Microstructure and hydriding characteristics of FeTi alloys containing manganese

Abstract

The effect of the substitution of manganese for iron, titanium or both iron and titanium in an FeTi alloy ( i.e. Fe 1− x TiMn x, FeTi 1− x Mn x and Fe 1−( x 2 ) Ti 1−( x 2 ) Mn x ( x = 0.1–0.3)) on the hydriding during the initial hydriding process, the pressure-composition isotherms and the microstructure was investigated. The partial substitution of manganese for iron, titanium or both iron and titanium yielded FeTi activation at 303 K after an incubation period without any activation treatment (with the exception of the Fe 0.9TiMn 0.1 alloy). Fe 1−( x 2 ) )Ti 1−( x 2 ) Mn x and FeTi 1− x Mn x alloys were composed of an FeTi phase and a second phase, (Fe 1− z Mn z ) 1.5Ti. Fe 1− x TiMn x alloys were predominantly composed of an FeTi phase with small amounts of second phases, (Fe 1− z Mn z ) 1.5Ti and Ti 1.5Fe 1− w Mn w ). However, after annealing, the second phases in the Fe 1− x TiMn x alloys disappeared and an activation treatment at 673 K with a hydrogen pressure of 4 MPa was then required. The results obtained in this study showed that the presence of a second phase in the FeTi alloy played a crucial role in shortening the incubation time. The incubation time for the initial hydriding was dependent not only on the number of interfaces between the FeTi phase and the second phase, but also on the composition of the second phase, i.e. if the second phase readily reacts with hydrogen to form its hydride, even a small amount of second phase shortens the incubation period. Manganese substituted for iron in the FeTi phase and its composition was approximately represented by (Fe 1− y Mn y )Ti. The plateau pressure decreased and the lattice parameter of the FeTi phase increased with increasing manganese content.