October_2021_AMP_Digital

A D V A N C E D M A T E R I A L S & P R O C E S S E S | O C T O B E R 2 0 2 1 5 0 Shape Memory and Super- elasticity: Advances in Science and Technology (SMJ) is the official journal of the Interna- tional Organization on Shape Memory and Superelastic Technologies (SMST), an af- filiate society of ASM Interna- tional. The journal publishes original peer-reviewed papers that focus on shape memory materials research with contri- butions from materials science, experimental and theoret- ical mechanics, physics with cognizance of the chemistry, underlying phases, and crystallography. It also provides a fo- rum for researchers, scientists, and engineers of varied disci- plines to access information about shapememorymaterials. These articles, from our published 2021 issues, were select- ed by Shape Memory Editor-in-Chief Huseyin Sehitoglu. SMJ is available through springerlink.com . For more information, visit asminternational.org/web/smst. March 2021 LATTICE DEFECTS GENERATED BY CYCLIC THERMO- MECHANICAL LOADING OF SUPERELASTIC NiTi WIRE Ondřej Tyc, Luděk Heller, and Petr Šittner Abstract cyclic instability of stress–strain–temperature functional responses of Nitinol is presumably due to the plastic deformation accompanying martensitic transforma- tion proceeding under external stress. To obtain systematic experimental evidence of this, the authors performed a se- ries of cyclic thermomechanical loading tests (10 cycles) on superelastic Nitinol wires with nanocrystalline microstruc- ture, evaluated accumulated unrecovered strains, and ana- lyzed permanent lattice defects created during the cycling by TEM. The accumulated unrecovered strains and density of lattice defects increased with increasing temperature and stress, at which the forward and/or reverse transfor- mation proceeded. It did not correlate with the tempera- ture and stress applied in the test as such. If the martensitic transformationproceededat lowstress (<100MPa), thecyclic stress–strain–temperature responses of the wire were found to be almost stable (only marginal accumulated unrecov- ered strain and few isolated dislocation loops and segments were generated during the thermomechanical cycling). This was the case in thermal cycling at low stresses or in cyclic shape memory test. If the forward and/or reverse marten- sitic transformation proceeded under large external stress (> 250 MPa), the responses were very unstable (large accu- mulated unrecovered strains and high density of dislocations and deformation bands). A scheme allowing for estimating the cyclic instability of functional behaviors of various Niti- nol wires in wide range of thermomechanical loading tests was introduced (Fig. 1). SMJ HIGHLIGHTS Fig. 1 — (a) Microstructure of the virgin austenitic 15 ms Nitinol wire. (b) Electrical resistivity thermal cycle across transformation range. There are no lattice defects observable by TEM in the recrystallized microstructure. 114 1 4 (a) (b)