Microstructural characterization of rapidly solidified Al-Ta alloys
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I.
INTRODUCTION
R A P I D solidification of metals and alloys has been found to result in the formation of metastable phases including supersaturated solid solutions, metastable crystalline phases, and metallic glasses. In addition, it also results in refinement of segregation patterns and grain size. 1-4 It has been demonstrated that rapid solidification processing techniques offer unusual potential for the development of newer alloys with improved properties for demanding applications. 5'6 Although metallic glasses have received considerable attention till recently, the discovery that quasicrystals with icosahedral symmetry can be produced by rapid solidification has further accelerated research in the area of microcrystalline alloys. 7 Several investigations have been conducted on the formation of metastable intermediate phases, their morphology, and transformation to the equilibrium phases in Al-base alloys rapidly solidified from the melt (see, for example, References 8 through 10). Some of these studies have undoubtedly helped in gaining an understanding of some interesting features of decomposition behavior in the Al-transition metal systems. Very little work has been reported on the metastable effects produced in rapidly solidified A1-Ta alloys, and hence a detailed electron microscopy study of binary A1-Ta alloys quenched rapidly from the liquid state was undertaken. This paper presents the microstructural features observed in the alloys both in the as-solidified as well as annealed conditions. The equilibrium solid solubility of Ta in A1 at room temperature is negligible, and the maximum solid solubility even at the peritectic temperature of 940 K is only 0.25 pct" (all compositions are expressed in weight percent throughout the text). At the Al-rich end of the A1-Ta phase diagram, the intermetallic phase in equilibrium with A1 at room temperature is A13Ta. Table I lists the crystal structure data of S.SINGH is Assistant Professor, Department of Metallurgical Engineering, Bihar Institute of Technology, SINDRI-828 123, India. S. LELE and C. SURYANARAYANA are Professors, Department of Metallurgical Engineering, Banaras Hindu University, VARANASI-221 005, India. Manuscript submitted May 27, 1986.
METALLURGICAL TRANSACTIONS A
the intermediate phases in the A1-Ta system. 12Schustefl3 has recently reported an orthorhombic structure for the A13Ta2 phase which undergoes a polymorphic transformation to another orthorhombic structure at 1498 K. However, the structures have not been completely solved. On rapid quenching from the melt a metastable A1Ta3 phase with a bcc structure and a = 0.3283 nm has been reported. ~4
1I.
EXPERIMENTAL PROCEDURE
Two Al-rich A1-Ta alloys containing 3 and 6 pct Ta were prepared from 99.99 pct pure A1 and Ta rods in an arcmelting furnace under a protective argon atmosphere. The alloys were subjected to repeated remelting to achieve homogenization and were cast in the form of buttons. X-ray diffraction patterns recorded from the annealed alloys showed that they consist of A1 solid
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