<p>Ni-based superalloys have been widely used for high temperature applications such as turbine blades for jet propulsion and power plants due to their excellent creep, fatigue, and corrosion resistance. But as the demand for higher temperature capability and strength increases, there remains a need to better understand high temperature deformation mechanisms and improve and strengthen superalloys at these elevated temperatures. Recently, a correlation has been observed between solute segregation at planar defects (stacking faults, antiphase boundaries, etc) and enhanced high temperature creep properties – known colloquially as phase transformation strengthening. Experimentally, regardless of alloy composition, strong Co segregation at planar defects along with Cr has been observed. In addition, it has been suggested by density functional theory work that Co would promote Cr concentration at stacking faults by forming strong Cr-Co bonds. Based on these findings, it was hypothesized the presence of Co provides a significant thermodynamic driving force for segregation to planar defects. </p><p>In order to further investigate the correlation between solute segregation and deformation mechanisms the fabrication of a planar front single crystal Ni-based superalloy and its microstructure, alloy composition, and microhardness properties of the as-zone melted and solution heat treated states were investigated and compared to the directionally-solidified state to study the effect of microsegregation on these alloy characteristics. Next, new Co-containing, Cr-free alloys are designed to optimize g-g’ volume fraction, size, and morphology to mimic microstructures observed in single crystal superalloys. The general alloy design strategy and approach are outlined, and the composition, microstructure, phase transformation temperatures, and mechanical properties of new Cr-free and Co-containing alloys are reported. A new set of Cr-free alloys have thus been designed, with modifications of Nb, Ta, and Ti additions ranging from 3 to 7 at.% to investigate the role of these elements on the phase transformation strengthening mechanism at elevated temperatures.</p><p></p>

The theory of the lamellar eutectoid reaction by volume diffusion has been extended to account for solute segregation within the product phases and the effect of dilute third element additions. It has been demonstrated for symmetric binary systems that the segregation can account for 10% or more of the free energy stored in the product phases and can lead correspondingly to a predicted lamellar spacings appreciably greater than those obtained when segregation is neglected. This segregation is relatively high in the systems Cu-In, Cu-Be and Ag-Cd and may account for the fact that a secondary coarse-grained pearlitic reaction follows the initial fine grained one in these systems. The binary theory has been-used to analyse the available data for the eutectoid reaction in the Cu-Al and Fe-C systems and satisfactory agreement is obtained. The theory for ternary systems, while complete in principle, proves to be. intractable in all but the simplest version of the solution thermodynamics. It is concluded that the effect of the third element on the binary eutectoid reaction is mainly through its effect on the phase diagram, the ternary cross effects in the diffusion matrix tending to cancel out. Any additional element which lowers the eutectoid temperature will retard the reaction. / Thesis / Doctor of Philosophy (PhD)