Events
Place: DPG Spring Meeting 2009, Dresden, Germany
Thomas Hammerschmidt
Bernhard Seiser, Department of Materials, University of Oxford, Oxford, United Kingdom
Ralf Drautz
David Pettifor, Department of Materials, University of Oxford, Oxford, United Kingdom
The formation of topologically close-packed (tcp) phases in Ni-based
superalloys leads to the degradation of the creep properties of the
alloys.
The precipitation of the tcp phases is attributed to refractory elements
that are
added in low concentration to improve creep resistance. We have compiled a
structure map of the occurrence of tcp phases in binary transition metal
(TM)
compounds. The structure map displays well-established structural trends
that
are driven by the d-band filling. It also shows that different sizes of the
elements tend to stabilize the Laves phases over other tcp phases. The same
trend is reproduced in our extensive density functional theory (DFT)
calculations
of the tcp phases A15, C14, C15, C36, μ, σ, and χ in
unary and
binary TM systems. We show that the structural trend can be understood
using the
canonical d-band tight-binding model. The small positive values of the
heats of
formation of all tcp phases suggests that entropy plays a key role in the
stabilization of the experimentally observed tcp phases in TM binary
compounds.
