Formation of ω phase in Ti–Mo alloys after aging and deforming

Abstract

Investigations of ω phases formed by isothermal aging in Ti–20 wt-%Mo alloy and by deforming in tension in Ti–14 wt-% Mo alloy were carried out using transmission electron microscopy (TEM). Four ω phase variants in the same region of Ti–20Mo alloy single crystal aged at 623 K for 28·8 ks were observed using dark field TEM imaging. The microstructure of the ω particles was observed using high resolution electron microscopy (HREM). Dark field images were taken by using the diffraction spot due to respective ω phase variants and with the incident beams parallel to [l01], [313], and [311]. It was found that all ω particles grew in the 〈111〉 direction and were ellipsoids. The growth direction was the same as the direction of motion of atoms for forming ω particles according to a model proposed by previous authors. This result was confirmed by HREM images of the ω particles observed using dark field TEM. The volume fractions of respective ω phase variants were estimated using a conventional method. The ω particles were also observed, using dark field imaging, in the plastically deformed Ti–14Mo alloy. Although fine ω particles were observed in the as quenched state of this alloy, it was found that particle growth occurred only in the twin bands that formed as a result of the tensile deformation.MST/1364