Fig. 1. XRD pattern of the Al-4.9Nb-1.2B master alloy with Rietveld refinement result.

Fig. 2. (a) Back Scattering Electron (BSE) microstructure of the Al-4.9Nb-1.2B alloy; (b) enlarged image of (a). The images were captured at the position 20 cm above the bottom of the ingot.

Fig. 3. Grain structures in the Al-10Si-0.1Ti-0.02B (a) and Al-10Si-0.1Nb-0.02B (b) ingots; (c) average grain sizes of α-Al.

Fig. 4. (a) Typical α-Al grain in the Al-10Si-0.01Nb-0.02B ingot; (b) enlarged image of the NbB2 nucleation particles in (a) (milled by Focus Ion Beam (FIB)).

Fig. 5. (a) High Angle Annular Dark Field (HAADF) image for the atomic structure of (0 0 0 1) NbB2 surface; (b) HAADF image for the atomic structure of (1 -1 0 0) NbB2 surface; (c1)-(c6) distributions of Nb, B, Al, Si and Fe around the (0 0 0 1) NbB2 surface; (d1)-(d6) distributions of Nb, B, Al, Si and Fe around the (1 -1 0 0) NbB2 surface.

Fig. 6. Nano Si particles adhering to the NbB2 surfaces: (a) bright field image viewing from [1 1-2 0] NbB2; (b) annular bright field image viewing from [0 0 0 1] NbB2.

Fig. 7. (a) HAADF image of the NbB2/α-Al interface; (b) HRTEM image of the NbB2/α-Al interface; (c) composition profiles across the NbB2/α-Al interface; (d) electron diffraction pattern of the area in (b) with [0 0 0 1] NbB2 zone axis; (e) possible unit cell for the Nb-rich compound at the interface (the Al atom substituted for Nb is colored in yellow); (f) simulated electron diffraction pattern with the proposed structure in (e) and the OR of (1 -1 0 0) [1 1-2 0] NbB2//(1 1 0)[1 1 0] NbAl3’.

Fig. 8. Al-rich corner of the isopleth section of the Al-Si-Nb-B system with constant 0.1 wt. %Nb and 0.02 wt. %B; The red line denotes the zero-phase fraction line of NbSi2.

Fig. 9. Evolution of the phase fraction in the Al-10Si-0.1Nb-0.02B system during the Scheil solidification from 800 °C.

Fig. 10. Element contents in NbB2 from 800 °C to 596 °C during the equilibrium (a) and Scheil (b) solidification of the Al-10Si-0.1Nb-0.02B system.

Fig. 11. (a) Supercell of Al-doped (0 0 0 1) NbB2 surface with two potential diffusion paths for Al atoms; (b) energy barriers for Al atoms in different diffusion paths; (c) and (d) are the geometries of transition states 1 and 2, respectively.

Fig. 12. (a) Supercell of Al-doped (1 -1 0 0) NbB2 surface with two diffusion paths for Al atoms; (b) energy barriers for Al atoms in different diffusion paths; (c) to (e) are geometries of the transition states 1 to 3, respectively.

Fig. 13. (a) Schematic of the surface structure of the NbB2 particle covered by NbAl3; (b) atomic structure of the (1 1 0) NbAl3/(1 -1 0 0) NbB2 interface viewing from [0 0 0 1] NbB2 direction; (c) atomic structure of the (0 0 1) NbAl3/(0 0 0 1) NbB2 interface viewing from [1 1-2 0] NbB2 direction.

Fig. 14. Interfacial energies of (1 1 0) NbAl3/(1 -1 0 0) NbB2, (0 0 1) NbAl3/(0 0 0 1) NbB2, and (1 1 2) NbAl3/(0 0 0 1) NbB2 along with the chemical potential of Nb (μNb(interface)-μNb(bulk)).

Fig. 15. Schematic for the grain refining and anti Si-poisoning mechanism of the Al-5Nb-B alloy.

Fig. 16. Equilibrium Al content in MB2 (M = Nb, Ti) of the Al-10Si-0.1M-0.02B system at 700 °C.