Chemistry Reference
In-Depth Information
max
min
r
-
r
,
(15.33)
D
=
max
r
where ρ
max
and ρ
min
are the greatest and the smallest density values. This
estimation gives ∆ρ ≈ 0.01. This value can be reasonable for free volume in-
crease, which is necessary for loosely matrix devitrification (accounting for
closeness of
T
and
T
′
), but it is obviously small if to assume as real micro-
cracks formation. As the experiments have shown, REP extrusion at ε
e
> 0.52
is impossible owing to specimen cracking during extrusion process. This
allows to suppose that value ε
e
= 0.52 is close to the critical one. Therefore,
the critical dilatation ∆δ
cr
value, which is necessary for microcracks cluster
formation, can be estimated as follows [40]:
(
)(
)
21
+
n
2 3
-
n
,
(15.34)
D=
-
d
cr
11
19
n
where n is Poisson's ratio.
FIGURE 15.32
The dependence of specimens density ρ on extrusion strain ε
e
for extruded
(1) and annealed (2) REP [62].
Accepting the average value n ≈ 0.35, we obtain ∆δ
cr
= 0.60, that is essen-
tially higher than the estimation ∆ρ made earlier. These calculations assume
that ρ decrease at ε
e
= 0.43 and 0.52 is due to instable nanoclusters decay and
to corresponding REP structure loosening.
