Structure and growth of core–shell nanoprecipitates in Al–Er–Sc–Zr–V–Si high-temperature alloys

Abstract

Lightweight Sc-containing aluminum alloys exhibit superior mechanical performance at high temperatures due to core-shell, L12-ordered trialuminide nanoprecipitates. In this study, the structure of these nanoprecipitates was studied, using different transmission electron microscopy (TEM) techniques, for an Al-Er-Sc-Zr-V-Si alloy that was subjected to a two-stage overaging heat treatment. Energy-dispersive X-ray spectroscopy of the spherical Al3(Sc, Zr, Er ,V) nanoprecipitates revealed a core-shell structure with an Sc-and Er-enriched core and a Zr-enriched shell, without a clear V outer shell. This structure is stable up to 72% of the absolute melting temperature of Al for extended periods of time. High-angle annular dark-field scanning TEM was used to image the {100} planes of the nanoprecipitates, demonstrating a homogeneous L12-ordered superlattice structure for the entire nanoprecipitates, despite the variations in the concentrations of solute atoms within the unit cells. A possible growth path and compositional trajectory for these nanoprecipitates was proposed using highresolution TEM observations, where different rod-like structural defects were detected, which are considered to be precursors to the spherical L12-ordered nanoprecipitates. It is also hypothesized that the structural defects could consist of segregated Si; however, this was not possible to verify with HAADF-STEM because of the small differences in Al and Si atomic numbers. The results herein allow a better understanding of how the Al-Sc alloys' core-shell nanoprecipitates form and evolve temporally, thereby providing a better physical picture for future atomistic structural mappings and simulations.

Phase Engineering of Nanostructural Metallic Materials: Classification, Structures, and Applications

“…The unique hierarchical-CNP structure resulted in an exaggerated precipitation-strengthening effect and raised the yield strength and ultimate strength to 1860 and 2500 MPa, respectively . Core–shell CNPs in Al–Zr–Sc–Er alloy systems have frequently been reported and can improve the mechanical performance of alloys at high temperatures (300–400 °C). , The addition of Er and Zr in Al–Zr–Sc–Er alloy system leads to the formation of spheroidal Al 3 (Sc, Zr, Er) CNPs with a core/double-shell structure comprising an Er-enriched core surrounded by an Sc-enriched inner shell and a Zr-enriched outer shell (Figure c). The core–shell structure is shown to be stable and difficult to coarsen even at a high temperature of 400 °C for 64 days.…”

“…Copyright 2018 American Association for the Advancement of Science. (c) Core–shell nanoprecipitates . Reproduced with permission from ref .…”

Newer Developments in Aluminium Alloys Through Ordered Precipitates and Segregation of Transition Elements

“…Few reports emphasised a two-step ageing process in Al-Cu-Sc based alloys modified with Zr, V, and Ti. It results in forming coarsening resistant Al 3 Sc precipitates by allowing the segregation of low diffusing transition elements (Zr, V or Ti) at the Al 3 Sc precipitate/matrix interface during ageing at higher temperatures [39][40][41]. The addition of Zr to Al-Cu-Sc alloys was reported to enhance the high-temperature performance of the alloys [42][43][44][45].…”

Microalloying and Modification of Cast Aluminum Alloys for Increasing their Level of Exploitation Properties at Elevated Temperatures. A Review

“…Simultaneous addition of titanium, zirconium and hafnium is also used to ensure the effective use of scandium as a modifier of aluminum alloys. This technique provides the formation of thermally stable phases of Al 3 (Sc,Zr/Ti/Hf), including -with a two-layer "coreshell" structure with a favorable structure of the L1 2 -type [39][40][41].…”