-up region of about 2590μm can be obtained when the moving speed of heater is 0.28μm/s using Ni-Cr-Si filler metal.

Microstructure and stress rupture properties of single crystal superalloy CMSX-2 under high thermal gradient directional solidification

The interface morphologies of single crystal superalloy CMSX-2 were studied over a range of cooling rate with large variations in withdrawal speeds in directional solidification. A superfine cellular structure was obtained under both high thermal gradient up to 1000 K/cm and fast withdrawal rate up to 1 mm/s. The high rate directional solidification results in reduction in primary and secondary dendrite arm spacing, refinement of γ′ phase, reduced microsegregation of alloying elements and smaller size of γ–γ′ eutectics. The rupture life and plasticity of fine structure samples produced in high thermal gradient directional solidification increase significantly than that in conventional directional solidification process at 1323 K.

Study of Microstructure in Surface-Melted Region of Ni-Base Single Crystal Superalloy CMSX-4

This work investigated microstructure in the surface-melted region of Ni-base single crystal superalloy CMSX-4 by using a diode laser beam as a heating source. Such processing parameters as laser power and scanning speed in laser surface melting were varied while defocusing distance and shielding gas (Ar) flow rates were fixed. Specimen surfaces were arranged parallel to the (001) of base alloy. The microstructure in the melted region was analyzed by optical microscopy and SEM. Crystal orientation of the melted region was analyzed using electron backscattered pattern analysis. The microstructure was remarkably changed when the heat input of surface melting was varied. The surface-melted region was found to solidify into a single crystal with directional dendrites that grew along the [001] directions under low heat input conditions. The surface-melted region was also a single crystal with disoriented dendrites that grew along the [100] or [010] directions under medium heat input conditions. In contrast, the melted region consisted of poly crystals with stray crystals under high heat input conditions. Such tendencies were also observed in the melted region with gas tungsten arc. These results demonstrate that the surface-melted region can solidify into a single crystal under conditions in which larger temperature gradient and higher solidification rates can be achieved.

Simulation of deformation and lifetime behavior of a fcc single crystal superalloy at high temperature under low-cycle fatigue loading

This work deals with the formulation of a three-dimensional crystallographic time-incremental lifetime rule for face-centered cubic (fcc) single crystals used for gas-turbine blade applications. The damage contribution rate of each slip system to the total damage is governed by the current values of the resolved shear stress and the slip rate on the corresponding slip system. The damage rule is combined with a crystallographic viscoplastic deformation model. For the nickel-base single-crystal superalloy CMSX4 at 950 °C, various strain- and stress-controlled uniaxial cyclic tests with and without hold-times can be described for different crystal orientations by one set of material parameters. For verification, simulation results for a single-crystal specimen with a notch have been compared with corresponding experimental results. The predicted lifetime is within the factor of two of the measured one.

A continuum model for the anisotropic creep of single crystal nickel-based superalloys

In this paper, we extend the constitutive theory developed by Prasad et al. [Prasad SC, Rao IJ, Rajagopal KR. A continuum model for the creep of single crystal nickel-base superalloys. Acta Mater 2005;53(3):669–79], to describe the creep anisotropy associated with crystallographic orientation in single crystal nickel-based superalloys. The constitutive theory is cast within a general thermodynamic framework that has been developed to describe the response of materials capable of existing in multiple stress free configurations (“natural configurations”). Central to the theory is the prescription of the forms for the stored energy and rate of dissipation functions. The stored energy reflects the fact that the elastic response exhibits cubic symmetry. The model takes into account the fact that the symmetry of single crystals does not change with inelastic deformation. The rate of dissipation function is also chosen to be anisotropic, in that it reflects invariance to transformations that belong to the cubic symmetry group. The model is used to simulate uniaxial creep of single crystal nickel-based superalloy CMSX-4 for loading along the 〈0 0 1〉, 〈1 1 1〉 and 〈0 1 1〉 orientations. The predictions of the theory agree well with the experimental data.

等温凝固による結晶制御肉盛補修に関する基礎的検討

The applicability of built-up repairing by isothermal solidification to Ni-base single crystal superalloy CMSX-4 was investigated using Ni-Cr-B filler metals. Base metal dissolution and isothermal solidification behaviors were examined under various heat treatment conditions. Microstructure of the repaired region could be classified into eutectic zone, normally solidified zone, isothermally solidified zone, precipitated zone in the base metal and substrate. Isothermally solidified zone and precipitated zone in the base metal were found to possess the identical crystal orientation with the base metal one. The dissolution width of base metal and the isothermal solidification width increased with repairing temperature and holding time. The single-crystallization could be achieved in the isothermally solidified zone of the repaired region. The built-up repairing up to 38μm in width could be made at the repairing condition of 1448K×176.4ks by using a Ni-Cr-B filler metal.