Processing map for controlling microstructure in hot working of hot isostatically pressed powder metallurgy NIMONIC AP-1
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The hot deformation behavior of hot isostatically pressed (HIP) NIMONIC AP-1 superalloy is characterized using processing maps in the temperature range 950 ~ to 1200 ~ and strain rate range 0.001 to 100 s -1. The dynamic materials model has been used for developing the processing maps which show the variation of the efficiency of power dissipation given by [ 2 m / (m + 1)] with temperature and strain rate, with m being the strain rate sensitivity of flow stress. The processing map revealed a domain of dynamic recrystallization with a peak efficiency of 40 pet at 1125 ~ and 0.3 s -r, and these are the optimum parameters for hot working. The microstructure developed under these conditions is free from prior particle boundary (PPB) defects, cracks, or localized shear bands. At 100 s -I and 1200 ~ the material exhibits intercrystalline cracking, while at 0.001 s -l, the material shows wedge cracks at 1200 ~ and PPB cracking at 1000 ~ Also at strain rates higher than 10 s -l, adiabatic shear bands occur; the limiting conditions for this flow instability are accurately predicted by a continuum criterion based on the principles of irreversible thermodynamics of large plastic flow.
I.
INTRODUCTION
IN recent years, the powder metallurgy ( P / M ) route has been found to be attractive for the manufacture of turbine components in nickel-base superalloys. While hot isostatic pressing (HIPing) and other techniques are developed for powder consolidation, the need for a subsequent deformation processing technique is well recognized, t~,2~ Deformation processing at elevated temperatures helps in mitigating the undesirable effects of prior particle boundaries (PPB) which are responsible for the poor mechanical properties of HIP compacts, pm While trial-and-error methods are generally employed in selecting the hot-working parameters, the recently developed technique of processing maps for hot working is found to be very useful in optimizing workability and controlling microstructure, ts,6j In particular, the domain of dynamic recrystallization (DRX) is of importance in eliminating the PPB defects, since this process reconstitutes the microstructure and enhances the hot workability. The aim of the present investigation is to develop a processing map for the hot working of NIMONIC AP- 1" superalloy HIP P / M compact with a view toward optimizing the hot workability and achieving microstructural control. NIMONIC AP-1 is a low-carbon M.C. SOMANI, E.S. BHAGIRADHA RAO, N.C. BIRLA, AND M.L. BHATIA, Scientists, are with the Defence Metallurgical Research Laboratory, Hyderabad 500 258, India. VAKIL 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 December 5, 1991. METALLURGICAL TRANSACTIONS A
modification of conventional ASTROLOY** and was *NIMONIC AP-1 is a trademark of Henry Wiggins & Co. Ltd., London, United Kingdom. **ASTR
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