Constrained incipient phase transformation in Ni-Mn-Ga films: A small-scale design challenge

• Published in: Materials & design 2023-09, Vol.233, p.112259, Article 112259
• Main Authors: Fareed, Adnan, Rosalie, Julian M., Kumar, Sourabh, Kar, Satyakam, Hickel, Tilmann, Fähler, Sebastian, Maaß, Robert
• Format: Article
• Language: English
• Subjects: Incipient plasticity; Nanoindentation; NiMaGa; Shape-memory alloys
• ISSN: 0264-1275
• DOI: 10.1016/j.matdes.2023.112259

[Display omitted] •Incipient phase transformations in NiMnGa thin films were probed across temperature.•Stress-induced phase transformations emerge as pop-ins for thinner films.•Non-modulated martensite forms during pop-ins at high transformation stresses.•Thickness constraints increase the phase-transformation stress. Ni-Mn-Ga shape-memory alloys are promising candidates for large strain actuation and magnetocaloric cooling devices. In view of potential small-scale applications, we probe here nanomechanically the stress-induced austenite–martensite transition in single crystalline austenitic thin films as a function of temperature. In 0.5 µm thin films, a marked incipient phase transformation to martensite is observed during nanoindentation, leaving behind pockets of residual martensite after unloading. These nanomechanical instabilities occur irrespective of deformation rate and temperature, are Weibull distributed, and reveal large spatial variations in transformation stress. In contrast, at a larger film thickness of 2 μm fully reversible transformations occur, and mechanical loading remains entirely smooth. Ab-initio simulations demonstrate how an in-plane constraint can considerably increase the martensitic transformation stress, explaining the thickness-dependent nanomechanical behavior. These findings for a shape-memory Heusler alloy give insights into how reduced dimensions and constraints can lead to unexpectedly large transformation stresses that need to be considered in small-scale actuation design.