Al 2 Ti precipitation in Al 64 Fe 8 Ti 28 alloy
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I.
INTRODUCTION
T H E D022-type ordered AI3Ti is very brittle because of a lack of a sufficient number of independent slip systems. However, because of a high melting point, low density, superior strength-to-weight ratio, and high oxidation resistance, it is a desirable material for hightemperature structural applications. However, because of its limited ductility at room temperature, improving the ductility of A13Ti is of great interest, t~-61 The D022ordered AI3Ti can be transformed into high-symmetry, cubic L12 structure by adding transition elements such as V, Cr, Mn, Fe, Co, Ni, or Cu, with a goal of better ductility, tT-~~ Although the converted L12 trialuminides have five independent slip systems, they remain brittle and fracture by transgranular cleavage, which has been attributed to either inhibited dislocation emission from crack tips or intrinsically low cleavage strength, tL2j The added transition elements are not stable over a wide range of temperatures. They will precipitate out during cooling, and therefore, the L12 phase field in the A1-Ti-Fe ternary phase diagram decreases as the temperature drops. 131 The composition without the added transition elements is close to the range of AI2Ti phase, so precipitated AI2Ti has been observed in transition metal modified Ti trialuminides. I~L~21 There are two A12Ti structures reported in the A1-Ti binary system, tl31 A12Ti~, which is stable either higher than 1250 ~ or lower than 700 ~ and AI2TiH, which is stable between 700 ~ and 1250 ~ Both have tetragonal structures and can be described as nonconservative antiphase boundary modulations along the c axis of the cubic L12 structure. Both AI2Ti~ and A12Ti~ have also been observed precipitating in Cu-modified A13Ti alloy, l~2J II.
EXPERIMENTAL
The alloy with composition A164FesTi28 was used in this study. The nominal composition was chosen at the geometric center of the LI2 phase field of the A1-Fe-Ti system at 1200 ~ Alloy ingots were prepared by arc melting pure starting materials (Ti, 99 pct; A1, 99.999 pct; Fe, 99 pct) in a purified Ar atmosphere. After T.Y. YANG, Research Assistant, and E. GOO, Associate Professor, are with the Department of Materials Science and Engineering, University of Southern California, Los Angeles, CA 90089-0241. Manuscript submitted September 13, 1993. METALLURGICAL AND MATERIALS TRANSACTIONS A
casting, the ingots were homogenized at 1200 ~ for 50 hours and at 1000 ~ for 50 more hours in Ar and then cooled slowly at a rate of about 46 ~ to room temperature. Some homogenized samples were reheated to 1210 ~ for 5 hours and quenched in water. By using the Instron Universal Testing Instrument model 4204 at a crosshead speed of 0.05 in./min, all the as-cast, homogenized, and quenched samples were compressed at liquid N temperature until they fractured. Homogeneity of the samples was examined under backscattered electron imaging and energy-dispersive spectroscopy (EDS) analysis. The X-ray diffractometer scans were done before and after the homogenization heat treatment for determin
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