Influence of the Starting Microstructure on the Hot Deformation Behavior of a Low Stacking Fault Energy Ni-based Superal
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INTRODUCTION
NICKEL-BASED superalloys are widely used in the hot sections of modern gas turbine engines, and are highly efficient in these regions due to their excellent resistance to creep, fatigue, and corrosion at elevated temperatures.[1–4] These characteristics can be attributed to the underlying microstructures possessed by many commercial Ni-based superalloys, which are composed of coherent intermetallic particles of Ni3Al (c¢) with an ordered L12 crystal structure contained within a disordered face-centered cubic (FCC) austenitic (c) nickel matrix.[5] Both the c and c¢ phases in Ni-based superalloys are capable of accommodating a broad range of major and minor alloying additions that serve to strengthen or enhance the environmental properties of these alloys.[6] The material studied in this investigation was an experimental Ni-based superalloy possessing elevated concentrations of cobalt. The cobalt concentration was elevated in an effort to reduce the stacking fault energy and induce the formation of high proportions of
JOSHUA MCCARLEY, B. ALABBAD, and S. TIN are with the Illinois Institute of Technology, Chicago, IL 60616. Contact e-mail: [email protected] Manuscript submitted July 14, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
P coherent 3 twin boundaries which are P formed upon annealing.[7] High densities of coherent 3 twin boundaries contained within polycrystalline materials have been noted to enhance the mechanical integrity of [8,9] Previous works materials by limiting P dislocation slip. have highlighted 3 twins as possessing the ability to simultaneously offer the lowest interfacial energy and the highest barrier to slip motion during plastic deformation when compared to other high-angle grain boundaries. This in turn serves to extend the life by increasing the materials resistance to high-temperature fatigue and creep.[10] Past studies have shown that cobalt additions effectively lower the stacking fault energy of Ni-based superalloys.[7] This can be attributed to the fact that elemental cobalt has a lower stacking fault energy than nickel, and cobalt additions to Ni-based superalloys preferentially substitute into the face-centered cubic (FCC) Ni phase as its concentration is enhanced.[11] Lowering the stacking fault energy of the experimental Ni-based superalloy directly enhances the materials ability to effectively obstruct dislocation climb and cross-slip during plastic deformation, which in turn allows for a more rapid development of intragranular dislocation structures that P contribute to the formation of the desirable annealing 3 twin boundaries.[12–14] Over the years, powder processing of polycrystalline Ni-base superalloys has enabled significant gains in the temperature capability these materials as higher concentrations of alloying additions could be incorporated
while simultaneously avoiding solidification defects. However, as advanced gas turbine designs are continuously requiring materials with even higher temperature capabilities, new material systems and mechanisms need to
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