Microstructure Analysis of Alloy X-750+ for Spacer Grid Application in BWR Reactor Core
Typ
Examensarbete för masterexamen
Program
Publicerad
2020
Författare
Ipin, Mas
Modellbyggare
Tidskriftstitel
ISSN
Volymtitel
Utgivare
Sammanfattning
Due to its durability in hostile environments, nickel-based superalloy X-750 has
been used in high temperature (and pressure) applications such as gas turbine, solar
thermal power plant, and steam turbine. In nuclear technology, this material is
utilized as springs, bolts, and spacer grids in the reactor core. This thesis focuses
on the utilization of alloy X-750 as spacer grid in BWR reactor. A previous study
showed that a higher iron content of alloy X-750 gives a better corrosion resistance
in simulated environment of BWR reactor (5% and 8% wt of Fe content comparison)
[7]. Then, a new type of nickel based superalloys was developed by Westinghouse
Electric Sweden AB, the so called X-750+ which has dramatically high iron content,
i.e. 18%wt. However, corrosion resistant analysis was not performed in this work.
Instead, basic microstructure analysis was carried out to give preliminary insight on
the physical properties especially in relation to the iron content. Two other samples
beside X-750+ were also analyzed as references which are alloy X-750 and 718.
All samples were prepared with two type of heat treatments namely the standard
heat and two stages heat treatment. The microstructure analysis was divided into
two parts, atom probe tomography and Secondary Electron Microscopy (SEM).
Samples for atom probe tomography were prepared using electropolishing method
to produce needle-shape samples suitable for LEAP 3000 Atom Probe. Meanwhile,
SEM analysis was done using JEOL JSM-7800 Prime SEM and LEO Ultra 5.5
SEM. Samples for SEM were polished using grinding paper and liquid diamond
complemented by electroetching step to reveal the secondary particles.
Results from atom probe measurement were analysed using proxigram, a 2-D particle
distribution calculation provided by IVAS software. This analysis suggests that both
X-750 and X-750+ differ slightly in term of particle volume fraction which might
indicating that the mechanical properties are also similar. Meanwhile, standard heat
treatment tends to give a slightly larger volume fraction for alloy X-750 but the opposite
fashion was observed for alloy X-750+. Furthermore, heat treatment difference
seemed do not affect the particle distribution across the particle interfaces. 1-D
analysis was done for alloy 718 since the tertiary particle (
00) mostly situated side
by side with secondary particle (
0). This analysis showed that both heat treatments
did not give any differences. Lastly, SEM analysis revealed the grain boundaries for
all samples. Accoriding to SEM analysis, all samples with standard heat treatment
revealed more prominent grain boundaries except for alloy 718. However, further
investigation should be performed to obtain the chemical compositions across the
grain boundaries, such as Transmission Electron Microscopy (TEM) analysis.
Beskrivning
Ämne/nyckelord
alloy X-750 , microstructure , atom probe tomography , SEM