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

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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 event of local pseudoelastic strain of 6% to 8%. As a result, cracks are initiated close to the materials surface for reasons that remain uncertain until present, but that have critical impact on the release of Ni, the corrosion resistance and presumably also on the adhesion of bacteria and cells. Additionally, the deterioration of the structural fatigue performance is expected. Since oxide layers and similarly brittle intermetallic phases cannot follow the pseudoelastic deformation, their occurrence close to the materials surface is considered a key component for the initiation of cracks. In the first stage, the proposed project aims for clarifying the so far insufficiently explored formation of structures and phases at the surface of NiTi in the presence of oxygen, particularly with regard to the formation of the oxide layer, the phase formation and microstructure of Ni3Ti at the interface of oxide layer and NiTi-matrix, and the dimension and microstructure of the Ni-enriched layer within the intermetallic NiTi below the oxide layer. Depending on the amount of Ni, the pseudoelastic phase transformation may be suppressed entirely in this region. Additionally, the formation mechanism of only recently observed pores close to the interface of oxide layer and intermetallic material will be investigated. Building upon those results, during the second stage of the proposed project the impact of local strain, thickness of the oxide layer and prevailing intermetallic phases and pores on crack initiation will be accessed quantitatively for the first time and used for identifying the underlying mechanism of crack initiation at the surface of NiTi due to a single event of pseudoelastic deformation. The concluding objective of the proposed project is to develop a strategy for minimizing the consequences of cracks initiated in the near-surface layers due to pseudoelastic deformation. A key aspect of the strategy will be the deflection and redirection of initiated cracks towards a path parallel to the materials surface along interfaces and pores. Depending on the specific path of the cracks, their consequences on release of Ni, corrosion resistance and structural fatigue shall become predictable.