Precipitation evolution in Al-Zr and Al-Zr-Ti alloys during aging at 450-600 °C

The transformation of Al3Zr (L1(2)) and Al-3(Zr1-xTix) (L1(2)) precipitates to their respective equilibrium D0(23) structures is investigated in conventionally solidified Al-0.1Zr and Al-0.1Zr-0.1Ti (at.%) alloys aged isothermally at 500 degrees C or aged isochronally in the range 300-600 degrees C. Titanium additions delay neither coarsening of the metastable L1(2) precipitates nor their transformation to the D0(23) structure. Both alloys overage at the same rate at or above 500 degrees C, during which spheroidal L1(2) precipitates transform to disk-shaped D0(23) precipitates at ca. 200 nm in diameter and 50 nm in thickness, exhibiting a cube-on-cube orientation relationship with the alpha-Al matrix. The transformation occurs heterogeneously on dislocations because of a large lattice parameter mismatch of the D0(23) phase with alpha-Al. The transformation is very sluggish and even at 575 degrees C coherent L1(2) precipitates can remain untransformed. Mechanisms of microstructural coarsening and strengthening are discussed with respect to the micrometer-scale dendritic distribution of precipitates. (C) 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.