Redox-mediated sustainable NiTi

Redox-mediated solid-state synthesis of NiTi — Expert summary and suggested edits

Core scientific content and mechanisms

This study demonstrates a one‑step, solid‑state route to synthesize NiTi from NiO + TiH₂ by coupling hydride decomposition, oxide reduction and diffusion‑driven alloying. Thermogravimetric analysis (TGA) and in‑situ synchrotron X‑ray diffraction (SXRD) resolve the kinetics and phase sequence: in pure Ar the conversion begins near ~275 °C, NiO reduction is first observed at ~328 °C, and the overall conversion plateaus at ≈70%; with Ar + 5% H₂ the reduction onset shifts to ~180–280 °C, complete NiO reduction occurs below ~370 °C, and the conversion reaches ≈92%. Hydride decomposition of the TiH₂ (8‑phase) proceeds in multiple steps, with decomposition signatures appearing between ~370–468 °C in Ar and shifted lower when H₂ is present. SXRD shows early Ni metal formation followed by progressive appearance of intermetallics (Ti₂Ni, Ni₃Ti) and Ti phases containing dissolved hydrogen (α‑Ti(H), β‑Ti(H)); TiO₂ (rutile) forms transiently and partially reduces at higher temperature. Quantitative phase analysis reports markedly different intermetallic fractions depending on atmosphere: in Ar the intermetallics reach approximately Ti₂Ni 24% and Ni₃Ti 17%, whereas in Ar + 5% H₂ these reduce to Ti₂Ni 9% and Ni₃Ti 2%. A validation run to 1150 °C produced a bulk sample with up to 33.7 vol.% NiTi in the center and, after dehydrogenation, the material exhibits thermoelastic martensitic transformation with DSC transformation temperatures near Aₛ ≈ −11 °C and Mₛ ≈ −20 °C. The data show that external H₂ accelerates NiO reduction and suppresses metallothermic oxide formation pathways, while internal hydrogen from TiH₂ controls Ti phase stability and transient β‑Ti(H) formation that mediates diffusion and alloying.