Solubility of Oxygen and Hydrogen and Diffusivity of Oxygen in the fcc Phase of the Al-fe-ni-h-o System with Application to the Formation of a Protective α-al2o3 Scale at High Temperatures

Open Access
Author:
Ross, Austin Joseph
Graduate Program:
Materials Science and Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
July 24, 2015
Committee Members:
  • Dr Zi Kui Liu, Thesis Advisor
Keywords:
  • High Temperature Oxidation
  • Ni-Base superalloys
  • Selective Oxidation
  • Al2O3 Scale
Abstract:
This thesis will focus on the formation of a protective α-Al2O3 layer on Ni-base superalloys during high temperature oxidation in dry and wet gaseous environments. Ni alloys have proven useful in energy and aerospace applications due to their high melting points and excellent mechanical properties at elevated temperatures. High operating temperatures in air will result in aggressive forms of gaseous attack – especially by oxygen. For temperatures in excess of 1000 ºC an external, slow growing layer of α-Al2O3 on the surface of the alloy system is necessary to prevent excessive oxidation. Designing an alloy which has the necessary amount of Al required to form an external scale of α-Al2O3 requires detailed knowledge of the O solubility, O diffusivity, and Al diffusivity for the system of interest. The present thesis develops a thermodynamic description of the Al-Fe-Ni-O-H system and a kinetic description of O and Al in the fcc phase of the Al-Fe-Ni-O system. Special attention is given to the solubility of H and O in the fcc phase in the developed thermodynamic models. Using the calculation of phase diagram (CALPHAD) method, these models are constructed from available experimental data and theoretical data calculated using density functional theory (DFT.) Information on O diffusion in the system will also be collected from experiments and calculated from first-principles so as to construct a description of the O and Al mobility. When α-Al2O3 forming alloys are oxidized in wet environments the amount of Al needed to form an external, protective layer increases. Both an increase in O solubility and a decrease in the driving force of α-Al2O3 due to the presence of dissolved H from H2O have been suggested to explain this effect. The thermodynamic models developed herein are used to test these hypotheses. The calculated effect of dissolved H is to decrease the solubility of O and does not significantly decrease the driving force for α-Al2O3. Two models which predict Al concentrations likely to form scales of α-Al2O3, using solubility and diffusivity information developed in this thesis as inputs, are tested in both the Ni-xAl and Ni-xFe-yAl fcc-solid solution strengthened systems. The predictions for external scale formation in each system are in good agreement with experimental findings for each model. These predictions are discussed with respect to the calculated O solubility in the Ni-O system.