Dynamic recrystallization during hot deformation of aluminum: A study using processing maps
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I.
INTRODUCTION
T H E mechanism of hot deformation in aluminum and its alloys has been studied extensively t~-7J using laboratory tests such as tension, compression, and torsion, as well as metalworking processes like rolling and extrusion. In pure aluminum, it is considered that the mechanism of softening is essentially dynamic recovery, which occurs readily due to the high stacking fault energy (SFE) of aluminum. In view of this, it is generally stated that aluminum does not dynamically recrystallize, as there is no direct microstructural evidence after hot deformation. However, dynamic recrystallization (DRX) is reported in aluminum deformed at large strains and elevated temperatures, tS-m McQueen and co-workers t~z,~31called this "geometric" DRX, the process which increases the largeangle grain boundary area without changing the total number of grains in the specimen. Recent studies of the influence of strain-rate changes at 450 ~ on the stressstrain behavior and microstructural development in commercial aluminum showed that the transient behavior is strain rate-dependent, and the strain required to obtain equilibrium subgrain structure is higher for lower strain rates, t~4,~51 In aluminum alloys, the occurrence of DRX is attributed to the lowering of SFE caused by solute additions, particularly magnesium. 15,6.71The presence of fme particles may also cause DRX in aluminum, t4~ The aim of the present investigation is to examine whether DRX occurs in aluminum and, if so, study its characteristics. Earlier investigations {4,6,16,171 have followed a kinetic approach for analyzing the deformation behavior using an Arrhenius-type rate equation. However, the stress exponent was found to be temperature- and strain ratedependent, and so the interpretation of the apparent activation parameter is complex. In view of this, a different approach based on processing maps has been folN. R A V I C H A N D R A N , Graduate Student, and Y.V.R.K. PRASAD, Professor, are in the Department of Metallurgy, Indian Institute of Science, Bangalore 560 012, India. Manuscript submitted October 30, t990. METALLURGICAL TRANSACTIONS A
lowed. In the processing map developed by Raj, t18] on the basis of atomistic mechanisms, a regime of dynamic recrystallization is depicted at high temperatures. A newer approach is that using the Dynamic Materials Model, ]191 which helps in mapping the characteristics of power dissipation by microstructural changes as a function of temperature and strain rate using the strain-rate sensitivity parameter. The power dissipation map helps in delineating the domains within which particular microstructural mechanisms dominate. As any recrystallization process is sensitive to the purity of the material, processing maps on aluminum of different purities have been established in this study. II.
EXPERIMENTAL
Aluminum of four different purities, namely, 99.999, 99.995, 99.94, and 99.5 pct A1, were used in this investigation, and these will be referred to as 5-9, 4-9, 3-9, and 2-9 purity aluminum, respectively. Extrud
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