Study of annealing effects on structural and sorption properties of low energy mechanically alloyed AB5´s by manifold methods

B.A. TALAGAÑIS; M.R. ESQUIVEL; G. MEYER

Ciudad de Buenos Aires

Congreso; ISMANAM 2008-XV International symposium on Metastable, Amorphous and Nanostructured Materials; 2008

Fernando Audebert-Laura Damonde-Facultad de Ingenieria-UBA

Mechanical alloying is a popular technique used to synthesize AB5´s applied to thermal compression of hydrogen. These compounds can be obtained from mixtures of either metals or intermetallics. But the processing leaves micro and nano-structured materials with highly distorted particles characterized by crystallites with both low size and high strain values.  Further annealing is needed to fit the intermetallic ready to use in thermal compression. In this work, different AB5 intermetallics were mechanically alloyed in a low energy device enough integrated milling time to reach final or completion stages [1]. Samples were annealed at different conditions, i.e time and temperature, by non isothermal and isothermal methods and analyzed by manifold techniques: X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), Differential Scanning Calorimetry (DSC), Energy Dispersive Spectroscopy (EDS) and hydrogen volumetric methods. Cell parameter values, crystallite sizes and strain, particles morphology and size distribution along with hydrogen sorption pressure composition curves values were obtained for different annealing conditions. It was found that from room temperature to 200 ºC no further changes occur in neither of structural and hydriding properties. Between 200 ºC and 400 ºC, the relaxation effects due to the release of strain produced by annealing on the microstructure lead to appreciable change in structural properties and quality increment in PCI curves  in comparison with those of as-milled. Strong recrystallization effects are present between 400 and 600 ºC. A marked increment and decrement in crystallite size and strain values are along with a qualitative improvement in PCI curves is observed. Despite this behaviour, no strong crystallization peak is observed in samples analyzed by DSC from room temperature to 600 ºC. This issue, the amount of energy supplied during MA and the correlation between the structural and sorption properties are discussed according to the governing mechanisms of recrystallization observed in each temperature range.  References: [1]  M.R. Esquivel, G. Meyer, Mat. Sc. Forum, 570,  2008, 72-77.