Samples of Ti-15Cr and Ti-15V-3Sn-3Al-3Cr (wt%) containing controlled additions of
carbon up to 0.2wt% and different oxygen contents have been quenched and aged at temperatures
between 400 and 600°C. Optical, scanning and analytical transmission electron microscopy have
been used to characterise the microstructures of the quenched and aged samples. Hardness testing
has been used to follow the kinetics and extent of age hardening, which are accelerated in Ccontaining
samples. The addition of carbon results in the formation of Ti(CxOy) precipitates which
pin grain boundaries in forged samples so that the grain size in the quenched C-containing samples
is about a factor of ten less than that in the C-free samples. In the C-free samples coarse grain
boundary alpha tends to form, but in the C-containing samples alpha precipitation is more uniform
throughout the beta grains. The extent of omega precipitation is very different in the two alloys; the
Ti-15Cr alloy forms athermal omega in the as-quenched samples and large omega precipitates are
formed on ageing at 400°C. No evidence for omega has been obtained in the Ti-15-3. The
hardening responses and microstructural observations are interpreted in terms of the different grain
boundary oxygen contents in the C-containing and C-free samples and the different roles of omega
and of carbon in the two alloys.