Design of LPSO Phases in Mg-Y-Ni Alloys to Impact Hydrogenation Kinetics

Abstract

A series of Mg-Y-Ni alloys with different volume fractions of long-period stacking-ordered (LPSO) phase were prepared, by controlling the alloy composition, heat treatment, and single-pass extrusion, to assess the influence of increasing LPSO phase volume fraction on the hydrogen absorption and desorption properties of the extruded alloys. The LPSO phase volume fraction in the alloys increased with increasing solute concentration, from ~24% LPSO in Mg97Y2Ni1 (at.%) to ~60% LPSO in Mg93Y4Ni3 (at.%) up to ~92% LPSO in Mg91Y5Ni4 (at.%). The most refined microstructure was obtained in the alloy with highest volume fraction of LPSO phase. After 100 s at 300 °C, the Mg91Y5Ni4 alloy absorbed 4.6 ± 0.2 wt.% H while the Mg97Y2Ni1 and Mg93Y4Ni3 alloys each absorbed 3.8 ± 0.2 wt.% H. After 10,000 s at 300 °C, all three alloys had absorbed a maximum of 5.3 ± 0.2 wt.% H with no further significant difference in hydrogen absorption kinetics. The Mg91Y5Ni4 alloy desorbed 1.8 ± 0.2 wt.% H after 100 s at 300 °C against a vacuum while the Mg97Y2Ni1 and Mg93Y4Ni3 alloys desorbed 0.8 ± 0.2 wt. H and 0.6 ± 0.2 wt.% H, respectively. After 10,000 s at 300 °C, the Mg91Y5Ni4 and Mg97Y2Ni1 alloys completely desorbed 5.2 ± 0.2 wt.% H and 5.4 ± 0.2 wt.% H, respectively, but the Mg93Y4Ni3 alloy desorbed only 3.7 ± 0.2 wt.% H. Hydrogen absorption and desorption kinetics were fastest in the Mg91Y5Ni4 alloy with the highest LPSO volume fraction, but no consistent trend with LPSO phase volume fraction was observed with the Mg93Y4Ni3 alloy, which showed the slowest absorption and desorption kinetics. The hydrogen pressures corresponding to metal–hydride equilibrium did not vary with LPSO phase volume fraction or alloy composition, indicating that the (de)hydrogenation thermodynamics were not significantly changed in any of the alloys. Hydrogen absorption experiments with thin foils, made of extruded Mg91Y5Ni4 alloy with the highest LPSO phase fraction, demonstrated that the LPSO structures decompose into Mg phase, Mg2Ni phase, lamellar Mg/Mg-Y structures, and YHx particles. This study shows that hydrogen kinetics can be impacted in Mg-Y-Ni alloys by controlling the LPSO phases using common metallurgical techniques.

Keywords

• Mg-RE alloys
• LPSO phase
• microstructure
• hydrogen absorption/desorption kinetics