High-temperature deformation mechanisms and constitutive equations for the oxide dispersion-strengthened superalloy MA 9
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I.
INTRODUCTION
O X I D E dispersion-strengthened superalloys have been the subject of intense investigation in recent years because of their good high-temperature creep, oxidation, and corrosion resistance. Many experimental investigations of the monotonic creep behavior of dispersionstrengthened alloys have been conducted, and the steady-state creep of ODS alloys has been phenomenologically characterized. However, although it is generally accepted that the high creep strength of ODS alloys is due to interactions between dislocations and the dispersed phase, the exact nature of these interactions is still widely debated. Early studies tl'2] of dispersion strengthening at high temperatures concentrated on model alloys such as CuSiO2 undergoing steady-state creep. These investigations produced the f'n'st mechanistic models of high-temperature dispersion strengthening. None of these early models included any strengthening effect from the dislocation substructure; they all assumed that the flow stress was just the stress required for dislocations to be able to bypass all of the dispersoids they encountered in their glide plane. A number of more recent studies have concentrated on more practical ODS alloys. Lund and Nix [3] and Hausselt and Nix, t4] among others, have studied the steadystate creep of Ni-20Cr-2ThO2; Whittenberger tS'6,TJ has studied the steady-state compressive and tensile creep and fracture behavior of MA 956 and MA 6000; Howson et al. [8] and a large number of others have investigated the steady-state creep of MA 754. The experimental resuits of all of these investigators have a number of feaM. HAGHI, Graduate Student, and L. ANAND, Associate Professor, are with the Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139. Manuscript submitted January 16, 1989. METALLURGICAL TRANSACTIONS A
tures in common. They all report apparent activation energies which are higher than that for matrix selfdiffusion, though in some cases, when the temperature dependence of the elastic moduli is taken into account, the activation energies become of order the self-diffusion energy. Also, they all report very high stress exponents in the high-temperature, low strain-rate regime. This observation is usually explained in terms of a threshold stress for creep deformation. Various authors have attributed this threshold stress to (1) repulsive dislocation-particle interactions where the dislocations either loop the particles or bypass them by climb of short (of order the particle diameter) or long (of order the interparticle spacing) segments of the dislocation, tl,2] (2) attractive dislocation-particle interactions, where the dislocations, attractively pinned at the departure side of the particles, require a minimum stress to overcome them, [9,ml or (3) backstress due to the dislocation substructure. [4] The first two classes of theories generally ignore any dislocation strengthening, while the third class usually predicts a very small or no threshold stress in the absence of a disl
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