Nickel-based superalloys are employed in a variety of gas turbine components
due to their excellent high-temperature properties. Unfortunately, several
of these superalloys have proved to shrink during long-term service/ageing
at intermediate temperatures (400-600°C). This study comprises an
investigation of the ageing effects of two nickel-based superalloys from
different classes of material: Haynes 230 and single crystal CMSX-4. The
work is divided into three parts:
I. Literature study. Haynes 230 and CMSX-4 were studied as well as reports
and articles concerning shrinkage and microstructural changes owing to
ageing in different nickel-based superalloys.
II. Mechanical testing. The ageing effects on mechanical and physical
properties were investigated in a series of experiments. The employed
methods are tensile testing, impact testing, low cycle fatigue testing and
hardness testing.
III. Microstructural examination. The underlying causes to the property
changes were investigated in a microstructural examination including carbide
size measurements, chemical analysis of the composition and analysis of
micro images taken with scanning electron microscope and light optical
microscope.
The results from the experimental testing show that the ageing has a marked
effect on the properties of both CMSX-4 and Haynes 230. The low cycle
fatigue properties of Haynes 230 are unaffected by the ageing while the
yield and tensile strength and hardness are improved. The ductility is
improved in both Haynes 230 and CMSX-4. The hardness of CMSX-4 was
unaffected by the ageing while the tensile strength was slightly impaired.
It was also found that the properties and the ageing effect depended on the
single crystal direction.
The study shows that the ageing has a significant effect on the
precipitation of secondary carbides at grain boundaries in Haynes 230, which
has a strengthening effect on the material. No distinct changes of the
microstructure explaining the property changes in CMSX-4 were found.
It is reported in other studies that shrinkage in other superalloys is
caused by short and long range ordering based on Ni2C...
