207
As with the nanocrystalline systems, the strength scales as d
−1/2
,
where d is now the bilayer period. The explanation is the same:
For deformation to take place, dislocations must sweep through the
layers, penetrating the boundaries as they do so. When d is large,
pileups form (as in Figure 7.6), magnifying the applied stress; the
smaller d becomes, the smaller the number of dislocations that can
be squeezed into a pileup and the smaller the magnification of the
applied stress.
amorphous materials
Suppose now that the crystal size shrinks further until it becomes of
atomic dimensions. The material is now completely disordered, as in
Figure 7.1b. Many amorphous materials are familiar. Ordinary glass
is amorphous, and for that reason materials that are amorphous are
commonly referred to as glasses, even when they are metallic and
have nothing but their disordered structure in common with ordinary glass. Many polymers are glasses, among them polycarbonate,
acrylic (Plexiglas) and polystyrene. The Burger’s vector of a dislocation—the “quantum” of deformation—is of atomic dimensions,
so dislocations interact strongly with the disordered parts of the
structures of Figures 7.1 and 7.2, giving amorphous materials high
hardness and strength. Polycarbonate and Plexiglas might not seem
that hard, but as Figure 4.9 of Chapter 4 shows, on a scale of how
hard they could be, they rank high.
Amorphous materials, as we’ve already said, are an extreme class of
nanostructured matter. Three pairs of figures bring out the exceptional mechanical properties of these and other nanostructured
materials. The first, Figure 7.9a, is a chart of modulus and density
for the materials of engineering. The colored envelopes enclose
material classes; individual bubbles within them describe materials. In Figure 7.9b, with the same axes, the class envelopes of
Figure 7.9a appear as shadows. Superimposed in bolder symbols
are the properties of polymer, metal and ceramic nanocomposites,
nanocrystalline metals, and nanofibers and nanotubes, identified
by their own envelopes. The comparison makes clear that nanostructuring has the capacity to create materials with substantially
enhanced stiffness. Several applications of amorphous materials
are discussed in Section 9.2.
The pairs of Figures 7.10 and 7.11 show a similar comparison, this
time based on the charts for yield strength and tensile strength and
density. The strongest engineering materials (Figures 7.10a and
7.11a) reach levels of about 2000 MPa. Bulk nanostructured and
amorphous materials (Figures 7.10b and 7.11b) push the strength
Mechanical Properties
As with the nanocrystalline systems, the strength scales as d
−1/2
,
where d is now the bilayer period. The explanation is the same:
For deformation to take place, dislocations must sweep through the
layers, penetrating the boundaries as they do so. When d is large,
pileups form (as in Figure 7.6), magnifying the applied stress; the
smaller d becomes, the smaller the number of dislocations that can
be squeezed into a pileup and the smaller the magnification of the
applied stress.
amorphous materials
Suppose now that the crystal size shrinks further until it becomes of
atomic dimensions. The material is now completely disordered, as in
Figure 7.1b. Many amorphous materials are familiar. Ordinary glass
is amorphous, and for that reason materials that are amorphous are
commonly referred to as glasses, even when they are metallic and
have nothing but their disordered structure in common with ordinary glass. Many polymers are glasses, among them polycarbonate,
acrylic (Plexiglas) and polystyrene. The Burger’s vector of a dislocation—the “quantum” of deformation—is of atomic dimensions,
so dislocations interact strongly with the disordered parts of the
structures of Figures 7.1 and 7.2, giving amorphous materials high
hardness and strength. Polycarbonate and Plexiglas might not seem
that hard, but as Figure 4.9 of Chapter 4 shows, on a scale of how
hard they could be, they rank high.
Amorphous materials, as we’ve already said, are an extreme class of
nanostructured matter. Three pairs of figures bring out the exceptional mechanical properties of these and other nanostructured
materials. The first, Figure 7.9a, is a chart of modulus and density
for the materials of engineering. The colored envelopes enclose
material classes; individual bubbles within them describe materials. In Figure 7.9b, with the same axes, the class envelopes of
Figure 7.9a appear as shadows. Superimposed in bolder symbols
are the properties of polymer, metal and ceramic nanocomposites,
nanocrystalline metals, and nanofibers and nanotubes, identified
by their own envelopes. The comparison makes clear that nanostructuring has the capacity to create materials with substantially
enhanced stiffness. Several applications of amorphous materials
are discussed in Section 9.2.
The pairs of Figures 7.10 and 7.11 show a similar comparison, this
time based on the charts for yield strength and tensile strength and
density. The strongest engineering materials (Figures 7.10a and
7.11a) reach levels of about 2000 MPa. Bulk nanostructured and
amorphous materials (Figures 7.10b and 7.11b) push the strength
Mechanical Properties