Phase relationships in the neodymium-magnesium alloy system
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I.
INTRODUCTION
IT is well known that magnesium alloys, possessing such properties as high strength-to-weight ratio, good dimensional stability, high damping capacity, low thermal stress modulus, high thermal diffusivity, and high electrical conductivity, have important technological applications, especially in aerospace programs. Rare earths constitute an important class of alloying elements to magnesiumbased alloys because of their ability to enhance the hightemperature properties and casting characteristics. Recently, our group studied the Sm-Mg systemtq as part of a work on the alloys of several metals with the rare earths. The systematics of the rare earth alloys is of particular interest also, because it can be considered a tool for the investigation of the influence of several elemental parameters on the alloying behavior. The effects of progressive variations in the component properties on the properties of the various phases formed may be studied by a systematic investigation of the series of phase diagrams formed by the different rare earths (R) with a given element (Me). In the lanthanide series, a number of properties, such as atomic size, electronegativity, etc., vary in a regular manner. As a consequence, by substituting the different trivalent lanthanide metals in a series of R-Me alloys, a number of empirical regularities can be deduced, and certain theories can be expressed. Various authors have reported data on this subject; a general discussion has been presented by Gschneidnert2] and by Colinet et al. [31 Moreover, several groups of rare earth alloys have been discussed in detail: see, for instance, the alloys with Ag, t41Ga, ~5]In, (6,71Sn, t3,8] Bi, O1and T1.[~~ Studies of the crystal structures and of the phase diagrams in the binary systems between rare earth metals S. DELFINO, Professor, A. SACCONE, Associate Professor, and R. FERRO, Professor and Director, are with the Istituto di Chimica Generale, Universitfa di Genova, Genova, Italy. Manuscript submitted November 3, 1989. METALLURGICAL TRANSACTIONS A
and magnesium are far reaching but far from complete. The following phase diagrams of magnesium with lanthanides have been studied in detail: La-Mg, tH,121 CeMg, [11'131 Sm-Mg, tq and Gd-Mg. t121 In this paper, we report the results obtained in the study of the phase equilibria existing in the Nd-Mg system. The main points made in previous investigations on the Nd-Mg system are as follows: (1) Terminal solid solubility of neodymium in magnesium: the maximum solid solubility, determined by Park and Wyman['4] using the parametric method, is 0.1 at. pct Nd at the 546 ~ eutectic temperature. (2) Terminal solid solubility of magnesium in a-neodymium: the maximum solid solubility of Mg in a-nexxiymium, determined by Joseph and Gschneidnex415] using the parametric method, is 8.2 at. pct Mg at the 551 ~ eutectoidal decomposition of r-neodymium. (3) Crystal structures of the intermediate phases: NdMg (cubic, cP2 CsC1 type), [16'17'18]NdMg2 (cubic, cF24 CuEMg type), [19'201NdMg3 (cubic, cF16 BiF3 t
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