Kinetics of Ordering in Fe 3 Al: Experiment
- PDF / 427,978 Bytes
- 6 Pages / 420.48 x 639 pts Page_size
- 81 Downloads / 211 Views
Kinetics of Ordering in Fe3AI: Experiment Brent Fultz, Zheng-Qiang Gao, and Lawrence Anthony California Institute of Technology, Dept. of Materials Science 138-78, Pasadena, California, USA ABSTRACT Alloys near the Fe 3 AI stoichiometry were prepared far from thermodynamic equilibrium by rapid solidification methods. These highly disordered materials were then annealed at low temperatures to develop B2 and D03 order. Both shortrange and long-range order (SRO and LRO) were measured by Mossbauer spectrometry and by x-ray diffractometry, respectively. The SRO was found to evolve in two stages: an initial quick growth of the SRO alone, followed by a slower growth of SRO along with LRO. As expected, at lower temperatures the absolute rates of growth of order were suppressed, but more interestingly, the relative rates of growth of B2 and D0 3 long-range order parameters were different at different temperatures of annealing. When the B2 order parameter is graphed against the D0 3 order parameter, at lower temperatures the "kinetic path" through this graph shows that B2 order evolves relatively more rapidly than D03 order. INTRODUCTION In thermodynamic equilibrium, alloys with the Fe 3 Al stoichiometry have B2 long-range order (LRO) below a critical temperature of about 800 'C. Below 550 'C, Fe 3A1 undergoes a secondary ordering transformation where D0 3 LRO appears within the B2 ordered structure. In thermodynamic equilibrium, the parameters of both B2 and D0 3 LRO increase with decreasing temperature. In thermodynamic equilibrium, the short-range order (SRO) also increases with decreasing temperature, but the equilibrium SRO remains non-zero even above the critical temperature for B2 order. The present paper is not concerned with the equilibrium state of order in Fe 3A1, but rather with the nonequilibrium states of order that evolve as a highly disordered alloy is annealed at low temperatures. The thermodynamic states of order lie at the end of these sequences of nonequilibrium states. Any interpretations of the actual sequence of nonequilibrium states, however, are within the field of kinetics, not equilibrium thermodynamics. The first goal of our investigation was to find the stages by which SRO and LRO evolve during the annealing of disordered, rapidly solidified Fe3 Al. (Perhaps, for example, the SRO evolves substantially before the LRO, or perhaps the SRO and LRO evolve together.) This goal was motivated in part by recent calculations of the kinetics of ordering in the pair approximation [1-4]. In these activated-state rate theories, disordered alloys were found to develop order in distinct stages. After an early, quick rearrangement of the vacancy populations, the first stage of ordering comprised a rapid growth of SRO without the growth of LRO. After a quiescent incubation time, the second stage comprised a slower growth of SRO with the growth of LRO. Some reasons to doubt the existence of the incubation time were given in Refs. [3,5]. The essential problem with rate theories in the pair approximation is their inc
Data Loading...
