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

Abstract

Platinum group metals (PGMs), such as Ru and Pd, have recently been explored 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 potentially attractive element, since it is expected to offer similar property improvement to Pd, but at much lower cost. In the present investigation, the crystalline parameters of Ti–48 at.% Al alloys with Ru or Pd additions were studied. Tetragonality, unit cell volume and site occupancy were measured using X-ray diffraction. The site occupancy of Ru was also examined experimentally using the atom probe technique. Both Ru and Pd additions were investigated theoretically using ab initio local density functional calculations. 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 additions substitute on the Al rather than the Ti site in TiAl, which is confirmed by the X-ray diffraction and atom probe results. Moreover, the ab initio calculations predict that Ru has a stronger preference for Al sites than Pd because of the stronger d–d bonding with the nearest neighbour Ti atoms.