arXiv subjects

Ondřej Tyc

Publications and source records attributed to Ondřej Tyc.

2 recordsLinked to original sources

arXiv2026

HVAF Spraying of NiTi Coatings: Microstructure, Phase Transformation and Shape Memory Behavior

Depositing various coatings on surface of engineering components with the aim to improve their performance concerning wear, corrosion, friction and thermal protection is already a standard practice. Depositing metallic NiTi shape memory alloy coatings may be a viable alternative for hard ceramic coatings. NiTi coatings offer additional benefits originating from unique functional thermomechanical properties. However, fabrication of thick NiTi coatings turned out to be difficult. Standard electroplating and laser cladding methods are not suitable for NiTi the most widely used plasma spray methods tend to produce chemically inhomogeneous coatings that do not transform martensitically, cold sprayed NiTi coatings suffer from poor adhesion to the substrates. In this work we report on first ever successful fabrication of thick NiTi coatings (100-300 um) that display functional thermomechanical properties and simultaneously show very good adherence to the substrate. We used high velocity air fuel thermal spray method to fabricate NiTi coatings deposited on mild steel using four different sets of processing parameters. Chemical composition, porosity, microstructure, phase transformation and functional thermomechanical properties of the NiTi coatings were evaluated. Although the coatings contain inhomogeneous microstructure, voids, oxide particles, high density of dislocation defects and internal stress, they undergo martensitic transformation upon cooling and or mechanical loading. As sprayed NiTi coatings need to be annealed to display functional thermomechanical properties. Despite their limited tensile strength, the coatings displayed thermal actuation in 3 point bending tests and shape memory effects in nanoindentation and scratch tests.

cond-mat.mtrl-sci↗

arXiv2026

Plastic Deformation of B19' Martensite: Where it Matters in NiTi Technology

Nitinol technology, besides utilizing the functional thermomechanical properties derived from the B2 cubic to B19' monoclinic martensitic transformation, also exploits the excellent plastic deformability of NiTi in the martensite state. It originates from the unique mechanism of plastic deformation of the B19' martensite by kwinking involving dislocation slip based kinking assisted by deformation twinning. Although the mechanism of plastic deformation of martensite by kwinking was revealed only very recently, various unusual phenomena that can only be rationalized by kwinking, have been reported in literature in the last 50 years. These phenomena include: 1) cold working with a high degree of reduction without introducing cracks, 2) excellent plastic deformability in the martensite state (plastic deformation up to~80% strain at stresses >1GPa), 3) refinement of austenitic microstructure to a quasi-amorphous state by tensile deformation, 4) observation of high density of {114} deformation bands in austenitic microstructures, 5) systematic ruptures of strengthened NiTi wires in tensile tests via necking at the onset of plastic yielding, 6) localized plastic deformation in tensile tests via propagation of Lüders band fronts with very large localized strain (~40%), 7) unusually long upper stress plateaus in superelastic tensile tests (>8% strain), 8) large plastic strains (> 20 %) generated in a single closed-loop cooling/heating cycle under constant stress, 9) shape setting of already annealed NiTi by heating under external constraint. Finally, we discuss how kwinking deformation was considered in constitutive modelling of thermomechanical behaviors of NiTi and, particularly, what is the role of the kwinking deformation in NiTi technology.

cond-mat.mtrl-sci↗