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 AB4-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)2Ni7 (49.2 wt.%). When annealed at 1173 K, solid LaNi5 phase reacted with a liquid phase from (La,Mg)2Ni7 converting into the (La,Mg)5Ni19 phase. Increasing temperature to 1223 K, the residual LaNi5 phase continued to react with a liquid phase from (La,Mg)5Ni19 forming a new (La,Mg)6Ni24 phase in the alloy. Field-emission scanning electron microscope (FE-SEM) analysis found that the (La,Mg)6Ni24 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)6Ni24 phase abundance. When the abundance of (La,Mg)6Ni24 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 (I0) and hydrogen diffusion coefficient (D) increased to 360.35 mA g−1 and 2.450 × 10−11 cm2 s−1, respectively.