| Summary: | Nickel-base superalloys have been an topic of active research for over five
decades and the findings from various researchers worldwide have had a
direct impact not only on different areas of materials science but also on
applications thereof, in particular the aircraft industry, i.e. the aeroengine.
One of the primary concerns of aircraft engine manufacturers has been the
problem of microsegregation in turbine blade alloys. This phenomenon which
is present in almost all cast products occurs due to the partitioning of solute
between the solid and the liquid phases during solidification and is
ultimately the source of several types of defects such freckling and white
spots in castings resulting in rejection of defective products. Since
microsegregation results in a heterogeneous distribution of alloying elements
in the cast product it is deleterious to the component's thermo-mechanical
properties as well as to its resistance to environmental attack. Thus, much of
the current work in superalloy technology has been directed towards gaining
a better understanding of microsegregation in different alloys through
experimentation coupled with efforts leading towards finding a means of
satisfactorily predicting its effects and also to a degree controlling it.
Over the years efforts have culminated in many models of microsegregation
having been developed for different alloy systems with reasonably acceptable
predictive capacities. Nearly all of these models consider the phenomenon of
back-diffusion in the solid which serves to reduce the degree of
microsegregation in the final cast product by redistributing the solute during
solidification. In Ni—base superalloys however, evidence for back diffusion is
scarce and only a handful of models of microsegregation are available.
The work contained in this thesis addresses the issue of obtaining
experimental proof for back diffusion in a Ni—base single crystal superalloy.
The study was also aimed at developing a suitable model to predict the
microsegregation in the alloy. Directional solidification experiments were
conducted in a Directional Solidification and Quench (DSQ) furnace for
producing cast single crystal specimens of the alloy RR-2100 which were
characterized using Differential Scanning Calorimetry (DSC) for the alloy's
solidification behaviour and range. Electron—probe microanalysis (EPMA)
experiments were conducted on specimens prepared in this way to obtain
compositional data in the mushy zone. The raw data from these experiments
were sorted and interpreted with the model of microsegregation. The
agreement between the model's predictions and the experimental data were
found to be reasonably good. The analysis revealed evidence for back
diffusion in RR-2100 nickel-base single crystal superalloy and the measures
of the diffusivities of the solutes obtained were found to be consistent with
previous findings in the literature. === Applied Science, Faculty of === Materials Engineering, Department of === Graduate
|
|---|
