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

本研究的目的是为了更好地了解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。我们打算制备不同微观结构的富镍镍钛合金，并研究这些合金的功能和力学性能。德国合作伙伴将生产这些合金，并用差示扫描量热法(DSC)和定量金相(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。德国和中国的合作伙伴将使用微型机械试件来表征材料的机械和功能特性，包括单轴拉伸和压缩试件、双剪试件、扭转试件、扭转/压缩试件和圆形缺口试件。将特别强调力学测试过程中应变和损伤累积的微观结构方面，以及单轴和多轴应力状态下应力诱发马氏体相变的微观结构细节。中国合作伙伴将使用有限元方法计算与多轴试件机械加载相关的应力和应变。首先，中国合作伙伴将实施最先进的本构方程，允许将形状记忆行为合理化到他的有限元程序(ABAQUS)中。稍后，中方合作伙伴将致力于改进本构定律，并将在现有实验结果的基础上进行验证，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.