THERMODYNAMIC MODELING AND FIRST-PRINCIPLES CALCULATIONS OF THE CR-HF-Y TERNARY SYSTEM

Open Access
Author:
Hasek, Bradley
Graduate Program:
Materials Science and Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
April 05, 2010
Committee Members:
  • Dr Zi Kui Liu, Thesis Advisor
Keywords:
  • CALPHAD
  • SQS
  • Cr
  • Hf
  • Y
Abstract:
This thesis describes the thermodynamic modeling of the ternary Cr-Hf-Y system. Chromium, hafnium, and yttrium are all used as alloying elements in nickel superalloys. Nickel superalloys usually contain many different alloying elements, so there becomes a need to understand the phase equilibria of large, multicomponent systems. This can be accomplished by using the CALPHAD method of modeling. CALPHAD modeling requires experimental data to better model the system. In the absence of experimental data, first-principles calculations can be used to predict thermochemical data. Before modeling the Cr-Hf-Y system, the binary Cr-Hf, Cr-Y, and Hf-Y systems must be modeled. In the Cr-Y and Hf-Y system, there is limited experimental data, so first-principles calculations are done on the BCC and HCP solid solution phases using special quasirandom structures (SQS’s) to model the disordered systems. The symmetry is also investigated to ensure symmetry is not lost during relaxation. Using the first-principles calculations, the enthalpy of mixing is used to model these systems. The Cr-Hf system is modeled with experimental data and with first-principles calculations done on the Laves phase. With the three binary systems modeled, the ternary system is then modeled without any experimental data present. Isothermal sections are presented from 1273 K to 2200 K.