Modeling of the Al-rich Region of the Al-Co-Ni-Y System Via Computational and Experimental Methods for the Development of High Temperature Al-Based Alloys

Open Access
Author:
Golumbfskie, William Joseph
Graduate Program:
Materials Science and Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
July 26, 2005
Committee Members:
  • Zi Kui Liu, Committee Chair
  • Maurice Francis Amateau, Committee Chair
  • Paul Raymond Howell, Committee Member
  • Jorge Osvaldo Sofo, Committee Member
  • Timothy John Eden, Committee Member
Keywords:
  • Alloy Development
  • Thermodynamic Modeling
  • First-Principles Calculations
  • Al-based Alloy
Abstract:
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, one of which was fabricated in this work, was determined experimentally with respect to changes in composition and processing parameters. Phase identification and microstructural analysis was performed via LOM, SEM, TEM and XRD. Mechanical property measurements for fracture toughness, tensile strength, yield strength and elongation were also obtained. Each alloy consists of three types of intermetallic particles: Al3Y, Al9Co2, and an Al-rich Al-Ni-Y particle, embedded in an Al matrix. Initially, two alloys were created with the same nominal composition. Elevated temperature (300°C) yield strength values of 165 – 190 MPa and ambient temperature elongation values of 2 – 3 percent were determined. Increased secondary processing temperatures caused significant coarsening of the intermetallic particles in the matrix, altering the mechanical properties. A third alloy was created having one-half of the alloying element content of the previous alloys. This reduction in alloying element content, coupled with careful control of the extrusion temperature resulted in a significant increase in ambient temperature elongation (13 percent) and moderate decrease in elevated temperature yield strength (145 MPa). To systematically study the phase relations in the quaternary Al-Co-Ni-Y 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. In the CALPHAD modeling, the Gibbs energies of the individual phases (pure elements, binary phases, ternary phases, etc.) of a given system are optimized to produce a self-consistent thermodynamic database. First-principles calculations, based on Density Functional Theory, have been performed to obtain relevant thermodynamic information to input into the thermodynamic model. The first-principles calculations are determined not only for structures relevant to the optimization of the Al-Y-Ni-Co quaternary system, but have been rigorously analyzed to determine the validity of current state-of-the-art calculations, when compared to known experimental values. 0 K total energy calculations for ten Al-rich Al-Ni-Y ternary compound show excellent agreement with the experimental results, differing by approximately 1 kJ/mol-atom. Finite temperature enthalpy and entropy of formation values were also determined for the ternary compounds. Their validity is based upon the agreement with the available experimental information of the pure elements. The results are implemented into the CALPHAD model for the quaternary system. In the present work, the quaternary thermodynamic database is created and model predictions of the three spray formed alloys are performed and compared with the relevant experimental results. The quaternary model is built upon the foundation of the existing binary models, including the complete re-modeling of the Co-Y binary system, performed in this work. The Al-Ni-Y ternary phase information was obtained through a combination of experimental calculations and experimental phase diagram approximations obtained from the literature and first-principles calculations performed in this work. With only minimal changes of the Al-rich Al-Ni-Y ternary compound model parameters based upon experimental work from the literature, 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. Solidification simulations were performed to determine the stable phases present in three alloy compositions and compared favorably with the corresponding experimentally determined results. 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.