Type of presentation: Poster

MS-4-P-2108 TEM observation of microstructure in the early stage of Mg-Gd-Y alloy aged at 473K

Matsuoka Y.¹, Matsuda K.², Watanabe K.¹, Nakamura J.³, Lefebvre W.⁴, Saikawa S.², Ikeno S.⁵

¹Graduate School of Science and Engineering for Education, University of Toyama,Toyama, Japan, ²Graduate School of Science and Engineering for Research, University of Toyama, Toyama, Japan, ³Tohoku University, Sendai, Japan, ⁴Universite de Rouen, France, ⁵Hokuriku Polytechnic College, Toyama, Japan

ikenolab@eng.u-toyama.ac.jp

Magnesium alloys containing rare earth elements are known to show good heat resistance.[1] The Mg-Gd alloy shows good age-hardenability, and the Mg-Gd-Y alloys have been developed for practical Mg alloys by Kamado et. al. to reduce the density of alloy and these alloys have a good creep resistance, even 523 - 573 K.[1] In our previous study, Mg-Gd-Y alloys show the mono-layer structure has been discovered before β” phase with DO₁₉ structure in the aged sample at 473 K, and the β” and β’ phases with bco structure co-existed at the peak aged condition.[2] Recently, Nishijima et al. detailed examinations on the precipitation behaviours of Mg-Gd alloy and Mg-Y alloy by high angle annular dark field - scanning transmission electron microscopy (HAADF-STEM) technique.[3] They concluded that the arraignment of bright dots indicates the short range ordered state and the β’ phase nuclide in the short range ordered structure. And they have a doubt for the existence of the β” phase with DO₁₉ structure. Moreover, they presented a new structure model for the β’ phase, an Mg₇RE-type structure different from the previously proposed Mg₁₅RE-type. In this study, the early stage of aging in Mg-2.9at.%Gd-0.8at.%Y alloy has been observed by high resolution transmission electron microscopy (HRTEM), HAADF-STEM and calculations of images and electron density and bond overlap population (BOP) by first principal to understand the origin of precipitation in this alloy.

Fig.1 shows HRTEM image in as-quenched specimen. In HRTEM image, some lines which has brighter ( or darker ) dots having spacing of 0.64 nm on the mono-layer of {1-100}_Mg plane in the as-quenched sample.

Fig.2 shows HRTEM image obtained for the alloy aged at 473K for 7.2ks. The β’ phase has four atomic layers periodicity in the [1-100]_Mg, and the arrangement of the bright dots with space of 0.64nm is observed.

At the under-aged condition, precipitates observed by HRTEM were classified as follows; mono-layer, a part of β”, β’. By HAADF-STEM observation, zig-zag structure, small hexagonal network, and β’ can be recognized. The small hexagon of 0.37 nm is the first precipitate in this alloy, and this is the evidence of short range ordering close to DO₁₉ structure. This is referred as the pre β”-phase. Finally, we concluded that the proposed precipitation sequence is as follows; S.S.S.S. → pre β” phase having DO₁₉ SRO → β” → β’.

[1] S. Kamado, Y. Kojima, S. Taniike, I. Seki and S. Hama : Proc. of the International Conf. on Magnesium Alloys and Their Applications, (1998), 169-174 .

[2] T. Kawabata, D. Nakagawa, S. Saikawa, J. Nakamura, S. Ikeno and K.Matsuda, Materials Transactions, Vol. 54, (2013), 225-230.

[3] M. Nishijima and K. Hiraga, Materials Transactions, Vol. 48, (2007), 10-15.

Fig. 1: HRTEM image obtained for as-quenched sample.

Fig. 2: HRTEM image obtained for aged sample at 473K for 7.2ks.