Martensitic Transformations in NiTi TEM Lamellae Induced by Manipulator-Driven Bending

Unintentional deformation and residual stresses are frequently imparted to Transmission Electron Microscopy (TEM) samples during routine lamella preparation. This can become a significant issue in material systems with mechanically induced phase transformations such as B2→B19′ in NiTi shape memory alloys. We demonstrate that a Focused Ion Beam (FIB) lift-out TEM sample micromanipulator can be used as a localized loading tool to apply controlled bending displacements, a practical means to investigate preparation-induced artifacts. NiTi lamella were prepared via FIB lift-out, with a prescribed 0.5 µm tip displacement applied before final thinning, producing localized bending stresses sufficient to activate stress-assisted martensitic transformation. Unstrained control foils remained austenitic, while strained lamellae developed aligned bands of martensite with 2 progressively refined twin spacing toward the manipulator contact. HRTEM analysis confirms dense {201 ̅ } twin pairs form within the transformed regions. Finite-element modeling reproduces the bending-induced stress gradients and identifies peak stress pathways that spatially correlate with the observed martensite variant alignment and twinspacing gradients. These results show that manipulator-driven deformation during lift-out can not only be used to assess the potential for preparation-induced artifacts, but can potentially provide a simple, geometry-defined method to reproducibly induce martensitic twin architectures in NiTi lamellae without specialized mechanical testing holders.