Experimental studies of the phase separation mechanism in Ti-15at%A1

The phase decomposition mechanism operating in a binary Ti-Al alloy in the two-phase alpha + alpha(2) region has been investigated. A Ti-15at%Al alloy was aged in the range 550-750 degrees C for various times to produce mater ial at different stages of phase separation. The resulting microstructures have been examined experimentally using three-dimensional atom probe field ion microscopy and neutron diffraction. Results indicate that significant o rdering occurs in the system prior to the onset of chemical phase separatio n which occurs via a spinodal reaction. The mechanism is believed to lie in the class of 'conditional spinodal' mechanisms in which one stage of the p hase separation process is thermodynamically contingent upon another. (C) 1 999 Elsevier Science S.A. All rights reserved.