| In this thesis, a few sets of ternary phase diagrams of the system RE-Ti-Al (RE = Pr, Nd, Gd, Dy, Ho and Er), La-Ni-M (M = Fe, Nb, Cr, Si and Sn) and Pr-Mg-Ni have been experimentally determined by X-ray diffraction, scanning electron microscopy and differential thermal analysis. The synthesis, hydrogen-storage behavior and electrochemical properties of LRE-Mg-Ni alloys have also been investigated. The results are mainly as following:There are two ternary compounds of the type RETi2Al20 and RE6Ti4Al43 in the isothermal section (773K) of the RE-Ti-Al (RE = Nd, Gd, Dy, Ho and Er) system except for Pr-Ti-Al system in which only PrTi2Al20 was found. In the RE-Al binary systems, the compounds with atomic ratio RE:Al = 3:1, 2:1, 1:1, 1:2, 1:3 , 3:11 and 2:1, 3:2, 1:1, 1:2, 1:3 are formed for RE = LRE and HRE, respectively, indicating that the compounds with 3:1 and 3:11 are only in LRE-Al system, whereas the compounds with 3:2 only in HRE-Al system. For most compounds, substitution of Ti for Al in RE-Al leads a solubility line paralleled to the boundary of Ti-Al., the largest solid solubility being about 16.5at%Ti occurred in the REAl2 compound with cubic structure. Adding RE to the Ti-Al system results in formation of RE-Ti-Al ternary compounds in the Al-rich region but none in the Ti-rich region, however, in the intermediate region around the TiAl alloy, a variety of RE-Al binary phases are formed in the RE-Ti-Al alloys. The distribution of phase regions and phases relation are basically similar in the isothermal sections (773K) of RE-Ti-Al (RE = Pr, Nd, Gd, Dy, Ho and Er) ternary alloy phase diagrams.There is no ternary intermetallic compounds exist in the La-Ni-M (M = Fe, Nb, Cr) systems. While for the La-Ni-Si and the La-Ni-Sn system there are fourteen and eight ternary compounds, respectively, where both La14Ni6Si11 compound with Ce14Ni6Si11 structure-type and LaNiSn3 compound were found in our research work. The investigation of the structure of LaNiSn3 compound is now in progress.There are two ternary compounds, PrMgNi4 with cubic structure, lattice constants a = 0.71237nm and PrMg2Ni9 with hexagonal structure, lattice constants a = 0.48868nm, c = 2.3854nm, in the Ni-rich region of Pr-Mg-Ni ternary system. The isothermal section (1123K) of Pr-Mg-Ni consists of 8 single-phase regions, 14 two-phase regions and 7 three-phase regions, whereas the section at 773K consists of 11 single-phase regions, 21 two-phase regions and 11 three-phase regions. PrMgNi4 and PrMg2Ni9 compound possess the hydrogen-storage behavior, where amorphous PrMgNi4 single-phase alloy exhibits much better electrochemical characteristics than Mg2Ni in the charge /discharge capacity and the lifetime of discharge cycling.REMgNi4 (RE = La,Ce, Pr and Nd) compounds have much high hydrogen storage capacity, especially in the amorphous phases. Their charge /discharge capacities vary with the rare-earth elements, in the sequence of LaMgNi4 > NdMgNi4 >CeMgNi4 >PrMgNi4 for the investigated series of amorphous REMgNi4 obtained by ball-milling, in which the maximum value is about 350 mAh/g and the minimum is 250 mAh/g. However, for crystalline REMgNi4 obtained by ball-milling and sintering, their charge /discharge capacities are totally different, the order arranged in NdMgNi4 > PrMgNi4 LaMgNi4 > CeMgNi4 where the maximum is about 200mAh/g.The electrochemical properties of Mg-Ni based alloys are closely related with their crystalline structures and morphologies. Thus, the milling condition of mechanical alloying, including milling time, rotational speed of the milling machine, weight ratio of steel balls to powder and steel ball diameter, etc., has significant effect on the formation of alloys as well as their microstructure and electrochemical properties. In general, high revolution and high weight ratio of steel balls to powder are favourable for the formation of an amorphous phase and, using ball milling process together with sintering, it will speed up the synthesis of alloy phases, hence shortening the milling time.The rare-earth elem...
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