Intermetallics V-20Ti and FeTi. A transmission electron microscopy study of hydriding reactions and thermodynamic stability

Article abstract of DOI:10.1016/S0022-5088(06)80012-3

V-20Ti samples heat treated in an H₂ atmosphere at about 1000 mbar for different times were inspected by transmission electron microscopy for the occurrence of hydrides. Hydriding between 500 and 600°C leads to thin plates of a b.c.t. hydride presumably identical to the ε phase in the work of Hagi et al. Hydriding between 650 and 700°C produces a b.c.c. hydride with a unit cell approximately 7% larger than that of the matrix. This hydride is richer in titanium than the matrix and may be identified as Hagi et al.'s β phase. Hydriding at 750°C did not produce any hydrides or phase separation. Thus, to avoid embrittlement problems, one has to proceed with caution when annealing V-20Ti alloys in H₂ gas in the intermediate temperature range. We also expect hydride precipitation when V-20Ti is exposed to atomic hydrogen, as will occur in a fusion device. Contrary to thermodynamic considerations, long-term annealing of FeTi in H₂ does not result in TiH₂ precipitation.