Читать книгу Introduction to Nanoscience and Nanotechnology - Chris Binns - Страница 20

Mechanical properties such as the strength of metals can also be greatly improved by making them with nanoscale grains. Several basic attributes of materials are involved in defining their mechanical properties. One is strength, which includes characteristics with more precise definitions but basically determines how much a material deforms in response to a force. Others are hardness, which is given by the amount another body such as a ball bearing or diamond is able to penetrate a material, and wear resistance, which is determined by the rate at which a material erodes when in contact with another. These properties are dominated by the grain structure found in metals produced by normal processing. An example of the grains structure of a “normal” piece of metal is shown in Figure 1.13a, which is an electron microscope image showing the grain structure of tin. Each grain is a single‐crystal with a typical size of about 20 μm (20 000 nm). The mechanical properties listed above are due to grains slipping past each other or deforming, so clearly what happens at the grain boundaries is very important in determining properties such as strength, etc. It is possible by various techniques including nanoparticle deposition (Figure 1.11), electro‐deposition, and special low‐temperature milling methods to produce metal samples in which the grains are a few nanometers across. An example is shown in Figure 1.13b, where, on the same scale as Figure 1.13a, the grain structure disappears to show a homogenous material. Blowing up the magnification a further 15 000×, however, (Figure 1.13c) reveals the new nanoscale grain structure. In this image, the individual planes of atoms are indicated by the sets of parallel lines and the boundaries are where the lines suddenly change direction as highlighted for one of the grains. Whereas in the coarse‐grained metal shown in Figure 1.13a, about one atom in 100 000 is at a grain boundary, in the nano‐grained equivalent, about a quarter of the atoms are at a grain boundary. Clearly, this change is going to have a marked effect on the mechanical properties of the material. Changes in mechanical properties with grain size were quantified over 50 years ago by Hall and Petch [12, 13], but the modern ability to control the grain size right down to the nanometer scale can produce significant increases in performance.

Figure 1.13 Grain size in nanostructured materials. Electron microscope images showing a comparison of the grain structure in conventional and nanostructured materials. (a) Conventionally processed material (tin) showing a typical grain size of about 20 μm. (b) Nanovate™ nanostructured Ni‐based coating produced by Integran Technologies Inc. On the same scale as (a) the material appears homogenous. (c) Increasing the magnification by a factor of 15 000 reveals the nano‐sized grains. The lines in the picture are atomic planes and the edges of the grains are revealed by changes in the direction of the planes as indicated for one of the grains.

Source: Reproduced with permission from Integran Technologies Inc. (http://www.integran.com).

Figure 1.14 Yield strength of aluminum alloys. Comparison of Deformation (Strain) vs. Load (Stress) for aluminum alloys with different grain sizes. A is a normal aluminum alloy (coarse‐grained). B – D Nanostructured aluminum alloy containing grains of size ~30 nm produced by various processes. The plastic limit occurs at the point where the slope changes and the nanostructured materials have a value that is up to four times higher than the conventional alloy.

Source: Reproduced with the permission of Elsevier Science from K. M. Youssef et al. [14].

The most dramatic improvement is seen in the “yield strength,” which quantifies the load a material can tolerate before it becomes permanently deformed. All metals are elastic under a small load, that is, when the load is removed they return to their original shape, while beyond a certain load they deform plastically and remain permanently changed. Figure 1.14 shows a plot of the strain (relative elongation of a sample) vs. stress (load) for various nanostructured aluminum alloys compared to normal (coarse‐grained) aluminum alloy [14]. The plastic limit or yield strength occurs at the point where the slope changes, and it is seen that nanostructured materials have a value that is up to four times higher than the conventional material. This is a dramatic increase in strength but even higher values have been found in other metals, for example, a 10‐fold increase in copper [15]. A problem with nanostructured materials is also revealed by the plot, however, and that is that they fail (break) at relatively low strains. Problems such as this are being addressed by improvements in processing [15].