Formation Of TCP Phases Research Articles

Characterization of the spin arc welding technique for dissimilar materials Hastelloy C-2000 and C-276

Hastelloy C-2000 and C-276 are primarily employed in the petrochemical and pharmaceutical industries. Finding an effective way to combine these alloys is still a complex task in industry. The main focus of this study is to investigate the potential efficacy of the spin arc welding process on Hastelloy C-2000 and C-276. The welding process was carried out under various conditions, such as weld current, spinning speed, and spin diameter. ERNiCrMo-4, with a diameter of 1.2 mm, was employed as a filler. Optical microscopy, XRD and FESEM were used for microstructural examination. The mechanical properties of the welded samples were critically analyzed. The results of the ultimate tensile strength (UTS), hardness value (HV), and impact tests of spin arc welding were compared with those of the multipass pulsed current gas tungsten arc welding process (MPCGTAW). In spin arc welding, Hastelloy C-2000 and C-276 exhibited higher UTS, HV, and toughness compared to MPCGTAW because there was no secondary TCP phase formation in the weld centre and hot cracking in spin arc weldments. Additionally, compared to MPCGTAW, the elongation with spin arc welding was noted to be lower. Moreover, microstructural analysis revealed no evidence of grain segregation, which may have contributed to the improved UTS and HV of the materials during spin arc welding. Thus, spin arc welding proves to be superior for dissimilar Hastelloy joints of C-2000 and C-276.

Stress-induced influence on the evolution of γ′ phase in dendritic area during long-term stress aging at 900 °C in Ni-based superalloy K444

The evolution of γ′ phase in the Ni-based superalloy K444 alloy is investigated after long-term aging for 5000 h (LTA5,000) at 900 °C under different stresses (0 MPa, 50 MPa, 65 MPa, 80 MPa, 100 MPa and 120 MPa). The role of applied stress in the LTA5,000 process is analyzed in detail from multiple aspects, including morphology, composition partition, the formation of TCP phase, lattice misfit (δ) and dislocations configuration. The results reveal that the stress promotes the γ′ phase directional coarsening when it exceeds the critical value. For this alloy, the rafting structure is obvious when the stress is higher than 80 MPa, while γ′ phases maintain cuboid shape when the stress is below 65 MPa. It is caused by the stress gradient in the γ channels which accelerates the γ′ forming elements diffuse from horizontal γ channels to the vertical channels and only sufficient stress can activate the directional diffusion behavior. Besides, the stress induces that Al and Ti strongly assign to γ′ phases and the concentration of Cr in γ matrix increases clearly. In addition, the σ-TCP is free until the applied stress raises to 120 MPa, it is attributed to the increase in Cr concentration in the γ matrix. Moreover, the value of |δ| increases as the applied stress increases, and reducing interfacial energy becomes the driving force for directional coarsening. Finally, the calculated results based on the threshold stress demonstrate that the dislocations climb over the γ′ particles within the range of 65 MPa ∼ 120 MPa, which is in agreement with the observed phenomena.

In Situ Investigation of TCP Phase Formation, Stress Relaxation and γ/γ′ Lattice Misfit Evolution in Fourth Generation Single Crystal Ni-Base Superalloys by X-Ray High Temperature Diffraction

In nickel-based superalloys, the lattice misfit between the γ and γ′ phases and the propensity to TCP phase formation at service temperatures critically influence the microstructural evolution that takes place and hence the resultant mechanical properties. In this work, the lattice misfits of a series of highly alloyed Ru-containing 4th generation Ni-base superalloys are investigated by in situ X-ray diffraction (XRD) at high temperature. While the lattice misfit values of all alloys range between − 0.3 and − 1.3 pct at room temperature, they show an atypical temperature dependence, becoming less negative above 900 °C. In situ XRD measurements at 1100 °C reveal that the majority of the internal coherency stresses are already relieved after two hours. This is particularly pronounced for the alloys that have both a lattice misfit larger than |0.6| pct at 1100 °C and are prone to TCP phase formation. However, throughout the relaxation of the internal coherency stresses the lattice misfit remains fairly constant. Due to the similar chemical compositions of the alloys studied, qualitative guidelines for an optimum lattice misfit magnitude are developed by comparing the lattice misfit values with previous creep experiments. Our results indicate that no universal optimal lattice misfit value exists for these alloys and the value strongly depends on the applied creep conditions.Graphical

Oxidation of Thin Nickel-Based Superalloy Specimens: Kinetics Study and Mechanical Integrity

The oxidation behavior of a nickel-based superalloy was investigated from 650 to 1,000 °C for up to 1,000 h in air. Samples with thicknesses ranging from 20 to 500 µm were used to document the evolutions of microstructure and oxide scale. Oxidation products and subsurface evolution of the metal microstructure were characterized by XRD and EDS analyses. Local breakaway was observed after 600 h at 800 °C and after 100 h at 900 °C due to the full consumption of Cr from the alloy. Room temperature tensile tests were performed on aged and pre-oxidized specimens with thicknesses ranging from 20 µm to 500 µm, at 800 °C. The results were compared to tensile tests performed on the as-received metallurgical state. Both size effects due to sample thickness reduction and to sample thickness/pre-oxidation width ratio were examined onto the mechanical behavior. Interestingly, the formation of TCP phases, the oxide layer and the subsequent Cr-depleted subsurface region from either aging or oxidation treatments impairs the mechanical integrity. Bulk and subsurface regions are impacted, especially for tens-of-micrometer thin samples.

Effect of long-term thermal exposure on microstructure and creep properties of DD5 single crystal superalloy

The effect of thermal exposure on the microstructure and creep properties of the Ni-based single crystal superalloy in different test conditions was studied. Long-term exposure was performed at 1,000 °C and 1,100 °C for 500 h prior to the creep tests. The creep lifetime is found to be improved after the long-term exposure at 1,000 °C for 500 h as a result of the formation of secondary M23C6 in the interdendritic region. The coarsening of γ′ precipitates accompanied by the formation of TCP phase lead to the degradation of alloy, which is responsible for the reduction of the creep lifetime of Ni-base single crystal superalloy after long-term exposure at 1,100 °C for 500 h. The creep lifetime of 1,000 °C thermally exposed sample under the conditions of 1,093 °C/137 MPa is lower than that of heat-treated state. Thermal exposure at 1,100 °C for 500 h causes the creep lifetime to drop drastically.

Diffusion Modelling on the Microstructure Evolution in MCrAlY-Superalloy System.

Elemental diffusion drives the microstructure development in the MCrAlY-superalloy systems at high temperature. In this paper, two diffusion models were built to simulate the diffusion behavior of elements in the coating or in the coating–substrate system. Firstly, a core–shell model was set up to investigate the thermodynamic and kinetic behavior of the localized microstructure. The results of the simulation successfully explained the mechanism of the formation of α(core)–γ′(shell) structure at lower temperature (750 °C) and γ(core)–β(shell) structure at higher temperature (1100 °C). Secondly, a coating–substrate planner model was used to simulate the interdiffusion of elements between the MCrAlY coating and the superalloy substrate. The simulation results in the Ni22Cr10AlY-superalloy system semiquantitatively agreed with the experimental observation. Furthermore, by applying the planner diffusion model, the effect of the MCrAlY coatings on the formation of TCP phases in the substrate was studied, and a GOODMAN map for designing TCP-limited MCrAlY coatings can be provided.

On the High-Temperature Oxidation Behavior of a Ta-Containing Quaternary Co-Base Model Alloy System with \u03b3/\u03b3\u2032-Microstructure - Influence of \u03b3\u2032-Volume Fraction, Surface State, and Heating Condition on Alumina Growth

The improvement of the high-temperature oxidation resistance remains an ambitious goal for the design of new γ/γ′-strengthened Co-base superalloys, since their oxidation resistance beyond 800 °C still ranks behind their Ni-base counterparts. To better understand the origin of the poor oxidation resistance at higher temperatures, this study focuses on early stages of oxidation of four quaternary (Co-Al-W-Ta system) Co-base model alloys with a two-phase γ/γ′-microstructure and varying γ′-volume fraction at 800 °C, 850 °C and 900 °C. Based on time-resolved isothermal gravimetric analysis (TGA) in synthetic air and detailed electron microscopic analysis, the role of the γ-channel width (or γ′-volume fraction), the surface preparation prior to exposure (polishing versus shot-peening), and the heating conditions (synthetic air versus argon) on protective alumina growth is elucidated. Firstly, for alloys of increased γ′-volume fractions slower oxidation kinetics prevailed. Secondly, the two-phase microstructure was found to decisively affect the propagation of the internal oxidation front at the early stages of oxidation. Thirdly, shot-peening prior to exposure together with a lack of oxygen availability during heating was identified to foster protective alumina growth, accompanied by TCP-phase formation in the substrate. The critical role of a high Al availability in the alloy for a rapid growth of protective alumina and the relating challenges in alloy development regarding, for example, phase stability in this relatively novel Co-base alloy class are discussed in detail.

Microstructure and stress-rupture property of DD32 nickel-based single crystal superalloy fabricated by additive manufacturing

Additive manufacturing (AM) technology is currently undergoing rapid development in the aerospace field. However, due to the intense cracking susceptibility, additive manufacturing of nickel-based single-crystal (SX) superalloys still presents major challenges. In this paper, the crack-free SX superalloy is fabricated directly using pulsed laser. The microstructure and stress rupture property of the additively manufactured SX superalloy after heat treatment are carefully investigated and compared with conventional cast alloy. The results show that the stress rupture life of the AM sample (66.4 h) is longer than that of the cast sample (59.1 h). This is related to the difference of microstructure between AM and cast samples. AM samples have a finer dendritic structure and lower inter-dendritic segregation, which is conducive to solution strengthening and avoids the formation of TCP phase. Besides, the carbides in the AM sample is finer and evenly distributed, which is conducive to precipitation strengthening; The fine carbides are enveloped within layers of γ′ in the AM samples, which avoids the formation of cracks caused by separation of carbides and the matrix. All of those are beneficial to improve stress rupture properties.

Influence of Si and La on Workability of GH536

Silicon can promote the formation of TCP phases, such as sigma, Laves and G in nickel-base superalloys due to its strong tendency of segregation during solidification. The effect of Si on workability of GH536 has been discussed in detail using optical microscope and scanning electron microscope. It shows that the workability of GH536 was degraded by Si impurity. Also, minor La additions were found to be in favor of the workability.

Study into the Role of Nickel Vapor on Surface Modification of a Third-Generation Single-Crystal Superalloy

A third-generation single-crystal (SX) superalloy was heat treated at 1350 °C for 24 hours while being exposed to nickel vapor. It is observed that a layer of 60-µm thickness deposited onto the alloy. This prevented the formation of TCP phases and the occurrence of melting, which were reported in previous studies. The interdiffusion between the alloy and deposited layer shows strong indications of cross-diffusion effects, as shown from the composition profile measurements and simulations.

Influence of Mo and Ru additions on the creep behavior of Ni-based single crystal superalloys at 1100 °C

In order to obtain more knowledge to replace or lower the content of expensive and heavy elements Re and Ru in advanced Ni-based single crystal superalloys, the role of Ru playing in the microstructure and creep performance at 1100 °C and 140 MPa was evaluated in this study by considering its individual effect and also synergizing it with Mo to maximize the beneficial effect from each element. A series of single crystal superalloys with varying alloying elements were investigated and their initial microstructure and microstructural evolution during the creep deformation were characterized. It was shown that both Mo and Ru could increase the γ/γ' lattice misfit and density of interfacial dislocations, that contributed to improve the creep resistance at high temperature and low stress condition. The inferior microstructural stability regarding to the TCP phase formation in alloys with more Mo addition lowered creep performance to some extent, however, that could be restrained by the addition of Ru. The high temperature and low stress creep performance could be enhanced by the co-addition of Mo and Ru since their synergistic effect could be able to enhance the γ/γ' interfacial strengthening and retain the microstructural stability.

Effect of multiple alloying additions on microstructural features and creep performance at 950 °C and 400 MPa in Ru-containing single crystal superalloys

Microstructural features, including γ channel width, γ' size, γ' volume fraction, γ-γ' lattice misfit, TCP phase as well as dislocation substructures had influence on the creep performance in Ni-base single crystal superalloys. However, relatively limited work has been conducted to investigate the effect of microstructural features based on various alloying additions on creep properties in Ru-containing single crystal superalloys. In this study, the creep tests were conducted at 950°C and 400MPa in experimental alloys with different levels Co (7.0wt% and 15.0wt%), Cr (3.5wt% and 6.0wt%), Mo (1.0wt% and 2.5wt%) and Ru (2.5wt% and 4.0wt%) additions, and the microstructures during creep were characterized in detail. Co and Ru were found to decrease the γ channel width, respectively. The high level of Cr addition decreased the γ' volume fraction and promote the TCP phase formation during creep. The addition of Mo decreased the γ channel width and also acted as a TCP former. The γ-γ' lattice misfit was increased to more negative by the individual additions of Co, Ru, Cr, and Mo, respectively. The joint addition of microstructural stabilizers Co and Ru inhibited the TCP phase formation during creep. The precipitation of TCP phases served as one of the main factor to decrease the creep property in alloys with high level of Cr and Mo additions, respectively. The synergistic effect of Mo and Ru additions in the alloy with high level of Co content was found to increase the γ-γ' lattice misfit and the amount of stacking faults in γ matrix during creep process, which improved the creep resistance at 950°C and 400MPa. This study is helpful to understand the effect of alloying elements additions and microstructures on creep performance and to get better improvement of physical metallurgy knowledge and alloy design in Ru-containing single crystal superalloys.

Segregation of alloying elements in directionally solidified Re–Ru-containing Ni-based superalloys

In this work, the influence of nonequilibrium conditions of directed solidification was investigated for a nickel-based superalloy containing rhenium and ruthenium. The segregation of alloying elements was researched both as microsegregation into dendrite cell and as macrosegregation along casting. The castings of alloy samples (diameter 20 mm, length 100 mm) were manufactured by slow directional solidification (∼6 mm/h) at a high-temperature gradient (~150°С/cm) by the Bridgeman technique. The alloy research was performed by differential thermal analysis and scanning electron microscopy together with local X-ray spectral analysis. The crystal lattice constants of the γ and γ′ phases were determined by X-ray diffraction analysis at room temperature. Alloying elements such as rhenium and ruthenium are pushed aside into the solid phase; both enrich the dendrite core and initial part of castings. Rhenium and ruthenium also increase the solidus temperature for nickel-based superalloys. On the other hand, alloying elements such as aluminum and tantalum are pushed aside into liquid phase, enriching the interdendritic region and final parts of castings. It is shown that formation of over-alloying local areas in single-crystal castings is a result of microsegregation of alloying elements, mainly rhenium. The over-alloying local areas of single-crystal castings could be a potential reason for formation of TCP phases during heat treatment or long-time high-temperature tests. The γ/γ′-lattice misfit (mismatch of γ-phase and γ′-phase crystal lattices) is not changed in the alloy for directional solidification castings along the full length. This is explained by compensation of the rhenium and ruthenium decrease by the tantalum and aluminum increase along the full length of castings.

Long Time High Temperature Stress Rupture Strength Behavior and Structure Stability Study on Ni-Base Superalloy Nimonic 80A

Nimonic 80A Ni-base superalloy can be used for high temperature steam turbine components.There is a critical requirement for turbine blade application that the stress rupture strength for 105 hrs at relevant high temperatures should be higher than 100MPa.On the other hand, it must keep good structure stability and no harmful phase such as-phase and the TCP phase formation at long time high temperature exposure. Therefore, the long time stress rupture tests at temperatures of 600, 650, 680 and 700°C for different stresses have been carried out. The long time structure stability has been also studied in detail. The results show that Nimonic 80A can meet the critical requirement for USC steam turbine blades at 600~700°C. Any detrimental phases have not been found at the long time stress rupture tests. The morphology of grain boundary carbides also has no apparent change. Based on above mentioned results, Nimonic 80A is recommended to be used for USC steam turbine components in the temperature range of 600~650°C.

A duplex nanocrystalline coating for high-temperature applications on single-crystal superalloy

A new duplex nanocrystalline coating is designed for high temperature oxidation and hot corrosion protection. This coating combines the advantages of traditional NiCrAlY and nanocrystalline coatings, i.e., providing high resistance to oxidation and hot corrosion simultaneously, while avoids any disadvantages that the traditional coatings have suffered from, such as scale spallation, element interdiffusion (along with the formation of harmful TCP phases). It gives a good choice as the bond coating of a TBC system.

DFG Research Training Group 1229 “Stable and Metastable Multi‐Phase Systems for Elevated Service Temperatures”

In this Special Section, nine papers from the German Research Foundation (DFG) Research Training Group 1229 “Stable and Metastable Multi-Phase Systems for Elevated Service Temperatures” have been collected. High-temperature materials are of extremely high technical and economic interest. They play a major role in engines for transportation and energy conversion. The materials and coatings used at elevated temperatures typically are nanoscaled multiphase or composite materials with complex microstructures. The main research idea of this graduate school is to combine research activities on different classes of materials like metals, ceramics, and metal–ceramic composites for applications at high temperatures and to derive common aspects and synergies between these typically separated fields. Correspondingly, the research program of the graduate school is divided into four fields: ceramics and composites, metallic alloys, oxidation and coatings, and new methods and simulations. In total, 14 projects were funded in two rounds over a total time period of 10 years with a total financial sum of close to 10 Mio € by the DFG. Etched γ′ microstructure of a Co-based superalloy (upper left) and 2D plane weave carbon fiber reinforced ceramic matrix composite (lower left). The research training group is a joint initiative between the Universities in Bayreuth and Erlangen, which are situated within close proximity in northern Bavaria. Both universities have a high expertise in materials science and engineering, and this research program has led to a strong intensification of cooperative research activities between both universities. Also, the interaction between students has been intensified by a joint educational program. “GradKo-Bus” with members of the research training group. The general topic of multi-phase high-temperature materials also allowed creating a very attractive educational program. Different material classes, processing techniques, analytical methods, and theoretical models have been taught to the graduate students. A number of world-wide contacts allowed the incorporation of numerous well accepted international scientists within the graduate program. Many PhD students spend longer time periods abroad during a foreign research stay of up to 6 months during their PhD work at cooperating Universities like UC Berkeley, John Hopkins University, University of Cambridge, Ohio State University, University of the Witwatersrand, and many others. This was an extremely and very important element in the graduate program and helped to intensify international cooperation. A very prominent aspect of the educational program has also been the yearly summer schools, which were held at different places in the nice Franconian surrounding. The summer schools were supported by the members of the industrial advisory board, which allowed strengthening the background with respect to industrial application. During a last summer school in Heiligenstadt, we will celebrate finalizing our joint research program in July 2015. The research results have been published over the years in more than 200 publications in international journals and conference proceedings. The nine papers of this AEM volume summarize some recent results of our research training group from different fields. In this volume, several papers cover new aspects in the field of superalloy development including oxidation, TCP phase formation, phase field modeling, and other topics. Two other papers describe new developments in the field of composite materials. Members of the Research training group with external guests from the University of Cambridge during the summer school 2011 at the Castle in Pommersfelden. Mathias Göken (left) and Uwe Glatzel joint chairmen of the research training group 1229. Mathias Göken Friedrich-Alexander-University Erlangen-Nürnberg Uwe Glatzel University Bayreuth

The structure and mechanical properties of single-crystal nickel alloys with Re and Ru after high-temperature holds

The special characteristics of structure and phase transformations in nickel based superalloys after long high-temperature holds are discussed. The formation of TCP-phases with different morphology is known to be the main feature of the investigated alloys. The residual short-term mechanical properties study after holds are determined. It is shown that the destruction of the specimen with different morphology TCP-phases occurs due to the formation and development of cracks along the γ′/TCP-interface. Nanophase hardening of single-crystal nickel alloys is considered.

On the Thermodynamics and Kinetics of TCP Phase Precipitation in Re‐ and Ru‐ Containing Ni‐base Superalloys

AbstractThree experimental Ni‐base superalloys of the second, third and fourth generation are studied in this work to clarify the effect of Re and Ru on the precipitation of TCP phases. Thermodynamic calculations using the CALPHAD method are performed based on the local chemical composition obtained with the use of electron probe microanalysis. The effects of microsegregation and γ′‐precipitation are taken into account. The results, complemented with scanning electron microscopy observations, show that Re increases, while Ru decreases the inclination of the material to precipitate TCP phases. The effect of both elements is assigned to thermodynamic rather than kinetic reasons. The CALPHAD‐calculations do predict the effect of Re on the TCP phase formation in the investigated alloys correctly, but not the effect of Ru.

Effect of carbon content on the microstructure and creep properties of a 3rd generation single crystal nickel-base superalloy

Effect of carbon content on the microstructure and creep properties of a 3rd generation single crystal nickel-base superalloy has been investigated by the scanning electron microscope, X-ray computed tomography and electron probe microanalyzer. With the increase of the carbon content, MC carbides evolve from octahedral to well-developed dendrite, which promotes the formation of microporosity. Moreover, the volume fraction of porosity increases in the experimental alloys after solution heat treatment. As a result, the increase in the size of MC carbides and the porosity has a detrimental effect on the low temperature and high stress creep behavior of the alloys. The specimen crept at 850°C and 586MPa with the carbon content of 430ppm shows the shortest rupture life due to the largest primary creep strain. However, the creep behavior of the alloy at 1120°C and 140MPa gets better as the carbon content increases from 88 to 430ppm. TCP phase is observed near the fracture surfaces of the alloys, which explores as a potential cause for the creep rupture. However, the formation of TCP phase is effectively suppressed for decreasing segregation of the alloying elements, which results in the improvement of the creep life in the alloy with 430ppm carbon at 1120°C and 140MPa.

INFLUENCE OF Pt ON THE CREEP RUPTURE PROPERTIES OF Ni-BASED SINGLE CRYSTAL SUPERALLOY

The Ni- based single crystal superalloys are considered to be the major materials for advanced areo- engine blades. In order to improve the high temperature properties of Ni- based single crystal superalloys,many refractory elements are introduced into this kind of alloys. Recently Pt has been suggested to be the alloying elements of advanced Ni-based single crystal superalloys. However, there are no researches for the effects of Pt on creep rupture properties of advanced single crystal superalloys. In this work, the influence of Pt element on the creep rupture properties of a Re-containing single crystal superalloy was investigated. The high-temperature creep rupture properties of the Pt-containing Ni-based single crystal superalloy at 1100 ℃, 180 MPa and 1000 ℃, 310 MPa were investigated. The deformation microstructure and the morphology of dislocations were studied by SEM and TEM.The results show that the creep rupture life of Pt-containing superalloy decrease slightly at 1100 ℃, 180 MPa and decreased obviously at 1000 ℃, 310 MPa. The fracture models of different alloys are all ductile fracture, and many irregular microviods and microcracks can be observed in the fracture surfaces. After high temperature creep deformation, regular dislocation networks formed at the g/g' interfaces. The differences of creep rupture properties among those alloys are that Pt element may promote the formation of TCP phase, and the interface between the TCP phase and g matrix may be favorite sites of the initiation of microvoids and microcracks.