Recovery and recrystallization of the deformed, orderable alloy (Co 78 Fe 22 ) 3 V
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M. Takeyama,b) J. A. Horton, and C.T. Liu Metals and Ceramics Division, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, Tennessee 37831-6115 (Received 6 June 1990; accepted 27 September 1990)
An alloy of composition (Co78Fe22)3V, which orders to an Ll 2 superlattice below a critical temperature {Tc) of 910 °C, was rolled to 25-50% reduction in the initially ordered condition and annealed at various temperatures above and below Tc and examined by hardness, tensile testing, optical and electron microscopy and dilatometry, in order to study the progress of recovery and recrystallization. Recrystallization was severely retarded on annealing below Tc; close to Tc, recrystallization was =300x slower in the ordered than the disordered range. Although recrystallization started promptly, predominantly at grain boundaries, very rapid recovery-softening of the unrecrystallized regions progressively reduced the driving force for recrystallization and slowed it down drastically. However, at 770° and 500 °C, recovery-softening was replaced by some recovery-hardening (i.e., strain-age hardening). Above Tc, recrystallization was complete in a few seconds and a special annealing method was needed to measure such times accurately. Dilatometric measurements showed that most of the order destroyed by rolling was restored long before recrystallization began, but the restoration was never complete unless the alloy was heated up through Tc and then slow cooled. Electron microscopy showed no sign of any antiphase domains in recrystallized grains except for a few isolated domain boundaries on annealing at 770 °C. A model is proposed to rationalize the incidence of recovery-softening or strain-age hardening at different annealing temperatures.
I. INTRODUCTION
The greatly expanded research in the last few years on the mechanical properties of potentially useful structural intermetallics such as Ni3Al, NiAl, Fe3Al, FeAl, and Ti3Al has not been matched by similar research on the response of deformed intermetallics to heat-treatment. Such researches as have been published have mostly been devoted to model systems with relatively low critical temperatures for disordering (Tc), such as Cu3Au, Ni3Fe, and FeCo, which have Tcs in the range 390-700 "C.1 The whole field of the response of deformed orderable alloys to annealing has recently been reviewed by Cahn.2 The principal generalization which emerged was the drastic reduction of grainboundary mobility when such an alloy is in the ordered condition so that recrystallization is severely inhibited by the presence of atomic order. It was also seen that the kinetic competition between recrystallization and
a)
Address correspondence to this author. Current address: National Research Institute for Metals, 2-3-12 Nakameguro, Meguro-ku, Tokyo 153, Japan.
b)
the reordering of an alloy which had been partly disordered by cold work can lead to complex behavior, such as the existence of a temperature range in which recrystallization does not occur, sandwiched between temperature ranges in whic
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