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A Comprehensive Study of Diffusion and Modulus of Binary Systems within the Ti-Mo-Nb-Ta-Zr System

Chen, Zhangqi
http://rave.ohiolink.edu/etdc/view?acc_num=osu1562224291399741

Abstract Details

2019, Doctor of Philosophy, Ohio State University, Materials Science and Engineering.
Titanium alloys containing β stabilizers such as Mo, Nb and Ta are highly desirable for biomedical implants applications due to their low elastic modulus and excellent corrosion and fatigue resistance. To find the relation between β-Ti alloy compositions and their properties, a set of Ti-based diffusion multiples were made to generate composition libraries/gradients within the Ti-Mo-Nb-Ta-Zr system. Various experimental and analytical tools are applied to the diffusion multiples to extract the composition dependence of diffusion and mechanical properties. Diffusion is one of the fundamental processes in nature. A fast and easy-to-use simulation tool can help researchers understand and optimize diffusion processes in materials. A new Python-coded algorithm was developed to perform diffusion simulation in a fast and robust way. The core mechanism in this algorithm includes dynamic meshing and moving boundary structure. The diffusion simulation algorithm was combined with forward simulation analysis (FSA) to efficiently extract diffusion coefficients from experimental diffusion profiles. These diffusion simulation and analysis tools are coded and packaged as “pydiffusion”, a free open-source Python library that was deposited into the GitHub software sharing platform along with a description of its usage in a publication in the Journal of Open Research Software. Three Ti-Mo-Nb-Ta-Zr diffusion multiples were annealed at 800 ℃, 1000 ℃ and 1200 ℃, respectively to perform interdiffusion experiments. Electron probe micro-analysis (EPMA) was employed to collect diffusion profile data of various binary systems within three diffusion multiple samples. FSA was then applied to extract interdiffusion and impurity diffusion coefficients of seven Ti-X and Zr-X (X=Mo, Nb, Ta) binary systems. The new experimental results provide the most systematic and reliable data for the bcc phase of these seven binary systems. Through FSA calculation, the extracted interdiffusion coefficients are orders of magnitude higher than those from tracer experiments at the slower diffusion side of the Ti-X and Zr-X (X = Mo, Nb, Ta) binary systems. X-ray generation (interaction) volume simulation and an analysis of experimental profiles show that the artificial broadening of the concentration profile in very steep gradient region is responsible for the much inflated diffusion coefficients. A recommendation is made to trust only diffusion coefficients from concentration gradients less than 1 at.% per micron. To further investigate the relation between Ti alloy compositions, elastic modulus and crystal structure, nanoindentation tests were performed on one Ti-Mo-Nb-Ta-Zr diffusion multiple sample. Combining nanoindentation results with composition profile data, composition dependent indentation modulus data were extracted for the Ti-Mo, Ti-Nb and Ti-Ta binary systems. Based on the modulus trends, several TEM foils were also extracted from the same diffusion multiple sample using focused ion beam (FIB). TEM characterization shows the α’-α”-β phase transformation as the β-stabilizer contents increases in the Ti-X (Mo, Nb, Ta) binary systems, with the ω phase also observed in certain Ti-Mo and Ti-Nb compositions. The FIB sample extraction of the exact compositions of interest on the diffusion multiple is far more effective than making individual alloy samples. For thermal activated diffusion, essentially all diffusion coefficients follow the Arrhenius law. However, self-diffusion in several bcc metals are anomalous, such as β-Ti and Mo. The origin of the anomaly is still poorly understood. In recent decades, first-principles calculations based on density functional theory (DFT) have been extensively used to calculate diffusion coefficients. These calculations are usually coupled with the transition state theory under the harmonic or the quasi-harmonic approximations. However, traditional DFT method cannot be applied to β-Ti which is unstable structurally at 0 Kelvin. A fast approach using large phonon displacement was successfully applied to the unstable bcc phases of group IVB metals. By taking into account of thermal expansion in the bcc lattice, the nonlinear Arrhenius behavior of self-diffusion in Mo was successfully reproduced. Furthermore, for the first time, the large displacement approach was successfully demonstrated to calculate self-diffusion coefficients in β-Ti, again revealing the anomalous behavior as observed in experimental data. According to the current calculation results, the anomaly of self-diffusion in Mo and β-Ti is contributed from the temperature dependence of both vacancy formation entropy ΔS_v and atom-vacancy migration entropy ΔS_m, while the enthalpy of vacancy formation (ΔH_v) and migration (ΔH_m) also slightly changes within the calculation temperature range. The methodology employed here can be used to examine the anomalous diffusion in other bcc systems.
Ji-Cheng Zhao, Dr. (Advisor)
Stephen Niezgoda, Dr. (Committee Member)
Vicky Doan-Nguyen, Dr. (Committee Member)
Patrick Woodward, Dr. (Committee Member)
226 p.
Materials Science
diffusion; simulation; diffusion coefficients; diffusivity extraction; diffusion couples; Ti alloys; first-principles calculation; elastic modulus;

Recommended Citations

Citations

  • Chen, Z. (2019). A Comprehensive Study of Diffusion and Modulus of Binary Systems within the Ti-Mo-Nb-Ta-Zr System [Doctoral dissertation, Ohio State University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=osu1562224291399741

    APA Style (7th edition)

  • Chen, Zhangqi. A Comprehensive Study of Diffusion and Modulus of Binary Systems within the Ti-Mo-Nb-Ta-Zr System. 2019. Ohio State University, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=osu1562224291399741.

    MLA Style (8th edition)

  • Chen, Zhangqi. "A Comprehensive Study of Diffusion and Modulus of Binary Systems within the Ti-Mo-Nb-Ta-Zr System." Doctoral dissertation, Ohio State University, 2019. http://rave.ohiolink.edu/etdc/view?acc_num=osu1562224291399741

    Chicago Manual of Style (17th edition)

osu1562224291399741
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© 2019, some rights reserved.Creative Commons License A Comprehensive Study of Diffusion and Modulus of Binary Systems within the Ti-Mo-Nb-Ta-Zr System by Zhangqi Chen is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 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.