The deformation rate during the experiments displayed in Figure 11.24 was
10
À3 s
À1 .
Over a quite narrow range of grain sizes and temperatures, both materials display
mechanical properties that allow plastic deformations of more than 100%. In both
cases, the specimens were produced by the severe plastic deformation of conventional materials. In spite of the essential differences between these two types of
material, their mechanical properties with respect to superplasticity have several
points in common. In both cases, superplasticity of the nanocrystalline material is
observed at lower temperatures as compared to their coarse-grained analogs. In
nanocrystalline metallic materials, superplasticity is often observed at the transition
from dislocation to grain boundary processes for plastic deformation. This is a very
general and very important point. When considering Figures 11.21 and 11.22, it is
apparent that the transition from dislocation to grain boundary processes proceeds,
with decreasing deformation rate, to larger grain sizes. Insofar, these findings are
equivalent, since in both examples shown in Figure 11.24 the temperatures of
superplastic deformation are relatively low and the nanocrystalline structure is
quite stable.
Superplasticity is found not only in metallic materials but also in ceramic
materials, especially in oxides. However, as the ductility of ceramic materials is
significantly lower than that of metals, such huge plastic deformations as are found
in metals would not be expected. In addition, ceramic parts are produced from
powders by a sequence of pressing and sintering such that, unavoidably, fully
densified bodies are not obtained, especially as the pressing behavior of nanoparticulate powders is extremely poor. These poor pressing properties are not
compensated by the excellent sintering behavior of nanoparticulate powders. The
individual pores of residual porosity act as failure points, where cracking starts. This
occurs because, around a flaw, stress concentrations are observed which depend on
the aspect ratio of that flaw. Hence, the strength of a material containing pores is less
than might be expected from a reduction of the bearing cross-section. In fact, the
maximal strength s flaw of a specimen containing flaws of the size c follows the
proportionality:
s flaw /
1
c 0:5
ð11:10Þ
Increasing the size of the flaws necessarily leads to premature cracking, with the
consequences of reduced strain and strength. The successful production of ceramic
specimens exhibiting superplasticity is a reference for excellent abilities in ceramic
technologies. Densities close to theoretical values may be obtained with specimens
produced from submicron powders and these materials have the potential for
superplastic deformation of a few hundred percent.
As a typical example of superplastic ceramics, Figure 11.25 shows a stress–strain
diagram of yttria-doped zirconia. In this example, 5% yttria was added to stabilize
the tetragonal phase of zirconia; otherwise, the material would transform into the
monoclinic phase. The investigations were performed as tensile tests and not (as is
usually done for ceramics) in compression. The density of the material ranged from
318j 11 Mechanical Properties of Nanoparticles
10
À3 s
À1 .
Over a quite narrow range of grain sizes and temperatures, both materials display
mechanical properties that allow plastic deformations of more than 100%. In both
cases, the specimens were produced by the severe plastic deformation of conventional materials. In spite of the essential differences between these two types of
material, their mechanical properties with respect to superplasticity have several
points in common. In both cases, superplasticity of the nanocrystalline material is
observed at lower temperatures as compared to their coarse-grained analogs. In
nanocrystalline metallic materials, superplasticity is often observed at the transition
from dislocation to grain boundary processes for plastic deformation. This is a very
general and very important point. When considering Figures 11.21 and 11.22, it is
apparent that the transition from dislocation to grain boundary processes proceeds,
with decreasing deformation rate, to larger grain sizes. Insofar, these findings are
equivalent, since in both examples shown in Figure 11.24 the temperatures of
superplastic deformation are relatively low and the nanocrystalline structure is
quite stable.
Superplasticity is found not only in metallic materials but also in ceramic
materials, especially in oxides. However, as the ductility of ceramic materials is
significantly lower than that of metals, such huge plastic deformations as are found
in metals would not be expected. In addition, ceramic parts are produced from
powders by a sequence of pressing and sintering such that, unavoidably, fully
densified bodies are not obtained, especially as the pressing behavior of nanoparticulate powders is extremely poor. These poor pressing properties are not
compensated by the excellent sintering behavior of nanoparticulate powders. The
individual pores of residual porosity act as failure points, where cracking starts. This
occurs because, around a flaw, stress concentrations are observed which depend on
the aspect ratio of that flaw. Hence, the strength of a material containing pores is less
than might be expected from a reduction of the bearing cross-section. In fact, the
maximal strength s flaw of a specimen containing flaws of the size c follows the
proportionality:
s flaw /
1
c 0:5
ð11:10Þ
Increasing the size of the flaws necessarily leads to premature cracking, with the
consequences of reduced strain and strength. The successful production of ceramic
specimens exhibiting superplasticity is a reference for excellent abilities in ceramic
technologies. Densities close to theoretical values may be obtained with specimens
produced from submicron powders and these materials have the potential for
superplastic deformation of a few hundred percent.
As a typical example of superplastic ceramics, Figure 11.25 shows a stress–strain
diagram of yttria-doped zirconia. In this example, 5% yttria was added to stabilize
the tetragonal phase of zirconia; otherwise, the material would transform into the
monoclinic phase. The investigations were performed as tensile tests and not (as is
usually done for ceramics) in compression. The density of the material ranged from
318j 11 Mechanical Properties of Nanoparticles