  The effects of ageing at 800 $^\circ$C on the properties of the IRIS alloy
(Ti$_{49.9}$Al$_{48}$W$_2$B$_{0.1}$) are studied. The initial microstructure of
this alloy densified by Spark Plasma Sintering (SPS) is mainly composed of
lamellar colonies which are surrounded by $\gamma$ grains. The evolutions of
the alloy strength and creep resistance resulting from this ageing treatment
are measured by the related mechanical tests. The microstructural changes are
investigated by scanning and transmission electron microscopies and by X-ray
diffraction. The main structural evolutions consist in a shrinkage of the
lamellar areas and in a precipitation of $\beta_0$ phase, which is accompanied
by a moderate segregation of tungsten and a decrease of the $\alpha_2$ lamellar
width. However, these evolutions are relatively limited and the microstructural
stability is found to result mainly from the low diffusivity of tungsten.
Conversely, a moderate effect of this ageing treatment on mechanical
properties, at room and high temperatures, is measured. Such experimental
results are interpreted and discussed in terms of the microstructural
evolutions and of the deformation mechanisms which are activated at different
temperatures under various solicitations.
