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IN their list of the top 50 ‘‘Greatest Moments in Materials Science and Engineering,’’ the Minerals, Metals, and Materials Society includes the 1926 patent by Paul Merica, which described the addition of small amounts of aluminum to a Ni-Cr alloy creating the first ‘‘superalloy.’’[1] Indeed the evolution of these alloys, including the advances in chemical composition and processing has revolutionized air transport and gas and steam turbine commercialization. One remarkable feature of nickel-based superalloys is the ability to form an ordered intermetallic phase (referred to as gamma prime) based on the L12 structure. These precipitates have a small lattice mismatch with the ductile and disordered FCC host phase (referred to as gamma). One result of this is the ability to form very high volume fractions of fine precipitates, which in modern airfoil alloys now approach volume fractions of 70 pct. The presence of these precipitates and the difficulty of transmitting dislocations through an ordered structure give superalloys some very unique mechanical properties. In particular, the variation of the yield stress with increasing temperature shows a regime at elevated temperature where yield stress actually increases with increasing temperature. This is illustrated in Figure 1, which shows the variation of yield stress with temperature in precipitation-hardened Inconel 718[2] and in Rene N4.[3] PAUL S. FOLLANSBEE, James F. Will Professor of Engineering Sciences, is with Saint Vincent College, Latrobe, PA 15650. Contact e-mail: [email protected] Manuscript submitted February 5, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS A

This behavior, which is at odds with the temperature dependence typically observed in metals, is larger in the higher Ni3Al containing alloy N4 than in the lower Ni3Al containing alloy Inconel 718. Interestingly, in pure Ni3Al, the behavior is quite pronounced; at 900 K (627 C), the critically resolved shear stress exceeds that at 300 K (27 C) by a factor of 6 (see Figure 12).[4] In superalloys, the enhanced hardening at elevated temperatures is known as the ‘‘stress anomaly.’’ The origin of the effect arises from a self-locking mechanism in dislocations cross-slipping onto nonglide cube planes. This is known as the Kear–Wilsdorf locking mechanism.[5] Detailed models for the dislocation interactions have been proposed and long-debated.[6] Another observation related to elevated temperature deformation of superalloys is the presence of dynamic strain aging (DSA). This was topic of an extensive study by Mulford and Kocks in Inconel 600.[7] These investigators used elevated temperature strain-rate change tests to probe the instantaneous strain-rate sensitivity. Ha¨nninen et al. measured elevated temperature stress– strain curves in Inconel 600 and Inconel 690 and observed jerky flow (serrations) over a wide range of conditions.[8] In addition to the appearance of jerky flow, which often accompanies DSA, another effect of DSA is to increase the flow stress levels during deformation. While the