Proton-Irradiation-Induced Structural Transition from Modulated Seven-Layered to Non-modulated Martensite in Ni-Mn-Ga Films

This study aims to elucidate irradiation-induced microstructural changes that significantly influence the performance stability of Ni-Mn-Ga shape memory alloy films in aerospace radiation environments. The structural evolution of seven-layered modulated (7M) martensite in Ni₅₂Mn₂₉Ga₁₉ films under 3-MeV proton irradiation was systematically investigated using X-ray diffraction (XRD) and a transmission electron microscope (TEM) across a range of proton fluences. Reorientation of 7M martensite variants was observed to initiate at a fluence of 5.0 × 10¹⁵ p/cm². With increasing fluence up to 2.0 × 10¹⁶ p/cm², unfavorably oriented variants gradually diminished, while favorably oriented variants grew through coalescence. A pronounced martensitic transformation from 7M to non-modulated (NM) martensite was observed, accompanied by a rearrangement of the two-layer atomic stacking that disrupts the modulated structure. At proton fluences > 5.0 × 10¹⁶ p/cm², the irradiation-induced reorientation of the 7M martensite persists under continuous irradiation, as confirmed by experimental observations, and is accompanied by a significant increase in the volume fraction of NM martensite. Reorientation among NM martensite variants was also observed. High-resolution TEM analysis revealed that the coalescent zones between NM variants contain dislocations within distorted lattice regions. These microstructural changes are attributed to localized stress fields induced by proton irradiation in the crystal lattice. The findings provide valuable insights into the irradiation tolerance and microstructural stability of Ni-Mn-Ga films, supporting their potential application in aerospace actuation systems operating in radiation-exposed environments.