Creep deformation of materials in light water reactors (LWRs) - Materials' Ageing and Degradation in Light Water Reactors - page 20

T 3 > T 2 > T 1
σ 3 >

σ 2 >

σ 1

T 3 ,

σ 3

T 2 , σ 2

T 1 , σ 1

ε

t

3.3 Illustration of the effect of stress and temperature on creep behavior
of a material.

The effect of stress and temperature is clearly illustrated by Fig. 3.3. With
increasing stress and temperature, the instantaneous strain at the time of
stress application increases, the steady-state creep rate is increased and the
rupture lifetime is diminished.

3.2.2 Effect of microstructure

The analysis of any creep data is made by assuming the microstructure to
be constant. Some of the microstructural features that could change during
the course of a test are phase composition, precipitate size and distribution,
and grain size. Thus to estimate the different creep parameters and to deter-
mine the mechanism of creep, it is necessary to keep the microstructure
constant. To this end, materials are usually heat treated at temperatures
higher than the test temperature. Even though thermal stabilization estab-
lishes a constant microstructure during the course of a test, stress-assisted
processes altering the microstructure cannot be ruled out. Non-equilibrium
structures, namely nanocrystalline materials undergo stress-assisted micro-
structural changes that prevent the attainment of a constant creep micro-
structure. 14 Creep tests on such materials should be carried out at stresses
lower than the critical stress at which microstructural changes could be
initiated. 15
The most important microstructural parameter that plays a major role in
controlling the creep properties of a material is the grain size. The depen-
dence of the strength of a material on its grain size can be understood
through the Hall-Petch relationship which states that materials with fi ner
grain sizes possess greater strength than materials with larger grain size 3 :

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