Effects of Nanocrystalline Grain Size and Hydrogen on Stress-Induced Martensitic Transformation in NiTi Shape Memory Alloys: A Phase-Field Study

Despite advances in modelling NiTi shape memory alloys, it is still not trivial to model and understand the convoluted impact of variants, grain size, hydrogen amount and thermomechanical coupling in hydrogen-containing systems. This work introduces a thermodynamically consistent phase-field framework integrating Calphad-assessed Ni–Ti thermodynamics with DFT-derived NiTi–H energetics from Tang [1] and Holec et al. [2], respectively, to compute phase-specific Gibbs free energies and driving forces for B2 $\to$ B19${}^{\prime }$ transformation under dilute hydrogen conditions. Our simulations show stabilisation of austenite by nanocrystalline grain size and hydrogen during stress-induced martensitic transformation. In hydrogen-free NiTi, increasing the grain size lowers the critical applied stress for martensite nucleation, sharpens the transformation plateau, and raises maximum global martensite volume fraction. Hydrogen consistently increases nucleation stress and slope, delays the onset of the forward plateau, and reduces the martensite volume fraction, effects which are amplified at smaller grains. These predictions demonstrate that nanocrystalline grains enhance hydrogen-induced hindrance to martensite nucleation and propagation, leading to incomplete superelastic recovery. They align with experimental results [3], offering mechanistic insights for hydrogen-resistant nanostructured shape memory devices. Finally, we will present the effect of heterogeneous hydrogen distributions, which are encountered for instance during hydrogen loading of the samples, on the onset of the transformation.

References

1. W. Tang, Thermodynamic study of the low-temperature phase B19′ and the martensitic transformation in near-equiatomic Ti-Ni shape memory alloys, Metallurgical and Materials Transactions A, 28(3):537–544, 1997, https://doi.org/10.1007/s11661-997-0041-6.
2. D. Holec, M. Friák, A. Dlouhý, and J. Neugebauer, Ab initio study of point defects in NiTi-based alloys, Physical Review B, 89(1), 2014, https://doi.org/10.1103/physrevb.89.014110.
3. A. Baturin, A. Lotkov, V. Grishkov, I. Rodionov, and V. Kudiiarov, The effect of hydrogen on martensite transformations and the state of hydrogen atoms in binary TiNi-based alloy with different grain sizes, Materials, 12(23):3956, 2019, https://doi.org/10.3390/ma12233956.