Structural phase transformation and electrochemical features of La-Mg-Ni-based AB4-type alloys

In this work, we studied the structural phase transformation and electrochemical characteristics of AB(4)-type La-Mg-Ni-based alloys through zoning annealing of La0.78Mg0.22Ni3.90 polymorphic alloy. The as-cast La0.78Mg0.22Ni3.90 alloy contained two phases of LaNi5 (50.8 wt.%) and (La, Mg)(2)Ni-7 (49.2 wt.%). When annealed at 1173 K, solid LaNi5 phase reacted with a liquid phase from (La, Mg)(2)Ni-7 converting into the (La, Mg)(5)Ni-19 phase. Increasing temperature to 1223 K, the residual LaNi5 phase continued to react with a liquid phase from (La, Mg)(5)Ni-19 forming a new (La, Mg)(6)Ni-24 phase in the alloy. Field-emission scanning electron microscope (FE-SEM) analysis found that the (La, Mg)(6)Ni-24 phase showed diffuse distributing in the alloys and this particular morphology provided plenty of phase boundaries which offered more hydrogen diffusion routes. The high rate dischargeability (HRD) of the alloy electrodes increased with higher (La, Mg)(6)Ni-24 phase abundance. When the abundance of (La, Mg)(6)Ni-24 phase raised to 62.0 wt.%, the HRD of the alloy electrode at a discharge current density of 1500 mA g(-1)(HRD1500) reached 68.0%. Meanwhile, the surface exchange current density (I-0) and hydrogen diffusion coefficient (D) increased to 360.35 mA g(-1) and 2.450 x 10(-11)cm(2) s(-1), respectively. (C) 2016 Elsevier Ltd. All rights reserved.