Atom probe tomographic study of a friction-stir-processed Al-Mg-Sc alloy

Nhon Q. Vo*, David C. Dunand, David N. Seidman

*Corresponding author for this work

Research output: Contribution to journalArticle

19 Scopus citations

Abstract

The microstructure of a twin-roll-cast Al-4.5Mg-0.28Sc at.% alloy after friction-stir processing, performed at two tool rotational rates, was investigated by atom probe tomography. Outside the stir zone, the peak-aged alloy contains a high number density (∼8.0 × 10 23 m -3) of ∼1.5 nm radius Al 3Sc (L1 2) precipitates with a minor Mg content, providing an increase of ∼600 MPa in the Vickers microhardness. In the stir zone of the sample processed at 400 rpm rotational rate, the microhardness increase is mainly due to grain refinement, rather than precipitate strengthening, because the Al 3Sc precipitates, with spherical lobed cuboids and platelet-like morphology, grow and coarsen to a 10-20 nm radius. The Sc supersaturation across the stir-processed zone has a concentration gradient, which is higher on the retreating side and lower on the advancing side of the friction-stir tool. Hence, after aging at 290°C for 22 h, the microhardness increase within the stir zone also displays a gradient due to precipitate strengthening with varying precipitate volume fractions. In the stir zone for the sample processed at 325 rpm rotational rate, the microhardness increase is also predominantly due to grain refinement, as coarse Al 3Sc precipitates form heterogeneously at grain boundaries with a platelet-like morphology. The hardness remains unchanged after a 290°C aging treatment. This is because the Al 3Sc precipitates are highly heterogeneously distributed due to a combination of a small Sc supersaturation (0.05 at.%) in the matrix, the existence of dislocations, and a large area per unit volume of grain boundaries (∼4-6 × 10 6 m -1).

Original languageEnglish (US)
Pages (from-to)7078-7089
Number of pages12
JournalActa Materialia
Volume60
Issue number20
DOIs
StatePublished - Dec 1 2012

Keywords

  • Al-Mg-Sc alloy
  • Atom probe tomography
  • Friction-stir process
  • Strengthening

ASJC Scopus subject areas

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

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