Cast Ni-base Superalloy Research Articles

A Monte-Carlo approach for crack initiation modeling of cast superalloys informed by crystal plasticity

Crystal plasticity simulations of materials to assess the fatigue response on the microscale are becoming increasingly popular in industrial application. However, in the case of parts made from cast Ni-base superalloys, the amount of pores in a part combined with the fine spatial discretization around the pores needed due to strong mechanical gradients quickly leads to a computationally impractical number of mesh elements. In this paper we show an alternative to explicit crystal plasticity modeling of part-scale porosity by introducing a Monte-Carlo submodel that recombines the fatigue response of single pores predicted by crystal plasticity into the fatigue response of the pore agglomerate. The model can be applied in realtime due to the use of precomputed crystal plasticity results. We demonstrate that fatigue indicator parameters predicted by the Monte-Carlo submodel agree well with those predicted by explicit crystal plasticity simulations. Lastly, we apply the proposed model to study the influence of different porosity volume percentages, pore sizes and pore morphologies on the fatigue indicator parameters of a cast MAR-M247 superalloy.

Hot Deformation Behavior and Microstructure of Cast Ni-Based Superalloy IN-100 Based on the Processing Map

Hot Deformation Behavior and Microstructure of Cast Ni-Based Superalloy IN-100 Based on the Processing Map

Evolution of microstructure and 800 °C/294 MPa stress rupture property of cast Ni-based superalloys during long-term thermal exposure: Role and behavior of primary MC carbide

The role and behavior of primary MC carbide and their effects on the microstructure and stress rupture property of the cast Ni-based superalloys during long-term thermal exposure at elevated temperature are systemically investigated in this work. It is found that the chemical compositions of primary MC are proportional to the Nb/Ti and (Ti + Nb)/(Mo + W) ratios of alloys. During long-term thermal exposure, the thermal stability of primary MC can be quantitatively evaluated by its decomposition degree defined as Dt=(V0-Vt)/V0 × 100%, which is closely associated with the Nb/Ti and (Mo + W)/Cr ratios of alloys. The decomposition process, degree, form and mechanism of primary MC are revealed in detail. The STEM observations with the EDS analyses show that primary MC decomposition is an inter-diffusion process of elements between the primary MC carbide and γ matrix, which can increase the volume fraction of γ' phase, restrain the precipitation of topologically close-packed (TCP) phases and promote the coarsening of grain boundaries (GBs). The combination of these microstructural evolutions, together with the appropriate Al + Ti + Nb and W + Mo contents in the matrix, eventually results in a remarkable improvement in the stress rupture property of long-term thermally exposed cast Ni-based superalloys.

The role of the strengthening phases on the HAZ liquation cracking in a cast Ni-based superalloy used in industrial gas turbines

This work presents the influence of microstructural constituents on liquation crack formation in the cast Ni-based superalloy, René 108. The investigation was divided into three parts: characterisation of the material's microstructure in pre-weld condition, hot ductility studies and analysis of liquation cracking induced by the gas tungsten arc welding process. Using advanced electron microscopy techniques it is shown that the base material in pre-weld condition is characterised by a complex microstructure. The phases identified in René 108 include γ matrix, γ' precipitates, MC and M23C6 carbides, and M5B3 borides. Based on Gleeble testing, it was found that René 108 is characterised by high strength at elevated temperatures with a maximum of 1107 MPa at 975 °C. As a result of constitutional liquation, the superalloy’s strength and ductility were significantly reduced. The nil strength temperature was equal to 1292 °C, while the nil ductility temperature was 1225 °C. The low ductility recovery rate (32.1), ratio of ductility recovery (36.2) and hot cracking factor (Rf = 0.05) values confirmed the low weldability of Renѐ 108. In the heat-affected zone (HAZ) induced by welding, constitutional liquation of mainly γ' precipitates, with a contribution of M23C6 carbides and M5B3 borides, was observed. The thin non-equilibrium liquid film, which formed along high-angle grain boundaries, led to crack initiation and their further propagation during cooling. The eutectic γ–γ' re-solidification products are visible on the crack edges.

A review on the Cyclic Oxidation Behavior and Mechanical Properties of the Pt–Al-Coated Cast Ni-based Superalloys

A review on the Cyclic Oxidation Behavior and Mechanical Properties of the Pt–Al-Coated Cast Ni-based Superalloys

Effects of Re and Co additions on lattice parameters and lattice misfit in cast Ni-based superalloys

Abstract Morphological evolution of gamma prime precipitates was governed by an elemental partitioning ratio between the gamma and gamma prime phases. In the present study, Ni-based MGA 1400 alloy was modified by adding 4 wt% of Re and 6.4 wt% of Co by vacuum arc melting. The effects of Re and Co additions on lattice parameters of gamma and gamma prime phases were studied by TEM and XRD techniques. Calculated lattice misfit values obtained from XRD and TEM techniques were used to identify the shape and morphology of gamma prime precipitated particles after long-term exposure at 1173 K for 1800 ks? After long-term exposure, gamma prime particles became much coarser. The addition of Re retarded the coarsening rate of the gamma prime precipitates, resulting in smaller gamma prime particles at the same annealing times. Even though, both Re and Co atoms are strongly partitioned in the gamma matrix, the effect of Re and Co additions on lattice misfits was contradictory. The addition of Re produced rounded corner cuboidal morphology of gamma prime precipitates due to higher magnitude with negative lattice misfit (−0.3857%), while Co addition provided a more rounded shape of gamma prime particles due to the lattice misfit value approach to zero (+0.0936%).

Temperature and ΔK dependence of grain boundary effect on fatigue crack propagation in a two-dimensional polycrystalline Ni-base superalloy

Effects of grain boundaries (GBs) on fatigue crack propagation (FCP) in a cast Ni-base superalloy were experimentally investigated using “two-dimensional” polycrystalline specimens extracted from a directionally solidified material. FCP tests were conducted at room temperature (RT), 700 °C and 900 °C under the stress intensity factor range, ΔK, increasing conditions, and distinctive FCP behavior both in grain interiors and around the GBs was observed in-situ. At RT, the crack propagated in the shearing mode along slip planes. At 700 °C, the cracking mode transitioned from the opening mode to the shearing mode with the increasing ΔK. At 900 °C, only the opening mode cracking occurred. The shearing cracks at RT and in the high ΔK region at 700 °C were retarded near the GBs, whereas the opening mode crack in the low ΔK region at 700 °C was not affected by the GBs. The retardation was attributed to a transition from the shearing mode to the opening mode induced by the slip plane incompatibility at the GBs, and the degree of the retardation showed a good correlation with the size of the shearing crack plane. At 900 °C, the crack propagated along high angle GBs with higher FCP rates than those in grain interiors. The acceleration was greater at the GBs with larger misorientation, which was attributed to higher GB energy. In high ΔK regions, secondary cracks were nucleated along high angle GBs ahead of main crack tips possibly because of external stress normal to the GBs and/or the strain incompatibility at the GBs.

On the creep resistance in cast Ni-base superalloys

Abstract In this paper, we attempt to apply the modified hardening theory to estimate the creep resistance, which is considered as a key factor for controlling the creep deformation mechanism in cast nickel-base superalloys. It is suggested that, when the applied stress is high enough for the dislocations to cut into the γ′ particles, the creep resistance is almost a constant and independent of applied stress. At low applied stress, creep deformation is mainly controlled by dislocations climb, where the creep resistance has two components of a threshold stress and a friction stress. The model is verified for two cast nickel-base superalloys DZ17G and IN738. The results of detailed calculations are in good agreement with the experimental data.

Comparison of erosion rate of EB-PVD and plasma sprayed TBC

ABSTRACT A thermal barrier coating (TBC) consisting of platinum-aluminide bond coat and 7wt-% yttria stabilized zirconia (7YSZ) ceramic top coat deposited by electron beam physical vapour deposition (EB-PVD) system cast Ni-base superalloy CM-247LC. The erosion behaviour of the EB-PVD and air plasma sprayed (APS) TBCs are compared. The APS TBC consists of 7YSZ top coat and CoNiCrAlY bond coat on Inconel 738 substrate. Erosion responses of TBCs have been examined as a function of temperature. The erosion rate of EB-PVD TBC was significantly lower than that of APS TBC. Erosion rate of EB-PVD TBC decreased with test temperature. APS TBC exhibits mostly temperature independent erosion rate. The material removal mechanism of EB-PVD TBC was by the formation of parallel cracks in the ceramic layer close to its surface and their subsequent joining. The material is removed by propagation of subsurface cracks along the splat boundaries in case of APS TBC.

The thin-wall debit of a typical cast polycrystalline M951 alloy

The effect of specimen thickness on the rupture properties of cast Ni-based polycrystal superalloy M951 was investigated in this paper. The results of the rupture tests in air under 980 ​°C/90 ​MPa revealed that the rupture life and elongation obviously decreased with decreasing specimen thickness. The thin-wall effect for polycrystal superalloy M951 is associated with several factors. First, the surface oxidation and internal nitridation induce the reduction in the load-bearing section, which accelerates the rupture fracture of thin specimens. Second, the combined effect of oxidation and stress at the surface grain boundary facilitates the premature initiation and propagation of the surface cracks in thin specimen. Third, the through-grain boundaries introduce by specimens machining are detrimental for rupture property. A direct comparison of rupture properties of thin-wall samples with different grain morphologies indicate that thin-wall effect can be diminished by avoiding through-grain boundary introduction.

Segregation Phenomenon of As-Cast and Heat Treatment Microstructures in Investment Casting of IN738LC Superalloy

The effect of solidification rate on micro-segregation in investment casting of IN738LC superalloy was studied. In Ni-based superalloys, the micro-segregation of solute atoms is formed due to limited diffusion during cast and solidification. The microstructure of cast Ni-based superalloys is largely divided into dendrite core of initial solidification and interdendrite of final solidification. In particular, mosaic shaped eutectic γ/γ’ and carbides are formed in the interdendrite of the final solidification region in some cases. The micro-segregation phenomena formed in regions of dendrite core and interdendrite including eutectic γ/γ’ and carbides were analyzed using OM, SEM/EDS and micro Vickers hardness. As a result of analysis, the lack of (Cr, W) and the accumulation of Ti were measured in the eutectic γ/γ’, and the accumulation of (Cr, Mo) and the lack of Ti were measured in the interdendrite between dendrite and eutectic. Carbides formed in interdendritic region were composed of (Ti, W, Mo, C). The segregation applied to each microstructure is mainly due to the formation of γ’ with Ni3(Al,Ti) composition. The Ni accumulation accompanied by Cr depletion, and the Ti accumulated in the eutectic region as a γ’ forming elements. The Mo tends to diffuse out from the dendrite core to the interdendrite, and the W diffuse out from the interdendrite to the dendrite core. Therefore, the accumulation of Mo in the interdendrite and the deficiency of W occur in the eutectic region located in the interdendrite. Heat treatment makes the degree of the micro-segregation decrease due to the diffusion during solid solution. This study could be applied to the heat treatment technology for the micro-segregation control in cast Ni-based superalloys.

Strain Localizations in Notches for a Coarse-Grained Ni-Based Superalloy: Simulations and Experiments.

Alloys used for turbine blades have to safely sustain severe thermomechanical loadings during service such as, for example, centrifugal loadings, creep and high temperature gradients. For these applications, cast Ni-based superalloys characterized by a coarse-grained microstructure are widely adopted. This microstructure dictates a strong anisotropic mechanical behaviour and, concurrently, a large scatter in the fatigue properties is observed. In this work, Crystal Plasticity Finite Element (CPFE) simulations and strain measurements performed by means of Digital Image Correlations (DIC) were adopted to study the variability introduced by the coarse-grained microstructure. In particular, the CPFE simulations were calibrated and used to simulate the effect of the grain cluster orientations in proximity to notches, which reproduce the cooling air ducts of the turbine blades. The numerical simulations were experimentally validated by the DIC measurements. This study aims to predict the statistical variability of the strain concentration factors and support component design.

New alloy development from modified cast Ni-base superalloy GTD-111 with additions of Al, Ni and/or Co prepared by vacuum arc melting process

Abstract The aim of this research is to study and evaluate the effects of cobalt and nickel additions on heat-treated and thermal-exposed microstructures in cast nickel-base superalloy grades based on GTD-111 with cobalt and nickel modifications. In the study, 1 wt.-% aluminium, as well as various cobalt and nickel contents, was added to all the modified alloy. After standard heat treatment, it was found that increasing the 1 wt.-% aluminium content affected the morphology of gamma prime particles, causing them to become more cuboidal in shape and larger in size. On the other hand, the size of gamma prime in all modified alloys decreased slightly with increasing cobalt contents. However, cobalt and nickel additions did not strongly affect the shape of the gamma prime particles. After long-term exposure at temperatures of 1173 K and 1273 K for 1440 ks, these gamma prime particles were much larger in size than those of heat-treated ones. The higher temperature caused extreme increases in the average size of thermal-exposed gamma prime particles. Also, the gamma prime particles became nearly spherical in shape and had lower coarsening rate with higher cobalt and lower nickel contents. Moreover, the addition of cobalt reduced the coalescence of gamma prime particles. Samples with cobalt content higher than 13.1 wt.-% were able to maintain the cubic shape of the particles even after long-term exposure at a temperature of 1273 K for 1440 ks. No topologically close-packed phase was found in any analyzed specimens.

Microstructural evolution during exposure in air and oxidation behavior of a nickel-based superalloy

Microstructural evolution and isothermal oxidation behavior of a cast Ni-based superalloy have been investigated at 800 °C, 900 °C and 1000 °C in air. Results show that the oxidation kinetics of the Ni-based superalloy follows parabolic law at elevated temperatures. Oxide scales after oxidation can be divided into external layer, continuous Cr2O3 layer linked with internal TiO2, discontinuous Al2O3 layer and γ′-depletion zone. Due to the segregation of alloying elements, densely granular zone and porous zone are formed at 800 °C. External oxide layer is a mixed oxides zone, which mainly consists of TiO2 and Cr2O3 at 800 °C and 900 °C, NiO and NiCr2O4 at 1000 °C. The formation condition of NiO and NiCr2O4 is mainly after 50 h oxidized at 1000 °C. Thickness growth rate of Cr2O3 and Al2O3 layers decrease gradually during oxidation process, indicating that external oxides can effectively prevent the inward diffusion of oxygen. The oxidation activation energy of the experimental Ni-based superalloy is confirmed to be 205.3 kJ mol−1 and the oxidation process is controlled by diffusion of oxygen, Ti and Cr ions.

New alloy development from modified cast Ni-base superalloy GTD-111 with additions of Al, Ni and/or Co prepared by vacuum arc melting process

Abstract The aim of this research is to study and evaluate the effects of cobalt and nickel additions on heat-treated and thermal-exposed microstructures in cast nickel-base superalloy grades based on GTD-111 with cobalt and nickel modifications. In the study, 1 wt.-% aluminium, as well as various cobalt and nickel contents, was added to all the modified alloy. After standard heat treatment, it was found that increasing the 1 wt.-% aluminium content affected the morphology of gamma prime particles, causing them to become more cuboidal in shape and larger in size. On the other hand, the size of gamma prime in all modified alloys decreased slightly with increasing cobalt contents. However, cobalt and nickel additions did not strongly affect the shape of the gamma prime particles. After long-term exposure at temperatures of 1173 K and 1273 K for 1440 ks, these gamma prime particles were much larger in size than those of heat-treated ones. The higher temperature caused extreme increases in the average size of thermal-exposed gamma prime particles. Also, the gamma prime particles became nearly spherical in shape and had lower coarsening rate with higher cobalt and lower nickel contents. Moreover, the addition of cobalt reduced the coalescence of gamma prime particles. Samples with cobalt content higher than 13.1 wt.-% were able to maintain the cubic shape of the particles even after long-term exposure at a temperature of 1273 K for 1440 ks. No topologically close-packed phase was found in any analyzed specimens.

Solidification behavior and segregation characteristics of high W-content cast Ni-Based superalloy K416B

The solidification behavior of high W-content cast Ni-Based superalloy K416B has been investigated by DTA measurement, isothermal solidification experiment, and Thermo-calc simulation. The element segregation characteristic during solidification was also analyzed by EPMA. The results show that the solidification path of K416B can be summarized as follows: L → L + γ (1371 °C); L → L + γ + M6C (1370 °C); L → L + γ +MC (1332 °C); L → L + eutectic (γ + γ′) (1286 °C); L → MC (High Hf) (1147 °C). During the crystallize of primary γ dendrite, W and Co were enriched in the solid phase, while Cr, Al, Ti, Nb and Hf were enriched in the residual liquid. The positive segregation elements piled up at the remaining liquid promoted the formation of MC carbides and eutectic (γ + γ′). MC carbides precipitated at the interdendritic region and broke the residual liquid network, eutectic (γ + γ′) seemed instability and easily translated into eutectic γ′ by the remelting of γ phase and the coarsening of γ′ phase during subsequent cooling. The solidification terminated by the formation of MC (High Hf) phase.

Effect of alloying additions and presence of water vapour on short-term air oxidation behaviour of cast Ni-base superalloys

Ni-14Cr-9Co (wt.%) based alloys with additions of Al, Ti and Ta were exposed in dry and wet (+20 vol.%H2O) air at 950 °C for up to 100 h and characterized with GD-OES and SEM/EDX. Ni-14Cr-9Co-alloy exhibited initially Cr2O3-scale formation followed by breakaway Ni-oxidation. 3%Al addition to Ni-14Cr-9Co stabilized the Cr2O3-scale and largely suppressed Ni-oxide formation. Ni-14Cr-9Co-3Al-5Ti formed a fast growing Cr2O3-scale attributed to p-type Ti-doping, but Ni-oxide formation was completely inhibited. 5%Ta addition to Ni-14Cr-9Co-3Al promoted formation of a thin alumina scale. Addition of H2O to the test gas increased growth of Ni-rich oxides and shortened time to breakaway for the Al-free alloy.

Characterization of γ′ Precipitates in Cast Ni-Based Superalloy and Their Behaviour at High-Homologous Temperatures Studied by TEM and in Situ XRD

In situ X-ray diffraction and transmission electron microscopy has been used to investigate René 108 Ni-based superalloy after short-term annealing at high-homologous temperatures. Current work is focused on characterisation of γ′ precipitates, their volume fraction, evolution of the lattice parameter of γ and γ′ phases and misfit parameter of γ′ in the matrix. Material in the initial condition is characterised by a high-volume fraction (over 63%) of γ′ precipitates. Irregular distribution of alloying elements was observed. Matrix channels were strongly enriched in Cr, Co, W and Mo, whereas precipitates contain large amount of Al, Ti, Ta and Hf. Exposure to high-homologous temperatures in the range 1100–1250 °C led to the dissolution of the precipitates, which influenced the change of lattice parameter of both γ and γ′ phases. The lattice parameter of the matrix continuously grew during holding at high temperatures, which had a dominant influence on the more negative misfit coefficient.

Magnetic Property Changes of CoNiCrAlY Coating Under Cyclic Oxidation and Hot Corrosion

In this paper, a non-destructive method for evaluation of a CoNiCrAlY coating applied to gas turbine blades has been studied. The specimens from cast Ni-base superalloy IN738LC were coated with Co–34Ni–18.6Cr–8.7Al–0.5Y by low pressure plasma spray (LPPS) technique. The XRD peaks indicated that CoNiCrAlY coatings are composed of β-NiAl and γ-Co (Ni, Cr) matrix with small amounts of Al2O3 and γ′-Ni3Al. The surfaces of the coated specimens were covered with thin salt mixture of Na2SO4 + l0 wt% NaCl. Afterward, the specimens were maintained at 850 °C for 24 h, cooled to room temperature (as one cycle) and then reintroduced in the furnace. The magnetic susceptibility of coating, as one of the magnetic properties, was periodically determined after each cycle. The results showed that the magnetic susceptibility of Co–34Ni–18.6Cr–8.7Al–0.5Y coating is related to the phase compositions and microstructures. The magnetic susceptibility typically increases with depletion of β-NiAl phase and decrease in the coating thickness during cyclic oxidation and hot corrosion. Therefore, measurement of magnetic susceptibility can be employed as a non-destructive test (NDT) method to estimate the thickness or residual lifetime of CoNiCrAlY coatings.

The SEM Metallography Analysis of Vacuum Cast ZhS6K Superalloy Turbine Blade after Various Working Hours

A vacuum cast Ni-base superalloy ZhS6K is used for the production of turbine blades whose are working in the hot section of turbine in former USSR DV-2 jet engine. The usual working temperature for this kind of alloy is designed for 800/1050°C. Turbine blades with protective alitized layer made from this alloy were evaluated at the starting stage, then after 600, 1000, 1500 and 2000 hours of regular working. The SEM methods of quantitative metallography were used for structural characteristics evaluation. The microstructural analysis shows increasing of the medium distance of secondary dendrite arms D, what is related to chemical changes in dendritic structure, and the intermetallic strengthening gamma prime phase coarse and turn into spherical shape what leads to decreasing of dislocation and precipitation strengthening mechanism and further decreasing of hot temperature properties of the alloy. The method used at quantitative evaluation and techniques of colour optical microscopy and SEM is useful when microstructure degradation of superalloys has occurred.