Irreversible transformation in as-cast FeAl B2-ordered alloy obtained by melt spinning
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R. Montanari INFM-Dipartimento di Ingegneria Meccanica, Universita´ di Roma “Tor Vergata,” via di Tor Vergata 110, I-00133 Roma, Italy
C. Testani Centro Sviluppo Materiali, via di Castel Romano 100-102. I-00129-Roma, Italy
G. Valdre` INFM-Dipartimento di Fisica, Universita´ di Bologna, viale Berti Pichat 6/2, I-40127 Bologna, Italy and INFM-Dipartimento di Scienze della Terra e Geo-Ambientali, P.P. San Donato 1, Universita` di Bologna, I-40126 Italy (Received 20 March 1998; accepted 13 December 1999)
The aim of the work described in the present paper was to investigate the microstructural stability during annealing treatments of a Fe–Al alloy obtained by melt spinning. To this purpose internal friction (IF) and dynamic modulus (Md) measurements were employed, and the results correlated with x-ray diffraction, optical microscopy, and scanning and transmission electron microscopy observations. In particular, the B2-ordered Fe–38A1–2Cr–0.015C–0.003B (in at.%) alloy was studied during repeated heating runs from room temperature to 823 K by IF and Md. The modulus exhibited a broad maximum (in the range of 600–800 K) only in the first run. On the basis of transmission electron microscopy and x-ray diffraction analysis, the irreversible transformation was explained by considering a two-stage process that occurs when vacancies in supersaturation move toward dislocations. The first stage is connected to dislocation locking; the second one is due to annihilation of some vacancies by dislocation climb.
I. INTRODUCTION
The ordered Fe–Al compounds, in particular those with D03 (Fe3Al) and B2 (FeAl) structure are of high commercial interest because of their excellent resistance to oxidation, low density, high corrosion resistance, in addition to a good mechanical strength at intermediate temperature if compared to the majority of the iron- or nickel-based alloys. These properties make them candidate materials for several applications.1 However, the development of these materials has been limited by at least two factors: low ductility at room temperature and low mechanical strength and creep resistance above 850–900 K.2– 4 The deleterious environmental effect on ductility can be decreased by increasing the aluminium content to near to the equiatomic stoichiometry, nevertheless stoichiometric Fe–Al fractures before yielding.5,6 Two approaches are commonly followed to improve the ductility of iron-aluminides: the first one consists in the fine control of the grain boundary cohesion, by using microalloying methods; the second one is based on procJ. Mater. Res., Vol. 15, No. 3, Mar 2000
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esses which reduce the crystallite size and shape by employing different methods or treatments.1,2 In particular, grain refinement was recently achieved by using various synthesis techniques. These include powder extrusion with or without second phase particles,7 rapid solidification,8,9 and mechanical alloying.10,11 Rapid solidification processes have been extensively employed to enhance st
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