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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]

In this work, hot compression behaviors and deformation mechanisms of a Ni–Co-based superalloy prepared using directional solidification (DS) were investigated by carrying out the isothermal compression tests within the temperature range of 1080–1190 °C and the strain rate range of 0.001–1 s-1 under the true strain of 0.693. The effects of temperature and strain rate on dynamic recrystallization (DRX) were analyzed, and constitutive equations were established. The activation energies for γ+γʹ dual-phase and the γ single-phase regions of Ni–Co-based superalloy with columnar grains were 1190 kJ mol-1 and 415 kJ mol-1, respectively. The results showed that the dominant deformation mechanisms were closely related to the strain rate and temperature. DRX was accelerated at low strain rate conditions, while the DRX process was apparently sluggish at strain rates higher than 1 s-1. The undissolved γʹ phase interacted with dislocations and accelerated the DRX of the matrix during γʹ sub-solvus temperature (1080–1120 °C) deformation. Continuous dynamic recrystallization (CDRX) nucleation proved to play a dominant role. During γʹ super-solvus temperature (1150–1190 °C) deformation, the dominant dynamic softening mechanism of the alloy is discontinuous dynamic recrystallization (DDRX). Local migration of grain boundaries (GBs) causes an increase of misorientation within the deformed grains and induced the formation of subgrain boundaries near the GBs, which subsequently evolved into high angle grain boundaries (HAGBs) and participated in DDRX. Furthermore, during high-temperature deformation in Ni–Co-based superalloys, the relatively rare microtwins (MTs) were detected under compression at 1100 °C/1 s-1. • Constitutive equations were built segmentally considering the γ′ phases dissolution. • The undissolved γʹ particles promote DRX at γʹ sub-solvus temperature. • The MT formation mechanism at high temperatures was related to low SFE and LAGBs. [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]

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 microstructural evolution, tensile behavior and deformation mechanism of a Fe–Ni-based superalloy for advanced ultra-supercritical coal-fired power plant during long-term isothermal exposure at 700–800 °C for 500–3000 h were investigated. The coarsening rate constant of γ′ phase in the alloy exposured at 700 °C, 750 °C and 800 °C were about 47.543 nm3/h, 277.963 nm3/h and 2201.937 nm3/h, respectively. The activation energy for γ′ growing is around 250.24 kJ/mol. The results showed Al, Ti and Ni elements primarily gathered into γ′ precipitates, while Cr and Fe mainly partitioned into γ matrix in Fe–Ni base alloy. After long-term aging, there was no precipitation of harmful phases such as the η and σ phases, but the decomposition of MC + γ → M 23 C 6 + γ′ was observed. The microcracks were prone to initiate and propagate around pores formed by MC decomposition and lamellar M 23 C 6 carbide, which might result in the premature fracture failure of the alloy. The yield strength first remained stable with a slight increase in the γ′ phase diameter and then decreased after further thermal exposure. The variation of tensile strength with precipitate size could be rationalized by the transformation between shearing model and Orowan looping mechanism of γ′ precipitates with increasing the γ′ precipitate size. • Microstructural evolution of a Fe–Ni-base alloy for A-USC is investigated. • The γ′ phase is stable in the new alloy with over 40% Fe content during aging. • The transition of γ′ phase between shearing and Orowan looping is about 30–40 nm. • MC decomposition promotes initiation and propagation of cracks in tensile test. [ABSTRACT FROM AUTHOR]