boundaries in between will be eliminated. In this way a new extended path for
dislocation movement is opened, leading to a softening of the material [17]. The
mechanism involved is depicted in Figure 11.23.
11.2.3
Superplasticity
The phenomenon of superplasticity is found especially in connection with small
grain sizes. Superplastic materials allow deformation in tensile tests of a few
hundred percent; indeed, in special cases, deformations of more than 1000%
were found in metals. Although restricted to a certain, material-dependent range
of grain sizes and deformation rates, superplasticity is typically found at around
0.5T m , where T m is the melting temperature in Kelvin. Superplasticity is observed in
metals, alloys, and ceramic materials. Superplastic specimens neither narrow locally
nor form internal cavities, as both are local sites where fractures start. To date, no
generally accepted theory has been proposed of superplasticity that explains all of the
available experimental results. As with other deformation processes, the deformation rate of a superplastic specimen can be described by:
_
e /
s
n
d
2
ð11:9Þ
In Eq. (11.9) the stress exponent n is 1 for diffusion and grain boundary processes
or 2 for dislocation processes.
In order to demonstrate superplasticity in nanocrystalline metals, two different
types of material were selected. Figure 11.24 displays the stress–strain curves of
superplastic Ni 3 Al, an ordered intermetallic compound, and of Ti 6 Al 4 V, an alloy. The
intermetallic compound Ni 3 Al had a grain size in the range from 80 to 100 nm,
whereas for the Ti 6 Al 4 V alloy the grain size was smaller, in the range of 30–50 nm.
0
0.5
1
1.5
2
0
250
500
750
1000
Material
Ti 6 Al 4 V @950 K
Ni 3 Al @920 K
strain Δl/l
stress [MPa]
Figure 11.24 Stress–strain diagram for superplastic Ni 3 Al [18] and Ti 6 Al 4 V [19]. The large strain,
exceeding 1 (>100%) is remarkable.
11.2 Bulk Metallic and Ceramic Materials j317
dislocation movement is opened, leading to a softening of the material [17]. The
mechanism involved is depicted in Figure 11.23.
11.2.3
Superplasticity
The phenomenon of superplasticity is found especially in connection with small
grain sizes. Superplastic materials allow deformation in tensile tests of a few
hundred percent; indeed, in special cases, deformations of more than 1000%
were found in metals. Although restricted to a certain, material-dependent range
of grain sizes and deformation rates, superplasticity is typically found at around
0.5T m , where T m is the melting temperature in Kelvin. Superplasticity is observed in
metals, alloys, and ceramic materials. Superplastic specimens neither narrow locally
nor form internal cavities, as both are local sites where fractures start. To date, no
generally accepted theory has been proposed of superplasticity that explains all of the
available experimental results. As with other deformation processes, the deformation rate of a superplastic specimen can be described by:
_
e /
s
n
d
2
ð11:9Þ
In Eq. (11.9) the stress exponent n is 1 for diffusion and grain boundary processes
or 2 for dislocation processes.
In order to demonstrate superplasticity in nanocrystalline metals, two different
types of material were selected. Figure 11.24 displays the stress–strain curves of
superplastic Ni 3 Al, an ordered intermetallic compound, and of Ti 6 Al 4 V, an alloy. The
intermetallic compound Ni 3 Al had a grain size in the range from 80 to 100 nm,
whereas for the Ti 6 Al 4 V alloy the grain size was smaller, in the range of 30–50 nm.
0
0.5
1
1.5
2
0
250
500
750
1000
Material
Ti 6 Al 4 V @950 K
Ni 3 Al @920 K
strain Δl/l
stress [MPa]
Figure 11.24 Stress–strain diagram for superplastic Ni 3 Al [18] and Ti 6 Al 4 V [19]. The large strain,
exceeding 1 (>100%) is remarkable.
11.2 Bulk Metallic and Ceramic Materials j317