On the crystallography and composition of topologically close-packed phases in ATI 718Plus®

Robert Krakow, Duncan N. Johnstone, Alexander S. Eggeman, Daniela Hünert, Mark C. Hardy, Catherine M.F. Rae, Paul A. Midgley

Research output: Contribution to journalArticlepeer-review


ATI 718Plus® is a nickel-based superalloy developed to replace Inconel 718 in aero engines for static and rotating applications. Here, the long-term stability of the alloy was studied and it was found that topologically close-packed (TCP) phases can form at the γ-η interface or, less frequently, at grain boundaries. Conventional and scanning transmission electron microscopy techniques were applied to elucidate the crystal structure and composition of these TCP precipitates. The precipitates were found to be tetragonal sigma phase and hexagonal C14 Laves phase, both being enriched in Cr, Co, Fe and Mo though sigma has a higher Cr and lower Nb content. The precipitates were observed to be heavily faulted along multiple planes. In addition, the disorientations between the TCP phases and neighbouring η/γ were determined using scanning precession electron diffraction and evaluated in axis-angle space. This work therefore provides a series of compositional and crystallographic insights that may be used to guide future alloy design.

Original languageEnglish
Pages (from-to)271-280
Number of pages10
JournalActa Materialia
Early online date23 Mar 2017
Publication statusPublished - 15 May 2017


  • Electron diffraction
  • Intermetallic phases
  • Ni-based superalloys
  • Orientation relationship
  • Phase composition


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