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

在金属材料中，伪弹性镍钛合金表现出由于应力诱导相变而实现高可逆变形的独特适用性。在材料应用过程中的一个通用特征，例如微创植入物，因此至少有6%至8%的局部假弹性应变的单一事件。因此，裂纹在靠近材料表面的地方产生，其原因至今仍不确定，但对Ni的释放、耐腐蚀性以及细菌和细胞的粘附性都有关键影响。此外，结构疲劳性能的恶化是预期的。由于氧化层和类似脆性的金属间相不能跟随伪弹性变形，它们在材料表面附近的出现被认为是裂纹萌生的关键因素。在第一阶段，所提出的项目旨在澄清迄今为止尚未充分探索的在氧气存在下NiTi表面结构和相的形成，特别是关于氧化层的形成，氧化层和NiTi-基体界面处Ni3Ti的相形成和微观结构，以及氧化层以下金属间NiTi内富ni层的尺寸和微观结构。根据Ni含量的不同，该区域的伪弹性相变可能被完全抑制。此外，还将研究最近观察到的靠近氧化层和金属间材料界面的孔隙的形成机制。在这些结果的基础上，在拟议项目的第二阶段，将首次定量地访问局部应变、氧化层厚度、主要金属间相和孔隙对裂纹起裂的影响，并用于确定NiTi表面由于单一伪弹性变形事件而起裂的潜在机制。提出的项目的最后目标是制定一种策略，以尽量减少由于伪弹性变形而在近地表层中引发的裂缝的后果。该策略的一个关键方面将是初始裂纹沿界面和孔隙向平行于材料表面的路径的偏转和重定向。根据裂纹的具体路径，它们对Ni的释放、耐腐蚀性和结构疲劳的影响将变得可预测。

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.