Abstract:Phase transformations and lattice expansions of MlNi3.75Co0.75Mn0.3Al0.2during the electrochemical hydriding-dehydriding process were investigated using in-situ X-ray diffraction. An intermediate hydrideγphase between the hydrogen solid solutionαphase and fully hydridedβphase can be observed during the cycling. The formation ofγphase is related to the diffusion of hydrogen in the crystal grains. The lower the charge rate is, the higher the content ofγphase is. The phase transformations during the hydriding-dehydriding process can be described as α α+γ+β β γ+α α. The lattice expansion fromαtoβis discrete, while that fromγtoβis continuous. The formation ofγphase can reduce the discrete lattice expansion fromαtoβby 30%.

Key words: in-situ X-ray diffraction; phase transformation; lattice expansion; intermetallic alloys; hydrides