Characterization of Plastic Flow Pertinent to the Evolution of Bulk Residual Stress in Powder-Metallurgy, Nickel-Base Su
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THE final heat treatment of nickel-base superalloys typically comprises high-temperature solution treatment to dissolve all or most of a strengthening phase (such as gamma prime), cooling via water or oil quenching or forced convection, and then lower-temperature aging.[1,2] In large components of these materials, bulk residual stresses are developed due to small but nonuniform plastic strains associated with non-uniform cooling following the solution treatment step. In such instances, the surface cools rapidly and tends to shrink, but it is constrained by the hotter interior of the
S.L. SEMIATIN, Senior Scientist, Materials Processing/Processing Science, and R.E. DUTTON, Chief, Manufacturing and Industrial Technologies Division, are with the Air Force Research Laboratory, Materials and Manufacturing Directorate, Wright-Patterson Air Force Base, OH 45433. Contact e-mail: [email protected] P.N. FAGIN, Technologist, is with the Materials and Processes Division, UES, Inc., 4401 Dayton-Xenia Road, Dayton, OH 45432. R.L. GOETZ, Materials Engineer, is with Rolls-Royce Corporation, DSE Materials and Process Modeling, MC-S3-02, 546 South Meridian Street, Indianapolis, IN 46225-1103. D.U. FURRER, Senior Fellow Discipline Lead, is with Pratt & Whitney, Materials and Process Engineering, M/S 114-43, 400 Main Street, East Hartford, CT 06108. Manuscript submitted February 12, 2015. METALLURGICAL AND MATERIALS TRANSACTIONS A
component. The tensile stresses thus developed at the surface relax somewhat by creep-like flow at moderateto-high temperatures. Subsequently, the interior cools and tends to shrink from the surface, thereby tending to give rise to largely compressive stresses at the surface which are balanced by tensile stresses at the interior. The residual stresses so developed may be reduced during the subsequent aging treatment, but usually are not eliminated. Furthermore, depending on the location of the finished component within the heat-treatment envelope, clamping, cutter paths, etc., residual-stress patterns may also undergo changes during final machining. Because of the complex geometry of typical parts and the spatially-non-uniform nature of the temperature transients, stress patterns, and plastic flow, the quantitative prediction of residual stresses following solution heat treatment typically relies on finite-element-method (FEM) analysis.[3–9] In addition to part geometry, initial temperature, and orientation relative to the quench medium, inputs to such models include the thermophysical properties of the part/quench medium, interface heat transfer coefficients, and the constitutive response of the workpiece, each of which must be input as a function of temperature. A number of methods have been utilized to quantify the constitutive behavior used in FEM simulations of the formation of bulk residual stress. These techniques include conventional (isothermal) tension, compression,
creep, and stress-relaxation (SR) tests.[10–13] To quantify the often non-equilibrium nature of material behavior and microstructur
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