Polycrystalline Ni-based Superalloys Research Articles

Investigation of Oxide Bifilms in Investment Cast Superalloy IN100: Part I. Mechanical Properties

Oxide bifilms are a proposed casting inclusion reported to have been observed in vacuum investment cast polycrystalline Ni-based superalloys. Ongoing research seeks to determine if current superalloy casting practices can result in the formation of oxide bifilms, and subsequently if it is possible to observe and characterize this phenomenon. The effects of casting atmosphere, turbulence, filtering, hot isostatic pressing (HIP), and heat treatment have been investigated to identify the critical parameters that have been reported to result in bifilm formation in Ni-based superalloys. Room temperature tensile and room temperature fatigue testing are used to identify the effects of each casting and processing parameter on casting defect formation and the resultant effects on mechanical properties. Characterization of mechanical test specimens seeks to identify the role of casting defects and microstructural features on the fracture mechanisms of the specimen conditions analyzed, and in particular, evidence of bifilm formation and the chemical composition(s) of oxide bifilms. Analyzed tensile and fatigue data did not indicate an influence of bifilms on the tensile or fatigue strength of vacuum processed IN100. Bifilms were not observed, via the characterization methods utilized, to be an active mechanism in tensile or fatigue fracture.

Two different small size testing technique for the creep residual life evaluation of an equiaxed Ni-based superalloy for GT blade and vane application

Two miniaturised creep testing techniques have been applied to a polycrystalline Ni-based superalloy employed for gas turbine blades and vanes. The test campaign was conducted on as-delivered material to assess both testing techniques and evaluate their effectiveness in determining creep residual life and creep residual properties of operated material. The first technique involved small size uniaxial creep testing, conducted on samples with a diameter lower than 3 mm. The diameter effect was deeply studied and a calibration was done to correlate results from standard creep test pieces with those from small diameter samples. The second technique involved the small punch creep test, and the assessment on virgin material enabled determination of the correlation factor, ksp, to compare small punch test results with uniaxial creep tests. All the collected results were related to the coarse grain macrostructure that characterises this class of alloys and that increases the scatter of the collected experimental results.

1117 多結晶Ni基超合金のTMF微小き裂の発生・進展に及ぼす位相角の影響(OS8-4 材料力学と計算力学の新展開)

1117 多結晶Ni基超合金のTMF微小き裂の発生・進展に及ぼす位相角の影響(OS8-4 材料力学と計算力学の新展開)

Small Creep Crack Growth Interacting with Microstructural and Mechanical Factors in a Polycrystalline Ni-Based Superalloy

Behavior of small crack propagation during creep was studied, in comparison with that of physically long crack propagation behavior, based on non-linear fracture mechanics approach. Through the work special attention is paid to understand: (1) microstructural aspect on small creep crack propagation interacting with grain boundaries and dendrite boundaries, (2) difference between small and macroscopic long creep cracks, (3) role(s) of stress concentration and relaxation with creep time in the crack propagation behavior emanating from a notch introduced as a simulation of cooling holes in blades and vane applications. From the experimental results, a specific crack growth behavior was found at the beginning of early crack growth stage (small crack), where the crack growth rate decreased with increasing Jc,st under both the isothermal creep (ITC) and thermal gradient creep (TGC). Also, crack under the TGC showed different crack growth rate than that under the ITC condition, where the higher and lower rates were achieved at lower and higher temperature side, respectively, under a given value of Jc,st.

Oxidation behaviour of a next generation polycrystalline Mn containing Ni-based superalloy

A prototype next-generation superalloy containing 1at.% Mn was oxidised in air at 800°C for 100h, and compared with a commercial Ni-based superalloy. The oxide scale consisted of a multi-phase layered structure measured by Atom Probe Tomography as uppermost NiCr2Mn2O4, followed by an inhomogeneous mix of Cr2O3, spinel MnCr2O4 and rutile (Ti,Cr)O2. The Mn did not form a homogeneous, surface passivating oxide layer. Despite this, the alloy showed a 3× reduction of oxide thickness compared to a commercial polycrystalline Ni-based superalloy.

Modeling the effect of thermal–mechanical processing parameters on the density and length fraction of twin boundaries in Ni-base superalloy RR1000

Enhancement of the physical and mechanical properties of polycrystalline Ni-base superalloys may be achieved through control of the grain boundary structure and is dependent on the optimization of the thermal–mechanical processing parameters. Superalloys containing grain boundary networks that are comprised with a sufficiently high fraction of Σ3/twin boundaries have been reported to exhibit enhanced creep and fatigue properties. In this report, the density and length fraction of twin boundaries in annealed samples of powder processed Ni-base superalloy RR1000 were quantified and expressed as a function of the average grain diameter. The results were found to be consistent with classical models relating density and length fraction to grain size. The effects of varying hot deformation parameters on twin density and length fraction were also quantified and modified models were derived to describe the relationships.

Comparison of thermodynamic database models and APT data for strength modeling in high Nb content γ–γ′ Ni-base superalloys

In response to the increasing need for higher operating temperatures in advanced gas turbine engines, new alloying concepts are required to develop novel nickel-base superalloys with enhanced temperature capabilities. Recent studies have shown that polycrystalline Ni-base superalloys containing elevated levels of Nb additions exhibit superior properties at elevated temperatures when compared to existing commercial Ni-base superalloys. In order to design, develop and fully exploit this innovative class of superalloys, an understanding of how alloying elements partition to each phase is essential. Using atom probe tomography (APT), compositions of the constituent phases were measured in four high Nb content γ–γ′ Ni-base superalloys and the results were compared to thermodynamic database models from Thermo-Calc. Results were also used in predicting the solid solution strength behavior of the four alloys. The differences in phase composition predictions from thermodynamic models resulted in dissimilarities between the generated strength behavior curves and those from the experimental work.

Multiscale modeling of the mechanical behavior of IN718 superalloy based on micropillar compression and computational homogenization

A multiscale modeling strategy is presented to determine the effective mechanical properties of polycrystalline Ni-based superalloys. They are obtained by computational homogenization of a representative volume element of the microstructure which was built from the grain size, shape and orientation distributions of the material. The mechanical behavior of each grain was simulated by means of a crystal plasticity model, and the model parameters that dictate the evolution of the critical resolved shear stress in each slip system (including viscoplastic effects as well as self and latent hardening) were obtained from compression tests in micropillars milled from grains of the polycrystal in different orientations suited for single, double (coplanar and non coplanar) and multiple slip. The multiscale model predictions of the compressive strength of wrought IN718 were in good agreement with the experimental results.

Atomic imaging of the interface between M23C6-type carbide and matrix in a long-term ageing polycrystalline Ni-based superalloy

Besides the well-known cube-on-cube orientation relationship (OR) between M23C6 carbide and matrix, we have determined a new OR named as the twin-related OR in a long-term ageing Ni-based superalloy on the basis of the extensive and detailed electron diffraction analyses. Furthermore, by means of atomic-resolution high angle annular dark-field imaging technique which is implemented in the aberration-corrected scanning transmission electron microscope, we elucidated the interfacial characteristics between M23C6 and matrix for above two types of ORs. Taking into account of the interfacial characteristics, we propose that the twin-related OR possesses a higher total interfacial energy. Thus, its frequency of occurrence is lower than that of the cube-on-cube OR though both ORs are usually seen in the long-term ageing samples.

J0310203 多結晶Ni基超合金における空孔拡散融合巨視力学による切り欠き近傍のクリープ損傷形成挙動解析

Polycrystalline Ni-base superalloys are used for turbine blades in power plants and jet engines. In order to ensure safety in these systems, clarifying the behavior and mechanism of creep damage formation is highly in need. Thus in this study, elastic-plastic-creep finite element analysis combined with vacancy diffusion analysis was conducted for polycrystalline Ni-base superalloy model in order to mechanically examine the creep damage formation behavior. FEM and vacancy diffusion were combined in this analysis in the interest of considering both macro and micro damages. As the result of the analysis, damage caused by plastic deformation and vacancy, which forms voids when coalesced, were found to accumulate at grain boundary, which eventually lead to intergranular cracking. Furthermore, by combining FEM and vacancy diffusion analyses, macro damage region was detected by FEM analysis, and micro damage region within the macro damage region was detected by vacancy diffusion analysis.

An atom probe tomography study of the oxide–metal interface of an oxide intrusion ahead of a crack in a polycrystalline Ni-based superalloy

Intergranular oxide intrusions formed ahead of a crack in a Ni-based superalloy at 650 °C have been examined using three-dimensional atom probe tomography. Of two scans undertaken, a thin (∼5 nm) alumina layer was observed next to the alloy substrate in one, overlaid by a chromia layer. In the second, chromia penetrated to the alloy interface. The chromia layer contained Ti in solution as well as discrete particles of TiO 2 .

Microstructural Stability and Hot Deformation of γ–γ′–δ Ni-Base Superalloys

Nickel-base superalloys exhibit excellent high-temperature mechanical and physical properties and remain the first choice for structural components in advanced gas turbine engines for the aerospace propulsion and power generation applications. In response to the increasing demand for more efficient solutions and tighter requirements linked to gas turbine technologies, the properties of nickel-base superalloys can be improved by modification of their thermo-mechanical and/or compositional attributes. Recent investigations have revealed the potential use of ternary eutectic γ–γ′–δ Ni-base superalloys in advanced gas turbines due to high temperature mechanical properties that are comparable to state-of-the-art polycrystalline Ni-base superalloys. With properties largely dependent on microstructural strengthening mechanisms, both the composition and thermo-mechanical processing parameters of this novel class of alloys need to be optimized concurrently. The hot deformation characteristics of four γ–γ′–δ Ni-base superalloys with varying levels of Nb were evaluated at temperatures and strain rates between 1353 K and 1433 K (1080 °C and 1160 °C) and 0.01 to 0.001/s, respectively. Evidence of dislocation-based plasticity was observed following deformation at low temperatures and high strain rates, while high temperatures and low strain rates promoted superplasticity in these alloys. The extent of the microstructural changes and the magnitude of the cavitation damage which occurred during deformation was found to vary as a function of the alloy composition.

The effects of Ta on the stress rupture properties and microstructural stability of a novel Ni-base superalloy for land-based high temperature applications

A novel polycrystalline Ni-base superalloy was developed for land-based high temperature applications, such as isothermal forging dies and industrial gas turbines. The alloy possessed surprisingly high stress rupture life of 52h at 1100°C/118MPa which is comparable to the first generation single crystal (SC) superalloy and exhibited good microstructural stability. The effects of Ta addition on the phase change, stress rupture properties and microstructural stability of the alloy were investigated. The results indicated that Ta is a γ′-former and promotes the formation of eutectic γ′. The alloys with ∼7vol.% eutectic γ′ possess higher stress rupture life at 1100°C/118MPa than the alloys with higher ∼20vol.% eutectic. However, ∼20vol.% excessive eutectic phases will enhance the stress rupture life at intermediate temperature of 760°C for 686MPa but weaken high temperature stress rupture properties. The (Al+Ta) content over 14.4at.% led to the formation of large amounts of eutectic γ′ and exceeded the solubility of W and Mo in the residue liquid pool, which then promoted the precipitation of primary α-(W,Mo) and M6C phases. Tantalum was also found as a strong MC carbides forming element. The order of ability to form monocarbide decreased from NbC to TaC to TiC. 6Al–0Ta (wt.%) alloys possessed good microstructural stability with no harmful topologically close-packed (TCP) phases being observed after thermal exposure at 850°C/3000h, 900°C/1000h. Only trace amounts of secondary plate-like M6C carbides appeared in Ta-free and 5Al–4Ta (wt.%) alloys at 1100°C/100–500h. However, excessive Ta addition will destabilize the alloy and large amounts of secondary plate-like M6C carbides precipitated after thermal exposure at 1100°C. The transmission electron microscopy (TEM) and selected area electron diffraction (SAED) results showed the existence of the plate-like M6C carbides.

Hierarchical crystal plasticity FE model for nickel-based superalloys: Sub-grain microstructures to polycrystalline aggregates

A hierarchical crystal plasticity constitutive model, comprising three different scales for polycrystalline microstructures of Ni-based superalloys, is developed in this paper. Three scales, dominant in models of polycrystalline Ni-based superalloys, are: (i) the sub-grain scale of γ-γ′ microstructure, characterized by γ′ precipitate size and their spacing; (ii) grain-scale characterized by the size of single crystals; and (iii) the scale of polycrystalline representative volume elements. A homogenized activation energy-based crystal plasticity (AE-CP) FEM model is developed for the grain-scale, accounting for characteristic parameters of the sub-grain scale γ-γ′ morphology. A significant advantage of this AE-CP model is that its high efficiency enables it to be effectively incorporated in polycrystalline crystal plasticity FE simulations, while retaining the accuracy of detailed sub-grain level representative volume element (SG-RVE) models. The SG-RVE models are created for variable morphology, e.g., volume fraction, precipitate shape and channel-widths. The sub-grain crystal plasticity model incorporates a dislocation density-based crystal plasticity model augmented with mechanisms of anti-phase boundary (APB) shearing of precipitates. The sub-grain model is homogenized for developing parametric functions of morphological variables in evolution laws of the AE-CP model. Micro-twinning initiation and evolution models are incorporated in the single crystal AE-CP finite element models for manifesting tension–compression asymmetry. In the next ascending scale, a polycrystalline microstructure of Ni-based superalloys is simulated using an augmented AE-CP FE model with micro-twinning. Statistically equivalent virtual polycrystals of the alloy CMSX-4 are created for simulations with the homogenized model. The results of simulations at each scale are compared with experimental data with good agreement.

Evolution of secondary phases in alloy ATI 718Plus®during processing

ATI 718Plus® is a polycrystalline Ni-base superalloy for aero-engine disc applications, which relies on the precipitation of η-phase (Ni6 Nb[Al,Ti], D024 ) at grain boundaries for resistance to intergranular failure and grain size control. The nucleation and evolution of the η particles have been examined throughout the manufacturing process. During hot working below the η solvus, existing precipitates lose their orientation relationship with the matrix and become aligned with the forging flow. During heat treatment, a dual morphology develops by the uniform growth of those incoherent precipitates into coarse plates, together with the new nucleation of thin lamellae from selected grain boundaries. The balance between the two morphologies in the final component can be achieved by controlling precipitation and dissolution processes during forging.

Hot deformation characteristics of a polycrystalline γ–γ′–δ ternary eutectic Ni-base superalloy

Nickel-base superalloys possess an unparalleled combination of mechanical and physical properties for elevated temperature applications and are likely to remain the first choice for structural components in turbine engine applications in the future. In order to keep pace with increasingly demanding design requirements associated with advanced gas turbine technologies, the properties of Ni-base superalloys can be greatly enhanced via thermo-mechanical and/or compositional changes. Recent studies have revealed that ternary eutectic γ–γ′–δ Ni-base superalloys exhibit promising high temperature mechanical properties that may potentially be superior to state-of-the-art polycrystalline Ni-base superalloys that are currently used in advanced gas turbines. As the properties of this novel class of alloys are largely dependent on microstructural strengthening mechanisms, both the composition and thermo-mechanical processing parameters need to be optimized concurrently. Deformation at temperatures and strain rates between 1040–1140°C and 0.1–0.001/s resulted in a transition of dominant deformation mechanisms from dislocation based plasticity to grain boundary rotation and sliding. The governing deformation mechanism was found to influence the alignment of δ phase precipitates and the degree of anisotropy that occurs during hot deformation of γ–γ′–δ Ni-base superalloys. Coupling high deformation temperatures with low strain rates promotes grain boundary sliding and rotation leading to less internal cavitation damage and a more isotropic microstructure.

Quantifying Three-Dimensional Residual Stress Distributions Using Spatially-Resolved Diffraction Measurements and Finite Element Based Data Reduction

Residual stress can play a significant role in the processing and performance of an engineered metallic component. The stress state within a polycrystalline part can vary significantly between its surface and its interior. To measure three-dimensional (3D) residual stress fields, a synchrotron x-ray diffraction-based experimental technique capable of non-destructively measuring a set of lattice strain pole figures (SPFs) at various surface and internal points within a component was developed. The resulting SPFs were used as input for a recently developed bi-scale optimization scheme McNelis et al. J Mech Phys Sol 61:428–1007 449 (2013) that combines crystal-scale measurements and continuum-scale constraints to determinethe 3D residual stress field in the component. To demonstrate this methodology, the 3D residual stress distribution was evaluated for an interference-fit sample fabricated from a low solvus high refractory (LSHR) polycrystalline Ni-base superalloy.

Microstructure and mechanical properties of diffusion bonded single crystal to polycrystalline Ni-based superalloys joint

Bonding of a single crystal Ni-based superalloy to a polycrystalline Ni-based superalloy was carried out through diffusion bonding and the microstructure and mechanical properties of the joint were investigated. A diffusion zone with a width of 35μm was formed due to elements diffusion. From the single crystal substrate to the diffusion zone, the shape of γ′ phase changed from cuboidal to columnar. The hardness of the diffusion zone was a bit higher, which can be attributed to the precipitation of the fine γ′-strengthening phases. The average shear strength of 433MPa was obtained and the fracture presented a plastic characteristic.

Creep properties of different γ′-strengthened Co-base superalloys

Abstract The influence of various alloying elements on the creep properties of polycrystalline Co-base superalloys hardened by a ternary L1 2 compound, Co 3 (Al,W) (γ′-phase), was investigated. A Ti containing quaternary alloy shows creep strength similar to Ni-base superalloys IN100 and IN713C at 850 °C and strongly superior to conventional Co-base superalloys as Haynes188. The activation energy for creep between 850 and 950 °C is similar to the polycrystalline Ni-base superalloy IN100 in the same temperature range. Strengthening of the grain boundaries by third phase precipitates was found to be crucial for the mechanical properties. This can be achieved either by precipitation of borides or by additional intermetallic phases which precipitate due to oversaturation. During compressive creep at 850 °C only a slight tendency for directional coarsening occurs, while at 950 °C distinct γ/γ′-rafts perpendicular to the external compressive stress axis are formed which indicate a positive lattice misfit even at 950 °C.

Microstructural evolution of a Ni-based superalloy (617B) at 700 °C studied by electron microscopy and atom probe tomography

We report on the microstructural evolution of a polycrystalline Ni-based superalloy (Alloy 617B) for power plant applications at a service temperature of 700°C. The formation of secondary M23C6-carbides close to grain boundaries (GBs) and around primary Ti(C,N) particles is observed upon annealing at 700°C, where γ′ is found to nucleate heterogeneously at M23C6 carbides. Using atom probe tomography, elemental partitioning to the phases and composition profiles across phase and grain boundaries are determined. Enrichments of B at γ/M23C6 and γ′/M23C6 interfaces as well as at grain boundaries are detected, while no B enrichment is found at γ/γ′ interfaces. It is suggested that segregation of B in conjunction with γ′ formation stabilizes a network of secondary M23C6 precipitates near GBs and thus increases the creep rupture life of Alloy 617B. Calculations of the equilibrium phase compositions by Thermo-Calc confirm the chemical compositions measured by atom probe tomography.