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To promote the wide application of the G115 steel in ultra-supercritical (USC) power plants, creep deformation, microstructure evolution and rupture mechanism of the G115 welded joint were systematically investigated. The fabricated G115 welded joint exhibits superior creep strength to the P92 or MARBN steels. The deformation behaviors are analyzed by the Norton's power law, proving that the deformation mechanism is closely associated with the stress levels. During short-term creep under higher stress, the weld metal (WM) and heat-affected zone (HAZ) exhibit outstanding microstructure stability, and the rupture occurs within the base metal (BM) due to the lath fracture. While, during long-term creep under lower stress, the brittle Type IV cracking occurs within the inter-critical heat-affected zone (ICHAZ) or fine-grained heat-affected zone (FGHAZ) due to the formed micro-voids along boundaries, obviously deteriorating the creep strength. The weld strength reduction factors are calculated under different stresses, and prove that synergistic action of back stress and dislocation motion can enhance the creep resistance of the G115 welded joint. Additionally, the allowable service stress of G115 weldments should be designed according to the creep strength of the welded joints under expected performance life. • The fabricated G115 welded joint displays superior creep strength to the P92 and MARBN steels, and the deformation mechanism is varied with the applied stress. • As the stress is lower than 180 MPa, brittle Type IV cracking occurs within the FGHAZ or IGHAZ in the G115 welded joint, which can be ascribed the coarsening of M 23 C 6 particles and the rapid recovery of dislocations. • Synergistic actions of the back stress and dislocation motions in equilibrium condition can reinforce the creep strength of the G115 welded joint. [ABSTRACT FROM AUTHOR]

In DELTA processing of Inconel 718 alloy, δ phase plays a vital role in final grain refinement, whose precipitation during intermediate thermal cycles is tailored by prior wrought structure. Therefore, it is necessary to renew insights into the effect of wrought structure on the morphology and quantity of δ phase. From this point, this work was conducted to investigate the δ precipitation behaviors in wrought Inconel 718 alloy. The results show that the δ precipitation behaviors are tailored by prior dynamic recrystallization (DRX) degree. Under partial DRX, δ precipitation exhibits twofold characteristics of preferential GB-precipitation accompanied by sluggish intragranular precipitation; while modified into predominant homogeneous GB-precipitation by fully DRX, in spite of the presence of few intragranular δ precipitates. After hot deformation, the equilibrium δ content is improved and inversely proportional to DRX degree. After 24 h aging at 900 °C, the volume fraction of δ phase is 38.6% and 27.9% for partial and fully DRX, respectively, compared to 15.1% for strain-free state. The GB-δ nucleation process is accelerated due to the occurrence of DRX, and firstly finished in few minutes; differently, the intragranular δ nucleation is grain-dependent, that promoted within unrecrystallized grains but inhibited within DRX grains. The “encounter” phenomena are observed during δ growth, and some particular coalescence mechanisms are proposed. [ABSTRACT FROM AUTHOR]

The roles of the alloying elements, Ti and Ta, on the γ' strengthening phase and the corresponding high temperature deformation responses of the CoNi-based superalloys have been systematically investigated. The results indicate that addition of either 4 at.% Ti (4Ti) or 2 at.% Ta (2Ta) can significantly increase the volume fraction of the γ' phase. Detailed analysis of the deformation mechanisms and Gibbs free energies of each phase in the two alloys at elevated temperatures were carried out by using the transmission electron microscopy (TEM) characterization and the thermodynamic calculation, respectively. The dislocations shearing into the γ' precipitates is the primary deformation mechanism in both alloys when deformed at 600 °C. However, when increasing the deformation temperature to 800 °C, the dislocation cross-slipping and by-passing mechanism dominants the 4Ti alloy's deformation, while the additional stacking faults (SFs) were introduced to the 2Ta alloy due to the lower barrier energy of the SFs formation. Continuing increasing the deformation temperature to 1000 °C will promote the dislocation-based mechanisms by lower the dislocation propagation resistance in both alloys. The process will also induce the mechanical twins in the 2Ta alloy which can further strengthening the alloy. The alloying induced deformation mechanism transition and the related high-temperature mechanical performance of the CoNi-based superalloys have been further discussed. • Adding Ti or Ta to the CoNi-based alloy will improve its γ' volume fraction to 60% • Ti and Ta will improve the flow strength at medium and high temperature, respectively • Ta can reduce the energy barrier for stacking fault formation at high temperature • Deformation mechanisms at elevated temperatures can be altered by the alloy elements [ABSTRACT FROM AUTHOR]

To investigate the hot deformation behaviour of Al-3%Mg 2 Si, isothermal compression tests were carried out in the temperature range of 300–500 °C and strain rate range of 0.01–5 s−1. The deformed microstructures were characterized by optical microscope, scanning electron microscope, transmission electron microscope and electron backscatter diffraction. The results show that the flow stress curves reveal a typical dynamic recovery characteristic. A modified Arrhenius-type constitutive equation was built to describe the flow behaviour of Al-3%Mg 2 Si alloy. Furthermore, based on the processing map of Al-3%Mg 2 Si alloy established under the strain of 0.8, microstructure evolution was analyzed. The high efficiency of power dissipation regions is mainly due to the softening mechanism of dynamic recovery and partial dynamic recrystallization. Mg 2 Si particles also play a positive role in dynamic recrystallization. The damage mechanism of unstable domains is the fracture and debonding of Mg 2 Si particles. • A modified constitutive model was built to describe the rheological properties of Al-3%Mg 2 Si alloy. • The softening mechanism is dominated by DRV with partial dynamic recrystallization. • Mg 2 Si particles play a positive role in dynamic recrystallization. • The damage mechanism of unstable domains is fracture and debonding of Mg 2 Si particles. [ABSTRACT FROM AUTHOR]