TCP Phase Research Articles

Effect of Ru on γ/γ' microstructural evolutions and precipitation of TCP phases during thermal exposure at 1100 °C in a single crystal superalloy

The effect of 2 wt% Ru on γ/γ' microstructural evolutions and precipitation of TCP phases were investigated in two experimental alloys. The two alloys were subjected to unstressed thermal exposure at temperatures of 1100 °C for periods of 100, 500 and 1000 h. The microstructures of the samples before and after thermal exposures were characterized. Directional coarsening of γ' phases and the precipitation of TCP phases were observed in the both alloys after long-term exposure. The experimental results show that Ru has a very significant effect on the suppression of TCP phases by influences the partitioning of elements. Detailed analysis showed that Ru increases the lattice misfit strain, resulting in more dense and stable interfacial dislocation networks, which can promote the stability of γ/γ' microstructures by delaying topological inversion. Ledges and grooves at γ/γ' interfaces associated with the dislocations and strong segregations of Cr, Co and Re along the dislocation networks were both observed, under which a detailed explanation of the coarsening process during thermal exposure was given. • Ru increases the microstructural stability. • Ru increases the density and stability of dislocation networks. • Ru promotes the stability of γ/γ' microstructures by delaying topological inversion. • Ru reduces the nucleation of TCP precipitates.

The microstructure and properties of Fe55Cr15Ni(30-x)Nbx eutectic high-entropy alloys

The emergence of eutectic high entropy alloys (EHEAs) provides new insights for the design of the next generation structural alloys. In this paper, a set of low-cost Co-free Fe55Cr15Ni(30-x)Nbx (x = 0, 3, 6, 9, 12 and 15 at.%) EHEAs was successfully designed and prepared by vacuum arc melting. With the increase of Nb content, the solidification structure of the EHEAs change from single-phase FCC (x = 0) to divorced eutectic structure (x = 3) containing FCC and Laves phase, hypoeutectic structure (x = 6 and 9) and eventually becomes hypereutectic structure (x = 12 and 15). The FCC phase is rich in Fe, Cr and Ni, and the Laves phase is rich in Nb. The addition of Nb result in the increase of the volume fraction of the hard Laves phase which improved the room temperature strength. Solid solution strengthening and second phase strengthening are the main factors affecting the properties of the alloys. In this study, Fe55Cr15Ni21Nb9 alloy shows the best comprehensive mechanical properties, the compressive fracture strength, fracture strain and yield strength are 1216 MPa, 32% and 749 MPa, respectively. Phenomenologically, when the Nb content is in the range of 0–15 at.%, the yield strength of the alloy is a function of the Nb content: σ0.2=550×ex/15.81−256. For (Co)CrFeNi-based high-entropy alloys, a new phase prediction criterion is proposed in this paper. When the d-orbital energy level of the alloy is Md ≤ 0.92, the atomic radius difference is δ ≤ 2.88 or the mixing enthalpy meets −4.16≤ΔHmix≤0, single-phase FCC will be formed. If this is not the case, the FCC + TCP phase will be formed.

Effect of long-term aging on microstructure stability of a novel nickel base powder superalloy

The microstructure stability, evolution law and identification of carbide and harmful phase TCP of nickel-base powder superalloy aged at 800 ℃ for 100 h —10000 h were studied by means of field emission scanning electron microscope, atomic force microscope and electron backscatter diffraction. The results show that with the increase of aging time, the grain size changed little, the grain boundary coarsened and the twin boundary Σ3 increased first and then decreased. γ' phase coarsened obviously from 500 h aging, and the coarsening rate increased first and then decreased The coarsening mechanism is mainly the combination of adjacent γ' phase by AFM analysis. Lath-like σ phase CrCo precipitated at the grain boundary first after aging for 1000 h. When the aging time was up to 2000 h, the needle-like μ phase Co7(W, Mo)6 was observed in the crystal. With the further increase of aging time, the total content of TCP phase at grain boundary and in crystal increased obviously. However, at 10000 h, the TCP phase in the crystal and at the grain boundary almost disappeared, mainly because the atoms in the TCP phase diffused into the γ' phase to form a new solid solution and deposited on the grain boundary, which further coarsened the grain boundary.

Mutation in TCP phases and superior stress rupture life led by W/Mo ratio in Ni-based single crystal superalloys

Balancing the strengthening effect and microstructure instability led by refractory elements is crucial for superalloys. In this work, comparative study was conducted on two second-generation Ni-based single crystal superalloys which have similar components and total content of W + Mo except for different W/Mo ratios. It has been evidenced that μ phase precipitated in the high W/Mo ratio alloy after isothermal exposure for 1,500 h at 950 °C, whereas only sub-micron P phase particles formed in the low W/Mo ratio alloy even after 10,000 h of isothermal exposure. Striking increase in stress rupture life was achieved for the low W/Mo ratio alloy experienced 10,000 h of isothermal exposure. The mutation in TCP phases and enhanced stress rupture life were ascribed to the dendrite segregation of W and Re, and partitioning behavior of Cr and Re in γ and γ′ phases induced by W/Mo ratio.

Microstructure evolution and interdiffusion behaviors of (Ni,Pt)Al coating with and without Re-diffusion barrier on IC21 substrate at 1100°C

The degradation of high temperature oxidation resistance and mechanical properties caused by the high temperature interdiffusion between the coating and the single crystal intermetallic superalloy substrate has become a heavily researched topic within aeronautical materials research in recent years. In order to study the microstructural evolution and interdiffusion behaviors of the coating and superalloy, the (Ni, Pt)Al coating with and without Re-diffusion barrier (Re-DB) was prepared on the Ni3Al based superalloy (Grade: IC21). The thermal exposure tests were carried out on the specimens in air at 1100 °C for 200 h. The results revealed that the Al content in the coating gradually reduced during thermal exposure, and both coatings underwent phase transformation from β-(Ni, Pt)Al to Ni3Al. The coating with Re-DB had a lower phase transformation ratio. Moreover, the coating with Re-DB had smaller and fewer TCP phases precipitated in the inter-diffusion zone (IDZ) and secondary reaction zone (SRZ), showing that the Re-DB contributed to a lower extent of interdiffusion. Based on the actual measured composition-distance data, the interdiffusion coefficients between the (Ni, Pt)Al coating and the IC21 substrate with and without Re-DB were determined using HitDIC software, which verified that Re-DB impeded the diffusion of Re, Mo and Cr from substrate to the coating.

Influence of Cr/Mo ratio on microstructure and mechanical properties of the Ni-based superalloys fabricated by laser additive manufacturing

From the perspective of mechanical properties and weldability, four Ni-based superalloys with varying Cr/Mo ratio were designed using cluster model, and fabricated by laser additive manufacturing and subsequent solution treatment and aging. The influence of Cr/Mo ratio on microstructure, hardness, and tensile properties of the alloys in both as-deposited and post heat-treated states were investigated. The results show that the microstructure of all as-deposited alloys is featured by γ dendrites with fewer interdendritic phases. With the decrease of Cr/Mo ratio, the interdendritic phases evolve from lath and eutectic Laves phases to blocky P phases. The P phase can deteriorate the ductility, while the hardness and tensile strength monotonously increase due to enhanced solid solution strengthening. Post heat treatment results in dissolution of TCP phases and uniform precipitation of γ" phases, which enhances remarkably the hardness and tensile strength of as-deposited alloys, but slightly decreases the ductility. At nearly the same level of ductility (23.4–25.8%), the post heat-treated Ni56.3Cr15.2 Mo3.52Fe18.8 Al1.56Nb3.13Ti1.56 alloy with the Cr/Mo ratio of 13:3 shows the highest hardness (470 HV) and tensile strength (1063 MPa) among the studied alloys.

TEM and EBSD investigation of a Re-doped β-NiAl coating on a Ni-based single crystal superalloy

To figure out the microstructure and precipitation phases of a Re doped β-NiAl coating, the electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) were both used. Re-doped β-NiAl coating was composed of outer coarser β-NiAl grains with nanometer sized γ-Re precipitates, inner tiny β-NiAl grains with submicron sized α-W phases and bottom interdiffusion zone (IDZ, consisted of strip TCP and β-NiAl phases). Moreover, these γ-Re and α-W nanoparticle phases were not only aggregated at β-NiAl grain boundary but also appeared within the grains.

Stress-induced formation of µ-TCP phase during the early-stage interdiffusion process in the NiCrAlY/NiAlCoCrW model coating/superalloy system at ambient temperature

In this paper, two model NiCrAlY/NiAlCoCrW coating/superalloy diffusion couples, cp0 and cp15, were prepared and subjected to annealing at 1173 K for 40 h under no stress and a uniaxial compressive stress of 15 MPa, respectively. No noticeable microstructural evolution occurred in cp0, while apparent interdiffusion zones and especially a number of TCP phases developed across the interface in cp15. The TCP precipitates, majorly consisting of Ni, Cr, Co and W, were identified as μ phase. Moreover, μ-TCP phase precipitated as small granular particles along the interface perpendicular to the loading axis, while as large needle-like ones on both coating and superalloy sides parallel to the loading direction. The formation of μ-TCP phase with complex morphologies at such early stage of interdiffusion process due to the coupled effect of compressive stress and ambient temperature may seriously reduce the creep lifetime of the coating/superalloy system.

Dual effects of Ru on the microstructural stability of a single crystal superalloy

Generally, Ru can inhibit the precipitation of TCP phase in most single crystal superalloys. However, in this study, we found that the inhibition is related to the location in the exposed specimen relative to the specimen surface: Ru inhibits the precipitation of TCP phase inside the sample but promotes it in the near surface region. The promotion of the TCP phase was proven to be related to oxidation. Microstructural analysis suggested that Ru degrades the oxidation resistance by disturbing the barrier property of the oxide scale. More Ni atoms are consumed due to the poor oxidation, resulting in the concentration gradient toward the interior of the samples, which changes the driving force of TCP precipitation.

Co content dependence on the microstructure characteristic and creep performance at elevated temperature in Ru-containing single crystal superalloys

The composition design and optimization of Ru-containing fourth generation single crystal superalloys, which are currently considered as the most advanced and practical materials for manufacturing high-pressure turbine blades of aero-engines, have been in increasing and urgent demands. In this work, significant adjustments to the pivotal alloying element Co were made and the synthetic effect of Co on the microstructure as well as high-temperature creep property in Ru-containing alloys were systematically evaluated. The results illustrated that the size of γ′ phase was inversely proportional to the Co content but the volume fraction of γ′ precipitates stayed the same in different alloys. The TCP phases were more likely to precipitate in alloys with lower or extremely high Co addition during long-term aging, while the morphology of these particles transformed from long needle-like to short rod-like or blocky as Co content increased. It was noted that the element diffusion as well as the resultant coarsening and rafting of γ/γ′ structures were favorably facilitated by the increasing Co addition. The alloy with 12 wt% Co exhibited the longest stress rupture life at 1140 °C/137 MPa, regardless of the moderate precipitation of blocky TCP particles during creep. Moreover, the TCP phases in different alloys were analyzed to be typical μ phase but the specific element distributions varied with Co addition. Moreover, plenty of 2nd γ′ precipitates were observed after creep rupture, while their nucleation and precipitation could be affected by the Co addition in alloys. Ultimately, the comprehensive Co-effect was discussed and the optimum Co content was determined, in order to provide further guidance in the design and development of Ru-containing single crystal superalloys.

A scale-bridging study of the influence of TCP phases on the mechanical properties of an additive manufactured Ni-base superalloy combining microcompression testing, X-ray nanotomography and TEM

A scale-bridging study of the influence of TCP phases on the mechanical properties of an additive manufactured Ni-base superalloy combining microcompression testing, X-ray nanotomography and TEM - Volume 27 Issue S1

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.

3D printing of HAp/xSiO2 ceramic scaffolds for potential bone graft reconstruction

HAp has drawn great attention in recent years for its similarity to the inorganic component of bone and can be used as a biocompatible implant. However, the application is limited for its poor shape ability and low degradation ability. With the help of Digital Light Processing 3D printing technology, this study proposed a method for the fabrication of bone remodeling scaffolds with high complexity. In addition, this study found that the silica addition will help densify the HAp ceramic and make it transform to a more biodegradable TCP phase at low sintering temperature.

The effect of service temperature on the impact strength and fracture toughness of GTD-111 superalloy

The study of the impact resistance of gas turbine blades due to exposure to internal and external damage can prevent human and economic disasters. In this study, a GT 13-E2 General Electric gas turbine blade was used, which has been exposed to heating, hot gases, the material to a temperature of 850–900 °C in 10,000 operating hours. Microstructural examinations of the two sections of the airfoil and the root confirmed the microstructural damage of the airfoil due to more severe working conditions. Impact test was performed at temperature 25 and 900 °C for both airfoil and root sections. Impact test results showed that the fracture toughness for blade root in both temperatures of 25 and 900 °C was 4.7 and 6.2 J, respectively. This value was 2.4 and 3.3 for the airfoil blade, respectively. This was the formation of hard phases that were formed during service in airfoil due to unwanted aging. These phases included M23C6 carbide, γ′ nanometer particles and the TCP phase.

Stress-Induced Formation of μ-TCP Phase During the Early-Stage Interdiffusion Process in the NiCrAlY/NiAlCoCrW Model Coating/Superalloy System at Ambient Temperature

In this paper, two model NiCrAlY/NiAlCoCrW coating/superalloy diffusion couples, cp0 and cp15, were prepared and subjected to annealing at 1173 K for 40 h under no stress and a uniaxial compressive stress of 15 MPa, respectively. No microstructural evolution occurred in cp0, while apparent interdiffusion zones and especially a number of TCP phases developed across the interface in cp15. The TCP precipitates, majorly consisting of Ni, Cr, Co and W, were identified as μ phase. Moreover, μ-TCP phase precipitated as small granular particles along the interface perpendicular to the loading axis, while as large needle-like ones on both coating and superalloy sides parallel to the loading direction. The formation of μ-TCP phase with complex morphologies at such early stage of interdiffusion process due to the coupled effect of compressive stress and ambient temperature may seriously reduce the creep lifetime of the coatings/superalloy system.

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.

Effects of microstructure and lattice misfit on creep life of Ni-based single crystal superalloy during long-term thermal exposure

Abstract According to the microstructural evolution during longterm thermal exposure at 1100 °C, the creep rupture life of Ni-based single crystal superalloys at 980 °C/270 MPa was evaluated. The microstructure was characterized by means of scanning electron microscopy, X-ray diffraction and related image processing methods. The size of γ’ precipitates and the precipitation amount of topologically close-packed increased with the increase in thermal exposure time, and coarsening of the γ’ precipitates led to the simultaneous increase of the matrix channel width. The relationship between the creep rupture life and the lattice misfit of γ/γ’, the coarsening of γ’ precipitate and the precipitation of TCP phase are systematically discussed. In addition, according to the correlation between γ’ phase evolution and creep characteristics during thermal exposure, a physical model is established to predict the remaining creep life.

Solidification characteristics and as-cast microstructures of a Ru-containing nickel-based single crystal superalloy

The solidification characteristics and as-cast microstructures of a Ru-containing nickel-based single crystal superalloy were systematically investigated through thermal analysis, Thermo-Calc simulation, the planar interface solidification experiment, and the directional solidification quenching experiment. The main solidification transition temperature, segregation behavior, and solidification path were analyzed, and the microstructure evolution and phase formation mechanism were also discussed. The solidification began with the formation of primary γ dendrites (L → γ). Then Ni, Al, Ta, and Ru were enriched in the residual liquid, resulting in the precipitation of β-NiAl phase (L → β-NiAl). As the β-NiAl phase grew, the content of Ta gradually increased while the content of Al gradually decreased. Thus, the peritectic γ′ phases were precipitated on the surface of β-NiAl phase and coarsened by the incomplete peritectic reaction (L + β-NiAl → peritectic γ′ + β-NiAlResidual). The Al content further decreased and the Ta content further increased with the precipitation of the peritectic γ′ phase, leading to the formation of γ/γ′ eutectics on the surface of the γ dendrites or directly from liquid (L → γ/γ′ eutectic). Since the precipitation of β-NiAl phase and the subsequent incomplete peritectic reaction, Cr, Co, Mo, W, and Re were rejected into the residual liquid in the vicinity of β-NiAl phase, providing conditions for the nucleation of the TCP(R) phases (L → R). The wide freezing range of the alloy might be the cause of the severe micro-segregation and the precipitation of some secondary phases in the interdendritic regions.

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.

Microstructural evolution of a nickel-base single-crystal superalloy during high-temperature homogenisation

Microstructural evolution of a nickel-base single-crystal superalloy during homogenisation at ultra-high temperature has been investigated using scanning electron microscope and electron probe microanalyzer. The results show that incipient melting occurs and all γ/γ′ eutectics are re-melted into liquid after 1 h homogenisation at 1350°C. The volume fraction of IM decreases gradually with increasing homogenisation time. The IM disappears totally after 14 h homogenisation. Moreover, the formation of the TCP phase is suppressed during long-term thermal exposure at 1100°C for the reduction of element segregation, especially the segregation of refractory element Re was reduced during ultra-high-temperature homogenisation. The microstructure evolution of IM was discussed based on the element diffusion during homogenisation.