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Figure 6.2. Distribution of sizes of Fe-Cu nanoparticles made by hot compaction methods described in the text. [Adapted from L. He and E. Ma, J. Mater. Res. 15, 904 (2000).]

number of particles having 40 nm sizes. Figure 6.3 shows a stress-strain curve for this case. Its Young's modulus, which is the slope of the curve in the linear region, is similar to that of conventional iron. The deviation from linearity in the stress-strain curve shows there is a ductile region before fracture where the material displays elongation. The data show that fracture occurs at 2.8 GPa which is about 5 times the fracture stress of iron having larger grain sizes, ranging from 50 to 150 pm. Significant modifications of the mechanical properties of disordered bulk

Figure 6.2. Distribution of sizes of Fe-Cu nanoparticles made by hot compaction methods described in the text. [Adapted from L. He and E. Ma, J. Mater. Res. 15, 904 (2000).]

Figure 6.3. Stress-strain curve for bulk compacted nanostructured Fe-Cu material, showing fracture at a stress of 2.8 GPa. [Adapted from L. He and E. Ma, J. Mater. Res. 15, 904 (2000).]

materials having nanosized grains is one of the most important properties of such materials. Making materials with nanosized grains has the potential to provide significant increases in yield stress, and has many useful applications such as stronger materials for automobile bodies. The reasons for the changes in mechanical properties of nanostructured materials will be discussed below.

Nanostructured materials can be made by rapid solidification. One method illustrated in Fig. 6.4 is called "chill block melt spinning." RF (radiofrequency) heating coils are used to melt a metal, which is then forced through a nozzle to form a liquid stream. This stream is continuously sprayed over the surface of a rotating metal drum under an inert-gas atmosphere. The process produces strips or ribbons ranging in thickness from 10 to 100 nm. The parameters that control the nano-structure of the material are nozzle size, nozzle-to-drum distance, melt ejection pressure, and speed of rotation of the metal drum. The need for light weight, high strength materials has led to the development of85-94% aluminum alloys with other metals such as Y, Ni, and Fe made by this method. A melt spun alloy of Al-Y-Ni-Fe consisting of 10-30-nm A1 particles embedded in an amorphous matrix can have a tensile strength in excess of 1.2 GPa. The high value is attributed to die presence of defect free aluminum nanoparticles. In another method of making nanostructured

Figure 6.4. Illustration of the chid block melting apparatus for producing nanostructured materials by rapid solidification on a rotating wheel. (With permission from I. Chang, in Handbook of Nanostructured Materials and Nanotecbnobgy, H. S. Nalwa, ed., Academic Press, San Diego, 2000, Vol. 1, Chapter 11, p. 501.)

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Figure 6.4. Illustration of the chid block melting apparatus for producing nanostructured materials by rapid solidification on a rotating wheel. (With permission from I. Chang, in Handbook of Nanostructured Materials and Nanotecbnobgy, H. S. Nalwa, ed., Academic Press, San Diego, 2000, Vol. 1, Chapter 11, p. 501.)

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Figure 6.5. Illustration of apparatus for making droplets of metal nanoparticles by gas atomiza-tion. (With permission from I. Chang, in Handbook of Nanostructured Materials and Nanotech-notogy, H. S. Nalwa, ed., Academic Press, San Diego, 2000, Vol. 1, Chapter 11, p. 501.)

materials, called gas atomization, a high-velocity inert-gas beam impacts a molten metal. The apparatus is illustrated in Fig 6.5. A fine dispersion of metal droplets is formed when the metal is impacted by the gas, which transfers kinetic energy to the molten metal. This method can be used to produce large quantities of nanostructured powders, which are then subjected to hot consolidation to form bulk samples.

Nanostructured materials can be made by electrodeposition. For example, a sheet of nanostructured Cu can be fabricated by putting two electrodes in an electrolyte of CuS04 and applying a voltage between the two electrodes. A layer of nanostructured Cu will be deposited on the negative titanium electrode. A sheet of Cu 2 mm thick can be made by this process, having an average grain size of 27 nm, and an enhanced yield strength of 119MPa.

6.1.2. Failure Mechanisms of Conventional Grain-Sized Materials

In order to understand how nanosized grains effect the bulk structure of materials, it is necessary to discuss how conventional grain-sized materials fail mechanically. A britde material fractures before it undergoes an irreversible elongation. Fracture occurs because of the existence of cracks in the material. Figure 6.6 shows an example of a crack in a two-dimensional lattice. A "crack" is essentially a region of a material where there is no bonding between adjacent atoms of the lattice. If such a material is subjected to tension, the crack interrupts the flow of stress. The stress accumulates at the bond at the end of the crack, making the stress at that bond very high, perhaps exceeding the bond strength. This results in a breaking of the bond at die end of the crack, and a lengthening of the crack. Then the stress builds up on the

next bond at the bottom of the crack and it breaks. This process of crack propagation continues until eventually the material separates at the crack. A crack provides a mechanism whereby a weak external force can break stronger bonds one by one. This explains why the stresses that induce fracture are actually weaker than the bonds that hold the atoms of the metal together. Another kind of mechanical failure, is the brittle-to-ductile transition, where the stress-strain curve deviates from linearity, as seen in Fig. 6.3. La this region the material irreversibly elongates before fracture. When the stress is removed after the brittle to ductile transition the material does not return to its original length. The transition to ductility is a result of another kind of defect in the lattice called a dislocation. Figure 6.7 illustrates an edge dislocation in a two-dimensional lattice. There are also other kinds of dislocations such as a screw dislocation. Dislocations are essentially regions where lattice deviations from a regular structure extend over a large number of lattice spacings. Unlike cracks, the atoms in the region of the dislocation are bonded to each other, but the bonds are weaker than in the normal regions. In the ductile region one part of the lattice is able to slide across an adjacent part of the lattice. This occurs between sections of the lattice located at dislocations where the bonds between the atoms

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