Stability of the hydrogen absorption and desorption plateaux in LaNi5-H .4. Thermal history effects

We consider the effects of elevated (uniform) temperatures on the pressure hysteresis of LaNi5-H, concentrating on two main aspects: (i) promotion of gamma-phase formation and (ii) annealing effects. Hysteresis data taken at temperatures up to 110 degrees C and at 30 degrees C after annealing at temperatures up to 400 degrees C are presented and discussed. A strictly ascending sequence of measuring and annealing temperatures was employed. From the discontinuities in the absorption and desorption plateaux, the formation of the gamma phase at 90 degrees C in absorption and 60 degrees C in desorption was inferred. Both these temperatures are lower than in the original studies during an ascending temperature sequence. Critical perusal of published data suggests that detailed thermal history, rather than measuring temperature: determines the temperature at which the gamma phase appears. Hydrogen cycling at 110 degrees C caused the gamma phase to also form during the immediately following absorption-desorption cycle executed at room temperature. It is tentatively proposed that the microstructural modification required to form the gamma phase at room temperature is an ordering of dislocations.