Spatial analysis of plasticity patterns in micron-sized samples

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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.