A number
of high-entropy alloys (HEAs) in the TiVZrNbHf system
have been synthesized by arc melting and systematically evaluated
for their hydrogen sorption characteristics. A total of 21 alloys
with varying elemental compositions were investigated, and 17 of them
form body-centered-cubic (bcc) solid solutions in the as-cast state.
A total of 15 alloys form either face-centered-cubic (fcc) or body-centered-tetragonal
(bct) hydrides after exposure to gaseous hydrogen with hydrogen per
metal ratios (H/M) as high as 2.0. Linear trends are observed between
the volumetric expansion per metal atom [(
V
/
Z
)
fcc/bct
– (
V
/
Z
)
bcc/hcp
]/(
V
/
Z
)
bcc/hcp
with the valence electron concentration and average
Pauling electronegativity (χ
p
) of the alloys. However,
no correlation was observed between the atomic size mismatch, δ,
and any investigated hydrogen sorption property such as the maximum
storage capacity or onset temperature for hydrogen release.
We have investigated the structure and hydrogenation properties of a series of Ti, V, Zr, Nb and Ta based high-entropy alloys (HEAs) with varying degree of local lattice strain by means of synchrotron radiation X-ray diffraction, scanning electron microscopy, thermogravimetric analysis, differential scanning calorimetry and manometric measurements in a Sieverts apparatus. The obtained alloys have body-centred cubic (bcc) crystal structures and form face-centred cubic (fcc) hydrides with hydrogen-to-metal ratios close to 2. No correlation between the hydrogen storage capacity and the local lattice strain δr is observed in this work. Both bcc and fcc unit cells expand linearly with the zirconium-to-metal ratio [Zr]/[M], and increased concentration of Zr stabilizes the hydrides. When heated, the hydrides decompose into the original bcc alloys if [Zr]/[M]<12.5 at.%. The hydrides phase-separate in a hydrogen-induced decomposition type process for [Zr]/[M]≥12.5 at.%. The result is then a combination of two bcc phases, one with a larger and
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