Effect of near-surface layers on crack initiation in NiTi due to single-cycle pseudoelastic deformation - Significance of oxide layer and intermetallic Ni3Ti

• Applicant: Professor Dr. Andreas Undisz
• Subject Area: Mechanical Properties of Metallic Materials and their Microstructural Origins ; Synthesis and Properties of Functional Materials
• Term: from 2017 to 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 387559234

• Final Report Year: 2023

Final Report Abstract

Among metallic materials, pseudoelastic NiTi-alloys exhibit a unique suitability for realizing high reversible deformation due to a stress-induced phase transformation. A generic feature during the application of the material, e.g., for minimally invasive implants, is thus at least a single cycle of local pseudoelastic deformation of 6% to 8%. Within the scope of the project, the details of structure and phase formation at the surface of NiTi in the presence of oxygen were clarified. In contrast to previous expectations, the formation of Ni-rich intermetallic phases below the oxide layer already occurs for heat treatments of <2 min (~500 °C) and must consequently be taken into account in the research and application of the material. A mechanism for the unexpectedly rapid phase transformation was derived by local rearrangement of atoms, bypassing the nucleation phase necessary in classical phase transformations. Similarly, the phase transformation to the thermodynamically stable Ni3Ti occurs via the metastable phase Ni4Ti3, which otherwise forms in NiTi alloys as precipitates, a few nanometers in size. With regard to crack initiation, the effects of pseudoelastic strain in combination with the thickness of the oxide layer as well as existing intermetallic phases and pores were assessed. It should be particularly emphasized that cracks in the oxide layer do not necessarily propagate into the underlying Ni-rich layer, but that, especially when tensile and compressive stresses are superimposed, crack deflection can occur along pores in the oxide. In addition, the Ni-rich intermetallic layer appears to exhibit at least some ductility, which leads locally to deflection of the cracks. For the propagation of the cracks into the NiTi base material, it was recognized that blunting of the crack tip often occurs, which counteracts the risk of formation of critical cracks into the base material.

Publications

• Corrosion Resistance of Nitinol Wires After Deformation. Shape Memory and Superelasticity, 5(4), 346-351.
  Zende, Stefan; Freiberg, Katharina E.; Dorner, Franziska; Feth, Nils-Agne & Undisz, Andreas
  
• Bridging the gap between high temperature and low temperature oxidation of 316 L. Corrosion Science, 175, 108884.
  Wonneberger, Robert; Lippmann, Stephanie; Abendroth, Barbara; Carlsson, Anna; Seyring, Martin; Rettenmayr, Markus & Undisz, Andreas
  
• Chemical effects during the formation of various types of femtosecond laser-generated surface structures on titanium alloy. Applied Physics A, 126(4).
  Florian, C.; Wonneberger, R.; Undisz, A.; Kirner, S. V.; Wasmuth, K.; Spaltmann, D.; Krüger, J. & Bonse, J.
  
• Evaluation Methods for Non-contact Bend and Free Recovery Tests of Thin NiTi Wires and Their Effects on Measured Transformation Temperatures. Journal of Materials Engineering and Performance, 29(8), 5435-5441.
  Apell, Jonathan; Rettenmayr, Markus & Undisz, Andreas
  
• Characteristics of cracks formed at the surface of NiTi during a single cycle of pseudoelastic deformation. IOP Conference Series: Materials Science and Engineering, 1147(1), 012024.
  Freiberg, K. E.; Wonneberger, R. & Undisz, A.
  
• Oxide Layer Formation, Corrosion, and Biocompatibility of Nitinol Cardiovascular Devices. Shape Memory and Superelasticity, 8(2), 45-63.
  Nagaraja, Srinidhi; Brown, Ronald; Saylor, David & Undisz, Andreas
  
• Early oxidation stages of austenitic stainless steel monitored using Mn as tracer. Corrosion Science, 223, 111434.
  Wonneberger, Robert; Kirste, Gloria; Seyring, Martin; Hafermann, Martin; Ronning, Carsten; Schaal, Maximilian; Otto, Felix; Fritz, Torsten & Undisz, Andreas
  
• Phase formation in the Ni-enriched zone below the surface oxide on NiTi. Intermetallics, 154, 107817.
  Freiberg, K.E.; Seyring, M.; Wonneberger, R.; Rettenmayr, M. & Undisz, A.
  
• Surface amorphization of bulk NiTi induced by laser radiation. Surfaces and Interfaces, 38, 102827.
  Wonneberger, Robert; Wisniewski, Wolfgang; Lippmann, Stephanie; Müller, Frank A.; Gräf, Stephan & Undisz, Andreas