Hiroshi Kimura and Takashi Uchino
Department of Mechanical Engineering, School of Systems Engineering, National Defense Academy, Yokosuka 239-8686
This article describes the system design and the development of the thermo-mechanical processing for the high-speed forging with multi-variable control, in which one can evaluate the superplastic flow and achieve a nanocrystalline control. This nanoprocessing consists of three discrete components, the thermal system and the pulse system equipped with the manipulative pulse current discharging and the mechanical system with servo-control hydraulic pressing. The process analysis for the isochronal forging at a constant displacement rate permits us to derive the strain rate sensitivity exponent(m) of approximately 0.4 in the constitutive equation of \dotεm=Aσ exp(-Q/kT) for the superplastic flow in full density nanocrystalline (Ti50Al50)90Fe10 with the average crystallite size of 20 nm. For the full density amorphous Ti50Al50 sample, the isochronal forging under the constant load leads to a relatively high plastic strain rate of approximately 1 × 10-2 s-1 at 977 K and the compressivility of 0.89 at the maximum in the die of this experiment, and exhibits the Newtonian viscous flow that is given by a relation of η = η 0 exp(Qv/kT) in a supercooled liquid. The apparent activation energy(Q) for nanocrystalline Ti50Al50 in isochronal forging is derived at 113 kJ⋅mol-1 that is a nearly quarter of the value(467 kJ⋅mol-1) of the amorphous alloy.
(Received April 7, 2003)
thermo-mechanical nanoprocessing, isochronal forging, constant displacement rate control, in process nanocrystalline control, strain rate sensitivity exponent, super cooled liquid, activation energy
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