Site preference of Ru and Pd additions to gamma-based TiAl intermetallics

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.