Hot deformation characteristics of INCONEL alloy MA 754 and development of a processing map
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I.
INTRODUCTION
INCONEL* alloy MA 754 is an oxide dispersion *INCONEL is a trademark of Inco Alloys International, Inc., Huntington, WV.
strengthened nickel-chromium alloy produced by mechanical alloying. The oxide dispersoids are yttrium aluminates formed by reaction between Y203, excess oxygen in the powder, and aluminum. The material is used for gas turbine components where high-temperature strength and creep resistance are required, m In general, the alloy is used in a large grained and directioually recrystallized condition and has a strong crystallographic texture. As this material is generally manufactured by hot rolling, it will be useful to study its hot deformation characteristics so that the process parameters may be optimized and the microstructure controlled during processing. The aim of the present investigation is to establish the constitutive flow behavior of MA 754 by developing a processing map for hot working. An aspect of particular interest is the s*~dy of the effect of the oxide dispersoids on the processes of dynamic recovery and dynamic recrystallization (DRX), both of which are considered to result in a damage-free microstructure and are therefore "safe" for processing321 The processing maps are developed on the basis of the principles of the dynamic materials model, t3] which is reviewed by Gegel e t a l . taJ and Alexander. 151 In this model, the workpiece under hot-working conditions is considered to be a dissipator of power. At any instant, the power dissipation M.C. SOMANI, K. MURALEEDHARAN, and N.C. BIRLA, Scientists, are with the Defence Metallurgical Research Laboratory, Hyderabad 500 250, India. V. SINGH, Professor, is with the Department of Metallurgical Engineering, Banaras Hindu University, Varanasi 221 005, India. Y.V.R.K. PRASAD, Professor, is with the Department of Metallurgy, Indian Institute of Science, Bangalore 560 012, India. Manuscript submitted June 2, 1993. METALLURGICAL AND MATERIALS TRANSACTIONS A
occurs through a temperature rise (G content) and a microstructural change (J co-content), and the power partitioning between these two is decided by the strainrate sensitivity (m) of flow stress (or). At a given temperature and strain, the J co-content is given by t31 o-- ~ . m J - - [1] m+l where, ~ is the strain rate. The J co-content of the workpiece, which is a nonlinear dissipator, is normalized with that of an ideal dissipator (m = 1) to obtain a dimensionless parameter called by efficiency of power dissipation: J 2m = -- = 17 Jmax m + 1
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The variation of ~/with temperature and strain rate represents the constitutive behavior of the material and constitutes a processing map. The various domains in the map may be correlated with specific microstructural control. The dynamic materials model has as its basis the extremum principles of irreversible thermodynamics as applied to large plastic flow described by Ziegler. t61 Kumar m and Prasad tS] developed a continuum criterion combining these principles with those of separability of power dissipation and have sho
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