Processing map for hot working of alpha-zirconium
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I.
INTRODUCTION
THE hot deformation characteristics of alpha-zirconium have been studied earlier ~ and analyzed using the kinetic rate equation. The apparent activation energy estimated from this analysis was found to be close to that for self-diffusion in this material, and the mechanism of hot deformation was suggested to be that involving dynamic recovery t2] in view of the high stacking fault energy of zirconium, t3'4]The data used for the above analysis were up to a strain rate of 3 x 10 -1 s -1, which is rather low when compared with that encountered in commercial metalworking operations. Further in the above kinetic analysis, an Arrhenius-type rate equation is used along with a power law relation between flow stress and strain rate. It is generally assumed that the stress exponent is independent of strain rate and temperature, and the experimental data obtained over a wide range showed that this assumption is not strictly valid, tS'6j The aim of this investigation is to use the recently developed Dynamic Materials Model tT,s,91 for analyzing the constitutive behavior of alpha-zirconium at high temperature in a wide range of strain rates. According to this approach, the power dissipation characteristics of the material through microstructural changes are evaluated with the help of strain-rate sensitivity and mapped as a function of temperature and strain rate. These processing maps are interpreted in terms of specific microstructural mechanisms that dominate in a given temperature-strain-rate regime. Using this approach, it is possible not only to optimize hot workability but also to delineate regimes of flow instability t9,1~ which should strictly be avoided in processing. J.K. CHAKRAVAR'ITY, Scientific Officer, is with the Metallurgy Division, Bhabha Atomic Research Centre, Bombay 400 085, India. Y.V.R.K. PRASAD, Professor, is with the Department of Metallurgy, Indian Institute of Science, Bangalore 560 012, India. M.K. ASUNDI, formerly Divisional Head, Physical Metallurgy Division, Bhabha Atomic Research Centre, is an Emeritus Scientist with the Naval Chemical and Metallurgical Laboratory, Tiger Gates, Bombay-400 023, India. Manuscript submitted March 14, 1990. METALLURGICALTRANSACTIONSA
II.
EXPERIMENTAL
The material used in the present study was commercially pure reactor-grade zirconium (containing approximately 1000 ppm of oxygen) obtained in the form of extruded and cold drawn rods of 15-ram diameter. Cylindrical compression specimens (10 mm in diameter and 15 mm height) were machined from the as-received rods. The machined samples were sealed in silica tubes u n d e r helium and beta-quenched from 1030 ~ after soaking for 30 minutes. Samples were deformed at constant true strain rates of 0.001, 0.01, 0.1, 1, 10, and 100 s -1 over a temperature range of 650 ~ to 850 ~ at 50 deg intervals to a true strain of about 70 pct. The compression testing was done on a computer-controlled servohydraulic test system (DARTEC, Stourbridge, West Midlands, United Kingdom) which has the facility for an exponential
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