MD study of Cu and Zr adlayer deposition on Cu 46 Zr 54 BMG: structural and vibrational properties

Transcription

1 Physics Department University of Ioannina (UOI) Greece Andrius Ibenskas MRTN-CT MD study of Cu and Zr adlayer deposition on Cu 46 Zr 54 BMG: structural and vibrational properties Metallic Glasses" 1

2 Collaborators Prof. G. A. Evangelakis Physics Department, University of Ioannina, Greece Dr. D. G. Papageorgiou Dr. Ch. E. Lekka Department of Materials Science and Engineering, University of Ioannina, Greece Metallic Glasses" 2

3 Outline Computational setup Surface structural evolution during Zr and Cu adlayer deposition on Cu 46 Zr 54 BMG: Zr and Cu content Atomic structure by Common Neighbor Analysis (CNA) Zr and Cu diffusion in deposited adlayers Vibrational properties Conclusions Metallic Glasses" 3

4 Computational setup 1. Semi-empirical model in analogy to the Tight Binding Scheme in the Second Moment approximation 2. System consisted of 128 atoms that initially were arranged in B2 type CuZr cubic lattice 3. Cu 46 Zr 54 BMG was prepared by substituting 4 % of Cu atoms with Zr, followed by heating to 2 K and cooling down to 3 K 4. Equations of motion were integrated by means of Verlet s algorithm and a time step 5 fs. 5. Zr atoms were randomly deposited on Face 1, and Cu atoms on Face 2, at first 1 Zr/Cu atoms, then 1 again, 2, and finally 6 atoms, in the microcanonical ensemble. Metallic Glasses" 4

5 1 Surface structural evolution: Zr deposition P (%) 8 6 Zr (Face 1) Cu (Face 2) Zr layering is clearly observed in the local density profile, Cu content in Zr adlayer is very small Number of deposited atoms Local density profile (1) (2) (3) (4) (5) (6) T=3K T=4K T=5K T=6K 2-8. Metallic Glasses" Distance along z direction (nm)

6 1 Surface structural evolution: Cu deposition P (%) Zr (Face 1) Cu (Face 2) Number of deposited atoms 8 P(n) function shows rapid migration of Zr atoms into the Cu adlayer, which leads to atomic mixing. Layering is less pronounced: T=3K T=4K T=5K T=6K (3) (2) (1) Local density profile 6 4 (4) 2 6. Metallic Glasses" Distance along z direction (nm)

7 Surface structural evolution by CNA: Zr deposition Number of atom pairs with index 421 increases substantially, while 322, 433 and others decrease. This is an evidence of HCP surface structure. At t=, percentage of 421 and 422 pairs is about the same, which means that bulk-adlayer interface exhibits HCP structure, too. Percent (%) CNA Common Neighbor Analysis Amorphous T=5 K MC-RTN 1 European School 2 "Bulk Metallic Glasses" Time (ps) 7

8 Surface structural evolution by CNA: Cu deposition Indices, corresponding to amorphous structure, and index 311 increase significantly. Other indices do not change much. Atomic order in Cu adlayer is most probably FCC-like truncated or even mixed, especially in bulk-adlayer interface, where 433 is the major CNA index. Percent (%) T=5 K Amorphous MC-RTN 1 European School 2 "Bulk Metallic Glasses" Time (ps) 8

9 Zr and Cu diffusion in deposited adlayers Diffusivity (mean square displacement) of both species is quite low and it rapidly reaches a steady state. <Δr 2 > (nm 2 ) Zr (Face 1) Cu (Face 2) T=5 K Green-Kubo relation for mean square displacement: N 2 1 r r δr i i N () τ = r () t r ( t + τ) i= 1 2 At 6 K, diffusion coefficients of Cu and Zr are of order 1-5 cm 2 /s and 1-6 cm 2 /s (not displayed here), respectively, which is consistent with experimental findings..5 1MC-RTN European School 1.5 "Bulk 2 Time (ps) Metallic Glasses" 9

10 Vibrational properties of Zr and Cu adlayers Vibration amplitudes <u 2 > of Zr and Cu atoms are larger, compared to those measured experimentally, but they decrease with depth Zr (Face 1) Cu (Face 2) <u 2 > (nm 2 ) MC-RTN 3European School 4 "Bulk Metallic Glasses" Layer number 1

11 Vibrational properties of Zr and Cu adlayers A steep increase in Cu atoms vibration amplitude above 5 K shows a structural transition of Face 2 into amorphous state:.3.25 Zr (Face 1) Cu (Face 2) <u 2 > (nm 2 ) Temperature Metallic Glasses" (K) 11

12 Conclusions MD simulation of Zr and Cu adlayers on Cu 46 Zr 54 bulk metallic glass revealed some interesting properties. Crystalline Zr layers are formed in case of Zr deposition Cu deposition leads to partially crystallized surface with mixed composition At the very early stage of deposition, very fast local atomic rearrangement occurs at both interfaces However, atomic diffusion is slow up to T=5 K Above 5 K, mixed adlayers undergo reconstruction that is driven by Cu diffusion in amorphous phase Metallic Glasses" 12

13 Thank you for your kind attention! Metallic Glasses" 13