Thermal stability of primary carbides and carbonitrides in two cast Ni-base superalloys

Thermal stability of primary MC carbide and M(CN) carbonitride in K452 and K446 alloys is investigated. In K452 alloy, primary MC is very unstable relative to M(CN) for the former contains a higher content of the weakeners (W and Mo) than the latter. The ratios of element contents (at.%) (W + Mo)/Cr, (W + Mo)/(Ti + Nb) and Nb/Ti are three important parameters, which together determine the thermal stability of primary MC. Also, primary MC degeneration plays an important role in the microstructural instability of K452 and K446 alloys.     

On the creep and phase stability of advanced Ni-base single crystal superalloys

The present article examines microstructure stability and creep resistance of a 5th generation superalloy, which has Cr content at 4.6 wt%, 6.4 wt% Re and 5.0 wt% Ru, in comparison with that of a 4th generation superalloy (3.2 wt% Cr, 5.8 wt% Re and 3.6 wt% Ru). The aim is to elucidate the implication of increasing Cr, Re and Ru contents for future alloy developments. Experimental results have concluded that high Re + Ru content could promote formation of hexagonal δ phase at 900 °C; additional Cr and Re could enhance the precipitation of TCP phase at 1100 °C. Although an increase in lattice misfit between γ and γ′ in the 5th generation superalloy could strengthen the alloy against creep deformation under conditions at high temperatures (≥1000 °C) and low stresses (≤245 MPa) whilst the microstructural stability remained, the tendency to raft should be avoided during creep at lower temperatures and higher stresses.     

Predicting freckle formation in single crystal Ni-base superalloys

Freckle formation has been investigated in thirty-two distinct experimental single crystal Ni-base superalloys with large compositional variations. In addition to variations in alloy chemistry, the components studied in this investigation consisted of cylindrical bars with different diameters and solid blade-shaped components unidirectionally solidified in industrial casting trials. The occurrence of freckles was statistically assessed in over four hundred and seventy single crystal components. Detailed analyses of the microstructures, segregation behavior and phase transformation temperatures are presented and utilized to develop regression models capable of predicting the occurrence of freckle formation during solidification. The significance of the physical parameters incorporated with the model, and the implication of the model on the approach for alloy design are discussed.     

Thermal stability of a platinum aluminide coating on nickel-based superalloys

An investigation was carried out to determine the thermal stability of a platinum aluminide coating on the directionally solidified alloy MAR M 002 and its single-crystal version alloy, SRR 99, at 800, 1000 and 1100°C. The morphology, structure and microchemical composition of the coating were characterized using scanning electron microscopy, transmission electron microscopy, energy dispersive X-ray spectroscopy and X-ray diffraction. In the as-deposited condition, the coating was found to consist of two layers. Most of the platinum was concentrated in the outer coating layer which consisted of a fine dispersion of PtAl₂ in a matrix of β-(Ni, Pt)Al containing other elements in solid solution, such as cobalt and chromium. The inner coating layer was relatively free of platinum and consisted essentially of β-NiAl. Exposure at 800°C was found to have no significant effect on the structure and composition of the coating on each alloy. At temperatures ?1000°C, however, PtAl₂ became thermodynamically unstable and significant interdiffusion occurred between the coating and alloy substrate. After exposure at 1000°C, the components of the outer coating layer were NiAl and Ni₃Al. However, after exposure at 1100°C, the outer coating layer consisted only of Ni₃Al. Also, after exposure at both temperatures, the composition of the outer coating layer approached that of the inner layer due to interdiffusion. Although the coating on both alloys exhibited similar structural stability at all temperatures investigated, the coating on alloy MAR M 002 was found to develop a more protective scale. This behaviour was correlated with differences in alloy substrate composition particularly rare-earth elements such as hafnium.     

Development of a low-expansion bond coating for Ni-base superalloys

Ternary NiCrAl alloys were modified by the addition of Ti and Si in order to adjust their coefficients of thermal expansion (CTE) to less than 15 × 10⁻⁶ K⁻¹ (from room temperature to 1000 °C) without sacrificing essential high temperature oxidation resistance. Vacuum induction melted cast alloy samples were investigated by dilatometry and thermogravimetry (TG). Oxidative TG was conducted isothermally at 900 and 1000 °C and cyclically between 600 and 1100 °C for 100 h respectively. The CTE is reduced mainly by Ti and secondly by Cr additions. Quinary alloys which showed optimal oxidation resistance essentially exhibit phase stability in the solid state at all temperatures. Approximately 4 at.% Si is needed to attain low oxidation rates and to prevent oxide spallation as well. The beneficial effect of Si on oxidation behavior is attributed hypothetically to its ability to initiate the formation of protective alumina and subsequently silica. High Cr contents lessen the beneficial effect of Si owing to the concurrent formation of chromia and/or titania.     

镍基单晶高温合金热机疲劳断裂特征

为了进一步提高镍基单晶高温合金的热机疲劳性能,通过微观结构解析研究了合金热机疲劳断裂特征.通过金相和扫描电子显微镜研究了热机疲劳断裂的断口特征和微观结构.研究表明:裂纹起源于形变孪晶与试样外表面的交截处,过程中的氧化有助于裂纹的长大;裂纹尖端的应力场诱发出大量形变孪晶,而形变孪晶的存在为裂纹进一步沿着孪晶界扩展提供了便利条件;镍基单晶高温合金的疲劳断裂主要是由于形变孪晶的形成以及裂纹沿孪晶界的扩展造成的.形变孪晶与高温合金疲劳断裂密切相关.     

Approaches to enhance elevated temperature erosion resistance of Ni-base superalloys

Erosive wear is also known as impact wear. Several industrial components are degraded due to solid particle erosion at high temperatures. Solid particle erosion of metallic materials at high temperatures is influenced by the nature of interaction between erosion and oxidation. The main objective of the present work is to critically examine how strength and oxidation behaviour can be tailored to enhanced resistance to solid particle erosion of Ni-base superalloys at high temperatures. It is noted that alloys which form Al₂O₃ scale are likely to have good erosion resistance. This study examines methods of improving the erosion resistance by enhancing the elevated temperature mechanical properties. Methods for forming various scales and improving their adhesion characteristics are also elaborated.     

Microstructure-sensitive notch root analysis for dwell fatigue in Ni-base superalloys

Macroscopic viscoplastic constitutive models for γ–γ′ Ni-base superalloys typically do not contain an explicit dependence on the underlying microstructure. Microstructure-sensitive models are of interest in many applications since microstructure can vary in components, whether intentional or not. In such cases, the use of experiments from one microstructure condition to fit macroscopic models may be too limiting. The principal microstructure attributes that can significantly affect the cyclic stress–strain response of γ–γ′ Ni-base superalloys are the grain size and γ′ precipitate volume fraction and size distributions. An artificial neural network (ANN) is used to correlate the material parameters of a macroscale internal state variable cyclic viscoplasticity model with these microstructure attributes using a combination of limited experiments augmented by polycrystal plasticity calculations performed on other (virtual) microstructures within the range characterized experimentally. The trained model is applied to an example of a component fatigue notch root analysis with dwell periods at peak load to demonstrate the methodology and explore the potential impact of microstructure-sensitive constitutive models on life prediction for notched structures subjected to realistic load histories.     

Oxidation resistance: One barrier to moving beyond Ni-base superalloys

The implementation of new high-temperature materials is often hampered by their lack of oxidation or environmental resistance. This failing is one of the strongest barriers to moving beyond Ni-base superalloys for many commercial applications. In practice, usable high-temperature alloys have at least reasonable oxidation resistance, but the current generation of single-crystal Ni-base superalloys has sufficient oxidation resistance that optimized versions can be used without a metallic bond coating and only an oxygen-transparent ceramic coating for thermal protection. The material development process often centers around mechanical properties, while oxidation resistance, along with other realities, is given minor attention. For many applications, the assumption that an oxidation-resistant coating can be used to protect a substrate is seriously flawed, as coatings often do not provide sufficient reliability for critical components. Examples of oxidation problems are given for currently used materials and materials classes with critical oxidation resistance problems.     

Microstructural characterization of the failures of thermal barrier coatings on Ni-base superalloys

Abstract

Typical thermal barrier coating (TBC) systems consist of a nickel-base superalloy substrate coated with a MCrAlY or diffusion aluminide bond coat, onto which is deposited a yttria-stabilized zirconia (YSZ) TBC. The bond coats are usually deposited via diffusion aluminizing processes or low pressure plasma spray processes (LPPS). The YSZ can be deposited by air plasma spraying (APS) or electron beam physical vapor deposition (EBPVD). A layer of thermally-grown oxide (TGO), which is usually alumina, forms between the bond coat and YSZ during TBC deposition and subsequent high-temperature exposure. The conventional wisdom is that APS coatings tend to fail in the YSZ and that EBPVD coatings tend to fail at the interface between the TGO and bond coat. However, current research has shown that the situation is much more complex and that the actual fracture path can be a function of the type of bond coat, the type of high-temperature exposure, and coating process parameters. This paper describes the results of a study of the failure of state-of-the-art EBPVD TBCs deposited on NiCoCrAlY and platinum-modified diffusion aluminide bond coats. The failure times and fracture morphology are described as a function of bond coat type. The failure times were found to be a strong function of temperature for both bond coats. The failure for NiCoCrAlY bond coats was found to initiate at defects in the coating, particularly at the TGO/YSZ interface, but the fracture propagated primarily along the TGO–bond coat interface. The failure times and morphologies for platinum-modified diffusion aluminide bond coats depended strongly on bond coat surface preparation. The mechanisms for failure of the two bond coats are described. Also, the effects of modifications to the bond coats and variations in processing parameters on these mechanisms are presented.     

钌和铼对先进单晶高温合金组织稳定性的协同影响

为研究高代单晶高温合金组织稳定性影响机制,制备含6%(质量分数,下同)Ru和4.5%Ru的两种单晶高温合金D1和D2,经完全热处理后在980℃下长期时效1000 h。观察不同尺度上的显微组织及合金元素分布,并结合热力学计算进行分析。结果表明:两种合金经完全热处理后仍有较高含量的高熔点合金元素偏析于枝晶干中,使枝晶干区域长期时效后均有较多TCP相析出;两种合金中,Ru和Re均为TCP相主要形成元素,Ru含量较高的D1合金中TCP相析出量多于Ru含量较低的D2合金;Ru和Re含量增加会使合金平均d轨道电子能级增大,增加合金TCP相析出倾向,但由于Ru可以降低Re在γ相中偏析程度,因此Ru含量增加又可以减少Re对合金组织稳定性的不良影响;在本研究中,Ru对TCP相析出的促进更为显著,因此,在980℃下长期时效1000 h后D1合金较D2合金析出更多TCP相。     

An analysis of measurement of solute segregation in Ni-base superalloys using X-ray spectroscopy

The extent of solute segregation in a multi-component Ni-base superalloy, IN713LC, is analysed using X-ray spectroscopy techniques. It is demonstrated in this study that such an analysis is strongly influenced by the volume averaging of γ and γ′ phases sampled by the electron beam; the γ′ having precipitated from the γ phase below the γ′ solvus temperature in the solid-state. It is found that the size of the precipitate (γ′) in relation to the interaction volume is crucial; volume averaging is dominant when the precipitate size (γ′) is comparable to the interaction volume. Accurate interpretation of the measurements is essential when they are used to infer the micro-segregation and solute partitioning during solidification. Implications of this analysis in the assessment of segregation during solidification in Ni-base superalloys are addressed.     

Anomalous deformation behavior and twin formation of Ni-base superalloys at the intermediate temperatures

The tensile behaviors of polycrystalline Ni-base superalloys have been studied in the temperature range of 25-980 °C. Anomalous increase of yield strength was observed in precipitation hardened superalloys at intermediate temperature. The alloy with high γ′ volume fraction showed a remarkable increase of yield strength at intermediate temperature. A peak of yield strength was observed in the alloy with low γ′ volume fraction at intermediate temperature while solid solution strengthened alloys did not have such peak. Abrupt decrease of ductility in the intermediate temperature regime was observed not only in the γ′ strengthened superalloys but also in the solid solution strengthened superalloy. This result implies that γ′ precipitation is not a substantial cause for the occurrence of the ductility minimum in the superalloys. It was found that twinning was an important deformation mechanism of the superalloys at intermediate temperature where ductility was abnormally low. Deformation twins formed easily in the superalloys whose reduction of stacking fault energy was high regardless of strengthening mechanisms because alloys with low stacking fault energy was prone to extend stacking faults.     

Effects of Re on surface eutectic formation for Ni-base single crystal superalloys during directional solidification

Effects of Re on the formation of surface eutectics have been investigated by using Ni-base single crystal superalloys with different Re additions. It was found that Re promotes the segregation of Al and Ta to the eutectic melt, leading to an increase of the surface and internal eutectics. In addition, the addition of Re also increased the freezing range, the local solidification time, and the permeability of the dendritic network within the mushy zone. These factors ultimately promoted the outflow of the interdendritic residual liquid with the action of solidification shrinkage, and led to the formation of more surface eutectics. In contrast, the addition of Re had no obvious influence on the surface eutectic microstructures.     

High temperature characteristics of Ir–Ta coated and aluminized Ni-base single crystal superalloys

High temperature oxidation and hot corrosion properties of Ir–Ta coated and aluminized Ni-base superalloys are presented. An Ir–Ta binary alloy, proposed as a novel metallic bond coat material, was coated on a Ni-base single crystal superalloy TMS-75 using electron beam physical vapor deposition, followed by a conventional low activity Al pack cementation process. Cyclic oxidation tests and hot corrosion tests revealed that these Ir–Ta coated and aluminized specimens showed reasonably good oxidation and hot corrosion resistance. In addition, it was found that the formation of TCP phases is suppressed by the presence of the Ir–Ta enriched layer. These results indicated that the Ir–Ta alloy system is promising as a new metallic bond coat material for high temperature structural materials.     

Effects of Re content and crystallographic orientation on creep behavior of aluminized Ni-base single crystal superalloys

Effects of Re content and crystallographic orientation on creep behavior of aluminized Ni-base single crystal superalloys have been investigated in the present study. The addition of Re seemed to be effective for the creep strength improvement of Ni-base single crystal superalloys. The creep strength was significantly decreased in aluminized specimens due to the change in microstructure under coating layer by the formation of the interdiffusion zone (IDZ) and substrate diffusion zone (SDZ). The specimens with {100} side-surface showed longer creep rupture lives than the specimens with {110} side-surface which means that the anisotropic creep behavior took place. The anisotropic creep behavior was more evident in aluminized CM186LC than in aluminized PWA 1480. This anisotropic creep behavior was mainly induced by the discrepancy of fracture modes due to the different arrangement of slip systems, the geometry as well as the depth penetration of topologically close-packed (TCP) phases between the two side-surface orientations.     

Modeling the microstructural evolution of Ni-base superalloys by phase field method combined with CALPHAD and CVM

A quantitative simulation with phase field method (PFM) in a real alloy system was performed with a new strategy of modeling the microstructure evolution of Ni-base superalloys, in which, the calculation of phase diagrams (CALPHAD) method and cluster variation method (CVM) are combined with the PFM. In this strategy, the four sub-lattice model is used to evaluate the chemical free energy density while CVM to calculate some parameters, such as the lattice misfit and gradient coefficients in phase field equations. With this strategy, the microstructure evolution of a Ni–Al binary alloy at the temperature of 1000 K has been simulated; the elastic energy due to the lattice misfit and elastic inhomogeneity between γ and γ′ phases has also been taken into account. Moreover, the directional coarsening phenomenon of Ni–Al alloy has been reoccurred when the various external stress conditions are applied, which shows good agreement with Pineau’s directional coarsening map.