title: Effect of crystallographic compatibility and grain size on the functional fatigue of sputtered TiNiCuCo thin films.
abstract: The positive influence of crystallographic compatibility on the thermal transformation stability has been already investigated extensively in the literature. However, its influence on the stability of the shape memory effect or superelasticity used in actual applications is still unresolved. In this investigation sputtered films of a highly compatible TiNiCuCo composition with a transformation matrix middle eigenvalue of 1±0.01 are exposed to thermal as well as to superelastic cycling. In agreement with previous results the thermal transformation of this alloy is with a temperature shift of less than 0.1 K for 40 cycles very stable; on the other hand, superelastic degradation behaviour was found to depend strongly on heat treatment parameters. To reveal the transformation dissimilarities between the differently heat-treated samples, the microstructure has been analysed by transmission electron microscopy, in situ stress polarization microscopy and synchrotron analysis. It is found that good crystallographic stability is not a sufficient criterion to avoid defect generation which guarantees high superelastic stability. For the investigated alloy, a small grain size was identified as the determining factor which increases the yield strength of the composition and decreases the functional degradation during superelastic cycling.This article is part of the themed issue 'Taking the temperature of phase transitions in cool materials'.
authors.affiliation: Institute for Materials Science, Faculty of Engineering, University of Kiel, Kiel, Germany.
authors.forename: C
authors.identifier: 0000-0001-9337-9846
authors.initials: C
authors.lastname: Chluba
authors.affiliation: Department of Materials Science and Engineering, University of Maryland, College Park, MD, USA.
authors.forename: W
authors.initials: W
authors.lastname: Ge
authors.affiliation: Institute for Materials Science, Faculty of Engineering, University of Kiel, Kiel, Germany.
authors.forename: T
authors.initials: T
authors.lastname: Dankwort
authors.affiliation: Acquandas GmbH, Kiel, Germany.
authors.forename: C
authors.initials: C
authors.lastname: Bechtold
authors.affiliation: Acquandas GmbH, Kiel, Germany.
authors.forename: R Lima
authors.initials: RL
authors.lastname: de Miranda
authors.affiliation: Institute for Materials Science, Faculty of Engineering, University of Kiel, Kiel, Germany.
authors.forename: L
authors.initials: L
authors.lastname: Kienle
authors.affiliation: Department of Materials Science and Engineering, University of Maryland, College Park, MD, USA.
authors.forename: M
authors.initials: M
authors.lastname: Wuttig
authors.affiliation: Institute for Materials Science, Faculty of Engineering, University of Kiel, Kiel, Germany eq@tf.uni-kiel.de.
authors.forename: E
authors.identifier: 0000-0002-2314-5179
authors.initials: E
authors.lastname: Quandt
keywords: NiTi; fatigue; shape memory alloys; superelasticity; thin films