Spatial analysis of plasticity patterns in micron-sized samples

如果对固体材料施加载荷，该材料首先会发生弹性变形（如果载荷解除，它会恢复到原来的形状）。在临界载荷以上，材料会永久改变其形状，即发生塑性变形。例如，考虑一个由某种材料制成的圆柱体，我们通过从上到下的推动来压缩它。如果我们施加一个载荷，使应力（单位面积的力）在圆柱体中的各处都是相同的，它将通过变短和变厚而均匀变形，并在我们移除了载荷后保持不变。如果圆柱体很大，我们将观察到，随着我们增加应力，变形会逐渐增加，并且获得给定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！）。但当变形的物体变得非常小时，我们观察到的可能会大不相同。几年前，美国的研究人员开始在非常小的圆柱体上做这样的实验，比如直径为1/100毫米，他们用一些金属的单晶块加工而成。他们观察到：(1)即使应力缓慢而稳定地增加，变形也不是逐渐增加，而是大跳跃。这些跳跃是随机发生的，并且在不同的样品中发生不同的应力-即使这些样品是由同一块加工而成的。(2)使不同试样变形到给定应变所需的应力分布很大，并且通常随着试样变小而增大。(3)尽管应力处处相同，但试样的变形却极不均匀。因此，圆柱体呈现出一种手风琴般的形状。前两个方面已经进行了一些详细的研究，但对于如此小的试件中变形的空间分布还不太了解。因此，我们与开创微变形方法的美国研究人员合作。我们想研究微米尺度试样的变形空间分布。我们计划制作一些金属和非金属晶体（一种盐）的微样品，进行不同程度的变形，然后通过高分辨率成像样品来研究变形的分布。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装置。通过分析这些图像，我们将看到变形是如何发生的细节，并可能获得关于是什么导致了变形爆发和这种小样本中变形的一般不可预测性的提示。为什么它很重要？想象一下，你想把一块金属片弯曲，比如说把它做成一个圆柱体，用来制作罐头。如果你施加一个相等的力，你会得到一个漂亮的圆柱形。然而，如果你尝试在一个非常小的范围内做同样的事情，结果可能看起来完全不同！因此，变形的随机性和局域化可能会影响我们将材料形成非常小的形状以及为微技术生产非常小的部件的能力。由于这些技术在未来几十年将变得越来越重要，我们应该获得在微观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.