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Simulated elastic and magnetic properties of Iron-based metallic glass and glass-derived crystals

Simulated elastic and magnetic properties of Iron-based metallic glass and glass-derived crystals. with Ganesh Panchapakesan, Marek Mihalkovic (CMU) Gary Shiflet, Joe Poon, Despina Louca, Slava Kazimirov (UVa). Outline: First-principles calculations Phase stability

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Simulated elastic and magnetic properties of Iron-based metallic glass and glass-derived crystals

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  1. Simulated elastic and magnetic properties of Iron-based metallic glass and glass-derived crystals with Ganesh Panchapakesan, Marek Mihalkovic (CMU) Gary Shiflet, Joe Poon, Despina Louca, Slava Kazimirov (UVa) • Outline: • First-principles calculations • Phase stability • An approach to glass formability • Novel glass-derived soft magnetic materials • Vibrational and elastic behavior • Measuring bond stiffness

  2. First-principles calculations • Solves the quantum problem of atomic interactions • Highly realistic, captures essential chemical properties • Yields energies, forces, band structure, ..... Simulated amorphous Fe64B6C15Mo15 X-ray partial PDF’s Neutron partial PDF’s

  3. Self-Consistent Charge Density Mo2@CFe3.oP16  Fe Mo C Mo Fe C

  4. B-C Density of states relates to EELS experiments (Reinke,Buettner)

  5. Prediction of phase stability • Cohesive energy predicts low temperature stability • Can combine FP energies with CALPHAD method at high T (with Shiflet and Gao) • FP can predict previously unknown crystal structures • Can predict trends in stability with atomic substitution  Pearson crystal notation: cF116  cubic Face-centered 116 atoms per unit cell Hypothetical Known stable Known high T Known metastable

  6. Glass formability C6Cr23.cF116 prototype is chief competitor to Fe-based metallic glass. Known as metastable in (B,C)6Fe23. Can destabilize with large atoms (e.g. Y or Er). C6Fe23Y0: E = 40 (meV/atom) C6Fe22Y1: E = 85 (meV/atom) C6Fe22Y2: E = 138 (meV/atom) Jason Wang & Gary Shiflet

  7. Two cF116 Structures!(Fe,Co)23Zr6 and (Fe,Co)23B6 Fe23B6 Fe23Zr6 Zr or B atoms occupy NaCl-type networks of small and large octahedra Depending on relative amounts of Zr,B one phase or other emerges as secondary crystallization of (Fe,Co)-Zr-B metallic glass. Can we stabilize these phases for applications as soft magnetic materials? (Paul Ohodnicki and Mike McHenry)

  8. Characterizing mechanical properties Elastic moduli (Bulk (K) and shear (G) Poisson Ratio  = (3K-2G)/(6K+2G) Ductility likely for high Poisson Ratio (low G/K)

  9. Characterizing mechanical properties Vibrational density of states relates to interatomic bonding Low frequency limit linked to elastic moduli through sound speed cF116 vibrational density of states

  10. Characterizing bond stiffness Compression/dilation strain=a/a Bond length L changes by L Strain Ratio == (L/L)/(a/a) -----------------Bond Ratio ----------------- CMo  0.55 BFe  0.91 CFe 0.83 BMo  0.89 BEr 0.98 FeFe 1.02FeMo 1.04 ErFe 1.15 CEr 1.24

  11. Conclusions and outlook FP methods accurately predict relevant properties Can address atomistic structure and stability of complex alloys Can predict electronic, magnetic and mechanical properties Linking properties to structure requires ingenuity Nondestructive evaluation depends on understanding interactions of matter with radiation, forces, etc. Atomic level modeling is necessary to achieve this understanding.

  12. Tempering Molecular Dynamics Swap temperatures of runs with probability P~exp(-E*(kBT)-1) Tl=1423, Tg=777, Tx=819

  13. Difference Between Structures Differentiated By the Decoration of the Octahedra With the TM (Fe,Co) Atoms Each structure has 4 unique TM sites (Fe,Co)23Zr6 (Fe,Co)23B6 By stabilizing the structure, we could investigate the magnetic properties of these cubic phases which could be potentially interesting for soft magnetic applications. Question: Can Zr be accommodated into the B-based structure and B into the Zr-based structure?

  14. Charge Transfer (* Interpolated value for Mo, standard = 2.16)

  15. C (s) + Fe (spd) C (p) Fe (spd) + Mo (d) Electronic Density of States

  16. COOP: Crystal Orbital Overlap Population (Hoffmann ~1983) COOP

  17. COHP: Crystal Orbital Hamilton Population (Dronskowski & Blöchl ~ 1993) COHP

  18. Energy-projected (differential) COHP Mo2@CFe3.oP16 (TB-LMTO) Bonding Antibonding

  19. Total (integrated) COHP Mo2@CFe3.oP16 (TB-LMTO)

  20. i-COHP values (eV/bond) in BC7Cr2Fe18Mo4.oP16 (TB-LMTO)

  21. Classical potentials via force-matching V(R)=(a/R)b+(c/Rd)*cos(e*R+f)

  22. Classical pair potentials V(R)=(a/R)b+(c/Rd)*cos(e*R+f)

  23. Comparison of potentials vs. full first-principles MD

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