Platinum group metals (PGMs), such as Ru and Pd, have recently been explore
d as new alloying elements in TiAl base intermetallics. These elements are
anticipated to increase the resistance to low pH environment as well as to
improve mechanical properties. Among PGMs, Ru is especially regarded as a p
otentially attractive element, since it is expected to offer similar proper
ty improvement to Pd, but at much lower cost. In the present investigation,
the crystalline parameters of Ti-48at.% Al alloys with Ru or Pd additions
were studied. Tetragonality, unit cell volume and site occupancy were measu
red using X-ray diffraction. The site occupancy of Ru was also examined exp
erimentally using the atom probe technique. Both Ru and Pd additions were i
nvestigated theoretically using ab initio local density functional calculat
ions. We find that Ru and Pd both slightly reduce the tetragonality and the
unit cell volume. The ab initio calculations predict that both Ru and Pd a
dditions substitute on the Al rather than the Ti site in TiAl, which is con
firmed by the X-ray diffraction and atom probe results. Moreover, the ab in
itio calculations predict that Ru has a stronger preference for Al sites th
an Pd because of the stronger d-d bonding with the nearest neighbour Ti ato
ms.