Crystal-Like Rearrangements of Icosahedra in Simulated Copper-Zirconium Metallic Glasses and their Effect on Mechanical Properties

Indications of the ${\mathrm{Cu}}_2\mathrm{Zr}$ Laves phase are observed in MD simulations of amorphous ${\mathrm{Cu}}_{64}{\mathrm{Zr}}_{36}$ upon isothermal holding just above the glass transition temperature. The structural evolution towards ${\mathrm{Cu}}_2\mathrm{Zr}$ is accompanied by an increase in the fraction of Cu-centered icosahedra, which demonstrates that a large icosahedral fraction does not just indicate structural relaxation. The crystal-like regions generate an increase in strength and Young’s modulus, and a stronger localized shear band. A universal relation between the fraction of full icosahedra and their interconnectivity is found, and both can be modified simultaneously via changes of temperature or strain.

Figure 1

(a) Slice of 1 nm thickness (Cu: red; Zr: blue) of the annealed sample. A crystal-like region oriented with its $c$ axis along the viewing direction is seen (black encircled), revealing the sixfold symmetry of the ${\mathrm{Cu}}_2\mathrm{Zr}$ Laves phase. Similar regions can be visually identified by tilting the sample box and adjusting the slice thickness. (b) The same atomic configuration, where only FI-center atoms are shown, colored according to their interconnectivity $n$ ($<4$: gray, 4: blue, 5: red, 6: green, 7: yellow). Atoms with high interconnectivity are arranged in crystal-like regions while atoms with $n<4$ occupy the space in between. (c) Total neutron structure factor $S(q)$ for the as-cast (blue) and annealed (red) samples at 50 K.

Figure 2

(a) Stress-strain curves of the as-cast (blue) and annealed (red) states. A larger Young’s modulus and greater yield strength are observed for the annealed sample. The insets show a selected crystal-like region (Cu: red; Zr: blue) within the SB region before and after deformation. (b),(c) SBs in as-cast and annealed ${\mathrm{Cu}}_{64}{\mathrm{Zr}}_{36}$ after deformation and unloading. Atoms are colored according to their local atomic shear strain ${\eta }_{\text{Mises}}$ (logarithmic scale) and only atoms with a local strain greater than 0.2 are shown. In the annealed state the strain is more strongly localized in the SB and less strain is carried by the matrix.

Figure 3

FI fraction $f_{\mathrm{icos}}$, local atomic shear strain ${\eta }_{\text{Mises}}$, and interconnectivity $n$ as a function of the distance from the lateral center of the SB for the as-cast (blue) and annealed (red) samples. The solid horizontal lines in the upper image represent the average FI fractions for the undeformed bulk samples. The SBs are characterized by a low FI fraction, large local strain, and low interconnectivity.

Figure 4

Average interconnectivity $n$ as a function of FI fraction for deformed as-cast (blue) and annealed (red) samples at 50 K. The undeformed data are taken from Ref. [9] and represent states at different temperatures and after different thermal treatments. All data collapse on a universal curve.