Precipitation of ordered α2 phase in Ti–6–22–22 alloy

Abstract Microstructural characterization of Ti–6Al–2Cr–2Mo–2Sn–2Zr (Ti–6–22–22) after various heat treatments has been performed using optical microscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and high resolution electron microscopy (HREM). The presence of very fine α 2 precipitates has been detected by electron diffraction in conventionally heat treated samples. The α 2 precipitates were observed within the α matrix of the “triplex” heat treated materials and also in samples cooled from the β phase field with cooling rates of less than 5.5°C/s. It was found that the α 2 precipitates were homogeneously distributed within the α phase with sizes of about 1–2 nm. Long time ageing at 650°C for 1000 h stabilized the α 2 precipitates which grew to 5–10 nm in diameter and 20–50 nm in length. Structural stability of the α 2 precipitates has been discussed in terms of chemical composition and lattice mismatch between α 2 precipitates and the α matrix.