On the role of microstructure surface roughness and multiaxial loading on martensitic transformations in NiTi memory alloys

本文提出的研究目的是为了更好地理解NiTi形状记忆合金的显微组织特征与功能/力学性能之间的关系。制备不同显微组织的富镍NiTi合金，并对其功能性能和力学性能进行研究。德国合作伙伴将生产合金，并根据差示扫描量热法（DSC）和定量金相法（光学显微镜、扫描电子显微镜和透射电子显微镜）对材料进行表征。德国和中国合作伙伴都将使用微型力学试样来表征材料的力学和功能特性，包括单轴拉伸和压缩试样、双剪切试样、扭转试样、扭转/压缩试样和圆形缺口试样。特别强调的是在机械测试过程中应变和损伤积累的微观结构方面，以及在单轴和多轴应力状态下应力诱导马氏体转变的微观结构细节。中方合作伙伴将使用有限元法（FEM）计算与多轴试件机械载荷相关的应力和应变。首先，中国合作伙伴将实施最先进的本构方程，使形状记忆行为合理化到他的有限元代码（ABAQUS）中。随后，中方合作伙伴将致力于改进本构律，该本构律将在现有实验结果的基础上得到验证，并将反映重要的微观结构特征。

The objective of the research proposed here is to contribute to a better understanding of the relations between microstructural features and functional/mechanical properties of NiTi shape memory alloys. We intend to produce Ni-rich NiTi alloys with different microstructures and to study the functional and the mechanical properties of these alloys. The German partner will produce the alloys and characterize the materials in terms of differential scanning calometry (DSC) and quantitative metallography (optical microscopy, scanning electron microscopy and transmission electron microscopy). Both the German and the Chinese partner will characterise the mechanical and functional properties of the materials using miniature mechanical specimens including uniaxial tensile and compression specimens, double shear specimens, torsion specimens, torsion/compression specimens and circular notched specimens. Special emphasis will be placed on the microstructural aspects of strain and damage accumulation during mechanical testing and to the microstructural details of stress induced martensitic transformations under uniaxial and multiaxial stress states. The Chinese partner will calculate stresses and strains associated with mechanical loading of multiaxial specimens using the finite element method (FEM). To start off with the Chinese partner will implement state-of-the-art constitutive equations which allow to rationalize the shape memory behaviour into his FEM code (ABAQUS). Later, the Chinese partner will work on improved constitutive laws which will be validated on the basis of the available experimental results and which will reflect the important microstructural features.