TEM Studies Of Devitrification Products in Al-Gd-Ni-(Fe) Alloys.
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TEM Studies Of Devitrification Products in Al-Gd-Ni-(Fe) Alloys. A.L. Vasiliev, M. Aindow, M.J. Blackburn, T.J. Watson1 Department of Metallurgy and Materials Engineering, Institute of Materials Science, University of Connecticut, Storrs, CT 06269-3136, USA. 1 Pratt & Whitney, Materials & Process Engineering, Structural Alloys & Processes, 400 Main Street, Mail Stop 114-40, East Hartford, CT 06108, USA. ABSTRACT Metallic glasses can form in certain Al-Gd-Ni alloys. Devitrificaion occurs on subsequent heating to intermediate temperatures and although some phases form as expected the crystal structures of others are less certain. This paper presents a summary of transmission electron microscopy (TEM) data acquired from four devitrified Al-Gd-Ni(-Fe) alloys. It is shown that the alloys exhibit nanocrystalline microstructures consisting of face-centered-cubic α-Al, binary Al3Gd, and ternary phases. An Al-(Gd,Fe)-Ni phase forms as rods and exhibits the orthorhombic Al19Ni5Gd3 structure. Plates of the rhombohedral phase Al23(Ni,Fe)6Gd4 are also present. Complex faulting is observed in both ternary phases. INTRODUCTION Glass-forming Al : transition metal : rare earth alloys were first reported in 1988 by two groups, Inoue et al. [1] and Shiflet et al. [2]. Such glasses are metastable and devitrify if reheated to intermediate temperatures, this occurs in several stages and result in a variety of microstructures. These range from nano-sized aluminum particles in an amorphous matrix to small intermetallic particles in fine-grained aluminum. Recently, Guo et al and Gao et al showed that some of the best glass forming characteristics in aluminum based alloys is found in the AlNi-Gd system [3.4]. It was proposed that the stability of the amorphous phase could be tailored by adjusting the specific composition since additions of Ni promote the formation of nanoscale α-Al whereas additions of Gd suppress it. In a related study it was shown that the addition of a second transition element especially iron could retard the kinetics of glass decomposition [5]. Gao and Shiflet [6-8] also studied details of the phase formation sequence in these alloys during devitrification. Two binary phases were observed: Al3Ni and Al3Gd, the former exhibited the orthorhombic Fe3C-type structure [9], but the structure of the latter was not defined. Two polymorphs have been reported for Al3Gd; one with the hexagonal Ni3Sn-type structure and another with the rhombohedral BaPb3-type structure [10,11]. The identification of the ternary phases is still controversial. Rykal et al [12] proposed a partial isotherm for the Al-Ni-Gd phase diagram at 800˚C and in this, and other studies [13-15], several ternary phases with Al contents of >66 at.% were identified. These are (in order of decreasing Al content): Al16Ni3Gd, Al15Ni3Gd2, Al15Ni4Gd2, Al19Ni5Gd3, Al23Ni6Gd4, Al9Ni3Gd, Al4NiGd. Gao et al [7,8] reported that the phase Al15Ni3Gd2 (denoted τ1 by these authors) is the most Al-rich ternary formed during devitrification of a series of Al100-x-yNixGdy a
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