Shear-induced chemical segregation in a Fe-based bulk metallic glass at room temperature

Shear-induced segregation, by particle size, is known in the flow of colloids and granular media, but is unexpected at the atomic level in the deformation of solid materials, especially at room temperature. In nanoscale wear tests of an Fe-based bulk metallic glass at room temperature, without significant surface heating, we find that intense shear localization under a scanned indenter tip can induce strong segregation of a dilute large-atom solute (Y) to planar regions that then crystallize as a Y-rich solid solution. There is stiffening of the material, and the underlying chemical and structural effects are characterized by transmission electron microscopy. The key influence of the soft Fe–Y interatomic interaction is investigated by ab-initio calculation. The driving force for the induced segregation, and its mechanisms, are considered by comparison with effects in other sheared media.

where the solid lines are calculated using the method proposed earlier [1], while fitted areas represent separate calculated peaks. Carbon can hardly be separated from surface-absorbed carbon, though traces of carbides were formed. The boron content was too low to be detected. Non-destructive chemical and phase depth-profiling of nano-sized films in this investigation was carried out using the standard method [1]. This method enables determination of the depth profiles with a sub-monolayer accuracy from XPS data. The determined chemical composition of the 1.7 nm thick oxide layer is shown in Table S1. All four constituent metals were found to be present in the form of surface oxides.

S2.1. Diamond tips
An example of the diamond tips used for the scratch-wear tests is shown in Fig. S3.

S2.2. Normalized pressure and temperature calculations
In order to check whether temperature rise is likely during the wear, we consider the wear maps [2]. The normalized sliding velocity (') is given by: where v is the sliding velocity used in the test (1 m s -1 ),  is the thermal diffusivity of the BMG (~2 mm 2 s -1 ) [3] and r is the radius of the circular nominal contact area (~25 nm). The resulting value of ~910 -9 lies firmly in the regime of mild wear.
The normalized pressure P ' is given by: where F is the applied normal load, A is the nominal contact area of the wearing surface, (2×10 3 nm 2 estimated from the scratch width), and H V is the room-temperature Vickers hardness (12 GPa]) of the BMG sample. The resulting normalized pressure at 1 µN average load is 5×10 -2 which corresponds to delamination wear conditions.
The contact surface temperature (T s ) is given by: where  is the coefficient of friction,  is close to one, T* is the equivalent temperature T*=aH v /K m , and K m is the thermal conductivity (~12 W m -1 K -1 [4]). According to the calculations, the normalized pressure is 10 -2 and the normalized speed 5×10 -8 . Substituting values into Eq. (2), the temperature rise is found to be negligible (~10 -7 K).

S2.3. Finite-element modeling
The stress distribution inside the BMG sample in three dimensions loaded with a diamond tip 50 nm in diameter was modeled with the finite-element modeling (FEM) software DEFORM.
The specimen was divided into 30,000 tetrahedral elements. The elastic properties of the BMG were taken to be: E, 210 GPa; G, 80 GPa; B, 200 GPa; HV, 12 GPa; Poisson ratio, 0.325. These values, together and the stress-strain curves [5,6], were used for the FEM calculations together with elastic properties of diamond.

S3. Comparison with a typical Zr-based BMG
No strain hardening or increase in the Young modulus E is found for a Zr 62 Cu 22 Fe 5 Al 10 BMG also tested in oscillating mode as Fe-based BMG (Fig. S4). No crystallization is found. The profile is evidently very different from that shown in the main text ( Fig. 2 and Fig. 3) for the Fe-based BMG in the present work.
S4. An atomic cell simulated using first-principles calculations Ab-initio simulation of BCC Y containing 128 atoms (4 Cr, 10 Fe, 6 Mo and 108 Y) confirmed that the lattice parameter of such a solution containing 16 at.% of the solute elements is 380 pm which is quite close to the experimentally measured value. The resulting atomic cell is shown in Fig. S6. We modeled shear deformation in a Fe 95 Y 5 model glassy alloy as described in the Methods 8 section. Corresponding video file is also attached to the manuscript.