Fig. 1 SEM images of Al powders with average diameter of a 1.4 μm, b 2.3 μm, c 4.6 μm and d 7.0 μm, and the size distribution of e 1.4 μm, f 2.3 μm, g 4.6 μm and h 7.0 μm Al powders

Fig. 2 OM images of the as-extruded (B4C + Al2O3)/Al composites: a AE 1.4, b AE 2.3, c AE 4.6, d AE 7.0

Fig. 3 STEM-BF images of a AP 1.4 and d AP 2.3, and the distribution of elements of b, c AP 1.4 and e, f AP 2.3; yellow arrows denote the original Al powder boundaries, and red arrows denote the GBs formed in the hot-pressing process

Fig. 4 a STEM-BF image and the element distribution of b Al, c O, d B, e C of AP 7.0; yellow arrows denote the original Al powder boundaries, and red arrows denote the GBs formed in the hot-pressing process

Fig. 5 TEM images of as-pressed (B4C + Al2O3)/Al composites: a AP 1.4, b AP 2.3, c AP 4.6 and d AP 7.0; the insets show Al2O3 with higher magnification

Fig. 6 TEM images of a AE 1.4, b AE 2.3, c AE 4.6 and d AE 7.0; the insets showing the corresponding statistical results of the grain size

Fig. 7 Engineering stress-strain tensile curves of as-extruded composites at a RT and b 350 °C

Fig. 8 SEM fractographs of a AE 1.4, b AE 2.3, c AE 4.6 and d AE 7.0 at RT

Fig. 9 SEM fractographs of a AE 1.4, b AE 2.3, c AE 4.6 and d AE 7.0 at 350 °C; white arrows denote Al2O3 particles

Fig. 10 TEM images showing the microstructure of a AE 1.4 and b AE 7.0 after tensile testing at 350 °C

Fig. 11 Schematic drawing of high-temperature strengthening effect of Al2O3: a AE 1.4 and b AE 7.0