Huang Kai, Deng Zhongzheng, Yin Hao
School of Civil Engineering, Wuhan University, Wuhan 430072, China.
Department of Mechanical and Aerospace Engineering, The Hong Kong University of Science and Technology, Hong Kong, China.
Nanomaterials (Basel). 2025 Jan 16;15(2):125. doi: 10.3390/nano15020125.
Fracture toughness is a critical indicator for the application of NiTi alloys in medical fields. We propose to enhance the fracture toughness of NiTi alloys by controlling the spatial grain size (GS) gradient. Utilizing rolling processes and heat treatment technology, three categories of NiTi alloys with distinct spatial GS distributions were fabricated and subsequently examined through multi-field synchronous fracture tests. It is found that the one with a locally ultra-high GS gradient (20 nm-3.4 μm) has significantly enhanced fracture toughness, which is as high as 412% of that of the normally distributed nano-grains with an average GS of 8 nm and 178% of that of the coarse-grains with an average GS of 100 nm. Theoretical analysis reveals that in such a gradient structure, phase transition in the coarse-grained matrix greatly absorbs the surface energy of subcritical and stable propagation. Meanwhile, the locally non-uniform GS distribution leads to deviation and tortuosity of the crack path, increasing the critical fracture stress. Furthermore, the nanocrystalline clusters distributed in the form of network nodes reduce the stress intensity factor due to their higher elastic modulus compared to the coarse-grained matrix. This work provides guidance for developing new gradient nanostructured NiTi alloys with high fracture toughness.
断裂韧性是镍钛合金在医学领域应用的关键指标。我们建议通过控制空间晶粒尺寸（GS）梯度来提高镍钛合金的断裂韧性。利用轧制工艺和热处理技术，制备了三类具有不同空间GS分布的镍钛合金，并随后通过多场同步断裂试验进行了检测。结果发现，具有局部超高GS梯度（20纳米 - 3.4微米）的合金具有显著提高的断裂韧性，高达平均GS为8纳米的正态分布纳米晶粒的412%，以及平均GS为100纳米的粗晶粒的178%。理论分析表明，在这种梯度结构中，粗晶基体中的相变极大地吸收了亚临界和稳定扩展的表面能。同时，局部不均匀的GS分布导致裂纹路径的偏离和曲折，增加了临界断裂应力。此外，以网络节点形式分布的纳米晶团簇由于其比粗晶基体具有更高的弹性模量，降低了应力强度因子。这项工作为开发具有高断裂韧性的新型梯度纳米结构镍钛合金提供了指导。
