Dilute Al-Sc-Zr-Si-based aluminum alloys retain their strength for long periods of time at elevated temperatures due to a dispersion of nanometric, coherent, and coarsening-resistant Al3(Sc,Zr) (L12 structure) nanoprecipitates that are formed upon aging. These alloys are attractive for replacement of dense steels and costly Ti alloys in the 250–400 °C range; however, the high cost of Sc in these alloys limits their commercial viability. In this thesis, fundamental scientific studies are performed on the effects of transition metals Hf, Cr, Mo, Ni, and Zn on the precipitation behavior and creep resistance of Al-Zr-Er-Si alloys strengthened by Al3(Zr,Er) L12 nanoprecipitates. Aging responses of alloys with these additions are assessed using Vickers microhardness to monitor the evolution of ambient-temperature strength and electrical conductivity to monitor the progression of precipitation. Detailed microstructural investigations are primarily performed using atom-probe tomography, giving precise compositions of the L12 nanoprecipitates. Alloy microstructure and L12 composition are then linked to the high-temperature mechanical properties using compressive creep studies at 276–400 °C. Additions of slow-diffusing Hf lead to sluggish precipitation kinetics due to strong incorporation into Al3(Zr,Hf,Er) L12 nanoprecipitates (4.5 at.%), but the coarsening resistance is not improved. Slow-diffusing Cr and Mo and fast-diffusing Ni have no effect on aging response and do not partition significantly to the L12 nanoprecipitates. However, Cr and Mo additions increase creep strength due to solid-solution strengthening and enhanced Er partitioning to the L12 nanoprecipitates, while additions of 3 at.% Ni provide enhanced strength due to formation of eutectic Al3Ni microfibers upon casting. Fast-diffusing Zn is strongly incorporated into the L12 nanoprecipitates (6–7 at.%) but does not affect the precipitation kinetics or coarsening resistance. Zinc additions decrease the creep strength due to decreased lattice parameter misfit of the (Al,Zn)3(Zr,Er) nanoprecipitates, as confirmed by density functional theory calculations. Through careful optimization of solutionizing parameters and Er and Si composition, an Al-Er-Zr-Si alloy with core-shell (Al,Si)3(Zr,Er) nanoprecipitates was developed, with comparable aging and creep behavior to similar Sc-containing alloys.

Creator

• Michi, Richard Andrew

DOI

• https://doi.org/10.21985/n2-ebkv-3v50

Subject

• Materials Science

Language

• en

Alternate Identifier

• etdadmin_upload_718569
• http://dissertations.umi.com/northwestern:14999

Keyword

• L12 nanoprecipitates
• Atom-probe tomography
• Coarsening
• High-temperature alloys
• Creep
• Aluminum alloys

Date created

• 2020-01-01

Resource type

• Dissertation

Rights statement

• In Copyright