Physical metallurgy of metastable bcc lanthanide-magnesium alloys for R = La, Gd, and Dy
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I.
INTRODUCTION
MOST of the
rare earth metals have a high temperature bcc polymorphic form. Although these phases have been known for over 30 years, little is known about their properties. In order to obtain a better understanding of the role of structure in determining the physical properties of metals, we have undertaken a study of the rare earth bcc phase stabilized by various alloying additions. Measurement of a property on a series of alloys and extrapolation to 100 pct rare earth in many cases will yield the value for the hypothetical bcc p h a s e , which can be compared with those of the other polymorphic forms of a given rare earth metal. The rare earth metals are a natural choice since they possess many other stable polymorphic forms: fcc, dhcp (a close-packed hexagonal structure with twice the normal c-axis length and an ABAC arrangement of close-packed planes, i.e., 50 pct fcc + 50 pct hcp), 6 or the Sm type (a close-packed hexagonal structure with 4.5 times the normal c-axis length and an A B A B C B C A C arrangement of the close-packed planes, i.e., 33 pct fcc + 67 pct hcp), and hcp. Another study which might prove interesting is the transformation kinetics of the decomposition of the bcc phase into one of the four above close-packed structures. Of these transformations, only the bcc ~ fcc (e.g., Fe) and bcc ~ hcp (e.g., Ti and Zi) have been thoroughly studied in nonrare earth metals. In earlier papers, we have described a systematic survey for bcc stabilizers using La as the solvent, rt,2~ Only Mg and Cd proved to be effective stabilizers. MagneJ.W. HERCHENROEDER, formerly Graduate Student, Ames Laboratory and Department of Materials Science and Engineering, Iowa State University, is with Delco Remy, Division of General Motors Corporation, 6435 S. Scatterfield Road, Anderson, IN 46013. P. MANFRINETIq, formerly Postdoctoral Associate, Ames Laboratory, is located at Via Acquistapace 4/5, 15067 Novi Ligure (AL), Italy. K.A. GSCHNEIDNER, Jr., Distinguished Professor and Senior Metallurgist, is with the Ames Laboratory and Department of Materials Science and Engineering, Iowa State University, Ames, IA 50011. Manuscript submitted October 6, 1988. METALLURGICAL TRANSACTIONS A
sium was chosen as the stabilizer over Cd for subsequent work, because Mg alloys are oxidation resistant, and some of the phase diagrams are known. In this paper, we report some of the quench factors that influence the final microstructure of bcc R-Mg alloys (R = La, Dy, and Gd). Because the resulting alloys are nonequilibrium systems, we have also tried to quantify their thermal stability and identify the mechanisms by which the metastable bcc phase reverts to the equilibrium c~ + RMg (CsCl-type structure) phases. II.
EXPERIMENTAL PROCEDURES
A. Alloy Preparation Alloys were made from high purity starting materials. The rare earth metals - - La, Gd, and D y - - w e r e produced by the Materials Preparation Center at the Ames Laboratory. Generally, the total transition metal impurity content was less than 30 p p m atomic. The to
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