A novel long-period stacking/order structure in Mg-Ni-Y alloys

Highlights

• A novel type of the long-period stacking/order structure has been determined.

• Combined use of electron diffraction/microscopy and first principles calculations.

• 12R-type stacking sequence and the in-plane order of approximately $7\times {\left(1\overline{2}10\right)}_{hcp}$.

• Robust L1₂-Ni₆Y₈ cluster rules the LPSO phase formation as a pseudo-binary system.

Abstract

We have identified a novel long-period stacking/order (LPSO) phase in a Mg-Ni-Y alloy, which provides novel LPSO structural features regarding both the stacking/order aspects; the 12R-type stacking sequence and the in-plane modulation of approximately $7\times {\left(1\overline{2}10\right)}_{hcp}$ with respect to the fundamental hexagonal-closed-packed Mg structure. The ideal 7 M model for the present 12R-type LPSO phase is constructed based on the ordered arrangements of the L1₂-type Ni₆Y₈ clusters embedded in the local ABCA stacking layers, resulting in the stoichiometry Mg₇₇Ni₉Y₁₂ (Mg_78.6Ni_9.2Y_12.2) that appears to be extremely close to the experimentally determined composition Mg_78.4Ni_9.0±0.2Y_12.6±0.3 (at.%). This in turn suggests the robustness of the solute cluster with a specific ratio (Ni₆Y₈), emerging the pseudo-binary formation behaviors of the LPSO phase in the Mg ternary alloys.

Introduction

Mg alloys containing a few atomic percent of transition metal (TM) and rare-earth elements (RE) have attracted attentions because of their excellent mechanical properties [1] provided by unique long-period structures [[2], [3], [4], [5], [6], [7], [8]], which are termed as long-period stacking/ordered (LPSO) phases [7,8]. Fundamental LPSO structures are stacking polytypes of an original hexagonal-closed-packed (hcp) Mg structure and constructed by a combination between the 2H-stacking (AB …) and the intrinsic-II type stacking-fault unit represented as the ABCA stacking [7]. To date, four stacking polytypes, 10H, 18R, 14H and 24R [7], have been identified for the LPSO phases. In the LPSO structures, TM/RE atoms distribute across the ABCA stacking layers that form local face-centered cubic (fcc) environments, where the TM/RE atoms form L1₂-type clusters (TM₆RE₈) to give rise a superlattice order with an ideal dimension of 6 × (1$\overline{2}$10)_hcp (6 M) [8,9]. We note in particular that, after the discovery of the cluster-based characteristics, tuning and/or exploring the novel LPSO phases [10,11] have been guided along the particular TM/RE ratio (3/4) line [8] in the ternary phase diagram.

During the LPSO and its related phase investigations in the Mg-Ni-Y alloys around the Mg-rich corner [12,13], we have found the novel LPSO phase with a 12R-type stacking structure (recently, it has been also reported in Ref. [14]). Furthermore, the present 12R-type LPSO phase also reveals a novel in-plane order of approximately 7M-type. In the present paper, we describe details of the structural characteristic of the novel 12R-LPSO phase.