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

由于单周期伪弹性变形，近表面层对 NiTi 裂纹萌生的影响 - 氧化层和金属间化合物 Ni3Ti 的意义

• 批准号: 387559234
• 负责人: Professor Dr. Andreas Undisz
• 金额: --
• 依托单位: Institut für Werkstoffwissenschaft und Werkstofftechnik
• 依托单位国家: 德国
• 项目类别: Research Grants
• 财政年份: 2017
• 资助国家: 德国
• 起止时间: 2016-12-31 至 2021-12-31
• 项目状态: 已结题
• 来源: https://gepris.dfg.de/gepris/projekt/387559234?language=en
• 关键词: Effect; near; surface; layers; crack

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.

在金属材料中，伪弹性NiTi合金由于应力诱导相变而表现出用于实现高可逆变形的独特适用性。因此，在材料的应用期间，例如用于微创植入物的一般特征是至少6%至8%的局部伪弹性应变的单个事件。因此，裂纹开始接近材料表面的原因仍然不确定，直到现在，但对镍的释放，耐腐蚀性和可能也对细菌和细胞的粘附有关键的影响。此外，预计结构疲劳性能的恶化。由于氧化物层和类似的脆性金属间相不能遵循伪弹性变形，它们靠近材料表面的出现被认为是裂纹萌生的关键组成部分。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存在下在NiTi表面形成结构和相，特别是关于氧化物层的形成，在氧化物层和NiTi基体的界面处的Ni3Ti的相形成和微观结构，以及氧化物层下方的金属间NiTi内的富Ni层的尺寸和显微结构。取决于Ni的量，在该区域中可以完全抑制伪弹性相变。此外，最近观察到的孔隙靠近氧化物层和金属间化合物材料的界面的形成机制进行了研究。在这些结果的基础上，在拟议项目的第二阶段，局部应变，氧化层的厚度和占主导地位的金属间相和孔隙对裂纹萌生的影响将被首次定量访问，并用于识别由于伪弹性变形的单一事件在NiTi表面裂纹萌生的潜在机制。拟议项目的最后目标是制定一项战略，尽量减少由于伪弹性变形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偏转和重定向的初始裂纹的路径平行于材料表面沿着界面和孔隙。根据裂纹的具体路径，其对镍释放、耐腐蚀性和结构疲劳的影响应是可预测的。