Martensitic phase transformations and twinning in epitaxially grown Nickel Titanium films

在本研究计划中，我们研究外延生长的镍钛形状记忆合金薄膜。这些薄膜（厚度范围从约100 nm到几微米）被用作马氏体相变的高精度基础研究的模型系统，例如在几个长度尺度上分析NiTi中的自相似马氏体微观结构。最重要的是，我们将考虑的关键问题，无论是在NiTi马氏体微观结构的形态是由热力学平衡过程或最容易获得的转变路径。计划的实验工作考虑在不同的MgO和Al 2 O3衬底上外延生长NiTi膜。此外，我们使用X射线衍射，原子力显微镜和电子显微镜的方法来分析孪晶组织和相应的织构的NiTi马氏体。在原位研究中，马氏体相变是通过冷却过程中的原子力显微镜测量记录的，特别关注成核机制。应力诱导的转变是触发使用纳米压痕和小规模的拉伸测试的独立的薄膜;激活的孪生系统进行了分析，通过微观力学和连续力学计算，考虑弹性各向异性的高，低温相。我们的调查定义良好的模型系统的实验和理论结果将允许在几个相关的长度尺度上描述在NiTi马氏体微观结构的形成。将考虑不同的孪晶边界能对形成分级结构马氏体的影响，并与多步成核模型进行比较，其中相变的动力学由具有最低能量势垒的路径确定。

In this research project we study epitaxially grown NiTi shape memory alloy films. These thin films (with thicknesses ranging from about 100 nm to several microns) are used as a model system for highly precise, fundamental investigations on martensitic transformations, for instance to analyze on several length scales the self-similar martensitic microstructures in NiTi. Most importantly, we will consider the key question whether the morphology of martensitic microstructures in NiTi is determined by thermodynamic equilibrium processes or by the most easily accessible transformation paths. The planned experimental work considers epitaxial growth of NiTi films on different MgO and Al2O3 substrates. Moreover, we use X-ray diffraction, atomic force microscopy and electron microscopy methods to analyze the twinned microstructures and the corresponding textures of NiTi martensites. In in-situ investigations, the martensitic phase transition is documented by atomic force microscopy measurements during cooling, with a particular focus on nucleation mechanisms. The stress-induced transformation is triggered using nanoindentation and small-scale tensile testing of free-standing thin films; the activated twinning systems are analyzed by means of micromechanical and continuum mechanical calculations considering elastic anisotropy of both high and low temperature phases. The experimental and theoretical results of our investigations on the well-defined model system will allow to describe formation of martensitic microstructures in NiTi across several relevant length-scales. The effect of different twin boundary energies on the formation of hierarchically structured martensites will be considered, and a comparison with the multi-step nucleation model, where the kinetics of the phase transition is determined by the pathway with the lowest energy barrier, will be performed.