Dislocation/precipitate interactions during coarsening of a plastically strained high-misfit nickel-base superalloy
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I.
INTRODUCTION
DisLOCATION/precipitate interactions in nickel-base superalloys have been of interest in recent years because of their relation to stress coarsening. Theories have been put forth for stress coarsening which are based both on elastic energy interactions j,2,3 and dislocation interactions4,5,6 with an applied stress field. The result of these interactions is a dramatic change in precipitate morphology during stress coarsening, with a corresponding change in the mechanical properties of the material.7'8 Although considerable research has been done on the stress coarsening phenomenon, little research has been done on how dislocations, in the absence of an applied stress field, affect the morphology and distribution of y' precipitates. 9 The present research attempts to address this subject by examining how dislocation structures introduced prior to aging can interact with precipitates to create novel precipitate morphologies and distributions in high-misfit nickel-base superalloys. Particular attention is devoted to how matrix dislocations are incorporated into networks at precipitate/matrix interfaces and how these resulting networks provide a mechanism for preferential coarsening. An important part of the analysis is to determine, via the O-lattice construction, the equilibrium dislocation content for the misfitting 3" particles.
II.
EXPERIMENTAL PROCEDURES
The material selected for this study was INCONEL X750, a polycrystalline wrought nickel-base superalloy.10 It was selected because of several inherent microstructural characteristics. First, this material has a considerable lat*INCONELis a trademarkof the INCO familyof companies. THOMAS G. FERENCE, Research Assistant, and SAMUEL M. ALLEN, AssociateProfessorof Physical Metallurgy,are with the Center for Materials Scienceand Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139. Manuscript submitted March 17, 1986. METALLURGICALTRANSACTIONSA
tice parameter misfit of 0.3 --_ 0.1 pct (at, > ay) as measured at room temperature by convergent-beam electron diffraction. H This large misfit causes intense coherency strain fields to be set up around the precipitates. Second, INCONEL X-750 has a relatively low y ' volume fraction of around 15 to 20 pct for the selected aging temperature. This yields relatively isolated particles on which interfacial dislocation structure is readily observed. Also, the cuboidal form of the precipitates simplifies geometrical analyses of the defect structures. Standard V4-20 tensile specimens with a gage diameter of 3.3 mm were machined from polycrystalline INCONEL X-750. These specimens were subjected to one of the two processing schemes listed in Table I. The first processing treatment is representative of those used in the precipitation hardening of most nickel-base superalloys. The second process differs from the first in that it incorporates a slight tensile plastic deformation of the material prior to aging. It should be emphasized that the purpose of the present work is to study the effects that t
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