Shearing of γ′ precipitates by a<1 1 2> dislocation ribbons in Ni-base superalloys: A phase field approach

V.A. Vorontsov et al

The phase field model of dislocations has been used to study the propagation of dislocation ribbons with an overall Burgers vector of a through a simulated Ni-base superalloy. The driving force for dislocation dissociation reactions and formation of planar faults is incorporated into the free energy functional using periodic functions specially fitted to ab initio γ-surface data. The model shows that the mechanism of cutting of the γ′ precipitates by these ribbons exhibits significant dependence on stress magnitude, orientation and precipitate shape. In the case of mixed screw–edge ribbons a change of shearing mode is observed, from stacking fault shear to anti-phase boundary shear, when the applied stress approaches the yield of the material. This transition is absent in pure edge ribbons.

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Title: Creep-resistant Al2O3-forming austenitic stainless steels

Authors: Y Yamamoto, M P Brady, Z P Lu, P J Maziasz, C T Liu, B A Pint, K L More, H M Meyer, and E A Payzant

Source: Science 20 April 2007, Vol. 316, No. 5823, pp. 433-436

Abstract: A family of inexpensive, Al2O3-forming, high–creep strength austenitic stainless steels has been developed. The alloys are based on Fe-20Ni-14Cr-2.5Al weight percent, with strengthening achieved through nanodispersions of NbC. These alloys offer the potential to substantially increase the operating temperatures of structural components and can be used under the aggressive oxidizing conditions encountered in energy-conversion systems. Protective Al2O3 scale formation was achieved with smaller amounts of aluminum in austenitic alloys than previously used, provided that the titanium and vanadium alloying additions frequently used for strengthening were eliminated. The smaller amounts of aluminum permitted stabilization of the austenitic matrix structure and made it possible to obtain excellent creep resistance. Creep-rupture lifetime exceeding 2000 hours at 750°C and 100 megapascals in air, and resistance to oxidation in air with 10% water vapor at 650° and 800°C, were demonstrated.