Processing and Characterization of Al 2 Ti/Al 3 Ti Two-Phase Alloys
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dition. A second button of each composition was solution heat-treated at 12000 C for 40 hr in argon and cooled to room temperature at 40°C/min to produce the cast & solution treated material condition. A third button of each composition was solution heat-treated at 1200 0 C for 2 hr, cooled to 9000 C at 40°C/min and then held at 900'C for 12 hr before cooling to room temperature at 40°C/min, all in argon, to produce the cast & 2-step heat treated material condition. Finally, one sample was cut from each of the three as-cast buttons and compressed to a 50% reduction in height at 1100IC at a displacement rate of 4 x 10-3 mrnis to produce the hot forged material condition. Samples of each of the four material conditions for each of the three compositions were polished and then analyzed using scanning electron microscopy and energy dispersive spectroscopy (SEM/EDS). Hardness values were determined using a Vickers indenter at a 200 g load. RESULTS AND DISCUSSION The overall composition for each sample was measured by EDS on the SEM. All of the samples have an Al concentration averaging about 1.5 at.% lower than the nominal composition. The average overall compositions for the three alloys were determined to be 68.5% Al and 31.5% Ti, 69.5 at.% Al and 30.5% Ti, and 70.2% Al and 29.8% Ti for the nominally 70%, 71% and 72% Al alloys, respectively. For the remainder of this paper, the three compositions will still be identified by their nominal composition, i.e. 70%, 71% and 72% Al. As-Cast
The microstructures of the three alloys in the as-cast material condition are fairly similar. A low magnification backscattered electron micrograph of as-cast Ti-7 1%Al is shown in Figure 1.The microstructure consists primarily of two matrix phases, one which appears light and one which appears dark in backscattered imaging mode. The main difference between the three compositions is the volume fraction of the darker phase, with a lower volume fraction of this phase in the 70% Al alloy and a larger volume fraction in the 72% Al alloy. Additionally, at higher magnifications, bright needle-like particles approximately 0.5 .m in diameter are discernable, as seen in Figure 2 for the 70% Al alloy. The composition of the light matrix phase is approximately 68% Al and 32% Ti, with perhaps a slightly higher Al concentration in the 72% Al alloy. The light matrix phase is more Tirich than the overall composition but not quite as high as the 33% Ti content expected for A12Ti. The dark matrix phase has a composition of about 72.5% Al and 27.5% Ti. This phase is more Al-rich than the overall composition, however it does not contain as much Al as stoichiometric A13Ti. The composition measured for the needle-like particles varied between 60 and 64% Al (40 and 36% Ti), which lies between the compositions for TiAl and Al 2Ti. However, it is reasonable to assume that the actual Ti concentration in these particles is higher than that measured by EDS since the particles are less than, or on the order of, 0.5 jum wide and it is likely X-rays collected by the
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