Lattice expansion as a measure of surface segregation and the solubility of hydrogen in α-FeTiH x

Abstract

The lattice expansion of α-FeTiH x as a function of hydrogen content was found to be anomalously small. Samples of α-FeTiH x were prepared by absorbing hydrogen in previously activated strain-relieved FeTi with a specific surface area of 0.5 m 2 g −1. The apparent partial molar volume V H of hydrogen was 0.1± 0.1 cm 3 (g atom) −1 (FeTiH < 0.03); this is much lower than the accepted range of 2.2-1.4 cm 3 (g atom) −1 previously reported for hydrogen-metal solutions. This behavior is attributed to preferential occupation by hydrogen of titanium-rich sites produced by surface segregation effects which are known to occur in activated FeTi. The α-( α + β) phase boundary is estimated to occur at an overall composition equivalent to FeTiH≈ 0.04. At this composition about 85% of the total hydrogen content is estimated to be associated with surface layers approximately 100 Å in depth. Unactivated (low surface area) FeTi behaved quite differently and it is clear that, in the low dilution region, surface segregation effects play a dominant role in the behavior of the system. We also present evidence that strain and dislocations produced by a hydriding-dehydriding cycle increase the hydrogen solubility and shift the phase boundary towards higher hydrogen contents.