Mechanical behavior and microcracking of cubic ternary zirconium trialuminides
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I.
INTRODUCTION
IT has recently been reported ~ that cubic (L12) modifications of tetragonal (D023) A13Zr intermetallic macroalloyed with Cu, Mn, and Cr can be successfully fabricated by induction melting. Microstructure of these materials is characterized by low porosity and relatively low volume fraction of second phase. The amounts of the material obtained allow us to carry out mechanical testing. The purpose of the present article is to describe and interpret experimental observations on the mechanical behavior of cubic A13Zr-base intermetallic alloys modified with Cu, Mn, and Cr. Mechanical testing of as-cast specimens was carried out by measuring Vickers microhardness at room temperature and by compression tests from room temperature up to 900 ~ at every 100 ~ interval. Also, microcracking development during deformation was studied by observing formation of microcracks in several specimens loaded to progressively increasing levels of stress. II.
EXPERIMENTAL PROCEDURE
The intermetallic alloys were prepared from pure elements: aluminum (99.99999 pct), zirconium (99.8 pct), copper (99.9 pct), manganese (99.9 pct), and chromium (99.99 pct). The present article will deal with the mechanical properties of AI-12.5Cu-25Zr, A1-9Mn-25Zr, and A1-8Cr-25Zr (at. pct) intermetallic alloys, referred to hereafter as 12.5Cu, 9Mn, and 8Cr, which were processed by induction melting under a high-purity argon atmosphere with a pre-evacuation of the induction melt-
I.S. VIRK, Graduate Student, and R.A. VARIN, Associate Professor of Materials Science and Engineering, are with the Department of Mechanical Engineering, University of Waterloo, Waterloo, ON, Canada N2L 3G1. Manuscript submitted March 22, 1991. METALLURGICAL TRANSACTIONS A
ing chamber. The details of the melting procedure are described in References 1 through 3 (designated as method # 3 in Reference 1). The microstructure (porosity and second phase) and composition of as-cast material were investigated by optical microscopy (Nomarski contrast), scanning electron microscopy, X-ray diffraction, and a full quantitative energy dispersive spectroscopy analysis. The results of these tests have already been reported in References 1 through 3 and will not be discussed here. However, for the sake of clarity, it should be mentioned that the as-cast 12.5Cu and 9Mn alloys contain around 2 pct porosity and about 2 pct second phase, and 8Cr alloy contains 1.4 pct porosity and 1.2 pct second phase. The latter alloy also exhibits slight dendritic segregation.t31 No substantial dendritic segregation was found in the as-cast 12.5Cu and 9Mn alloys, and their compositions were very close to the target values, u-3j Vickers microhardness tests were performed using loads from 25 to 2000 g at 15 seconds dwell time for all alloys. Surface of the test sample was first polished using a grit 600 emery paper followed by alumina powder 0.3 /xm. A minimum of five indentations were made at each load. Both diagonals were measured with an optical microscope using an automated Java image analysis p
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