Surface-controlled mechanical properties of nanoporous metals

表面在塑性和弹性变形中起着重要作用，特别是当比表面积较大时，特别是在晶体尺寸较小的情况下。对尺寸依赖强度的观察很好地证明了这一点，例如在最近关于纳米线变形的工作中。这项拟议的研究旨在帮助确定表面在机械行为中所起作用的过程。我们建议开发一种新的非常规方法：我们将引发表面状态的周期性变化，而不是改变样品的大小或比表面积。这将在力学测试期间在现场完成，并将记录对力学行为的影响。我们的主要关注点是本构塑性行为，如屈服和流动应力、加工硬化和应变率敏感性的参数化。将作为密切相关的补充问题寻求有关弹性反应的信息。作为具有大比表面积的合适的模型材料，我们将研究纳米多孔贵金属及其合金，使用最新的合成路线来生成毫米尺寸的纳米多孔金属样品，这些样品在压缩时表现出大的(&gt；50%应变)延性变形。表面修饰将使用i)电化学实验和ii)可逆气体吸附研究来选择性地循环表面化学(例如，表面氧化/还原或氢吸附/脱附循环)或表面电荷密度。通过这些变量，我们的方法提供了对两个相关的毛细作用力，表面张力和表面应力，以及表面扩散系数的控制。我们的目标是建立第一个关于塑性流动和超额弹性行为对表面态的响应的实验数据库，以及ii)精确定位表面影响力学的微观现象，例如位错成核或逸出、位错终点阻力、台阶边缘能量学、表面扩散或表面(超额)弹性。

Surfaces play a significant role in plastic and elastic deformation, especially at small crystal size when the specific surface area is large. This is well documented by observations on the size-dependent strength, for instance in recent work on nanowire deformation. The proposed research aims to contribute towards identifying the processes that underlie the role of the surface in mechanical the behaviour. We propose to exploit a new and unconventional approach: rather than changing the size or the specific surface area of the sample, we shall instigate cyclic changes of state of the surface. This will be done in-situ during mechanical tests, and the consequences for the mechanical behavior will be recorded. Our main focus is on the constitutive plastic behaviour, as parametrized by yield- and flow stress, work hardening and strain rate sensitivity. Information on the elastic response will be sought as a closely related supplementary issue. As suitable model materials with large specific surface area we will study nanoporous noble metals and their alloys, using recent synthesis routes that yield millimetersized nanoporous metal samples which exhibit large (>50% strain) ductile deformation in compression. The surface modifications will use i) electrochemical experiments and ii) reversible gas adsorption studies to cycle selectively the surface chemistry (for instance, superficial oxidation/reduction or hydrogen adsorption/desorption cycles) or the superficial electric charge density. Through these variables, our approach affords control over the two relevant capillary forces, the surface tension and the surface stress, and over the surface diffusivity. We aim at i) establishing a first experimental data base for the response of plastic flow and of excess elastic behavior to the surface state, and ii) pinpointing the microscopic phenomena via which the surface affects the mechanics, for instance dislocation nucleation or egression, dislocation endpoint drag, step edge energetics, surface diffusion, or surface (excess-) elasticity.