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Study of back-diffusion in the nickel-base single crystal superalloy RR-2100 Thirumalai, Anand

Abstract

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.

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