Phase transformations in Ti3Al based alloy
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A 'super a2' Ti3Al based alloy with additions of niobium, chromium and tantalum was studied with respect to phase transformation under different heat treatment and aging times and temperatures. As received samples were heated at temperatures ranging between 1000 degrees C ad 1300 degrees C and quenched to retain the high temperature microstructure. Quenched samples were aged between 500 degrees C and 850 degrees C for various times and transformation were studied using dilatometry, x-ray analysis, transmission electron microscopy and microhardness testing. It was found that at around 650 degrees C the transformation of Beta-a2 occurred after two hours of aging, while below this temperature, aging for up to 100 hours at 500 degrees C produced only the Beta-w transformation with very little a2. Also, an 'w-type' phase was observed at this aging temperature. Microhardness measurements and x-ray diffraction confirmed the above results. Quenching from high temperature showed the presence of lath-like features which were poor in Ta and Cr and had the B2 structure. Finally some regions with an orthorhombic structure were observed in the as quenched samples. Overall, peak hardness was obtained after aging the quenched Beta phase sample at 650 degrees C. This corresponded to a macrostructure of retained beta with a fine acicular structure of a2..
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