| Summary: | <p>Austenitic alloys are commonly used in the nuclear industry due to
their high corrosion resistance and excellent mechanical properties. De-
spite boasting an impressive service history, it is possible for critical
structural components made from these alloys to undergo stress cor-
rosion cracking (SCC) under pressurised water reactors (PWR) condi-
tions. As the primary method of material degradation, SCC has been
studied extensively for several decades, and many factors have been
shown to affect SCC susceptibility, these include cold-work, tempera-
ture and water chemistry. To develop a better mechanistic understand
of SCC further research is required. This body of research evaluates the
effect of hydrogen on mechanical deformation in SCC. Localised and
high-resolution testing has been employed successfully to characterise
key processes in SCC crack propagation. For this reason, nanoindenta-
tion, transmission Kikuchi diffraction (TKD), NanoSIMS and Thermal
desorption spectrscopy (TDS) have been used to characterise material
deformation on a length-scale relevant for SCC.</p>
<p>The main objective of this thesis was to correlate the SCC crack growth
rate’s (CGR) dependency on nickel content, first observed by Coriou,
with both the localised mechanical deformation and hydrogen up-take
at PWR temperatures. To satisfy this objective three alloys have been
systematically tested in every chapter, these include SS316L (12%-
Ni), A800 (32%-Ni) and A600 (72%-Ni) as they represent commercially
available alloys with low, intermediate and high nickel contents.
Room temperature micromechanical testing has been used to compare
the mechanical properties of austenitic alloys on a length-scale that is
relevant to SCC. The mechanical properties of all austenitic alloys are
highly comparable at room temperature, and show no deviation with
changing nickel content. High-magnification cross-sectional analysis
shows that the deformation mechanisms in A800 differ from SS316L
and A600 at room temperature. A800 does not undergo Σ3 defor-
mation twinning, but instead deforms by dislocation glide. This is
unexpected as the stacking fault energy of A800 is an intermediate of
the other alloys. Deformation is a main SCC mechanism, and the ob-
served alloy deformation correlates with Arioka’s reported SCC CGR
in PWRs, i.e the slowest CGR undergoes no twinning. As the me-
chanical properties of alloys with a 25-40% nickel content do not differ
significantly from other austenitic alloys, it is thought that their corro-
sion resistance is weighted towards diffusion-based SCC mechanisms.
Such an explanation is consistent with findings at hotter temperatures,
where diffusion-based mechanisms are more active. High temperature
micromechanical testing supports these observations, and can be used
to better understand the individual mechanism weights depending on
alloy composition and temperature.</p>
<p>NanoSIMS and TDS were used to quantitatively and qualitatively ex-
plore the hydrogen trapping in austenitic alloys exposed to pure heavy
water at PWR temperatures. Hydrogen is found trapped by various
microstructural features, such as carbides, grain boundaries and ox-
ides by NanoSIMS. The total hydrogen content of SS316L and A800
is five-times greater than that of A600, which correlates with the en-
thalpy of diffusion in the alloys. These results show an inverse trend
with the SCC failure reported by Coriou, and a weaker inverse correla-
tion to Arioka’s results. The weaker correlation with Arioka’s results is
due to a change in the corrosive medium from pure water, used in this
study and by Coriou, to simulated primary PWR water. It is suggested
that the inverse relationship could be due to the elastic shielding pro-
vided by the hydrogen enhancing dislocation motion, and preventing
the stress intensity factor reaching a critical values that will initiate
crack propagation.</p>
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