Environmental Engineering Reference
In-Depth Information
10
8
Al-3%Mg
T
=0.9
Tm
Harper-Dorn
10
7
Climb
Glide
10
6
10
5
Nobarro-Herring
10
4
Coble
10
3
10
-7
10
-3
10
-5
10
-4
10
-3
σ
/
G
3.18
Mohamed-Langdon deformation mechanism map describing
transitions in creep mechanisms as functions of grain size and stress.
80
climb occurring at lower stresses; this plot did not consider the climb region
at the higher stress-end as described earlier. Also missing is the GBS that is
expected between viscous and dislocation creep mechanisms.
3.6
Modeling creep life: extrapolation of strain and
rupture data
In the previous sections, we discussed the different mechanisms of creep
that have been observed in various materials. The stress, temperature and
microstructural dependence of each mechanism was described and the
steady-state strain rate of deformation of each mechanism was correlated
to these parameters. We also outlined the different regions, through defor-
mation mechanism maps, where a given mechanism would be dominant
over others. In all these sections, more emphasis was laid on the second-
ary creep region and the mechanism maps were also constructed taking
into account these steady-state creep-rates. However, such a methodology
would be based on the premise that the secondary creep stage accounts
for a signifi cant fraction of the useful creep life. While such a method is
not entirely wrong, it is unsuitable for several materials which tend to have
larger primary or tertiary creep regimes. For example, Ni-based superalloys
have been found to exhibit primary creep strains of the order of 1% or
more.
81
These alloys are used as materials for fan and compressor blades of
aero-engines. The dimensional tolerance for these components is very small
and plastic strains in the order of 1% are suffi cient to wreck the stability of
the engine. Hence under such conditions, modeling by considering only the
steady-state creep rates will grossly overestimate the useful creep life of the
material. Furthermore some of the mechanisms of creep, for example in
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