Microstructure and Mechanical Properties of Ll 2 -Structure Alloys Based on Al 3 Zr
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MICROSTRUCTURE AND MECHANICAL PROPERTIES OF L1 2 -STRUCTURE BASED ON Al 3 Zr
ALLOYS
J. H. Schneibel and W. D. Porter Metals and Ceramics Division, Oak Ridge National Laboratory, P. 0. Box 2008, Oak Ridge, TN 37831
ABSTRACT The tetragonal compound Al 3 Zr can be transformed into the cubic L1 2 structure by additions of Cr, Fe, Ni or Cu. This result is interpreted in terms of Pettifor's structure map. The transformation is accompanied by substantial softening. The significant porosity of cast buttons which develops during the homogenization required to produce the LI 2 structure is attributed to the Kirkendall effect. Al-5.5Fe-25Zr (at. %) specimens with the Li 2 structure can be deformed at room temperature in compression, and their yield stress has been determined as a function of temperature. Al-Fe-Zr is nevertheless a brittle material. This is illustrated for a Al-Fe-V-Zr L12-structure compound, the fracture toughness of which is only slightly higher than that of sintered SiC. Evidence is presented to suggest that L1 2 formation is accompanied by an increase in toughness. Significant improvements in the mechanical properties of Al 3 Zr-type alloys may possibly be achieved by improved processing.
INTRODUCTION Owing to the successful ductilization of polycrystalline Ni 3 AI [I], intermetallic compounds have been of great interest in recent years [2,3]. There are many intermetallic compounds which have higher melting points and lower densities than Ni 3 Al, suggesting the possibility of even more advanced materials in those structural applications, where temperature and density are critical. However, such materials, like most intermetallic compounds, tend to be brittle. If alloying does not result in any measurable ductility then the fracture toughness instead of ductility becomes the appropriate parameter characterizing the material. In fact, determination of the fracture toughness then becomes an important tool to guide alloy design. A1 3 Zr with the D02 3 structure is an example of a brittle intermetallic compound with high melting point (1853 K) and low 3 density (4.1 Mg/m ). The present work is an attempt to measure the fracture toughness of a cubic ordered (L1 2 ) modification of this compound. In addition, crystallographic and microstructural findings, as well as compression strength data are presented.
EXPERIMENTAL PROCEDURE Intermetallic alloys based on AI 3 Zr were prepared from pure elements (typical purity 99.9% or better) by arc-casting in argon, and homogenizing for 2 h at 1573 K in vacuum (10-4 Pa). Their compositions will be stated in at. %. Vickers hardnesses were determined with a Leco hardness tester. Compression testing of electrodischarge (ED) machined cube-shaped specimens with an edge length of 4 mm was carried out with a servo-hydraulic testing machine equipped with a vacuum furnace (10-4 Pa). Toughness measurements were performed with ED machined specimens 18 mm long and 3 mm x 4 mm in cross section. The 4 mm wide sides were mechanically polished in order to
Mat.Res. Soc. Symp. Proc.Vo
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