264 11 Mechanical Properties
The stress–strain diagrams plotted in Figure 11.18 show strains far beyond
100%. These nanocrystalline specimen were produced by severe plastic deformation of conventional materials. Specimens produced by powder metallurgy will
hardly reach such strains, as it is nearly impossible to obtain flawless parts by
these processes. Analyzing the properties of these two specimens, it is remarkable
that they show superplasticity at temperatures lower than observed in specimens
with larger grains. For these two specimens, the temperature was low enough to
inhibit grain growth. One may take as a rule that in nanocrystalline materials,
superplasticity is observed in a particle size–temperature–stress regime, where the
transition from dislocation to grain-boundary processes takes place.
Superplasticity is found in ceramic materials, especially in oxides, too. Certainly,
as the ductility of ceramic materials is significantly lower than the ductility of
metals, one may not expect huge plastic deformations, as are found in metals.
Furthermore, ceramic parts are produced from powders by a sequence of pressing
and sintering; therefore, as mentioned above, fully densified bodies are not possible. In particular, as the poor pressing behavior of nanoparticulate powders is
not compensated by the excellent sintering properties, therefore, the specimens
contain pores, which reduce the strength of a material by the reduction of the
bearing cross-section. Perhaps more important in reducing strength and plasticity
is the reduction of the maximal strength σ flaw of a specimen caused by flaws of the
size c, which is described by the proportionality
σ flaw ∝
1
0 5
c
.
.
(11.13)
The existence of pores or other flaws leads to premature cracking, with the consequences of reduced strain and strength. Successful production of ceramic specimen exhibiting superplasticity is a reference for excellent abilities in ceramic
technologies.
In spite of all these problems, it is possible to produce superplastic ceramic
parts. As typical example for superplastic ceramics, Figure 11.19 displays a
stress–strain diagram, taken in a tension test (the tension test must be noted, as
in general, ceramic materials are tested in compression), of zirconia doped with
5 wt% yttria to stabilize the tetragonal phase [15] (see also Chapter 7). The density
of the material was in the range from 84 to 94% of the theoretical density. The
grain size was in the range from 45 to 75 nm. This material was sintered at
1420 K.
Figure 11.19 shows the elongation of the specimen of 37 and 52%, values, which
are, for a ceramic material, extremely large deformations. The testing temperature
was not significantly lower than the sintering temperature, therefore, substantial
grain growth was observed during deformation. It could be that the superposition
of plastic deformation with grain growth has a distinctive influence on this result.
Grain-boundary sliding was identified as the deformation mechanism. Furthermore, these experiments showed that the deformation was directly connected to
grain growth and a reduction in density.
The stress–strain diagrams plotted in Figure 11.18 show strains far beyond
100%. These nanocrystalline specimen were produced by severe plastic deformation of conventional materials. Specimens produced by powder metallurgy will
hardly reach such strains, as it is nearly impossible to obtain flawless parts by
these processes. Analyzing the properties of these two specimens, it is remarkable
that they show superplasticity at temperatures lower than observed in specimens
with larger grains. For these two specimens, the temperature was low enough to
inhibit grain growth. One may take as a rule that in nanocrystalline materials,
superplasticity is observed in a particle size–temperature–stress regime, where the
transition from dislocation to grain-boundary processes takes place.
Superplasticity is found in ceramic materials, especially in oxides, too. Certainly,
as the ductility of ceramic materials is significantly lower than the ductility of
metals, one may not expect huge plastic deformations, as are found in metals.
Furthermore, ceramic parts are produced from powders by a sequence of pressing
and sintering; therefore, as mentioned above, fully densified bodies are not possible. In particular, as the poor pressing behavior of nanoparticulate powders is
not compensated by the excellent sintering properties, therefore, the specimens
contain pores, which reduce the strength of a material by the reduction of the
bearing cross-section. Perhaps more important in reducing strength and plasticity
is the reduction of the maximal strength σ flaw of a specimen caused by flaws of the
size c, which is described by the proportionality
σ flaw ∝
1
0 5
c
.
.
(11.13)
The existence of pores or other flaws leads to premature cracking, with the consequences of reduced strain and strength. Successful production of ceramic specimen exhibiting superplasticity is a reference for excellent abilities in ceramic
technologies.
In spite of all these problems, it is possible to produce superplastic ceramic
parts. As typical example for superplastic ceramics, Figure 11.19 displays a
stress–strain diagram, taken in a tension test (the tension test must be noted, as
in general, ceramic materials are tested in compression), of zirconia doped with
5 wt% yttria to stabilize the tetragonal phase [15] (see also Chapter 7). The density
of the material was in the range from 84 to 94% of the theoretical density. The
grain size was in the range from 45 to 75 nm. This material was sintered at
1420 K.
Figure 11.19 shows the elongation of the specimen of 37 and 52%, values, which
are, for a ceramic material, extremely large deformations. The testing temperature
was not significantly lower than the sintering temperature, therefore, substantial
grain growth was observed during deformation. It could be that the superposition
of plastic deformation with grain growth has a distinctive influence on this result.
Grain-boundary sliding was identified as the deformation mechanism. Furthermore, these experiments showed that the deformation was directly connected to
grain growth and a reduction in density.