In situ SEM microtensile testing

Microtensile coupons were prepared by focused ion beam (FIB) on an FEI Quanta 3D dual beam (SEM/FIB) using a semiautomated “lathe-milling” procedure described in detail in (40). The resulting microtensile coupons have a cylindrical gauge section with an average diameter of 4 μm and a length of 12 μm.

Uniaxial tension tests were performed under electron beam observation using a nanomechanical testing system (model FT-NMT03, FemtoTools, Buchs, Switzerland) at room temperature in displacement-controlled mode with a nominal strain rate of 1 × 10⁻³ s⁻¹. The force was measured by a micro-electro-mechanical system (MEMS)–based microforce sensor (model FT-S200’000), whose 50-μm by 50-μm flat probe tip was milled into a tensile grip by FIB (see fig. S8). A sample displacement was measured simultaneously using a piezo-based actuation system with subnanometer resolution. Platinum reference markers were electron beam deposited on both ends of the gauge section to enable relative displacement measurements in post processing. Marker positions were tracked frame-by-frame using a custom MATLAB script to calculate the sample elongation.