Experimental Determination of the Allotropic Transition Temperature Between Tetragonal and Orthorhombic Al 4 Sm Metastab
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rmations in aluminum-rare-earth alloys have been observed to involve competition between a large number of stable and metastable phases[1–19] that influence solidification and glass formation[20–23] as well as the evolution of various crystalline or partially crystalline devitrification nanostructures[24,25] with a
R.E. NAPOLITANO is with the Division of Materials and Engineering, Ames Laboratory, DOE, Ames, IA and also with the Department of Materials Science and Engineering, Iowa State University, Ames, IA. Contact e-mail: [email protected] S.H. ZHOU and F.Q. MENG are with the Division of Materials and Engineering, Ames Laboratory, DOE. X. YANG is with the State Key Laboratory of Powder Metallurgy, Central South University, Changsha, Hunan, P.R. China. Manuscript submitted October 5, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
range desirable properties.[25,26] Perhaps the most notable of the aluminum-rare-earth binary systems is Al-Sm, for which many complex transformation sequences have been observed,[2–6,9,14,16] including the formation of large-unit-cell intermetallics during devitrification of the alloy glass.[17–19] The highly competitive metastable phase landscape is also evident during solidification and subsequent annealing, during which b-Al4Sm (tetragonal) and c-Al4Sm (orthorhombic) phases compete with each other and with the equilibrium Al-fcc and d-Al3Sm phases.[3,4,9,14] The relevant portion of the Al-Sm phase diagram is shown in Figure 1.[27] Indeed, first principles calculations show that d-Al3Sm and Al-fcc are the low-temperature equilibrium phases, while c and b are metastable states for the Al4Sm stoichiometry,[28] with c being favored over b at zero Kelvin.[28] Experimentally, the b phase has been observed after solidification in the range of 8 to 14 at. pct Sm, with a transition to the c phase occurring upon subsequent low-temperature (< 873 K) annealing,[3,4,9,14] indicating a lower bound for the c-b transition temperature. In this communication, we report on the experimental determination of the c-b transition temperature. Toward the goal of building a comprehensive description of the thermodynamic landscape surrounding competitive crystallization in this system, we examine the relative stability of these two metastable phases and the corresponding allotropic transition. Experimental determination of the c-b equilibrium presents a challenge since both will spontaneously decompose to Al-fcc + d-Al3Sm in the relevant temperature regime. Our methods include arc melting and heat treatment of solidified structures, with subsequent analysis conducted through scanning electron microscopy (SEM), electron probe microanalysis (EMPA), differential scanning calorimetry (DSC), and X-ray diffraction (XRD). Alloy test specimens of Al-20 at. pct Sm were prepared by arc melting the elemental constituents (0.99999 Al and 0.999 Sm, by weight[29]) on a copper hearth in a high-purity (0.99999) argon atmosphere. Each alloy specimen, approximately 15 g, was arc-melted three times to ensure homogeneity. An example of t
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