Friday, July 12, 2013

1307.2996 (Julian M. Rosalie et al.)

On short-range order and stacking faults in γ' (AlAg2) precipitates    [PDF]

Julian M. Rosalie, Christian Dwyer, Laure Bourgeois
A detailed study has been carried out on \gamma' (AlAg2) precipitates in Al-Ag and Al-Ag-Cu alloys to reconcile the conflicting reports on short-range order and stacking faults in this phase. High angle annular dark field scanning transmission electron microscopy and convergent beam electron diffraction show no indication of short-range order in \gamma' precipitates in alloys aged at 473 K for 2-23 h. Precipitates adopted thicknesses in integer unit cell increments from 1-13 times the \gamma' unit cell height with no systematically absent thicknesses. Growth ledges with a step height of 1 unit cell height (0.46 nm) were directly observed. Genuine stacking faults were extremely rare and only observed in thicker precipitates. However, Ag strongly segregated to the broad, planar precipitate-matrix interfaces and this segregation is responsible for previous reports of stacking faults in \gamma' precipitates. The results indicate that the early stages of \gamma' precipitate growth are interfacially controlled
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