Modeling of the aluminum-rich region of the aluminum-cobalt-nickel-yttrium system via computational and experimental methods for the development of high temperature Al-based alloys

The present thesis focuses on the understanding and development of high temperature Al alloys based on the Al-Co-Ni-Y system through experimental and computational investigations. The phase relations of the phases present in the Al-Co-Ni-Y system have been studied with special emphasis on the Al-rich region, to produce an accurate thermodynamic model.; Initially, the structure---property relationship of three spray-formed alloys was determined experimentally with respect to changes in composition and processing parameters. Increased secondary processing temperatures caused significant coarsening of the intermetallic particles in the matrix, altering the mechanical properties. A new alloy was created with the percentage of alloying elements reduced by half. This reduction, coupled with careful control of the extrusion temperature resulted in a significant increase in ambient temperature elongation with only moderate decrease in elevated temperature yield strength.; To systematically study the phase relations in the quaternary system, a computational thermodynamics approach is employed. The thermodynamic model was created using the CALPHAD technique with the aid of first-principles calculations and available experimental information. The Al-Co-Ni-Y quaternary thermodynamic database is created and model predictions of the three spray formed alloys are performed and compared with the relevant experimental results. With only minimal changes of the Al-rich Al-Ni-Y ternary compound model parameters, a first approximation of the quaternary database is produced and capable of providing accurate predictions for the type and amount of phases present within the Al-rich region of this system.; This work provides a foundation for the coupling of the thermodynamic database with experimental knowledge to produce an Al-rich alloy with an enhanced combination of the elevated temperature yield strength and ambient temperature toughness for structural applications. Experimental analysis of the three alloys from this work defines a range of compositions and processing parameters from which an optimum combination of mechanical properties can be explored and further refined. Beyond the specific scope of this work, this procedure for alloy optimization via combined experimental and computational methods can be applied for many other alloy systems, since the fundamentals of this approach transcend the quaternary Al-Co-Ni-Y system.