Tian T. Li

A new class of castable cerium strengthened aluminum alloys has phenomenal high temperature properties without the need for heat treatment.

Decreasing microstructural length scales to the nanoscale is a proven way of increasing strength, but the intrinsic metastability of such structures typically makes them susceptible to thermally activated coarsening. Recent advances in additive manufacturing permit bulk-nanostructured materials to be produced through rapid solidification, but like other metastable materials the as-built structures typically coarsen rapidly with even modest thermal exposure. Here, selective laser melting is employed to produce an Al-Ce-based alloy with high mechanical strength arising from the as-built microstructure, which can be controlled by build conditions. In addition, the alloy exhibits extreme resistance to thermal coarsening up to 400 °C and superior strength retention compared to conventional Al alloys after extended thermal exposure. The near-zero solubility of Ce in Al and potent solid solution strengthening of Mg enable this behavior without requiring heat treatment. This result demonstrates that combining insoluble alloying elements with additive manufacturing is a viable method of producing exceptionally stable bulk nanoscale alloys.

Pitting corrosion in seawater is one of the most difficult forms of corrosion to identify and control. A workhorse material for marine applications, 316L stainless steel (316L SS) is known to balance resistance to pitting with good mechanical properties. The advent of additive manufacturing (AM), particularly laser powder bed fusion (LPBF), has prompted numerous microstructural and mechanical investigations of LPBF 316L SS; however, the origins of pitting corrosion on as-built surfaces is unknown, despite their utmost importance for certification of LPBF 316L SS prior to fielding. Here, we show that Mn-rich silicate slags are responsible for pitting of the as-built LPBF material in sodium chloride due to their introduction of deleterious defects such as cracks or surface oxide heterogeneities. In addition, we explain how slags are formed in the liquid metal and deposited at the as-built surfaces using high-fidelity melt pool simulations. Our work uncovers how LPBF changes surface oxides due to rapid solidification and high-temperature oxidation, leading to fundamentally different pitting corrosion mechanisms.