Experimental and theoretical investigations of the structure and the properties of Ti-Ta-X-Y and Co-Ni-Ga-X high-temperature shape Memory alloys: Syncrotron and Neutron diffraction and atomistic modelling

在子项目3中，结合对Ti-Ta和Co-Ni-Ga合金形状记忆效应的结晶学机制的研究，在原子水平上研究了Ti-Ta基高温形状记忆合金的结构和稳定性。所获得的见解被用来识别i)Ti-Ta基合金和在高温下提供稳定形状记忆行为的适当的形变处理，以及ii)在高温下具有超弹性应变循环稳定性的Co-Ni-Ga合金。为此，我们将密度泛函理论计算与高分辨衍射方法相结合。第二个资助期的基本方面是将理论方法扩展到三元系Ti-Ta-X(X=Al，Sn，Zr)，对有限温度和无序体系的一致处理，以及对扩散过程的研究。对无序系统和扩散的设想方法分别是特殊的准随机结构方法和微动弹性带方法。声子对自由能的贡献将通过从头算准谐和近似或分子动力学模拟来计算--取决于系统。衍射实验确定了中子和同步辐射的结构参数和相形成过程。在第二个资助期，根据TP1的合金发展情况，实验扩大到包括三元和四元合金Ti-Ta-(X-Y)，其中X，Y=Al，Sn，Zr。采用模拟和实验相结合的方法，研究了合金元素对稳定相和亚稳相的结构和热力学性质以及相形成(或抑制)动力学的影响。在Ti-Ta-(X-Y)体系中，这与(I)新发现的纳米β相及其与体系中形成的其他β相的关系，(Ii)omega相及其合金元素X，Y对其结构、稳定性和形成动力学的影响，以及(Iii)在富钛体系(Ti-20Ta-5Al)和TP2的蠕变实验中观察到的与多个不同Ta含量的β相共存的α-Ti的形成有关。此外，还研究了Ti-Ta和Co-Ni-Ga-系中马氏体孪晶的结晶学特征和相变行为。通过衍射法研究了改善形状记忆性能的有利显微组织对马氏体变型形成和残余应变的影响。这里，Co-Ni-Ga双晶的竹节结构(由TP5产生)和Ti-Ta-X-Y合金(由TP1/TP4产生)和Co-Ni-Ga系(由TP5产生)的形变处理是特别感兴趣的。TP2对相和微结构进行了补充的透射电子显微镜研究，并将得到理论见解和衍射实验的支持。

In Subproject 3 the structure and stability of Ti-Ta based HT-SMA is investigated on an atomistic level and combined with the study of crystallographic mechanisms of the shape memory effect in Ti-Ta- and Co-Ni-Ga alloys. The obtained insights are used to identify i) Ti-Ta-based alloys and appropriate thermomechanical treatments providing stable shape memory behaviour at high Ms temperatures and ii) Co-Ni-Ga alloys with cyclic stability of superelastic strain at high temperatures. For this we combine density-functional theory calculations with high-resolution diffraction methods. Essential aspects of the 2nd funding period are the extension of the theoretical methods to ternary systems Ti-Ta-X (X=Al, Sn, Zr), a consistent treatment of finite temperatures and disordered systems, and the investigation of diffusion processes. The envisaged approaches to disordered systems and diffusion are the Special Quasirandom Structures method and the Nudged Elastic Band method, respectively. Phonon contributions to the free energy will be calculated - depending on the system - by ab initio quasiharmonic approximation or molecular dynamics simulation. The diffraction experiments determine structural parameters and phase formation processes with neutron- and synchrotron-radiation. In the 2nd funding period the experiments are expanded to cover ternary and quaternary alloys Ti-Ta-(X-Y), with X,Y = Al, Sn, Zr, according to alloy development in TP1. Combining simulations and experiment the function of alloying elements in modifying the structure and thermodynamic properties of stable or metastable phases and the kinetics of phase formation (or inhibition, respectively) is addressed. In the Ti-Ta-(X-Y)-system this relates to (i) the newly discovered nano-beta-phase and its relationship to the other beta-phases forming in the system, (ii) the omega-phase and the influence of alloying elements X,Y on its structure, stability and kinetics of its formation, and (iii) the formation of alpha-Ti coexisting with several beta-phases of different Ta-contents, which are observed in Ti-rich systems (Ti-20Ta-5Al) and in the creep experiments of TP2. Furthermore, investigations of the crystallographic characteristics of the martensitic twinning and transformation behaviour in the Ti-Ta and Co-Ni-Ga-system are performed. By diffraction we investigate the effect of advantageous microstructures for improved shape memory behaviour on the martensite variant formation and residual strain. Here, the Bamboo-structure of Co-Ni-Ga bicrystals (produced in TP5) and thermomechanical treatments of the Ti-Ta-X-Y alloys (from TP1/TP4) and Co-Ni-Ga systems (from TP5) are of special interest. TP2 carries out complementary TEM-investigations on phases and microstructures and will be supported by the theoretical insights and diffraction experiments.