Hamouda Ghonem
University of Rhode Island, USA
Title: Deformation response of solid solution strengthened alloy 617 with non uniform grain boundary carbide distribution
Biography
Biography: Hamouda Ghonem
Abstract
Microstructural changes in grain boundary carbide precipitates have been correlated with stress during creep of solid solution strengthened by nickel based superalloy, alloy 617, at 950°C. The observed redistribution of carbides and carbide migration is explained in terms of dissolution of carbides under compressive stress and coarsening along boundaries subjected to tensile stress. The role of carbide, size, volume fraction and distribution on the deformation response has been examined on 3 microstructure variants of alloy 617 subjected to low cycle fatigue testing under strain controlled loading conditions at 760°C. The 3 microstructures are (1) as-rolled microstructure with uniform distribution of discrete carbide precipitates along all grain boundaries in addition to the presence of carbides in the matrix, (2) solution treated microstructure (1200°C for 2 hours and water quenched) with no carbides in the matrix or along grain boundaries, and (3) solution treated microstructure (1200°C for 2 hours and water quenched) subjected to creep testing at 950°C. This sequence results in redistribution of carbides, localized along boundaries perpendicular to creep load direction which are subjected to tensile stresses. Hysteresis loops associated with complex low cycle fatigue testing were used to determine the isotropic hardening, in terms of yield strength and cyclic hardening/softening, as well as, the kinematic hardening (long- and short-range) as a function of grain boundary carbide size and volume fraction. Results of this analysis are incorporated in a non-linear kinematic, internal state variable model to simulate the cyclic stress-strain response of alloy 617 as a function of the microstructure state with respect to the carbide distribution.