PRECIPITATION OF ORDERED ALPHA(2) PHASE IN TI-6-22-22 ALLOY

Microstructural characterization of Ti-6A1-2Cr-2Mo-2Sn-2Zr (Ti-6-22-22 ) after various heat treatments has been performed using optical micro scopy, scanning electron microscopy (SEM), transmission electron micro scopy (TEM), and high resolution electron microscopy (HREM). The prese nce of very fine alpha(2) precipitates has been detected by electron d iffraction in conventionally heat treated samples. The alpha(2) precip itates were observed within the alpha matrix of the ''triplex'' heat t reated materials and also in samples cooled from the beta phase field with cooling rates of less than 5.5 degrees C/s. It was found that the alpha(2) precipitates were homogeneously distributed within the alpha phase with sizes of about 1-2 nm. Long time ageing at 650 degrees C f or 1000 h stabilized the alpha(2) precipitates which grew to 5-10 nm i n diameter and 20-50 nm in length. Structural stability of the alpha(2 ) precipitates has been discussed in terms of chemical composition and lattice mismatch between alpha 2 precipitates and the alpha matrix. ( C) 1998 Acta Metallurgica Inc. Published by Elsevier Science Ltd. Ali rights reserved.