HOMOGENIZING BEHAVIOR AND PRESSURE-COMPOS ITION ISOTHERMS OF HYDROGENIN LANI4.55AL0.45 ALLOY DURING ANNEALING

Plateau flatness of the pressure-composition (P-C) isotherm of hydroge n in hydrogen-absorbing alloys is important for achieving high-perform ance application systems. In this study, the effect of annealing time t on the plateau flatness and micro-structure of an alloy was quantita tively studied. Samples were prepared by annealing an induction-melted LaNi4.55Al0.45 alloy in the range up to t = 259.2 ks in a vacuum at 1 323 K. The P-C isotherms at 353 K were analysed with the normal cummul ative distribution function model which we developed previously to eva luate the standard deviation sigma(P) as the plateau flatness paramete r. The homogenizing behaviour of the micro-structure through the annea ling was investigated with electron-probe micro analysis and X-ray pow der diffraction analysis. With increasing t, ln sigma(P) decreased lin early and the plateau area flattened. The plateau slope was caused by a heterogeneous Al distribution in the dominant phase with high Al con centration area around precipitated AlNi3 phase. A decrease of the hal f value of the X-ray powder diffraction profile through the annealing showed the homogenization of the micro-structure. The AlNi3 phase was stable through the annealing and therefore, it has no influence on the homogenization. The homogenizing behaviour was analysed with a non-st eady state Al diffusion model where the Al concentration profile in th e dominant phase was a cosine function. As a result, an equation to gi ve the linear relation between ln sigma(P) and t was found. This equat ion is helpful for determining manufacturing conditions for a variety of alloys because it includes mean precipitation distance 2l and annea ling conditions (time t and temperature T) explicitly.