Impact of hydrogen-assisted heat treatments on microstructure and transformation path in a Ni-rich NiTi shape memory alloy

We report results of differential scanning calorimetry and transmission electron microscopy experiments in which martensitic transformations and microstructure of Ti–50.9 at%Ni shape memory samples were investigated after heat treatments in controlled gaseous environments. The heat treatments were performed in mixtures consisting of hydrogen and helium with an overall pressure of 900 mbar. The cylindrical samples were heat treated either in a Regime I (850 °C/1800 s/WQ) or in a Regime II (the Regime I plus 450 °C/3600 s/WQ). DSC charts clearly document that the B2 → B19′ or R → B19′ martensitic phase transitions diminish after Regime I and Regime II treatments with increasing partial pressure of hydrogen. We show that the hydrogen rich environments do not cause variations in size and density of Ni4Ti3 particles. Conversely, the Regime I heat treatments assisted by hydrogen result in 1/2 〈1 1 1〉B2 diffuse intensity maxima in selected area electron diffraction patterns. These maxima were not observed after heat treatments in hydrogen deficient atmospheres. Therefore, we suggest that the gradual suppression of the martensitic transformation may be related to either chemical and/or strain modulations of the B2 lattice triggered by the presence of interstitial hydrogen atoms.

► We report on the suppression of the B2–B19′ MT in Ni-rich NiTi alloys.
► Heat treatments in gaseous environments containing hydrogen cause the suppression.
► Ni4Ti3 phase precipitation is not affected by the hydrogen environment.
► Diffuse maxima in TEM SAD patterns indicate a presence of lattice modulations.
► Related nano-domains may cause the extinction of the martensitic transformation.