Insights into the composition exploration of novel hydrogen storage alloys: evaluation of the Mg–Ni–Nd–H phase diagram

Abstract

In order to provide an efficient tool to explore alloy composition and processing conditions for Mg-based alloys with good hydriding/dehydriding (H/D) properties, investigation of the Mg–Ni–Nd–H quaternary system was carried out by experimental measurements and CALPHAD thermodynamic analysis combined with first-principles calculations. A new stable compound Nd₁₆Mg₉₆Ni₁₂ with the space group of Cmc2₁ was identified in the Mg–Ni–Nd system by synchrotron powder X-ray diffraction (SR-PXRD). The phase equilibria and phase transformation related to Nd₄Mg₈₀Ni₈, Nd₁₆Mg₉₆Ni₁₂, NdMg₅Ni and NdMg₂Ni were studied by scanning electron microscopy (SEM), X-ray diffraction (XRD) and differential scanning calorimetry (DSC). The formation enthalpies of these ternary compounds were calculated by the density functional theory. The existence of the quaternary hydrides NdMgNi₄H₄ and NdMg₂Ni₉H₁₂ was taken into consideration. Based on the obtained thermodynamic description of the Mg–Ni–Nd–H system, the hydrogen storage capacities and pressure–composition–temperature (P–C–T) curves were predicted and two new ternary compounds Nd₄Mg₈₀Ni₈ and Nd₁₆Mg₉₆Ni₁₂ were designed as hydrogen storage alloys with excellent properties. Meanwhile, the H/D mechanism of the Mg–Ni–Nd alloy was revealed.