Kinetic paths of B2 and DO 3 order parameters: Experiment
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Rapidly quenched powders of Fe3Al were subjected to thermal annealings at temperatures well below the critical temperatures for B2 and DO3 ordering. X-ray diffractometry was used to measure the subsequent evolution of B2 and DO3 long-range order. It was found that the relative rates of change of B2 and DO3 order parameters were temperature dependent; hence at different temperatures the alloy passed through different states of order en route to thermal equilibrium. These temperature dependences of "kinetic paths" can be understood in terms of a theory of kinetic paths based on the kinetic master equation. The theory indicates that the temperature dependence of the observed kinetic paths originates from having firstnearest-neighbor interactions that are stronger than second-nearest-neighbor interactions. This seems consistent with previous thermodynamic analyses of critical temperatures of Fe3Al.
I. INTRODUCTION In this paper we demonstrate a kinetic phenomenon involving the independent evolution of long-range order (LRO) parameters in Fe3Al. According to the equilibrium phase diagram of the Fe-Al system,1'2 Fe3Al is in the DO3 phase below about 550 °C. Above this temperature Fe3Al loses its DO3 LRO but continues to possess B2 LRO until about 800 °C. From about 800 °C to its melting point at about 1500 °C, Fe3Al exists as a solid solution of Al in cc-Fe. Because Fe3Al can assume two different types of LRO, two order parameters are needed to characterize fully the state of LRO in this alloy. In the present study we employed x-ray diffraction measurements of superlattice peak intensities to determine the relative amounts of B2 and DO3 LRO in Fe3Al. There have been a number of previous studies of the kinetics of ordering in Fe3Al. Many of these studies were concerned with the determination of the Fe-Al phase diagram in the region of 20-30 at. % Al.3"6 A theoretical and experimental study,7 more akin to the work reported here, investigated the rates of growth of both B2 and DO3 order parameters. This latter study reported variations in the amounts of B2 and DO3 order when Fe3Al was quenched from different high temperatures. These variations in B2 and DO3 order parameters reflect differences in the equilibrium degrees of B2 and DO3 order near the critical temperature for DO3 order. Otherwise when quenched materials were annealed at low temperatures, this previous study reported no evidence of an independent evolution of the two order parameters. In principle, there is no reason why the B2 and DO3 order parameters must evolve at the same relative rates at different low temperatures. In the present study we sought to determine if the choice of annealing temperature could alter the relative rates of evolution of B2 and DO3 order. We annealed our initially disordered materials at temperatures well below the critical temperatures of the B2 and DO3 order-disorder transformations. At these low tempera1140
tures the equilibrium values of both B2 and DO 3 order parameters should be nearly their maximum values for Fe3Al of 0.5 an
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