Challenges During Microstructural Analysis and Mechanical Testing of Small-Scale Pseudoelastic NiTi Structures

Abstract

Most investigations on NiTi-based shape memory alloys involve large-scale bulk material; knowledge about the martensitic transformation in small-scale NiTi structures is still limited. In this paper, we study the microstructures of thin NiTi layers and their mechanical properties, and we discuss typical challenges that arise when experiments are performed on small samples. A physical vapor deposition (PVD) process was used to deposit thin NiTi wires with a cross section of 15 9 15 lm 2 and dogbone-shaped samples 5 9 500 lm 2 . Microstructural properties were characterized by X-ray diffraction, electron backscatter diffraction, and scanning electron microscopy. Moreover, tensile tests were performed using optical strain measurements in order to observe martensite band formation during cyclic loading. The surfaces of the crystalline wires reflect the columnar growth of NiTi during deposition. The wires exhibit pseudoelastic material behavior during tensile testing. Fracture typically occurs along the columns because the column growth direction is perpendicular to the straining direction. Electropolishing removes these local stress raisers and hence increases fracture strains. Our results demonstrate that the pseudoelastic properties of the PVDprocessed materials agree well with those of conventional NiTi, and that they provide new opportunities to study the fundamentals of martensitic transformation in small-scale model systems.

Thin NiTi Films Deposited on Graphene Substrates

“…It is well known that NiTi thin films deposited on silicon substrates exhibit a random grain orientation fully in line with our observations as shown in Fig. 1a [14]. Such a randomly oriented microstructure is typically characterized by globular grains with grain sizes that are in the range of about 5-15 lm.…”

“…We also found that a Cu interlayer (with a thickness of a few nanometers) can be used to adjust the adhesion between graphene and NiTi and to improve the adhesion conditions in general. Comparing our samples to recent results obtained from experiments using silicon substrates [8,14] together with Ni, Ti, and Cu interlayers, we did not expect to observe any influence on the NiTi grain orientation and grain size. However, after introducing the additional Cu layer between graphene and NiTi, we observed a quite different behavior, as discussed below.…”

Growth, microstructure and thermal transformation behaviour of epitaxial Ni-Ti films

“…Second, a smooth film surface is essential, as only such a well-defined cut through the crystal allows distinguishing a martensitic microstructure from film morphology later on. Small scale mechanical testing also requires smooth surfaces to avoid crack formation in tensile tests [17] or adverse effects on nanoindentation measurements [18,19]. Both aspects can be examined best when the film is within the austenitic state at room temperature and therefore in the present study we focus on a film composition where the martensitic transformation occurs just below room temperature.…”