After machining, the material in the chip depicts a strong structure refinement. Figure 1 shows a channeling contrast focused ion beam (FIB)18 image of the cross-section of the chip. The surface being in contact with the tool is at the top of the image, red dashed line separates two areas with distinctly different structure. Under the line, the microstructure reveals a shear direction with a parallel distribution of the long axis of the crystals. Closer to the surface, where the effects of friction should be more evident the crystallites are very small and do not show signs of a particular shear direction. This structure is typical for the material undergone dynamic recrystallization (DRX) in FIN samples and is known as a “white layer” in corresponding literature19. Both sheared and recrystallized structures have been mechanically tested in the areas indicated by yellow boxes. The pillar compression tests covered both recrystallized and shared material, while beam bending was only performed at the very surface sub-100 nm grained material due to necessity for side access.
In Fig. 1a a certain gradient of grain sizes can be observed, from sub-micron at the boundary of the DRX layer and down to 80 nanometers in the top surface (Fig. 1b), which comparing to the original material structure denotes a two orders of magnitude reduction in grain size. Moreover, in contrast to the original material, which has pearlite colonies in 75% of the volume, the recrystallized material presents a lack of cementite lamellae and a few visible carbide particles (see red circles in Fig. 1b and Fig. 2). FEM simulations with similar conditions suggested that the recrystallized area develop strains over 300% (see details in Methods), introducing energy to initiate a DRX process.
DRX area has been analyzed by scanning transmission electron microscopy (STEM). Figure 2 (left) shows A STEM image of a few grains from DRX layer, and corresponding EDX maps for iron (center) and carbon (right) obtained from the same region. With the exception of a small particle, the chemical analysis revealed that carbon is homogeneously distributed without segregation in the grain boundaries.
The crystallographic structure of the recrystallized and sheared area has been further analyzed by (transmission) electron back scatter diffraction ((t)-EBSD) in a scanning electron microscope (SEM). Figure 3a shows t-EBSD map of the material at the very surface of DRX layer (depth increases from top to the bottom of the map), Fig. 3b shows corresponding kernel average misorientation (KAM) map. EBSD data on Fig. 3 (c) and (d) provide a broader view of DRX layer on top and shared region at the bottom. The sheared layer shows a structure with a prominent shear deformation, where grains/sub-grains with similar orientation are aligned in lamellae at oblique angle to the top surface. In contrast, the crystals in DRX layer have equiaxial shape without prominent signs of shear. Pole figures (Fig. 3e,f) show significantly more prominent crystal texture of the sheared layer as compared to DRX. KAM map is an illustrative indicator of the density of geometrically necessary dislocations20. Dislocation density is visually substantially lower in DRX layer (Fig. 3b,d) confirming structure reorganization (internal strain release) by re-crystallization.
For the mechanical tests, pillars of 6, 3 and 2 µm of diameter have been milled by FIB on the cross-section at the very surface region and down to 63 µm deep into the sheared layer. The pillars heigh for each case is 3 times the diameter. For each case, pillar strength has been calculated at a strain of 5%. Figure 4 shows the values of strength depending on the distance of the center of the pillar to the edge. The pillar diameter did not make a notable influence on the strength, indicating no size-effect on the measurements. In contrast, the strength strongly depends on the distance to the surface, i.e. on the grain size and on the strain state of the material. Figure 5 shows individual stress-strain plots of 2 µm diameter pillars located at different distances from the surface.
The curve representing the sheared layer (29 µm to the edge) demonstrates the maximum strength value around 1300 MPa, while at the top of the DRX layer (2.5 µm to the edge) the maximum strength may exceed 2000 MPa. FIB images of compressed pillars (Fig. 6) reveal the difference in deformation behavior of the pillars: while nanocrystalline pillar from DRX layer shows a regular structure of nano-grains that deform homogeneously, the pillar from the 29 µm depth contains the deformed pearlitic layers which determine the easy sliding plane.
One of the features observed on the curves on Fig. 5 is a light reduction of the elastic modulus in the pillars in the recrystallized area. Variation of elastic modulus can indicate a fundamental change in atomic structure and/or interatomic distances21, thus pointing to some redistribution of the components in the lattice.
However, determining the elastic modulus in micro-compression experiments leads hardly reproducible results, small misalignments use to change notably the results. In order to obtain an accurate value of the elastic modulus beam bending experiments were performed in the proximity of the edge, inside the area affected by DRX. The method of beam bending was previously demonstrated to be suitable for elastic modulus measurement at small scales22. It provides good sensitivity to modulus calculation and, contrary to pillar compression, it is relatively agnostic to small geometric misalignments. Beams with a width of 5 µm and 25 µm long have been tested by flexion in-situ in SEM (Fig. 7). Calculations of elastic modulus have been made following the procedure described by Demir et al.23. The set of beam bending tests has led to reproducible values of elastic modulus rounding 178 GPa, with a standard deviation of 5 GPa.