Fig. 1. Initial microstructure of the as-solidified Co-81.5at.%B eutectic alloy. (a) SEM-BSE image. (b) Higher magnification SEM-ETD image. (c) EBSD phase map and (d) the corresponding Y axis inverse pole figure (IPF) map (Y axis is parallel to the direction of gravity).

Fig. 2. Melt processing history and the corresponding solidification behavior of the Co-81.5at.%B eutectic alloy treated with a high overheating temperature of 1800 K. (a) The temperature and magnetization as a function of time. (b) The enlarged area of the dashed square in Fig. 2(a), showing the solidification process. (c) The corresponding temperature dependence of magnetization curve (M-T curve). (d) The corresponding inverse magnetization as a function of temperature curve (1/M-T curve). The field intensity and gradient at the sample position are 1.56 T and 23.235 T m-1, respectively. The heating and cooling rates are 10 K min-1.

Fig. 3. Melt processing history and the corresponding solidification behavior of the Co-81.5at.%B eutectic alloy treated with a low overheating temperature of 1470 K. (a) The temperature and magnetization as a function of time. (b) The enlarged area of the dashed square in Fig. 3(a), showing the solidification process. (c) The corresponding M-T curve. The inset figure is the enlarged area (dashed square) showing the anomalous magnetism. (d) The corresponding 1/M-T curve. The 1/M-T curve shown in Fig. 2(d) are taken as the reference. The field intensity and gradient at the sample position are 1.56 T and 23.235 T m-1, respectively. The heating and cooling rates are 10 K min-1.

Fig. 4. The corresponding macrographs and the surface morphologies after solidification. (a) and (b) correspond to the processing histories shown in Fig. 2(a) and Fig. 3(a), respectively. (a1) and (b1) are the SEM surface images. (a2) and (b2) are 3-dimensional images showing the surface roughness.

Fig. 5. Longitudinal (parallel to the direction of magnetic field) microstructure of the sample shown in Fig. 4(a). (a) Low-magnification overview. (b) The corresponding XRD pattern. (c) Medium magnification image. (d) The corresponding close-up views of the square region in Fig. 4(c).

Fig. 6. EBSD analysis of the microstructure shown in Fig. 5. (a) SEM-ETD image. (b) Phase map and (c) the corresponding IPF map. The small red rectangle in Fig. 6(b) indicates the location for preparing in-depth TEM lamella by FIB.

Fig. 7. TEM analysis of a FIB lift-out lamella prepared from the location in Fig. 6(b). (a) STEM bright-field image. (b, c) SAD patterns taken from the regions indicated by circles in Fig. 7(a). The indices in the bottom right corner indicate the corresponding zone axes.

Fig. 8. Longitudinal (parallel to the direction of magnetic field) microstructure of the sample shown in Fig. 4(b). (a) Low-magnification overview. (b) The corresponding XRD pattern. (c, d) Medium magnification images of the square regions in Fig. 8(a). (c1, d1) The corresponding close-up views of the squares in Fig. 8(c, d), respectively.

Fig. 9. EBSD analysis of the microstructure shown in Fig. 8. (a-a2) and (b-b2) correspond to the locations shown in Fig. 8(b) and Fig. 8(c), respectively. (a, b) SEM-ETD image. (a1, b1) Phase map and (a2, b2) the corresponding IPF map.

Fig. 10. The mechanical characterization of constituent phases of the two samples. (a) Representative load-displacement curves for a maximum load of 10 mN. (b) The nanohardness H and plastic energy ratio (PE/TE) for different phases as determined by nano-indentations. It can be concluded that a higher H often leads to a lower PE/TE as ‘plastic energy’ actually reflects the work done during indenting that is not recovered during retraction.

Fig. 11. The macroscopic tribological behavior of the two samples. (a) The friction coefficient as a function of time. (b) The wear rate and macroscopic hardness of the two samples. The WLI images (c, d) and the corresponding two-dimensional line scans (e) of the worn surface of two samples, respectively.

Fig. 12. SEM micrographs of the worn surfaces of the two kinds of samples: (a) sample I; (b) sample II. Locally magnified images of the worn surfaces are shown in (c) and (d), respectively. SEM observation at the tribolayer shows the local delamination and peeled-off of debris on the surface of wear scar, indicating the brittle fracture of sample II. On the contrary, multiple parallel ploughs can be observed rather than cracking and delamination behavior.

Fig. 13. SEM images and corresponding EDS mapping of the elemental composition on the worn surfaces for (a) sample I and (b) sample II. It is obvious that a compacted and stable oxide-layer is formed on sample I but only sparse oxide particles founded on the surface of sample II.

Fig. 14. The energy barrier ∆G* dependent on θ (θ=T/Tm-1) for nucleation of sample I with experiencing LLST and sample II without experiencing LLST (marked by the red pentagram). The case with magnetic field-induced LLST in this work and temperature-induced LLST in [33] are described by blue and green lines with symbols, respectively. θ0 is the critical transition point of LLST for Co-18.5at.%B alloy, which could be manipulated by the application of magnetic field.