Towards understanding the influence of Mg content on phase transformations in the La3-xMgxNi9 alloys by in-situ neutron powder diffraction study

The present work is focused on the studies of the phase-structural transformations in the La3-xMgxNi9 (x 1/4 1.0, 1.1 and 1.2) alloys as active materials of negative electrodes in the Nickel-Metal Hydride (Ni/MH) batteries. The phase equilibria and phase-structural transformations in the alloys were probed by in situ neutron powder diffraction (NPD) at the temperatures ranging from 300 K to 1273 K using the measurements of the equilibrated alloys at 8 setpoint temperatures of 300, 973, 1073, 1123, 1173, 1223, 1248 and 1273 K. Prepared by induction melting initial alloys were found to be multi-phase structured, containing up to 6 individual intermetallic compounds with different stoichiometric compositions. With the increase of the temperature and holding time, various transformations took place in the studied alloys. These included the formations and transformations of super-stacking intermetallics with variable ratios (La thorn Mg)/Ni, 1:3, 2:7 and 5:19. With increasing temperatures, several systematic changes took place. (a) Abundances of (La,Mg)(2)Ni-4 AB(2) and (La,Mg)Ni-3 AB(3) type intermetallics gradually decreased before they melted/decomposed above 1073 K; (b) The (La,Mg)(2)Ni-7 A(2)B(7) type intermetallics began to decrease in abundances above 1123 K; (c) The transformation in the (La,Mg)(5)Ni-19 intermetallics from 3R to 2H proceeded above 1223 K. The increase of Mg content had no obvious influence on (La,Mg)(2)Ni-4 and (La,Mg)(2)Ni-7 phases, and corresponding reactions R1 and R3 took place at the same temperatures as in the La-Ni system. However, with increasing Mg content the melting point of (La,Mg)(5)Ni-19 phase increased while the melting point of the (La,Mg) Ni-3 phase it decreased, leading to the variation of the reaction temperatures of the corresponding processes. The present study will assist in optimizing phase-structural composition of the alloys in the La-Mg-Ni system which contain Mg-modified layered structures by tailoring the high temperature annealing conditions.