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The influence of solution temperature on the microstructure evolution and creep behavior of a new Ni–Co base superalloy for turbine discs, including the grain morphology, grain boundary character distribution (GBCD), and grain boundary (GB) carbide morphology, was investigated using scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), and transmission electron microscopy (TEM). The results showed that the creep fracture life first increased and then decreased with increasing solution temperature. The alloy solution treated at 1140 °C exhibited the best creep properties, with a creep life of 209 h. The enhanced creep performance of this alloy can be explained as follows: the volume fraction of GB carbides increased with increasing solution temperature, the diffuse fine granular carbides relieved stress concentration and strengthened GBs, the coarse primary γ′ phases rapidly dissolved, and the volume fraction of the fine and dispersed secondary and tertiary γʹ phases significantly increased. The narrower channels in the γ matrix increased the resistance to dislocation motion. The coherent Σ3 twin boundaries (TBs) with low interface energy hindered the dislocations and stacking faults intersecting the TBs on different slip planes. The relative frequency of Σ3 TBs increased to 53.6% when the solution temperature was increased to 1140 °C. The high proportion of Σ3 TBs played a key role in improving the creep resistance of the alloy. • Microstructure and creep behaviors were studied at different solution temperatures. • Creep life increased at first and thereafter decreased as solution temperature increased. • The continuous rod-like morphology of the M 23 C 6 carbide degraded the creep properties. • The improved creep properties were determined by the increase of Σ3 TBs and fine γ′. [ABSTRACT FROM AUTHOR]

The under-aging, peak-aging, and over-aging microstructures of a precipitation-strengthened Fe–Ni-based alloy were designed by applying different heat treatment procedures. Tensile creep tests were conducted at 700 °C and 250 MPa on samples exhibiting different strengthening γ′ precipitate volume fractions and diameters. The creep life of the alloy in the under-aged state was increased by more than 10% compared to the peak-aged condition, which provided a new approach to designing the heat treatment regime that improve the creep rupture life and plasticity of precipitation-strengthened alloys. The improvement in creep properties of the under-aged state was attributed to the transition from shearing to Orowan looping of γ′ particles by matrix dislocations during the dynamic precipitation and coarsening of γ′ phase, increasing intragranular deformation resistance. The under-aged microstructure design allowed reducing the grain boundary sliding contribution to overall strain during creep. As a result, the steady-state creep stage of the alloy in the under-aged state was extended. Additionally, relationships between applied stress and nucleation, growth, and coarsening of the γ′ phase have been evidenced. • Proposing an approach for superalloys with favorable plasticity and prolonged life. • Under-aged state is a superior choice for γ′-strengthened alloys under low stress. • Effect of dynamic precipitation and coarsening of γ′ phase on creep is investigated. • Applied stress has limited effect on nucleation and coarsening of γ′ phase. [ABSTRACT FROM AUTHOR]

In this work, isothermal compression tests of a Ni-based superalloy were carried out at γ′ sub-solvus (1140 ℃) and γ′ super-solvus (1170 ℃) temperatures under a true strain 0.693. The effect of C contents (0.01 wt%, 0.04 wt% and 0.08 wt%) on the microstructural evolution and hot deformation behavior of the alloy was investigated. The formation of MC carbide is closely related to C content. Besides, the initial grain size decreased slightly with the increase of C content due to the effect MC carbides. During compression deformation at 1140 ℃, the peak stress increased with the increase of C content. When deformed at 1170 ℃, the 0.04 wt% C alloy has the lowest peak stress, because of the acceleration effect on dynamic recrystallization DRX by MC carbides. However, the 0.08 wt% C alloy has the highest peak stress, the dislocation density also increases with the number of MC carbides increase，which caused high working hardening rate and increased the peak stress in 0.08 wt% C alloy. The discontinuous dynamic recrystallization (DDRX) is the dominant mechanism during deformation at 1140 ℃, while the DDRX and continuous dynamic recrystallization (CDRX) occurred at the same time when deformed at 1170 ℃. • The addition of C content decreased the grain size slightly. • The DRX were accelerated by MC carbide when deformed at 1170 °C. • Appropriate C content can reduce the hot deformation resistance. [ABSTRACT FROM AUTHOR]

Tensile properties and deformation mechanisms of a Fe–Ni-based superalloy with different size of γ′ precipitates obtained through varied aging time are investigated at 700 °C. The γ′ phase size increases with aging time, the yield strength of the alloy increases first and then decreases, which is consistent with the change trend of critical resolved stress of deformation mechanism. The dominant deformation mode of the alloy with fine γ′ phase is weakly-coupled dislocation pairs with slip bands, and then changes to Orowan looping at a γ′ size of approximately 30 nm, resulting in the occurrence of peak value of strength. As the γ′ size is above 40 nm, the deformation mechanism is Orowan bowing along with stacking fault shearing, decreasing the yield strength of the alloy. When γ′ size is between 30 nm and 40 nm, the dominant deformation mechanism is Orowan looping with strongly-coupled dislocation pairs and the fracture mode is intergranular. Intermediate temperature brittleness occurs in the secondary aging treated alloy with a γ′ phase size of approximately 30 nm, which is attributed to the increase in critical resolved stress and strain localization caused by the transformation of tensile deformation mechanism, and stress concentration due to dislocation entanglement at grain boundary. The sub-aging state of the alloy used for advanced ultra-supercritical plants may be a good choice for obtaining simultaneously favorable strength and ductility. • The yield strength, elongation and deformation mechanism are γ′ phase size-dependent. • APB shearing transitions to Orowan looping along with SF shearing with the increasing γ′ size. • ITB of the alloy at 700 °C is attributed to change of deformation mechanism and dislocation pile-up at grain boundary. [ABSTRACT FROM AUTHOR]

In this study, the effect post weld heat treatment (PWHT) on microstructural evolution and mechanical properties of weld metal of HT700P alloy with Inconel 617 filler metal was investigated. After welding, columnar grain, dendritic microstructure and segregation of Mo, Co and Ti elements related to the equilibrium distribution coefficient were observed in the weld metal. The PWHT was conducted in both direct-aging treatment with different time and solution treatment with different cooling methods. The results showed that direct-aging treatment had a limit impact on homogenization of weld metal, while microstructural homogenization and elemental segregation were significantly improved after solution treatment and massive M 6 C and M 23 C 6 carbides precipitated at the grain boundaries and interdendritic region of weld metal. It was found that the elongation at 700 °C of welded joints after solution treatment was relatively increased, which was related to the decrease of residual stress and the precipitation of Mo-rich carbides at the grain boundaries. The fracture occurred in the weld metal presented interdendritic fracture characteristics, and the fracture mechanism for stress relaxation cracking was the strong grain interior induced by the precipitation of γ′ phase, leading to the stress concentration and nucleation of cavities at grain boundaries. After solution treatment, the creep lifetime of furnace cooling was much greater than that of water quenching. The superior creep properties of furnace-cooled joint could be attributed to the lower residual stress and the migration of carbon element in the fusion line, which inhibited the nucleation and aggregation of cavities in the partially melted zone. • Effect of PWHT on microstructure and properties of welded joints of a Fe-Ni-based alloy is studied. • The segregation of Mo, Co, Ti in the weld metal and fusion line is analyze. • The mechanism for stress relaxation cracking is illustrated after different PWHT. • Residual stress and carbon migration in fusion line dramatically affect creep life of joints. [ABSTRACT FROM AUTHOR]

The microstructure, mechanical and wear behaviour of a high chromium cast iron was investigated with various tempering temperatures (200 °C, 280 °C, 360 °C, 440 °C) by SEM, TEM and EBSD. Both of primary M 7 C 3 and secondary carbides increasingly coarsed as temperature increasing, contributing to the best mechanical properties as tempered at 200 °C. The highest strength and hardness produced highly deformed layers, including the SFs locks in M 7 C 3 and strong work-hardening in matrix, retarding dislocations movement and hindering materials peeling, thus improving the wear resistance as tempered at 200 °C. Besides, The wear mechanisms shifted from abrasive and adhesive wear to oxidative and fatigue wear as temperature raising. The optimal heat treatment was ascertained to provide further instructions for the development of advanced cast irons. [ABSTRACT FROM AUTHOR]

[Display omitted] • Coarse-grained superplasticity is rarely observed in materials that have grain sizes above 10 µm. • Superplastic elongation of high-strength alloy is reported, improving the workability of the Ni-Co-based superalloy. • DRX enhanced the thermal stability of the deformed microstructure and the coordinated elongation. A near-superplastic tensile elongation of up to 232% at 1140 °C and 0.001 s−1 for a coarse-grained Ni-Co-base superalloy with an average grain size of 85 µm is reported. Nevertheless, poor grain deformation coordination and low grain boundary bonding strength led to terrible thermoplasticity for the coarse-grained alloy below 1100 °C. Dynamic recrystallization (DRX) relieved the stress concentration and decreased the damage accumulation during superplasticity, improving the ductility with the increase in temperature within the γ + γ′ dual-phase regime. [ABSTRACT FROM AUTHOR]

The effects of deformation temperatures on the superplastic behavior, microstructure evolution, and failure mechanisms of Ni-Co-based superalloy were analyzed at temperatures ranging from 900 to 1140 °C. The superplastic-tensile temperatures have significant effects on flow behavior, which shows distinctive features of dynamic recrystallization (DRX). The alloy exhibits a striking increase in elongation with increasing the superplastic-tensile temperature to 1100 °C, reaching 1760 %. The deformed microstructure was characterized using SEM, EBSD, and TEM. As the superplastic-tensile temperatures increased, the DRX-grain fraction increased. The superplastic deformation of the alloy is primarily governed by discontinuous-DRX (DDRX) mechanism. Continuous-DRX (CDRX) had a significant effect at 900 °C and occurs as an assistant mechanism during superplastic deformation of the alloy at elevated temperatures. The fracture at 900 °C was initiated by a combination of intergranular and micro-void coalescences, while the pure intergranular fracture takes place above 1050 °C. Meanwhile, the micro-void coalescence has only a minor effect on the fracture behavior at 1140 °C. • Superalloys are usually sensitive to hot deformation parameters. • The flow behavior during hot deformation is complex. • The flow behavior affects the mechanical properties of superalloys, making microstructure control critical to optimization. • Microstructure evolution, deformation, and failure mechanisms are significantly affected by the deformation temperature. [ABSTRACT FROM AUTHOR]

• Superplasticity can occur in the Ni-Co-based superalloy at low temperatures and high strain rates. • The primary γ′-phase significantly contributes to the superplastic deformation. • DRX enhances the ability for coordinated deformation during superplasticity. The Ni-Co-base superalloy with a 750 °C service temperature contains a substantial number of refractory and γ′-phase forming elements that cause high deformation resistance, low thermoplasticity, and a limited hot-working window. The traditional deformation processing is carried out at low strain rates and high temperatures > 1100 °C, which is extremely strict on equipment working conditions. We coarsened the γ′-phase to facilitate the dynamic recrystallization (DRX) and inhibit grain growth, obtaining small grains of less than 10 µm. Ultimately, the superplasticity of the alloy can reach 1650% elongation at temperatures below 1000 °C and a high strain rate of 0.01/s. [ABSTRACT FROM AUTHOR]

The effect of strain rate on microstructure evolution of a Ni-Co-base superalloy during superplasticity was examined in this study. The experiments were conducted at 1050 °C with various strain rates. The strain rate had profound impacts on flow behavior and elongation that were connected to the characteristics of dynamic recrystallization (DRX). A comparatively high strain rate promoted the occurrence of DRX, and the elongation of >1335% prior to failure was obtained at 0.01/s. Scanning electron microscope (SEM), electron backscatter diffraction (EBSD), and transmission electron microscopy (TEM) measurements were used to investigate the deformed microstructures. A 0.01/s strain rate accelerated the recovery and dissolution within the primary γ'-phase, resulting in a reduction in its volume fraction and size, which enhanced the DRX process of the γ-matrix. The primary γ'-phase partly retarded the DRX process of γ-matrix grains, which was expedited by twin boundaries. Regardless of strain rate, discontinuous-DRX (DDRX) dominated the superplastic deformation. Meanwhile, continuous-DRX (CDRX) occurrence increased slightly at 0.1/s, which had a minor effect during the superplastic deformation of the Ni-Co-base superalloy. • The flow behavior during hot deformation is complex and significantly influenced by strain rate. • The complicity of flow behavior affects the microstructure evolution and then the mechanical properties of superalloys. • Microstructure evolution and the DRX process are significantly affected by the strain rate during superplasticity. [ABSTRACT FROM AUTHOR]