基于包套压缩冷塑性变形的多态结构镍钛基形状记忆合金裂纹萌生与扩展机理多尺度研究

NiTi-based shape memory alloys have been widely used in the engineering fields because of their shape memory effect and superelasticity. Fracture damage is one of the most frequent failure modes in the engineering application of NiTi-based shape memory alloys. It has been found in previous studies that the NiTi-based shape memory alloys possess poor plasticity at room temperature, while they can be subjected to greater extent of cold plastic deformation in the condition of canning compression, and thus polymorphic structural alloys containing coarse grains, twins, nanocrystallines and amorphous phase are able to be obtained. So far, the fracture mechanisms for this kind of polymorphic structural alloy during subsequent manufacturing and engineering application have been unclear. As a consequence, in the present study, plastic deformation process experiment, material fracture mechanics experiment, continuum mechanics finite element simulation, crystal plasticity finite element simulation, discrete dislocation dynamics simulation and molecular dynamics simulation are combined from the macroscale, mesoscale, microscale and nanoscale perspectives. The present investigation is aimed at revealing the basic laws of crack initiation and propagation in the polymorphic structural NiTi-based shape memory alloy obtained from cold plastic deformation based on canning compression, exploring the influence rules of coarse grains, twins, nanocrystallines, amorphous phase and various interface structures surrounding the crack tip on the crack propagation, and obtaining basic mechanisms of crack initiation and propagation in the alloy, which is able to lay scientific and theoretical foundations for preventing fracture damage of NiTi-based shape memory alloy in the engineering applications.

镍钛基形状记忆合金因其形状记忆效应和超弹性而在工程领域得到了广泛应用。断裂损伤是镍钛基形状记忆合金在工程应用中经常出现的一种失效模式。前期研究结果表明镍钛基形状记忆合金在室温条件下塑性较差，但在包套压缩条件下可以实现较大程度的冷塑性变形，获得含有粗晶、孪晶、纳米晶和非晶的多态结构合金。这种多态结构合金在后续加工和工程应用中的断裂损伤机制目前尚不清楚。因而本项目从宏观尺度、介观尺度、微观尺度和纳观尺度出发，将塑性变形工艺实验、材料断裂力学实验、连续介质力学有限元模拟、晶体塑性有限元模拟、离散位错动力学模拟和分子动力学模拟相结合，旨在揭示基于包套压缩冷塑性变形的多态结构镍钛基形状记忆合金裂纹萌生与扩展的基本规律，探索裂尖周围粗晶、孪晶、纳米晶、非晶及各种界面结构对裂纹扩展的影响规律，获得该合金中裂纹萌生与扩展的基本机理，为镍钛基形状记忆合金在工程应用中防止断裂损伤提供科学的理论基础。

镍钛基形状记忆合金经较大程度冷塑性变形可以发生局部剧烈剪切塑性变形，从而诱发局部纳米晶区和非晶区，导致其微观结构非常复杂，除了残留的粗晶和形成的纳米晶及非晶区外，还可能会产生变形孪晶和高密度位错。了解这种具有多态结构的镍钛基形状记忆合金材料在后续加工或使用过程中的裂纹萌生与扩展机制对其工程应用具有重要意义。本项目以二元镍钛形状记忆合金和三元镍钛铁与镍钛铜形状记忆合金为研究对象，以局部包套压缩大塑性变形工艺实验、热处理工艺实验、微观组织表征实验和力学性能实验为手段，结合宏观有限元模拟、晶体塑性有限元模拟和离散位错动力学模拟等多尺度模拟方法，揭示了镍钛基形状记忆合金包套压缩大塑性变形的机制和微观结构演变规律、后续热处理过程中的微观结构演变规律和相关力学行为，为镍钛基形状记忆合金的大塑性变形及后续热处理工艺提供了科学的理论基础。以二元马氏体镍钛形状记忆合金为研究对象，采用局部包套压缩和后续热处理实验制备出多态结构镍钛基形状记忆合金，采用实验和多尺度模拟方法研究了原始粗晶、多态结构和纳米晶镍钛形状记忆合金的力学行为及裂纹萌生与扩展机制。研究发现，冷塑性变形诱发的纳米晶区和非晶区均对镍钛基形状记忆合金裂纹萌生和扩展有影响，其中非晶的影响最为显著。当多态结构镍钛基形状记忆合金中存在非晶区时，更容易诱发裂纹的萌生，但裂纹很难穿过非晶区，即非晶区对裂纹扩展具有一定的抑制作用。上述成果为冷塑性变形镍钛基形状记忆合金在加工和应用中防止断裂损伤提供了科学的理论基础。