Evolution of microstructures in the nickel modified titanium trialuminides near the L1 2 phase field
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I.
INTRODUCTION
THE LI 2 trialuminides (based on the DO22 A13Ti intermetallic transformed to LI2 by macroalloying) have five independent slip systems; they are not particularly strong or hard at room temperature, and because they do not fracture intergranularly at ambient temperatures, they appear not to have weak grain boundaries, v-4j However, they without exception exhibit a poor fracture toughnessY~ Since the L12 trialuminides possess some attractive properties such as low density, high elastic moduli, and good oxidation resistance, t2,3,4jtheir poor toughness is the most important drawback to be overcome before they can find extensive engineering application as structural materials. One potential way of improving fracture toughness of the L12 trialuminides could be by producing the in situ composite alloys in which the reinforcing phase can be introduced via either solidification or solid state precipitation, tS~ However, for that purpose, the understanding of the pertinent phase diagrams becomes critical. In the present work, the microstructural evolution in the as-cast and homogenized multiphase trialuminide-base alloys, near the Al-rich comer of the ternary A1-Ti-Ni system, is reported. Some preliminary results on the microstructural evolution in several alloys in the preceding system have already been reported.t61 The major aim of the present work is to establish more accurately the exact phase and phase mixture fields in the near-L12 section of the A1-Ti-Ni system.
S. BISWAS, formerly Graduate Student, Department of Mechanical Engineering, University of Waterloo, is Process Engineer, Greening Donald Co. Ltd., Erin, ON, Canada NOB 1T0. R.A. VARIN, Professor of Materials Science and Engineering, is with the Department of Mechanical Engineering, University of Waterloo, Waterloo, ON, Canada N2L 3G1. Manuscript submitted October 13, 1994. METALLURGICAL AND MATERIALSTRANSACTIONS A
II.
EXPERIMENTAL PROCEDURES
The intermetallic composite alloys with the different target compositions, as shown in Table I, were fabricated via induction melting of pure elemental metals, aluminum (99.99999 pct superpurity grade), titanium (99.9 pct), and nickel (99.9 pct), followed by subsequent homogenization at 1000 ~ for 48 hours under a high purity argon atmosphere (fumace cooling to room temperature in about 12 hours). The details of the melting as well as homogenization procedures were described in a previous article. I6] Some loss of aluminum during melting was observed. As a result, the ingot was always found to be deficient in aluminum and enriched in titanium with no change in nickel content with respect to the target composition. It was, therefore, necessary to add extra aluminum to make the starting composition of the melting charge according to the stoichiometry of Alx+2Tiy_2Niz (at. pct) in order to attain the target composition AlxTiyNiz (at. pct) in the homogenized condition. For investigating the microstructural stability, alloy 4 was also homogenized at 1000 ~ for 100 hours, followed by furnace cooling to room
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