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First Principles Thermodynamics of Metallic Alloys

Rao, You
http://rave.ohiolink.edu/etdc/view?acc_num=osu1601932817906742

Abstract Details

2020, Doctor of Philosophy, Ohio State University, Materials Science and Engineering.
Ni-based superalloys and high entropy alloys are both promising materials for applications at elevated temperatures. Understanding the deformation mechanisms is crucial for further improvement of their mechanical properties. Interestingly, under high temperature conditions, it is often the thermodynamics that governs the mechanical behaviors. Phenomena such as segregation and phase transformation can have profound impacts on how the material responds to the applied stress. Therefore, it is important to study the thermodynamics of the materials. Here, we first present our newly developed Monte Carlo based method for efficient phase prediction for multicomponent systems and showcase a few successful examples of its application. Then it is applied to two high entropy alloys, the refractory high entropy alloy HfNbTaZr and the CoCrNi based high entropy alloy V1. Phase separation is predicted for both cases. Subsequent calculation on one of the resulting phases indicates that the phase transformation is the main reason for the deactivation of twinning in the alloy V1 instead of the altering of the stacking fault energy of the alloy itself by the addition of Al and Ti. For Ni-based alloys, we systematically perform DFT calculations to study the segregation behavior of common alloying elements to stable planar faults in γ′-Ni3Al. No obvious driving force for segregation to existing SISF, SESF or twin boundaries is found whereas segregation to these faults has been observed in experiments. This discrepancy is an indicator that these stacking faults do not form directly in the γ′-Ni3Al. Instead, their formations involve precursors that are attractive to solutes. Further investigation of the segregation propensity to the meta-stable transitional configurations confirms that the formation mechanisms are related to the concept of reordering instead of simply shearing of the superpartials. Then we look at the synergistic effects between the solutes combining DFT calculations and a segregation isotherm. We find that the segregation of Cr is greatly facilitated by the segregated Co atoms. Also, the supposedly small interaction energy is enough to bring sufficient Co to the stacking fault as observed in experiments, which gives rise to a new possibility of the deformation mechanisms in Ni-based superalloys. Finally, the local phase transformation is investigated. We show that the interfacial effects can change the type of phase that forms and the formation of η phase is theoretically more favorable at the stacking fault area in some cases than in the bulk region.
Maryam Ghazisaeidi (Advisor)
Wolfgang Windl (Committee Member)
Michael Mills (Committee Member)
191 p.
Materials Science

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Citations

  • Rao, Y. (2020). First Principles Thermodynamics of Metallic Alloys [Doctoral dissertation, Ohio State University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=osu1601932817906742

    APA Style (7th edition)

  • Rao, You. First Principles Thermodynamics of Metallic Alloys. 2020. Ohio State University, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=osu1601932817906742.

    MLA Style (8th edition)

  • Rao, You. "First Principles Thermodynamics of Metallic Alloys." Doctoral dissertation, Ohio State University, 2020. http://rave.ohiolink.edu/etdc/view?acc_num=osu1601932817906742

    Chicago Manual of Style (17th edition)

osu1601932817906742
88
© 2020, some rights reserved.Creative Commons License First Principles Thermodynamics of Metallic Alloys by You Rao is licensed under a Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. Based on a work at etd.ohiolink.edu.
This open access ETD is published by The Ohio State University and OhioLINK.