Solidification microstructure, aging evolution and creep resistance of laser powder-bed fused Al-7Ce-8Mg (wt%)

• Published in: Additive manufacturing 2022-07, Vol.55, p.102862, Article 102862
• Main Authors: Rakhmonov, Jovid U., Weiss, David, Dunand, David C.
• Format: Article
• Language: English
• Subjects: Additive manufacturing; Aging; Al-Ce alloy; Creep; Microstructure; Solidification
• ISSN: 2214-8604; 2214-7810
• DOI: 10.1016/j.addma.2022.102862

The solidification microstructure and the aging- and creep-resistances of a laser powder-bed fused (LPBF) hypoeutectic Al-7.3Ce-7.7Mg (wt%) alloy were investigated. Optimized process parameters allowed to produce specimens with > 99.7% relative density and high hardness (~1650 MPa), without solidification cracking despite a relatively coarse grain size (~ 30 µm). Two distinct solidification microstructures are observed within the solidified melt pools: (i) eutectic α-Al(Mg) and Al13CeMg6 (a metastable phase) at the bottom pool region, and (ii) eutectic α-Al(Mg) and Al11Ce3 (a stable phase) precipitates at the top pool region. The high hardness is predominantly attributed to Orowan strengthening and load-transfer effects from the Ce-rich precipitates, but solid-solution hardening from Mg is also quite pronounced, as most Mg resides in α-Al. Aging at 325 or 400 °C causes a continuous, slow hardness drop, but 85–90% of the initial hardness is retained after over 1000 h aging, consistent with the observation that the Ce-rich precipitates within the grains show very sluggish coarsening, making the alloy highly resistant to aging. The alloy exhibits two creep regimes at 300 ºC. At stresses below 18–20 MPa, diffusional creep is operating with signs of grain-boundary sliding inhibition due to Ce-rich precipitates at grain boundaries. At higher stresses, dislocation climb with either stress-invariant or stress-dependent subgrain structure is dominant, with additional load-transfer contributions and possibly a threshold stress due to dislocation-precipitate interactions. The alloy also exhibits much lower steady-state strain rates (e.g., two orders of magnitude lower at 20 MPa) as compared to a compositionally-similar, but conventionally-cast Al-Ce-Mg alloy with much coarser microstructure, demonstrating that LPBF Al-7.3Ce-7.7Mg alloy – because of its fine precipitates and despite its fine grain size - is well suited for long-term uses under stress at 300 ºC.