Strength and deformation of precious high entropy alloys

本研究旨在对单相、面心立方、多组分均质固溶体的变形和强化机制有更深入和全面的了解。这将在对Au-Cu-Ni-Pd-Pt高熵合金（HEAs）的结构-性能关系进行良好描述的基础上实现。这些合金已被证明是(i)单相的，（ii）可以得到均匀的固溶体，（iii）可能在整个浓度范围内都适用。Au-Cu-Ni-Pd-Pt体系是唯一已知的体系，它允许研究组成变化的影响，大概在整个范围内，而不需要考虑二次相结构变化的发生。单相微观结构是合理描述性能的决定性因素，因为它允许单独研究固溶体和晶界硬化等问题，而无需考虑任何二次相的影响。因此，精心设计并调整成分的合金系列将被表征，以可视化晶格失配参数对屈服强度的影响。该失配参数是目前广泛接受的多组分合金固溶强化模型中的关键参数。借助这样的合金系列，模型的另一个假设，即合金的剪切模量是根据维加德混合规则确定的，将得到验证（或证伪）。为了更好地描述合金的力学性能，还将研究晶粒尺寸和应变速率对合金力学性能的影响。机械孪晶应该在高应变速率下发生，它将显示孪晶是如何在高应变速率下轻松激活的。冷加工产生的高应变和剧烈的塑性变形以及机械孪晶的形成是细化组织的有效措施。这些加工方案将允许识别主动变形机制。结果将用于将HEAs的材料行为与单相传统合金的材料行为完全分离，并确定导致其特殊性能的HEAs的特殊问题。

The present proposal aims at developing a deeper and comprehensive understanding of the deformation and strengthening mechanisms of a single phase, face centred cubic, multi-component homogeneous solid solution. This will be achieved upon a sound description of the structure-property relationships in Au-Cu-Ni-Pd-Pt high entropy alloys (HEAs). These alloys have been proven to be (i) single phase, and (ii) a homogeneous solid solution can be obtained, which (iii) presumably holds for the entire concentration range. The Au-Cu-Ni-Pd-Pt system is the only known system that allows to investigate effects of compositional changes, presumably in the whole range, without the necessity to also consider the occurrence of structural changes of secondary phases. The single phase microstructure is the decisive factor for a sound description of the properties, as this allows to solely investigate issues such as solid solution and grain boundary hardening without the necessity to also consider any effect of secondary phases. Consequently, well designed alloy-series with deliberately adjusted compositions will be characterised to visualise the influence of the lattice misfit parameter on the yield strength. This misfit parameter represents the key parameter in the most widely accepted model of solid solution strengthening in multicomponent alloys. With the help of such alloy-series, another assumption of the model, that the shear modulus of an alloy is determined according to Vegard’s rule of mixture, will be verified (or falsified). I order to obtain a sound description of the mechanical properties of the alloys, an investigation of the influence of the grain size and the strain rate will also be done. Mechanical twinning is supposed to occur at high strain rates, and it will be shown how twinning is activated with ease at higher strain rates. High strains, as obtained from cold work and severe plastic deformation, as well as the formation of mechanical twins are efficient measures to refine the microstructure. These processing schemes will allow to identify the active deformation mechanism. The results will be used to separate the material behaviour of the HEAs cleanly from that of single phase conventional alloys and to identify which issues are special for HEAs leading to their peculiar properties.