Coarsening- and creep resistance of precipitation-strengthened Al-Mg-Zr alloys processed by selective laser melting

The coarsening behavior of Al3Zr precipitates during aging was investigated for two Al-Mg-Zr alloys (Al-3.6Mg-1.2Zr and Al-2.9Mg-2.1Zr, wt%) processed by selective laser melting (SLM). Scanning transmission electron microscopy (STEM) investigations of peak-aged (400 degrees C, 8 h) samples reveal both continuous (similar to 2 nm in diameter) and discontinuous (similar to 5 nm wide and hundreds of nanometers in length) coherent, secondary L1(2)-Al3Zr precipitates. In-situ STEM experiments showed that aging at 400 degrees C results in the appearance and growth of both grain-boundary Al3Zr precipitates, and intragranular nanometer-sized spherical Al3Zr precipitates in Zr-rich dendritic arms. Heating to 500 degrees C resulted in the disappearance of most Al3Zr precipitates and oxide particles. This microstructural evolution sheds light on the evolution of the alloy strength at elevated temperature. For short-term yield tests, as-fabricated samples displayed higher yield strengths than peak-aged samples at temperatures above 150 degrees C (e.g., 87 vs 24 MPa at 260 degrees C). This is attributed to coarsening of grain-boundary precipitates during aging, decreasing their ability to inhibit grain-boundary sliding (GBS) of the fine equiaxed grains (similar to 1 mu m). For longer term creep tests at 260 degrees C, both asfabricated and peak-aged samples displayed near-identical creep behavior during a long-duration (168 h) creep test; by contrast, during a shorter duration creep test (8 h), as-fabricated samples are more creep-resistant than samples previously aged at 260 degrees C (threshold stresses of similar to 40 vs. similar to 14 MPa, respectively). Again, the creep behavior is consistent with coarsening of grain-boundary precipitates, occurring now during long-duration creep tests at 260 degrees C. An exact creep mechanism could not be isolated due to microstructural changes during testing but is believed to be a combination of GBS and dislocation motion. (C) 2020 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.