Nanostructured amorphous Al2O3-La2O3 composites through controlled spark plasma sintering based on super-cooled liquid region

Nanostructured amorphous composites exhibit superior mechanical properties compared to crystalline materials due to their disordered atomic arrangement and high defect density. Spark plasma sintering (SPS) is a suitable technique for in-situ synthesis of nano-amorphous composites, allowing densification of amorphous powders via viscous sintering. In this work, amorphous nanopowders of Al 2 O 3 -La 2 O 3 (AL), Al 2 O 3 -La 2 O 3 -ZrO 2 (ALZ), and Al 2 O 3 -ZrO 2 (AZ) were prepared using sol-gel method, followed by densification into nanostructured amorphous composites via SPS in the super-cooled liquid region (SCLR). The AL sample demonstrates a uniform structure, achieving the maximum relative density of 98.1 % along with minimum crystallinity of 30.22 % after SPS at 890 °C. However, with increasing SPS temperature, the relative density of AL sample decreases instead of increasing. This phenomenon is due to the promotion of amorphous-to-crystalline phase transition at high temperatures, which inhibits the viscous flow of amorphous phase during SPS and hinders the densification. Attributed to the narrow SCLR of AZ and ALZ systems, they are faced with the competition between crystallization and structural densification. The crystallinity values are 64.86 % and 51.16 % for AZ and ALZ systems, while the maximum relative densities were 91.9 % and 96.6 %. The SCLR is confirmed as a primary factor to regulate the competition between relative density and crystallization of amorphous powders, and AL system with widest SCLR ( Δ T x  =  T x - T g  = 72 °C) is beneficial to the densification of nanostructured amorphous composites. Furthermore, STEM-BF and HRTEM reveal nanocrystalline precipitation from amorphous phase matrix during sintering process, and the nanoindentation tests indicate superior mechanical propriety of AL nanostructured amorphous composites.