High strength and plastic deformability of Mg-Li-Al alloy with dual BCC phase produced by a combination of heat treatment and multi-directional forging in channel die

Takahiro Mineta, Kaoru Hasegawa and Hiroyuki Sato
Department of Mechanical Science and Engineering, Graduate School of Science and Technology, Hirosaki University

Strengthening behavior of BCC Mg-Li alloys by multi-directional forging have been investigated. LA143 alloys with a high specific yield strength were prepared by heat treatment and multi-directional forging in channel die (MDFC, Fig. 1) which is one of severe plastic deformation.

It was found that

The Vickers hardness of the LA143 alloy depends on the heat treatment condition. The Vickers hardness of as-HT_{573K_1h} was higher than that of as-HT_{473K_1h} and as-HT_{673K_1h}. Moreover, it was revealed that the cold water-quenched LA143 alloy has a higher Vickers hardness than the air-quenched LA143 alloy, despite its larger grain size.
The Vickers hardness and yield strength of the as-HT_{573K_1h} LA143 alloy sample increased with the number of MDFC passes (Figs. 2 and Fig. 3). Moreover, MDFC LA143 alloy samples showed good deformability. The specific hardness and the specific yield strength of the LA143 alloy samples prepared by heat treatment andMDFC were significantly higher than major conventional lightweight structural materials.
Spinodal decomposition occurred in the as-HT_{573K_1h} LA143 alloy sample, and not in the as-rolled sample (Fig. 4). It is concluded that spinodal decomposition is the reason for the high strength of the as-HT_{573K_1h} sample.

Future work to be done includes investigating the strengthening mechanisms of this alloy in more detail, by measuring the spatial distribution of the various atomic species, the grain boundaries, and the dislocation density. This will help contribute to further improvements in the mechanical properties of Mg-Li-base alloys.

[Published in Materials Science and Engineering: A, Vol. 773 (2020), 138867]

Fig. 1 (a) Schematics of the MDFC process and (b) Photograph of the channel die. n is the natural number.

Fig. 2 Relationship between Vickers hardness of LA143 alloys and the total equivalent strain.

Fig. 3 Yield strength and specific yield strength of LA143 alloy samples as a function of the total equivalent strain.

Fig. 4 XRD patterns of as-HT₅₇₃ and as-rolled LA143 samples.