On chemical order and interfacial segregation in γ ′ (AlAg2) precipitates

Julian M. Rosalie, Christian Dwyer, Laure Bourgeois

Research output: Contribution to journalArticlepeer-review

18 Scopus citations


A detailed study has been carried out on γ′ (AlAg2) precipitates in Al-Ag and Al-Ag-Cu alloys to reconcile the conflicting reports on chemical ordering and stacking faults in this phase. High-angle annular dark-field scanning transmission electron microscopy and convergent beam electron diffraction show no indication of chemical ordering on alternate basal planes of γ′ precipitates in alloys aged at 473 K for 2-23 h. Precipitates were visible as Ag-rich regions with 1-13 face-centred cubic (fcc) → hexagonal close-packed stacking faults, corresponding to γ′ platelets with thicknesses ranging from 0.69 to 6.44 nm. There were no systematically absent thicknesses. Growth ledges with a riser height equal to the c-lattice parameter (0.46 nm) were directly observed for the first time. Genuine stacking faults within the precipitates were extremely rare and only observed in thicker precipitates. In precipitates with 1-3 stacking faults there was also substantial Ag in the surrounding fcc layers of the matrix, indicating that Ag strongly segregated to the broad, planar precipitate-matrix interfaces. This segregation is responsible for previous reports of stacking faults in γ′ precipitates. The results indicate that the early stages of γ′ precipitate growth are interfacially controlled.

Original languageEnglish (US)
Pages (from-to)224-235
Number of pages12
JournalActa Materialia
StatePublished - May 2014
Externally publishedYes


  • Aluminium alloys
  • Interface segregation
  • Ledgewise growth
  • Long-range order
  • Stacking faults

ASJC Scopus subject areas

  • Electronic, Optical and Magnetic Materials
  • Ceramics and Composites
  • Polymers and Plastics
  • Metals and Alloys


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