Topologically Close-packed Research Articles

The influence of post treatments on the microstructure and corrosion behavior of thermally sprayed NiCrMoAl alloy coating

The relationship between microstructure and corrosion behavior of arc-sprayed NiCrMoAl alloy coating after different post treatments has been studied. Two different post treatments were applied to improve the properties: heat treatment at 1100 °C for 60 min and flame remelting. The microstructural characteristics of the as-sprayed and post-treated coatings were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) and transmission electron microscopy (TEM). The corrosion behaviors of all the coatings were also studied. The results reveal significantly different microstructure, including morphology, distribution and percentage of the phases, in the different coatings. For both post-treated coatings, intermetallic compounds (MoSi2, MoNiSi, and topologically close-packed (TCP) phases) around γ-Ni phase were observed. Moreover, the microstructure of post-treated coatings contained more oxide phases at intersplat boundaries, leading to a decrease in the porosity after post treatments. Due to the microstructural change after post treatments, the corrosion rate was decreased. A decrease of open pores along with an increase of oxide phases at intersplat boundaries, significantly contributed to enhancing the corrosion resistance of the thermally sprayed NiCrMoAl alloy coating.

Microstructural evolution and creep mechanisms in Ni-based single crystal superalloys: A review

Nickel-based single crystal superalloys (Ni-SXs) are of great interests in aircraft industry due to their excellent high temperature creep strength deriving from the rigid gamma/gamma prime (γ/γ′) microstructure and the single crystal nature. The creep properties of Ni-SXs depend strongly on the degradation rate of the original rigid γ/γ′ microstructure with the dislocation movements in the microstructure. Since the applied temperature and stress strongly affect the dislocation movements, the different creep mechanisms are displayed to Ni-SXs during low temperature (∼750 °C), mid-temperature (∼950 °C) and high temperature (∼1100 °C) creep. The creep deformation during low temperature creep is mainly caused by the shearing of γ′ phase by a limited number of dislocations, while high temperature creep mainly occurs to the accumulation of creep strains with the increased dislocation activities in γ phase. To enhance the creep properties of alloy, a variety of strategies of the microstructural optimization need to be considered. One is to increase the volume fraction of γ′ phase to enhance the precipitation strengthening effects. But excessive amounts of γ′ phase are disadvantageous to low temperature strength of alloy. The coarsening of γ′ phase and the rafting process strongly correlates to dislocation movements. It is of importance to discuss the specific mechanisms of coarsening and rafting as well as their influence to the creep properties of alloy. The lattice misfit between the γ and γ′ phase plays an important role in influencing the coarsening of γ′ phase and the creep properties of alloy. A proper lattice misfit helps to form the cubic γ′ phase which is advantageous to alloy. But a too large lattice misfit can promote the coarsening of γ′ phase that damages the structural stability of alloy at high temperatures. The increased additions of refractory elements in alloy promote the formation of topologically close-packed (TCP) phase. TCP phase facilitates the nucleation and propagation of micro-cracks around it that damages the creep properties of alloy. This review aims to summarize some aspects of microstructural evolution during creep of alloy with considering their effects to creep properties. To guide the design of Ni-SXs in future, some perspectives about the microstructural optimization are properly provided.

Effect of ruthenium on microstructure and high-temperature creep properties of fourth generation Ni-based single-crystal superalloys

The effects of Ru on microstructure and creep properties of the two alloys were investigated in detail. According to the creep curves of the two alloys at 1140 °C/137 MPa, the creep rupture life was significantly improved with the increase of Ru and the mechanism of each stage during the creep deformation was different. Thus, the evolution of the γ' phase, the dislocation configuration, and the effect of Ru on solution strengthening were discussed. The γ/γ' lattice misfit of the initial microstructure presented increasingly negative as the content of Ru increased, which resulted in smaller and more regular γ' particles in the initial state. Meanwhile, more consistent and denser dislocation networks on the γ/γ' interface during creep deformation were examined. Hence, the addition of Ru decreased the minimum creep rate and prolonged the secondary creep stage. Moreover, the so-called “reverse partitioning” behavior and enhancement of γ matrix strength appeared with the increase of Ru. When the alloy contained 2.5 wt% Ru, the rapidly increasing in creep strain rate induced by the topological inversion appeared. When the alloy included 3.5 wt% Ru, the needle-like and rod-like topologically close-packed (TCP) phases were observed occasionally. The stress and the supersaturation of refractory elements resulted in the precipitation of the TCP phase. The dramatic increase of the creep strain rate was principally related to the propagation of micro-cracks around the casting and creep voids in the necked regions.

Effect of composition on the microstructure of Mg(100−x)Cux alloys during rapid solidification

A molecular dynamics simulation was performed to investigate the effect of content on the atomic structure of Mg(100−x)Cux (x = 10, 20, … 90) alloys during solidification at a cooling rate of 1 × 1010 K s−1. The Mg50Cu50 alloy was split into two composition ranges by various analytic methods. When the molar fraction of Cu (CCu) was less than 50%, the number of S555 bond-type and topologically close-packed (TCP) structures were greater than those in the other composition ranges and clearly changed. The formation of ordered local structure would decrease the configurational entropies. This work provides insights into glass formation in alloys from a cluster-level structural perspective and is expected to shed light on the development of additional metallic glasses.

First-principles investigations of effects of solute elements on stability and electronic structure of laves phase/matrix interface in Ni-based superalloys

It is acknowledged that solute elements like Ru exert great influence on precipitation of deleterious topologically close-packed (TCP) phases. In this work, by means of first principle calculation based on density functional theory (DFT), the interface structure between C14-laves phase and matrix γ phase has been established for the first time, and stable interfacial termination type was achieved. Effects of solute atoms like Ru on interface stability, electronic structure, and bond characteristics were further calculated. It has been proved that, unlike other elements, Ru can notably elevate the interfacial energy of C14-laves/γ system, lower interface stability, and increase the difficulty of nucleation of C14-laves phase; on the contrary, W will decrease the interfacial energy of C14-laves/γ and make it easier for C14-laves phase to nucleate; Re, Mo, and Cr will also increase the interfacial energy, but nowhere near the effect of Ru. Besides, in comparison with other solute elements, C14-laves/γ substituted by Ru has the smallest degree of interfacial charge accumulation, lowest interfacial atomic bonding strength, and poorest interfacial stability. All the reasons discussed above lead to the blocking effect of Ru on the nucleation of C14-laves phase.

A NEW WELDING MATERIAL FOR REGENERATION IN THE WELDING TECHNOLOGY BASED ON NICKEL. ANALYSIS OF THE PROCESSABILITY OF APPLICATION OF THE EXPERIMENTAL FILLER IN THE REGENERATION OF SUPERALLOY CASTINGS VIA TIG WELDING AND OTHER TECHNOLOGIES

This work involves an analysis of high-chromium high-temperature deformable wieldable nickel alloys for use in GTE repair assemblies. It is shown that the alloys EP868 (VZh98) and Haynes 230 can be used in welded assemblies with an operating temperature of 800-1100 °C. The alloys Nimonic 81, Nimonic 91, IN 935, IN 939, and Nicrotan 2100 GT also have a high potential for use in welded assemblies. They are characterized by a combination of good weldability, high-temperature strength, and resistance to scaling. There have been conducted studies on high-temperature salt corrosion of model nickel alloys. They allowed establishing the patterns of the impact of base metal alloying with chromium, aluminum, titanium, cobalt, tungsten, molybdenum, niobium, tantalum and rare earth metals on the critical temperature of the start of salt corrosion Tcor and the alloy mass loss. It has been established that alloys with a moderate concentration (13-16%) of chromium can possess satisfactory hightemperature corrosion resistance (HTC resistance) under the operating conditions of ship GTE. The HTC resistance of CrAl-Ti alloys improves upon reaching the ratio Ti/Al ˃ 1. Meanwhile, the ratio Ti/Al ˂ 1 promotes the formation of corrosion products with low protective properties. The positive effect of tantalum on the HTC resistance of alloys is manifested at higher test temperatures than that of titanium, and the total content of molybdenum and tungsten in alloys is limited by the condition 8Mo2 – 2W2 = 89. The presence of refractory elements stabilizes the strengthening phase and prevents formation of the ɳ-phase. However, their excess promotes formation of the embrittling topologically close packed (TCP) phases and boundary carbides of an unfavorable morphology. Based on the studies of the HTC resistance, there has been identified a class of model high-temperature corrosionresistant nickel alloys with a moderate or high chromium content (30%), Ti/Al ˃ 1, and a balanced content of refractory and rare-earth elements.

MD study on topologically close-packed and configuration entropy of Mg40Al60 metallic glasses under rapid solidification

Herein, the effects of cooling rate during rapid solidification of Mg40Al60 metallic glasses are studied using the molecular dynamics (MD) simulation. The system energy, pair distribution function (PDF), local structures, configuration entropy, and three-dimensional visualization are used to systematically analyze the evolution of microstructure with temperature. Results indicate that the lower the cooling rate, the lower the average atomic energy. With the decrease of cooling rate, increases the height difference between two splitting subpeaks of the second peak on PDF curves; while decreases the onset temperature of glass transition. The effect of the cooling rate on the topologically close-packed (TCP) clusters is similar to the local five-fold structures of S555: the lower the cooling rate, the more the amount of TCP clusters, and the lower the configuration entropy. In addition, most TCP clusters are centered the smaller Al atoms.

Phosphorous behavior and its effect on secondary phase formation in high refractory content powder-processed Ni-based superalloys

The behavior of phosphorous and its effect on the microstructure of a powder-processed, high refractory content, Ni-based superalloy was studied and compared to a P-free variant. Transmission electron microscopy and atom probe tomography were used to identify the crystal structure of the five topologically close packed (TCP) and geometrically close packed (GCP) phases that formed, assess phase compositions, and determine any elemental segregation to phase boundaries. The results revealed strong phosphorous segregation to grain boundaries, moderate solubility in TCP and MC phases, limited solubility in the γ matrix and no solubility in the γ′ phase or any other Ni3X type GCP phases. The phosphorus segregation to the grain boundaries of the P-doped alloy lowered the incipient melting temperature, thereby eliminating the processing window, and led to the formation of (Ni,Co,Cr,P)2(Nb,Mo) laves phases along grain boundaries during supersolvus heat treatment. Interestingly, APT revealed segregation of P to Al2O3 oxide surfaces, explaining the frequent observation of laves precipitates nucleating on such impurities. Due to the low stability of the C14–Ni2Nb laves phase, it was prone to phase separation, and served as the nucleation point for μ, intergrown η/δ, and Nb3P phosphide phases.

Abnormal increase of TCP phase during heat treatment in a Ni-based single crystal superalloy

Generally, elevated solution heat treatment temperature decreases the residual dendritic segregation of refractory elements (i.e. Re and W) and suppresses the precipitation of deleterious topologically close-packed (TCP) phases in Ni-based superalloys. However, in the present study, elevated solution temperature resulted in an abnormal increase of TCP phases in a Ni-based single crystal superalloy after long-term thermal exposure at 900 °C. Microstructural analysis suggested that higher supersaturation in γ phase which promoted the formation of TCP phases was induced by the increased γ′ precipitation after heat treated at elevated solution temperature.

First-principles theoretical and experimental studies of effects of ruthenium on precipitation behavior of μ phase and μ/matrix interface stability in Ni-based single crystal superalloys

Precipitation of deleterious topologically close-packed (TCP) phases worsens the mechanical behavior of Ni-based single crystal superalloys. It is acknowledged that Ru will inhibit precipitation of TCP phase. In this paper, precipitation behaviors of TCP in alloy with and without Ru have been investigated through experimental characterization means, combined with the first principles calculations. For the first time, the interface structure between precipitated μ phase and matrix γ phase has been established, and stable interfacial termination types in different orientation relationships were obtained. Effects of solute atoms like Ru on interface stability and site substitution behaviors of different solute elements in two phases were further calculated, and compared with precipitation behavior and composition information of TCP phases in two alloys. It is verified that Ru is different with other elements, which can remarkably increase the interfacial energy of μ/γ interface, decrease interface stability, and rise the difficulty of nucleation of μ phase; conversely, W will lower the interfacial energy of μ/γ, making it easier for μ phase to nucleate; the effects of Co and Cr are not that obvious; the effects of Re and Mo on interfacial energy and nucleation of μ phase vary with orientation relationships. Also, Re, Cr, and Ru have larger trends to diffuse from matrix γ to μ phase during growing process of μ phase. The above reasons give rise to blocking effect of Ru on the nucleation of TCP phase, but Ru has trifling influence on inhibition of precipitation growth.

A NEW WELDING MATERIAL FOR REGENERATION IN THE WELDING TECHNOLOGY BASED ON NICKEL. DEVELOPMENT OF A NEW HIGH-TEMPERATURE AND CORROSION-RESISTANT WELDING MATERIAL FOR THE REGENERATION OF DEFECTS IN HNA CASTINGS. ANALYSIS OF EXISTING FUELLING STATIONS TYPES AND VEHICLES USING HYDROGEN, HYDROGEN PRODUCTION AND STORING METHODS

This work involves an analysis of high-chromium high-temperature deformable wieldable nickel alloys for use in GTE repair assemblies. It is shown that the alloys EP868 (VZh98) and Haynes 230 can be used in welded assemblies with an operating temperature of 800-1100 °C. The alloys Nimonic 81, Nimonic 91, IN 935, IN 939, and Nicrotan 2100 GT also have a high potential for use in welded assemblies. They are characterized by a combination of good weldability, high-temperature strength, and resistance to scaling. There have been conducted studies on high-temperature salt corrosion of model nickel alloys. They allowed establishing the patterns of the impact of base metal alloying with chromium, aluminum, titanium, cobalt, tungsten, molybdenum, niobium, tantalum and rare earth metals on the critical temperature of the start of salt corrosion Tcor and the alloy mass loss. It has been established that alloys with a moderate concentration (13-16%) of chromium can possess satisfactory hightemperature corrosion resistance (HTC resistance) under the operating conditions of ship GTE. The HTC resistance of CrAl-Ti alloys improves upon reaching the ratio Ti/Al ˃ 1. Meanwhile, the ratio Ti/Al ˂ 1 promotes the formation of corrosion products with low protective properties. The positive effect of tantalum on the HTC resistance of alloys is manifested at higher test temperatures than that of titanium, and the total content of molybdenum and tungsten in alloys is limited by the condition 8Mo2 – 2W2 = 89. The presence of refractory elements stabilizes the strengthening phase and prevents formation of the ɳ-phase. However, their excess promotes formation of the embrittling topologically close packed (TCP) phases and boundary carbides of an unfavorable morphology. Based on the studies of the HTC resistance, there has been identified a class of model high-temperature corrosionresistant nickel alloys with a moderate or high chromium content (30%), Ti/Al ˃ 1, and a balanced content of refractory and rare-earth elements.

Microstructural evolution and compression property of a novel γʹ-strengthened directionally solidified CoNi-base superalloy

The microstructural evolution and compression property of a novel γʹ-strengthened CoNi-base superalloy have been investigated. The directionally solidified CoNi-base superalloy possesses a lower density of 9.03 g/cm3 and a higher γʹ solvus temperature of 1166 °C, compared with those of the Co–Al–W-base superalloys with high level of W element. After aging at 900–1100 °C, the alloy displays stable γ/γʹ two-phase microstructure with high volume fraction of the γʹ phase, and without detrimental topologically close-packed (TCP) phase. The CoNi-base superalloy shows good compression property. In the process of compression, the alloy exhibits positive temperature dependence of the 0.2% flow stress above 600 °C. With increasing temperature, the alloy displays a maximum of 0.2% flow stress of 607 MPa at 850 °C. The major deformation mechanisms of the CoNi-base alloy are pairs of a/2<110>-type dislocations shearing γʹ precipitates from room temperature to 850 °C and unpaired dislocations bypassing the γʹ precipitates above the peak temperature of 850 °C. In this alloy, high content of Ni improves the γʹ solvus temperature and microstructural stability, which leads to a good compression property at high temperatures.

Abnormal Increase of TCP Phase During Heat Treatment in a Ni-Based Single Crystal Superalloy

Generally, elevated solution heat treatment temperature decreases the residual segregation of refractory elements (i.e. Re and W) and suppresses the precipitation of deleterious topologically close-packed (TCP) phases in Ni-based superalloys. However, in the present study, elevated solution temperature resulted in an abnormal increase of TCP phases in a Ni-based single crystal superalloy after long-term thermal exposure at 900 oC. Microstructural analysis suggested that higher supersaturation in γ phase which promoted the formation of TCP phases was induced by the increased γ' precipitation after heat treated at elevated solution temperature.

A NEW WELDING MATERIAL FOR REGENERATION IN THE WELDING TECHNOLOGY BASED ON NICKEL. PRINCIPLES OF DOPING OF HIGH-TEMPERATURE NICKEL ALLOYS RESISTANT TO HIGH-TEMPERATURE CORROSION

This work involves an analysis of high-chromium high-temperature deformable wieldable nickel alloys for use in GTE repair assemblies. It is shown that the alloys EP868 (VZh98) and Haynes 230 can be used in welded assemblies with an operating temperature of 800-1100 °C. The alloys Nimonic 81, Nimonic 91, IN 935, IN 939, and Nicrotan 2100 GT also have a high potential for use in welded assemblies. They are characterized by a combination of good weldability, high-temperature strength, and resistance to scaling. There have been conducted studies on high-temperature salt corrosion of model nickel alloys. They allowed establishing the patterns of the impact of base metal alloying with chromium, aluminum, titanium, cobalt, tungsten, molybdenum, niobium, tantalum and rare earth metals on the critical temperature of the start of salt corrosion Tcor and the alloy mass loss. It has been established that alloys with a moderate concentration (13-16%) of chromium can possess satisfactory hightemperature corrosion resistance (HTC resistance) under the operating conditions of ship GTE. The HTC resistance of CrAl-Ti alloys improves upon reaching the ratio Ti/Al ˃ 1. Meanwhile, the ratio Ti/Al ˂ 1 promotes the formation of corrosion products with low protective properties. The positive effect of tantalum on the HTC resistance of alloys is manifested at higher test temperatures than that of titanium, and the total content of molybdenum and tungsten in alloys is limited by the condition 8Mo2 – 2W2 = 89. The presence of refractory elements stabilizes the strengthening phase and prevents formation of the ɳ-phase. However, their excess promotes formation of the embrittling topologically close packed (TCP) phases and boundary carbides of an unfavorable morphology. Based on the studies of the HTC resistance, there has been identified a class of model high-temperature corrosionresistant nickel alloys with a moderate or high chromium content (30%), Ti/Al ˃ 1, and a balanced content of refractory and rare-earth elements.

The short-range order in liquid and A15 crystal of zirconium

Molecular dynamic simulations have been carried out to investigate the crystallization mechanism during rapid cooling and isothermal relaxation for Zirconium (Zr) under a pressure of 100 GPa. The properties of the simulated system are analysed in terms of the system energy, the pair distribution function (PDF), and the largest standard cluster analysis. It is found that the so called topologically close-packed (TCP) local structures dominate in number in liquid, supercooled liquid and the final solid. Both solidification and relaxation crystallize into an A15 phase rather than the simple bcc or hcp crystals, the metastable and stable crystal of Zr under the ambient condition. Interestingly on PDF curves for liquids and super-liquids, left to the first major peak (at around r = 3.0 Å) there is a mini-peak (at around r = 1.87 Å) that decreases with the decrease of temperature and vanishes when crystallization ends. Further analysis reveals that the mini-peak originates from dynamical compact atomic pairs that are however almost nothing to do with TCP local structures. In addition, the multi characteristic lengths of the A15 unit (composed of Z12 and Z14 local structures with ratio of 1:3) result in the distinct split of the first and second peaks on the PDF curves.

Stress-induced formation of TCP phases during high temperature low cycle fatigue loading of the single-crystal Ni-base superalloy ERBO/1

The microstructural evolution in the single crystal Ni-base superalloy ERBO/1 (CMSX 4 type) is investigated after load controlled low cycle fatigue (LCF) at 950 °C (load-ratio: 0.6, tensile stress range: 420–740 MPa, test frequency: 0.25 Hz, fatigue rupture life: about 1000 - 3000 cycles). Bulk topologically close packed (TCP) phase particles precipitated and were analyzed by three-dimensional focus ion beam slice and view imaging and analytical transmission electron microscopy. The particles did not precipitate homogenously but at locations with enhanced levels of local stresses/strains, such as isolated γ-channels subjected to cross channel stresses, shear bands and in front of micro cracks. The influence of stress/strain is furthermore apparent in the spatial arrangement and the shape of the TCP phase particles. Only μ-phase TCP particles were found by electron diffraction. Results of a structure-map analysis suggest that most of these TCP particles observed after LCF testing would not precipitate in thermodynamic equilibrium. In order to rationalize this effect, the atomic volume was analyzed that transition-metal (TM) elements take in unary fcc and in unary μ-phase crystal structures and found that all TM elements except Zr and V take a larger volume in a unary μ phase than in a unary fcc phase. This trend is in line with the observed localized precipitation of TCP phases that are rich in Ni and other late TM elements. The experimental and theoretical findings suggest consistently that formation of TCP particles in LCF tests is considerably influenced by the local tensile stress/strain states.

A NEW WELDING MATERIAL FOR REGENERATION IN THE WELDING TECHNOLOGY BASED ON NICKEL. ANALYSIS OF THE COMPOSITION AND PROPERTIES OF DEFORMABLE HEAT-RESISTANT HIGH-CHROMIUM NICKEL-BASED ALLOYS FOR WELDED PARTS. ANALYSIS OF EXISTING FUELLING STATIONS TYPES AND VEHICLES USING HYDROGEN, HYDROGEN PRODUCTION AND STORING METHODS

This work involves an analysis of high-chromium high-temperature deformable wieldable nickel alloys for use in GTE repair assemblies. It is shown that the alloys EP868 (VZh98) and Haynes 230 can be used in welded assemblies with an operating temperature of 800-1100 °C. The alloys Nimonic 81, Nimonic 91, IN 935, IN 939, and Nicrotan 2100 GT also have a high potential for use in welded assemblies. They are characterized by a combination of good weldability, high-temperature strength, and resistance to scaling. There have been conducted studies on high-temperature salt corrosion of model nickel alloys. They allowed establishing the patterns of the impact of base metal alloying with chromium, aluminum, titanium, cobalt, tungsten, molybdenum, niobium, tantalum and rare earth metals on the critical temperature of the start of salt corrosion Tcor and the alloy mass loss. It has been established that alloys with a moderate concentration (13-16%) of chromium can possess satisfactory hightemperature corrosion resistance (HTC resistance) under the operating conditions of ship GTE. The HTC resistance of CrAl-Ti alloys improves upon reaching the ratio Ti/Al ˃ 1. Meanwhile, the ratio Ti/Al ˂ 1 promotes the formation of corrosion products with low protective properties. The positive effect of tantalum on the HTC resistance of alloys is manifested at higher test temperatures than that of titanium, and the total content of molybdenum and tungsten in alloys is limited by the condition 8Mo2 – 2W2 = 89. The presence of refractory elements stabilizes the strengthening phase and prevents formation of the ɳ-phase. However, their excess promotes formation of the embrittling topologically close packed (TCP) phases and boundary carbides of an unfavorable morphology. Based on the studies of the HTC resistance, there has been identified a class of model high-temperature corrosionresistant nickel alloys with a moderate or high chromium content (30%), Ti/Al ˃ 1, and a balanced content of refractory and rare-earth elements.

Formation of topologically closed packed phases within CMSX-4 single crystals produced by additive manufacturing

The homogeneity of single crystals produced by additive manufacturing (AM) is much higher than that of single crystals produced by the standard investment casting process. Aim of this paper is to investigate the influence of this homogeneity on the formation kinetics of topologically closed packed (TCP) phases. CMSX-4 single crystals produced by selective electron beam melting and investment casting are homogenized and annealed at 870 °C and 1050 °C for up to 480 h. The formation kinetics of TCP phase formation is analyzed by electron microscopy.

Determination of solubility limits of refractory elements in TCP phases of the Ni-Mo-Cr ternary system using diffusion multiples

The Ni-Mo-Cr system is critical for the design of nickel-based superalloys. This system stabilizes different topologically close packed (TCP) phases which precipitate in many of the commercially used superalloys. A diffusion multiple approach was applied to map the phase diagram of the ternary system in a highly efficient manner. Two isothermal sections of the Ni-Mo-Cr system at 1100°C and 1250°C were constructed from the results obtained from diffusion multiples using scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and electron backscatter diffraction (EBSD). At both temperatures two TCP phases were observed, namely the tetragonal σ- and the orthorhombic P-phase. In the range of this study the σ-phase was found to be widely stable towards the Cr-poor side, whereas the P-phase was detected on the Ni-rich side. The phases were characterized mechanically by microhardness measurements. Determined values range from 8,5 to 9,2 kN/mm2 for σ- and P-phase. By analysing interdiffusion zones of quaternary systems the solubility limits of Re, Ru and W were determined and compared for both TCP phases in order to assess their stabilising impact. Maximum solubilities of up to 12,7 at.% were detected, whereas the P-phase exhibits higher solubility limits than the σ-phase. In both phases solubility limits decrease with increasing temperature. In contradiction to previous experience in nickel-based superalloys, the observed stabilising effect on TCPs in quaternary systems is stronger for Ru than for Re.

Effect of post-weld heat treatment on the microstructure and tensile properties of electron-beam-welded 21st century nickel-based super alloy 686

This work examines the influence of post-weld heat treatment (PWHT) on the fusion zone microstructure, and mechanical properties of electron beam (EB)-welded alloy 686. Comparative studies have been made on weld microstructure and tensile properties of the weldments both in as-welded and post-welded heat-treated conditions. PWHT consists of direct aging (DA/480°C for 3 h) and solution treatment (ST, 980°C for 1 h) followed by ageing and finally, homogenizing treatment (HT, 1200°C for 1 h) followed by DA and ST. The secondary topologically closed packed (TCP) phases formation, their distribution and microsegregation characteristics are studied with the aid of scanning electron microscope (SEM) analysis. Energy-dispersive X-ray spectroscopy (EDS) is also performed to estimate the microsegregation of alloying elements in the dendritic core and interdendritic regions of the weldments. The result shows that there is no significant change in the microstructure of DA and solution-treated sample as compared to the as-welded sample. The microstructure of HT sample was entirely different from those of as-welded and other HT samples. The SEM/EDS analysis illustrates the presence of secondary TCP phases (σ, P and μ) in the as-weld, DA and ST condition, whereas HT weldments did not show the presence of TCP phases. Tensile test results show higher tensile strength in ST condition whereas homogenization samples show higher ductility compared with others.