Structural and microstructural study of AB5 intermetallics processed by mechanical milling and mechanical alloying

M.V. BLANCO; M.R. ESQUIVEL

Ciudad de Buenos Aires

Congreso; VI Reunión de la Asociación Argentina de Cristalografía; 2010

Asociación Argentina de Cristalografía

The mechanical milling (MM)  and mechanical alloying (MA) of AB5 intermetallics is currently under study. Samples obtained from these processes present unique hydrogen sorption properties. Despite all the reference data reported on the subject, the mechanisms that govern these processing is not completely understood. Therefore, it is worth to study the processing of these materials by these techniques and to characterize the products. In this work, the microstructural and structural parameters of AB5 intermetallics were analyzed after processing by mechanical milling and alloying. LaNi5 was processed by MM and La0.25Ce0.52Nd017Pr0.06Ni5 was synthesized by MA of La0.25Ce0.52Nd0.17Pr0.06 and Ni. An AB5 intermetallic was also synthesized by MA from a mixture of these intermetallics (0.40 mass % La0.25Ce0.52Nd017Pr0.06Ni5-0.60 mass % LaNi5). Processing was done under Ar atmosphere keeping a constant ball-to-sample mass ratio (6.28). Particle size distribution and morphology were observed by Scanning electron microscopy (SEM). Composition was analyzed by Energy Dispersive Spectroscopy (EDS). Structural and microstructural parameters were established by X-ray diffraction (XRD). Stages of MM of LaNi5 were identified and characterized. Each stage corresponds to different governing process including Fracture and Cold Welding. The structures of the intermetallics were analyzed by XRD. P6/mmm space group was assigned to LaNi5 and La0.25Ce0.52Nd017Pr0.06Ni5. Lanthanides were distributed in Wyckoff positions (1a) and Ni was distributed in Wyckoff positions (2c, 3g). The structural parameters of LaNi5 before and after mechanical milling were determined and related to the mentioned processes.  Starting cell parameters were a = 5.02 ± 0.05 Å  , c = 3.98 ± 0.05 Å and Vc =  86.79 ± 0.5 Å3. According to observations in LaNi5 samples, milling resulted in a broadening of the X-ray diffraction peaks owing to the introduction of microstrains and reduction in crystallite size. The cell volume increased near 0.35 Å3.  Microstructural lattice parameters a and b experimented a slight decrease with increasing milling time , while c increased with increasing milling time. Finally, crystallite size decreased significantly (50%) as a result of mechanical milling. Initial results also indicate that the structure is unstable on milling and Ni is segregated from the AB5 matrix..   The structural and microstructural parameters of La0.25Ce0.52Nd017Pr0.06Ni5 were analyzed after mechanical alloying and after thermal annealing at 600 °C for 48 h. The cell parameters of annealed La0.25Ce0.52Nd017Pr0.06Ni5 were  a = b = 4.91 ± 0.05  Å, c = 3.97 ± 0.05 Å and Vc = 83.06 ± 0.5 Å3. The AB5 obtained from the mixture of intermetallic was also characterized after milling and after annealing. The structural parameters were obtained. Values were a = b = 4.97 ± 0.05  Å, c = 3.98 ± 0.05  Å and Vc = 85.18 ± 0.05  Å3 . The microstructural parameters were also obtained. Crystallite size decreased near (35 %) while cell parameter strain increased close to ( 100 %). Results presented are discussed in order to correlate the effect of lanthanides in the structural parameters of the structure. These effects are relevant to determine the hydrogen sorption properties of the intermetallics and its relation with the structural parameters of these intermetallics. These two goals aimed the elaboration of this work.     KEY WORDS: mechanical alloying, mechanical milling, XRD, AB5