Nanoindentation response of substrate-attached and freestanding single-crystalline Fe7Pd3 ferromagnetic shape memory thin films around the martensite transition: The impact of constraints and beyond

Abstract Ferromagnetic shape memory alloys offer great potential in the fields of engineering and medical sciences as integrated actuators or sensors. However, their physical properties, when miniaturized and connected to a substrate or mounted as active elements, are still insufficiently understood. The present work explores the impact of miniaturization and external boundaries on one of the most central features, namely twin boundary mobility. By measuring the nanoindentation response of substrate-attached films and freestanding foils around the austenite ↔ martensite transformation temperature in classical indentation, as well as dynamical quasi-continuous stiffness-measurement mode, dramatic softening and increasing recovery after film lift-off are discovered. The atomistics of these findings are explored with the help of classical multimillion-atom molecular dynamics simulations on indentation into martensite and austenite films on rigid or flexible substrates, as well as freestanding or mounted thin foils. They clearly demonstrate how substrate or lateral constrains hinder twin boundary motion, while complete untwinning only prevails in the presence of a flexible substrate or completely free foils. Experimentally observed pop-in events can be rationalized as local austenite → martensite transitions. Surface softness, which is observed by low indentation moduli, when compared to predictions from the bulk elastic constants, might indicate a more fundamental scenario close to the martensite transformation.