Experimental and computer simulation determination of the structural changes occurring through the liquid–glass transition in Cu–Zr alloys

Abstract

Molecular dynamics (MD) simulations were performed of the structural changes occurring through the liquid–glass transition in Cu–Zr alloys. The total scattering functions (TSF), and their associated primary diffuse scattering peak positions (K _p), heights (K _h) and full-widths at half maximum (K _FWHM) were used as metrics to compare the simulations to high-energy X-ray scattering data. The residuals of difference between the model and experimental TSFs are ∼0.03 for the liquids and about 0.07 for the glasses. Over the compositional range studied, Zr_{1− x} Cu _x (0.1 ≤ x ≤ 0.9), K _p, K _h and K _FWHM show a strong dependence on composition and temperature. The simulation and experimental data correlate well between each other. MD simulation revealed that the Cu–Zr bonds undergo the largest changes during cooling of the liquid, whereas the Cu–Cu bonds change the least. Changes in the partial-pair correlations are more readily seen in the second and third shells. The Voronoi polyhedra (VP) in glasses are dominated by only a few select types that are compositionally dependent. The relative concentrations of the dominant VPs rapidly change in their relative proportion in the deeply undercooled liquid. The experimentally determined region of best glass formability, x _Cu ∼ 65%, shows the largest temperature dependent changes for the deeply undercooled liquid in the MD simulation. This region also exhibits very strong temperature dependence for the diffusivity and the total energy of the system. These data point to a strong topological change in the best glass-forming alloys and a concurrent change in the VP chemistry in the deeply undercooled liquid.

Keywords:

• molecular dynamics simulation
• X-ray diffraction
• liquid metal
• amorphous alloy
• liquid–glass transition

Acknowledgements

Work at the Ames Laboratory was supported by the Department of Energy, Office of Basic Energy Sciences, under Contract No. DE-AC02-07CH11358. The high-energy X-ray work at the MUCAT sector of the APS was supported by the US Department of Energy, Office of Science, Basic Energy Sciences under Contract No. DE-AC02-06CH11357. Ames Laboratory is operated for the US Department of Energy by Iowa State University under Contract No. DE-AC02-07CH11358. The work at Washington University was partially supported by the National Science Foundation under Grant Nos. DMR-0606065 and DMR-0856199, and by NASA under Contract No. NNX07AK27G.