Superalloys are being employed in more extreme conditions requiring higher strength, which requires producers to forge products to finer grain sizes with less grain size variability. To assess grain size, crystallographic texture, and substructure as a function of forging conditions, frictionless uniaxial compression testing characteristic of hot working was performed on INCOLOY 945 (Special Metals Corporation, Huntington, WV), which is a newly developed hybrid of alloys 718 and 925, over a range of temperatures and strain rates. The microstructure and texture were investigated comprehensively using light optical microscopy, electron backscatter diffraction (EBSD), electron channeling contrast imaging (ECCI), and transmission electron microscopy (TEM) to provide detailed insight into microstructure evolution mechanisms. Dynamic recrystallization, nucleated by grain/twin boundary bulging with occasional subgrain rotation, was found to be a dominant mechanism for grain refinement in INCOLOY 945. At higher strain rates, static recrystallization occurred by grain boundary migration. During deformation, duplex slip along {111} planes occurred until a stable h110i fiber compression texture was established. Recrystallization textures were mostly random but shifted toward the compression texture with subsequent deformation. An exception occurred at 1423 K (1150°C) and 0.001 seconds À1 , the condition with the largest fraction of recrystallized grains, where a h100i fiber texture developed, which may be indicative of preferential growth of specific grain orientations.

The effects of alloy 718 microstructure on hydrogen embrittlement susceptibility and tensile fracture mode were assessed through slow strain rate tensile testing and fracture surface analysis. Alloy 718 was annealed and aged to produce microstructures with variations in grain size and amount of grain boundary precipitates. Furthermore, the different ageing conditions likely resulted in differences in volume fractions and sizes of γ′ and γ′′ precipitates. The extent of grain boundary precipitation had the strongest effect on hydrogen embrittlement susceptibility, while grain size did not have any significant effect. Hydrogen embrittlement susceptibility was also correlated with differences in strength level, which was primarily controlled by the γ′ and γ′′ precipitate populations.

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Nickel-based superalloys possess an excellent combination of mechanical properties and environmental resistance at elevated temperatures, and have been widely used in challenging environments prevalent in aircraft engines and land-based power generation gas turbines, as well as in nuclear power systems and chemical plants. The ever-increasing demand for higher operating temperatures to achieve better fuel efficiency has been driving the development of the next generation of superalloys, in which the higher temperature capability has been achieved by increasing additions of refractory elements, optimizing processing conditions, and through the application of coatings. However, there is a strong need for materials that can make a disruptive change in temperature capability beyond that provided by current materials.The symposium ''Materials for High Temperature Applications: Next Generation Superalloys and Beyond'' was held during the 2017 TMS annual Meeting from February 27 to March 2, 2017 in San Diego, California, with the sponsorship from the TMS High Temperature Alloys Committee and TMS Refractory Metals Committee. The goal of this symposium was to discuss (i) challenges that current Ni-based superalloys are facing and (ii) the recent progress in the development of next generation of superalloys and high temperature material systems beyond Ni-based superalloys. The symposium was composed of a total of seven sessions covering current and next generation Ni-based superalloys, refractory metal-based materials, intermetallic-based materials, coatings, Ir-based alloys, and ceramics composites and emerging materials. The organizers would like to thank all the presenters and the participants for their excellent contribution to the success of this symposium. There were in total 60 oral presentations, including 2 keynote presentations and 18 invited presentations, and 6 poster presentations. This special issue presents selections of 10 high quality research papers submitted to this symposium. We would like to express our appreciation to Prof.

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