Spatial analysis of plasticity patterns in micron-sized samples

如果将载荷施加到实心材质，则该材质首先会弹性变形(如果移除载荷，则会恢复到其原始形状)。在临界载荷以上，材料永久地改变其形状，即经历塑性变形。例如，考虑一个由某种材料制成的圆柱体，我们通过自上而下的推动来压缩它。如果我们施加一个载荷，使得圆柱体中的应力(单位面积的力)在任何地方都是相同的，它将通过变得更短和更厚而均匀地变形，并在我们移除载荷后保持不变。如果圆柱体很大，我们将观察到变形随着应力的增加而逐渐增加，获得给定的相对变形所需的应力将不取决于圆柱体的大小(力是这样，但应力不是！)。但当变形体变得非常小时，我们观察到的可能会完全不同。美国的研究人员几年前开始在非常小的圆柱体上做这样的实验，比如直径1/100毫米，他们用一些金属的单晶块加工而成。他们观察到的是：(1)即使应力缓慢而稳定地增加，变形也不是逐渐增加的，而是以大的跳跃形式增加的。这些跳跃是随机发生的，并且在不同的试件中以不同的应力发生--即使这些试件是从相同的块中加工出来的。(2)将不同的试件变形到给定应变所需的应力分布很大，通常随着试件变小而增加。(3)尽管各处的应力都相同，但试件的变形是非常不均匀的。因此，圆柱体呈现出一种手风琴状的形状。前两个方面已经进行了一些详细的研究，但对于这样小的试件中变形的空间分布还知之甚少。因此，我们与开创了微变形方法的美国研究人员合作。我们想要研究微米级样品中变形的空间分布。我们计划制作一些金属和非金属晶体(盐)的微观样品，对它们进行不同程度的变形，然后通过高分辨率成像来研究变形的分布。为此，我们将使用原子力显微镜和一种称为扫描白光干涉仪的光学设备。通过分析图像，我们将详细了解变形是如何发生的，并可能获得关于是什么导致变形爆发的线索，以及在如此小的样品中变形的一般不可预测性。为什么它很重要？想象一下，你想要弯曲一块金属片，比如把它做成一个圆柱体来生产罐头。如果你施加相等的力，你会得到一个漂亮的圆柱形。然而，如果您尝试在非常小的范围内执行相同的操作，结果可能会完全不同！因此，变形的随机性和局部化可能会影响我们将材料形成非常小的形状和生产用于微技术的非常小的部件的能力。由于这种技术在未来几十年将变得越来越重要，我们应该获得在微尺度上处理成形过程所需的知识和专业知识。我们的研究希望为获得这一目的所需的一些基本信息做出贡献。

If a load is applied to a solid material, the material deforms first elastically (it reverts to its original shape if the load is removed). Above a critical load, the material changes its shape permanently, i.e., it undergoes plastic deformation. Consider, for example, a cylinder made of some material which we compress by pushing from top to bottom. If we apply a load such that the stress (the force per unit area) is everywhere the same in the cylinder, it will deform homogenously by getting shorter and thicker, and remain so after we have removed the load. If the cylinder is large, we will observe that the deformation increases gradually as we increase the stress, and the stress needed to obtain a given relative deformation will not depend on the size of the cylinder (the force does, but the stress does not!). But when the deforming body becomes very small, what we observe might be quite different. Reserachers in the US have a few years ago started do do such experiments on very small cylinders, say, 1/100 of a millimetre in diameter, which they machined out of single cystal blocks of some metals. What they observed was: (1) Even if the stress is increased slowly and steadily, the deformation does not increase gradually but in large jumps. These jumps occur randomly, and take place at different stresses in different specimens - even if those specimens are machined out of the same block. (2) The stress required to deform different specimens to a given strain scatters hugely, and in general increases as the specimens become smaller. (3) Even though the stress is everywhere the same, the specimens deform in a very inhomogenous manner. As a consequewnce, the cylinders assume a kind of accordeon-like shape.The first two of these aspects have been studied in some detail, but not much is as yet known about the spatial distribution of deformation in such small specimens. Therefore, we have teamed up with the US researchers who pioneered the microdeformation method. We want to investigate the spatial distribution of deformation in micron-scale specimens. We plan to fabricate micro-specimens of some metal and, for comparison, a nonmetallic crystal (a salt), to deform them to different degrees, and then to investigate the distribution of deformation by imaging the specimen with great resolution. To this end we will use an atomic force microscope and an optical device called a scanning white-light interferometer. By analysing the images we will see how in detail the deformation has occurred, and possibly gain hints as to what causes the deformation bursts and the general unpredicatbility of deformation in such small specimens.Why is it important? Imagine you want to bend a sheet of metal, say for making it into a cylinder for producing a can. If you apply an equal force, you will get a nice cylindrical shape. However, if you try to do the same on a very small scale, the result might look quite different! So, randomness and localization of deformation may affect our ability to form materials into very small shapes and to produce very small parts for microtechnologies. As such technologies will become more and more important in the next decades, we should gain the knowledge and expertise needed to handle forming processes on the microscale. Our research wants to make a contribution to obtain some of the basic information needed for this purpose.