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7 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

Intermetallic particle heterogeneity controls shear localization in high-strength nanostructured Al alloys

The mechanical behavior of two nanocrystalline Al alloys, Al-Mg-Y and Al-Fe-Y, is investigated with in-situ micropillar compression testing. Both alloys were strengthened by a hierarchical microstructure including grain boundary segregation, nanometer-thick amorphous complexions, carbide nanorod precipitates with sizes of a few nanometers, and submicron-scale intermetallic particles. The maximum yield strength of the Al-Mg-Y system is measured to be 950 MPa, exceeding that of the Al-Fe-Y system (680 MPa), primarily due to a combination of more carbide nanorods and more amorphous complexions. Both alloys exhibited yield strengths much higher than those of commercial Al alloys, and therefore have great potential for structural applications. However, some micropillar specimens were observed to plastically soften through shear banding. Post-mortem investigation revealed that intermetallic-free deformation pathways of a few micrometers in length were responsible for this failure. Further characterization showed significant grain growth within the shear band. The coarsened grains maintained the same orientation with each other, pointing to grain boundary mechanisms for plastic flow, specifically grain rotation and/or grain boundary migration. Here, the presence of intermetallic particles makes it difficult for both matrix and intermetallic grains to rotate into the same orientation due to the different lattice parameters and slip systems. Therefore, we are able to conclude that a uniform distribution of intermetallic particles with an average spacing less than the percolation length of shear localization can effectively prevent the maturation of shear bands, offering a design strategy for high-strength nanocrystalline Al alloys with both high strength and stable plastic flow.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Bulk nanocrystalline Al alloys with hierarchical reinforcement structures via grain boundary segregation and complexion formation

Grain size engineering, particularly reducing grain size into the nanocrystalline regime, offers a promising pathway to further improve the strength-to-weight ratio of Al alloys. Unfortunately, the fabrication of nanocrystalline metals often requires non-equilibrium processing routes, which typically limit the specimen size and require large energy budgets. In this study, multiple dopant elements in ternary Al alloys are deliberately selected to enable segregation to the grain boundary region and promote the formation of amorphous complexions. Three different fully dense bulk nanocrystalline Al alloys (Al-Mg-Y, Al-Fe-Y, and Al-Ni-Y) with small grain sizes were successfully fabricated using a simple powder metallurgy approach, with full densification connected directly to the onset of amorphous complexion formation. All the compositions demonstrate densities above 99% with grain sizes <60 nm following consolidation via hot pressing at 585 °C. The very fine grain structure results in excellent mechanical properties, as evidenced by nanoindentation hardness values in the range of 2.2-2.8 GPa. Detailed microstructural characterization verifies the segregation of all dopant species to grain boundaries as well as the formation of amorphous complexions, which suggests their influential role in aiding effective consolidation and endowing thermal stability in the alloys. Moreover, nanorods with a core-shell structure are also observed at the grain boundaries, which likely contribute to the stabilization of the grain structure while also strengthening the materials. Lastly, intermetallic particles with sizes of hundreds of nanometers form in all systems. As a whole, the results presented here demonstrate a general alloy design strategy of segregation and boundary evolution pathway that enables the fabrication of multiple nanocrystalline Al alloys with hierarchical microstructures and improved performance.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on YMgAl by Materials Project

MgYAl crystallizes in the hexagonal P-62m space group. The structure is three-dimensional. Mg is bonded in a 4-coordinate geometry to four Al atoms. There are two shorter (2.85 Å) and two longer (2.89 Å) Mg–Al bond lengths. Y is bonded in a 5-coordinate geometry to five Al atoms. There are four shorter (3.16 Å) and one longer (3.19 Å) Y–Al bond lengths. There are two inequivalent Al sites. In the first Al site, Al is bonded in a 9-coordinate geometry to three equivalent Mg and six equivalent Y atoms. In the second Al site, Al is bonded in a 9-coordinate geometry to six equivalent Mg and three equivalent Y atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Y2MgAl by Materials Project

MgY2Al is Heusler structured and crystallizes in the cubic Fm-3m space group. The structure is three-dimensional. Mg is bonded in a body-centered cubic geometry to eight equivalent Y atoms. All Mg–Y bond lengths are 3.19 Å. Y is bonded in a body-centered cubic geometry to four equivalent Mg and four equivalent Al atoms. All Y–Al bond lengths are 3.19 Å. Al is bonded in a body-centered cubic geometry to eight equivalent Y atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Y(Mg4Al3)4 by Materials Project

Y(Mg4Al3)4 crystallizes in the cubic I-43m space group. The structure is three-dimensional. there are two inequivalent Mg sites. In the first Mg site, Mg is bonded in a 12-coordinate geometry to seven Mg and five equivalent Al atoms. There are a spread of Mg–Mg bond distances ranging from 3.02–3.16 Å. There are a spread of Mg–Al bond distances ranging from 2.86–3.17 Å. In the second Mg site, Mg is bonded in a 10-coordinate geometry to three equivalent Mg, one Y, and six equivalent Al atoms. The Mg–Y bond length is 3.27 Å. All Mg–Al bond lengths are 3.16 Å. Y is bonded in a 12-coordinate geometry to four equivalent Mg and twelve equivalent Al atoms. All Y–Al bond lengths are 3.22 Å. Al is bonded in a 11-coordinate geometry to seven Mg, one Y, and three equivalent Al atoms. There are one shorter (2.70 Å) and two longer (2.78 Å) Al–Al bond lengths.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Y(Mg4Al3)4 by Materials Project

Y(Mg4Al3)4 is gamma-brass-derived structured and crystallizes in the trigonal R3m space group. The structure is three-dimensional. there are five inequivalent Mg sites. In the first Mg site, Mg is bonded in a 10-coordinate geometry to four Mg and six Al atoms. There are a spread of Mg–Mg bond distances ranging from 3.05–3.16 Å. There are a spread of Mg–Al bond distances ranging from 3.07–3.12 Å. In the second Mg site, Mg is bonded in a 3-coordinate geometry to five Mg, one Y, and five Al atoms. There are two shorter (3.16 Å) and two longer (3.18 Å) Mg–Mg bond lengths. The Mg–Y bond length is 3.59 Å. There are a spread of Mg–Al bond distances ranging from 2.87–3.20 Å. In the third Mg site, Mg is bonded in a 9-coordinate geometry to three equivalent Mg, one Y, and twelve Al atoms. The Mg–Y bond length is 3.32 Å. There are a spread of Mg–Al bond distances ranging from 3.19–3.24 Å. In the fourth Mg site, Mg is bonded in a 1-coordinate geometry to four equivalent Mg, one Y, and five Al atoms. There are two shorter (3.12 Å) and two longer (3.14 Å) Mg–Mg bond lengths. The Mg–Y bond length is 3.11 Å. There are a spread of Mg–Al bond distances ranging from 2.82–3.16 Å. In the fifth Mg site, Mg is bonded in a 12-coordinate geometry to seven Mg and five Al atoms. Both Mg–Mg bond lengths are 3.07 Å. There are a spread of Mg–Al bond distances ranging from 2.94–3.29 Å. Y is bonded in a 12-coordinate geometry to seven Mg and nine Al atoms. There are six shorter (3.22 Å) and three longer (3.25 Å) Y–Al bond lengths. There are three inequivalent Al sites. In the first Al site, Al is bonded in a 12-coordinate geometry to eight Mg, one Y, and three Al atoms. There are one shorter (2.68 Å) and two longer (2.77 Å) Al–Al bond lengths. In the second Al site, Al is bonded in a 11-coordinate geometry to eight Mg and three Al atoms. Both Al–Al bond lengths are 2.77 Å. In the third Al site, Al is bonded in a distorted q6 geometry to seven Mg, one Y, and three Al atoms. There are one shorter (2.71 Å) and one longer (2.80 Å) Al–Al bond lengths.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on YMgAl4 by Materials Project

MgYAl4 is Hexagonal Laves-derived structured and crystallizes in the trigonal P-3m1 space group. The structure is three-dimensional. Mg is bonded in a 12-coordinate geometry to three equivalent Mg, one Y, and twelve Al atoms. All Mg–Mg bond lengths are 3.36 Å. The Mg–Y bond length is 3.31 Å. There are three shorter (3.15 Å) and nine longer (3.23 Å) Mg–Al bond lengths. Y is bonded in a 12-coordinate geometry to one Mg, three equivalent Y, and twelve Al atoms. All Y–Y bond lengths are 3.42 Å. There are a spread of Y–Al bond distances ranging from 3.20–3.27 Å. There are three inequivalent Al sites. In the first Al site, Al is bonded to six equivalent Mg and six equivalent Al atoms to form AlMg6Al6 cuboctahedra that share corners with twelve equivalent AlY3Mg3Al6 cuboctahedra, edges with six equivalent AlMg6Al6 cuboctahedra, and faces with twenty AlY6Al6 cuboctahedra. All Al–Al bond lengths are 2.75 Å. In the second Al site, Al is bonded to six equivalent Y and six equivalent Al atoms to form AlY6Al6 cuboctahedra that share corners with twelve equivalent AlY3Mg3Al6 cuboctahedra, edges with six equivalent AlY6Al6 cuboctahedra, and faces with twenty AlMg6Al6 cuboctahedra. All Al–Al bond lengths are 2.77 Å. In the third Al site, Al is bonded to three equivalent Mg, three equivalent Y, and six Al atoms to form AlY3Mg3Al6 cuboctahedra that share corners with eighteen AlMg6Al6 cuboctahedra, edges with six equivalent AlY3Mg3Al6 cuboctahedra, and faces with eighteen AlMg6Al6 cuboctahedra. There are two shorter (2.70 Å) and two longer (2.82 Å) Al–Al bond lengths.

36 MATERIALS SCIENCE↗