Search

• Correlation between ageing temperature and creep failure was clarified firstly. • γ′ size and γ/γ′ lattice misfit increased as primary ageing temperature raised. • As primary aging temperature was 1080 °C, creep life reached to the highest 180 h. • Increased elements diffusion rate reduced formation of vacancies and micro-voids. • Superislocations were impeded to cut into γ' as ageing temperature was 1080 °C. The heat treatment design and optimization of Re-free low-cost single crystal (SX) superalloys, which are commonly regarded as the most practical and economic alloys to manufacture guide vanes and high-pressure turbine blades of aero-engines, are in great demand. In this study, significant adjustments to the primary ageing temperature (PAT) were made, and the synthetic impact of PAT variations from 1050 °C to 1100 °C on the microstructure and creep behaviours in novel Re-free SX alloys was investigated in detail. The results showed that the γ′ size and γ/γ′ lattice misfit increased with increasing PAT, resulting in a reduction in the γ/γ′ interface area and a shift in the morphology of γ' from spherical to cubical. Notably, element diffusion and the resultant coarsening of γ/γ′ structures were facilitated by increasing the PAT. The alloy with 1080 °C PAT showed the longest creep life at 1100 °C/137 MPa, which was approximately 1.5 and 1.2 times that of alloys with PATs of 1050 °C and 1100 °C, respectively. Moreover, as PAT increased, the number of microvoids decreased due to increasing the diffusion rate of elements and hindering the formation of vacancies. When the PAT was too low or too high, the "inland shapes" of the γ′ phase and topological inversion phenomenon occurred during creep, which were detrimental to the creep properties. Furthermore, increasing the PAT resulted in the morphology of γ/γ′ interfacial dislocation networks shifting from sparse waves to fine hexagons and then to coarse hexagons. The dense interfacial dislocation networks in the sample with 1080 °C PAT remarkably impeded superdislocations cutting into γ′ phases. Ultimately, the thermodynamics and kinetics of γ′ formation and growth were also discussed, and the optimum PAT was ascertained to provide further guidance to design and develop Re-free low-cost SX superalloys. [ABSTRACT FROM AUTHOR]

An evaluation was conducted on the impact of a Pt–Al bond coat on the creep behaviours of a second-generation Ni-based single crystal superalloy at 750 °C/820 MPa, 850 °C/630 MPa, 1038 °C/137 MPa and 1100 °C/112 MPa, respectively. As revealed by the creep results, the creep behaviours of the coated superalloy were inferior to those of the bare superalloy to some extent under all testing conditions. The maximum deterioration in the strain-to-fracture and time-to-rupture was observed at 1100 °C/112 MPa (7.3%) and 750 °C/820 MPa (74.7 h), respectively. Following the creep test, deconvolution calculations were performed, which indicated that the creep stress for all core superalloys was as low as approximately 1.7–1.9% of the applied stress while the bond coat stress was 4.0–10.4% that at the minimum creep rate. Moreover, TEM analyses revealed that within the superalloy substrate adjacent to the bond coat, there were fewer stacking faults and an abundance of dislocation debris observed within γ′-Ni 3 Al at 750 °C/820 MPa. The misfit of the γ/γ′ phase was insignificant and the dislocation networks became irregular at 1100 °C/112 MPa. With an increase in temperature from 750 °C to 1100 °C, the fracture mechanism of both the substrate and bond-coat was found to shift from quasi-cleavage to micro-void coalescence. The quasi-cleavage cracks occurring within the bond coat increased the number of connections with creep cracks to cause spreading from the interior of the samples, thus resulting in premature failure at 750 °C/820 MPa and 850 °C/630 MPa. However, the degradation at 1038 °C/137 MPa and 1100 °C/112 MPa was ascribed to the increasing thickness of the inter-diffusion zone, as well as the volume of the blocky γ′-Ni 3 Al phase inside the bond coat and topologically close-packed phases in the superalloy near the bond coat. [ABSTRACT FROM AUTHOR]

Low-cost Ni-based single crystal superalloy with high performance attracted much attention to the researchers in recent years. A Ru-free low-cost fourth-generation single crystal superalloy was designed in this study, which was rarely reported previously. This study mainly focused on the effect of solid solution heat treatment duration on the microstructure evolution and high temperature and low stress creep properties on the experimental superalloy. By the metallographic method, it was revealed that the dendritic segregation was gradually eliminated with the increasing solid solution heat treatment duration. Segregation of Al and Ta were well homogenized as well as W. Segregation of Re could still be observed after 32 h solid solution heat treatment. Quantity and fraction of pores increased, which has a significant influence on the creep properties. Compositional variation between dendritic and inter-dendritic region was the main factor that caused the qualitative difference in the size and volume fraction of the γ' phase. Gap of the γ' size and volume fraction between dendritic cores and inter-dendritic region after solid solution heat treatment became minor. Homogeneous dislocation networks formed during the creep test and the ruptured dislocation networks of the specimen subjected to 32 h solid solution heat treatment were found. In the tertiary stage of the creep test, the primary creep deformation mechanism is the super-dislocations with Burgers vector of a 〈010〉 shearing the rafted γ' phases. The specimen subjected to 16 h solid solution heat treatment has the optimal creep performance, followed by those subjected to 32 h and 8 h, respectively. • Ru-free Ni-base fourth-generation single crystal superalloy was rarely reported. • Temperature enduring capability increased by 20 °C compared to last generation. • Dislocation configuration under each solid solution time was analyzed in detail. • Comprehensive influence of solid solution heat treatment time was discussed. [ABSTRACT FROM AUTHOR]

A balance between the mechanical performance and castability of Ni-based single crystal superalloys is expected to be achieved by doping with appropriate carbon content. In this study, the influences of carbon content on the tensile properties of a novel low-cost 3rd-generation Ni-based single crystal superalloy at 760 °C and 1120 °C were investigated. Results showed that minor carbon addition could slightly increase the strength and plasticity of the experimental alloy at 760 °C, whereas it led to the decrease of strength and plasticity at an elevated temperature of 1120 °C. This variability was discussed in terms of the microscopic resistance for dislocation movement and the macroscopic deformation capacity associated with damage accumulation. It was found that the solid solution strength and interface strength were enhanced with increasing carbon content, however, the strength at high temperature was compromised by the enormous consumption of refractory elements, even though the precipitation of blocky M 6 C carbides provided additional hindrance. As for the macroscopic deformation capacity, interdendritic carbides could hinder the extension and/or trigger the activation of slip bands at 760 °C. Nonetheless, the initiation tendency and propagation behavior of microcracks at 1120 °C were facilitated due to the strong stress concentration and severe deformation incompatibility of interdendritic carbides. Overall, future compositional optimization should focus on reducing the formation of Ta-enriched MC carbides to minimize the loss of refractory element reinforcement and the reduction of deformability at high temperatures. • Primary MC carbides altered the extension and/or activation of slip bands at 760 °C. • Primary MC carbides caused the shift of local fracture from pure shear to cleavage fracture. • Interdendritic carbides promoted the generation and expansion of microcracks at 1120 °C. • This work guided future optimization of carbon content in Ni-based SX superalloys. [ABSTRACT FROM AUTHOR]

The effects of γ′ forming elements Ta + Al on the microstructural evolution of two Ru-containing single crystal (SX) superalloys deposited with γ′ + β NiAl coatings were investigated after thermal exposure at 1140 °C. TCP precipitation, recrystallization, and elemental interdiffusion were particularly investigated to illustrate the influence of Ta + Al on the compatibility between the coating and substrate. After exposure at 1140 °C, two distinct zones were formed under the IDZ, where the SRZ in the higher Ta + Al alloy contained a large number of rod-like and needle-like TCP phases. The other was the SDZ in the lower Ta + Al alloy, which possessed fewer and smaller granular TCP phases. Noteworthily, this discrepancy was attributed to the enhanced tendency of recrystallization in alloy with higher Ta + Al, since higher Ta + Al would result in more negative misfit of γ/γ′ and lower Al partitioning in γ phase, which provided more driving force for recrystallization. Meanwhile, the elemental distribution in the diffusion zone of the alloy with higher Ta + Al was more heterogeneous, and even the Ru Al phase was observed in the coating. As the thermal exposure time prolonged, the thickness of the SRZ exceeded that of the SDZ, which significantly reduced the effective load-bearing area of alloy with higher Ta + Al. It could be summarized that a worse compatibility between the coating and the substrate would be introduced with the increase of Ta + Al in the substrate. This study provided valuable insights into the design of coatings and alloy compositions, thereby facilitating the development and application of advanced Ru-containing SX superalloys as well as their corresponding coatings. • Microstructural evolutions below the coating/substrate interface were investigated. • The higher Ta + Al promoted the formation of SRZ with numerous rod-like TCP phases. • The mechanism of higher Ta + Al promoting recrystallization formation was analyzed. • The effect of different Ta + Al on elemental interdiffusion was discussed. [ABSTRACT FROM AUTHOR]

The advanced fourth-generation single crystal superalloys have great service performance at high temperature, including outstanding fatigue, creep and oxidation-resistance. Low cycle fatigue (LCF) behaviors and microstructure evolution of a fourth-generation single crystal superalloy under different strain amplitudes at 800 °C were investigated in this study. As the strain amplitude increased, the cyclic hardening level of the experimental superalloy increased. The crack initiation could be attributed to the exfoliation of the oxidation layers. Furthermore, within strain amplitude range of 0.65% to 1.0%, slipping bands sheared both γ and γ' phases. The process of super-dislocations shearing into the γ' precipitates contributed to cyclic softening and resulted in the severe plastic deformation, thereby reducing the LCF life. Dense stacking faults (SFs) with "straight feature" and "zigzag feature" in the γ channels were observed under strain amplitude of 0.9% and 1.0%. The continuous Lomer-Cottrell lock (L-C lock) is the strong obstacle of dislocation movement, which resulted in cyclic hardening. This investigation and the resulting experimental data serve as a valuable reference for the engineering application of the fourth-generation single crystal superalloy. The findings can provide insights into the behavior and properties of the alloy, aiding in its practical use and development in various engineering applications. • LCF test for a fourth-generation single crystal superalloy was conducted at 800 °C. • Fatigue fracture mechanism was discussed from both macroscopic and microscopic perspectives. • Formation mechanism of the dislocation configuration was analyzed in detail. • Comprehensive influence of different dislocation configurations was summarized. [ABSTRACT FROM AUTHOR]