ICAMS / Interdisciplinary Centre for Advanced Materials Simulation
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Structural stability of topologically close-packed phases in transition metals

Date: 23.03.2009
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.

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