Nanostructured amorphous materials represent a novel class of materials characterized by nanostructured amorphous particles and amorphous interfaces between particles. Due to the nanometer scales, amorphous disorder, controllable atomic and electronic structure, this kind of material exhibits numerous unique properties distinct from conventional materials. Recently, scholars discovered that composites composed of nanostructured amorphous materials as the matrix with precipitated nanocrystals exhibited excellent mechanical properties. However, densification of both nanostructured amorphous materials and nanostructured amorphous composites was primarily achieved through ultra-high-pressure forming. During ultra-high-pressure forming, the plastic deformation of nanostructured amorphous powder under high pressure enabled amorphous particles to contact each other, extrude pores, and diffused through interfaces to form new interfaces. However, these studies required high-pressure forming equipment, and the associated costs constrained further applications.

In contrast to the ultra-high-pressure molding mentioned earlier, Rosenflanz et al. employed viscous sintering to consolidate amorphous powder within the supercooled liquid region. This method utilized amorphous powder with low viscosity above the glass transition temperature (T_g) and its viscous flowed under external pressure to fill the pores between particles, thereby achieving densification. This approach only required 34 MPa pressure, effectively circumventing the unrealistically high pressures previously required for amorphous material preparation. However, crystallization during viscous sintering may impede the viscous flow of amorphous powder, necessitating a sufficiently wide kinetic window between the glass transition temperature (T_g) and crystallization temperature (Tx) to achieve densification. To address this issue, Rosenflanz et al. again employed an alumina-rare earth oxide system (Al₂O₃-RE₂O₃) to modulate the polyhedral structure of Al-O, regulated the glass formation ability of the system, substantially widen the kinetic window of the oxide system, and fabricated a range of dense amorphous ceramics. Nonetheless, viscous sintering process has never been reported in the realm of nanostructured amorphous materials.

Amorphous powder of 57.3% Al₂O₃-19.7 %La₂O₃-23 %ZrO₂(ALZ) was synthesized via sol-gel method and subsequently calcined at 800 ℃ for 1 h. The ALZ amorphous powder was then densified using the SPS system under vacuum conditions. Uniaxial compression was applied using graphite punches at the top and bottom, with a loading pressure of 50 MPa. The heating rate from room temperature to sintered temperatures (Ts) was set at 100 ℃/min. The sintered temperatures ranged between 875 ℃ and 1 000 ℃, with a holding time of 5 min. The obtained sample was cylindrical with a diameter of 15 mm and a thickness of 1 mm.

The powders calcined at 800 ℃ for 1 h was amorphous with an average particle size of 11.6 nm, The glass transition temperature (T_g) and crystallization onset temperature (Tx) of the obtained powders are 844 ℃ and 914 ℃, respectively. Numerous pores can be observed in samples sintered at 875 ℃, indicating an incomplete densification at this temperature, whereas no pores were detected in samples sintered at 900 ℃. The relative densities of samples sintered at 875, 900, 927 ℃ and 1 000 ℃ were 87.6%, 98.3%, 96.6% and 91.5%, respectively. The main diffraction peaks in XRD pattern for the sample sintered at 875 ℃ corresponds to La_0.28Zr_0.72O_1.8 with a grain size of 14.2 nm, when sintered at 900 ℃, diffraction peaks of samples corresponded to La₂ (Zr₂O₇) with a grain size of 9.9 nm, which suggested that high degrees of crystallization of La_0.28Zr_0.72O_1.86 sintered at 875 ℃ impeded the viscous flow of the amorphous powder, resulting in a formation of numerous pores. However, the relatively low crystallinity of La₂ (Zr₂O₇) phase sintered at 900 ℃ facilitated the viscous flow of the amorphous powder, and densification was achieved only under a pressure of 50 MPa. Thus, the phase transition formed La_0.28Zr_0.72O_1.86, which owned a higher crystallinity, to La₂(Zr₂O₇) with lower crystallinity was advantageous for the densification of the amorphous powder via viscous flow. A further increase of crystallinity of samples sintered at 927 ℃ and 1 000 ℃ was not conducive to the viscous flow of the amorphous powder, resulted in the formation of numerous pores. STEM-BF images of sample sintered at 900 ℃ exhibited noticeable light-dark contrast. Upon further magnification, distinct lattice fringes can be observed at the interfaces of dark regions, while no lattice fringes were observed in the bright regions. The precipitated crystals were composed of La(AlO₃), La_0.28Zr_0.72O_1.86, and La₂(Zr₂O₇) phases, being consistent with that of XRD results. Both EDS line scanning and mapping revealed that the bright areas were composed of Al-enriched materials, whereas the interfaces between dark areas were Zr and La-enriched materials. Furthermore, the HRTEM image revealed that the nanocrystalline phase precipitates from the amorphous phase matrix.

Nanostructured amorphous ALZ materials with a relative density of 98.3% can be prepared through spark plasma sintering at 900 ℃. Nanocrystalline phases precipitated from the amorphous matrix were observed via STEM-BF and HRTEM, revealing the presence of La(AlO₃), La_0.28Zr_0.72O_1.86, and La₂(Zr₂O₇) phases. The average hardness and Young's modulus of samples were 12.0 GPa and 159.9 GPa, respectively. Meanwhile, samples sintered at 875, 927 ℃ and 1 000 ℃ existed notable pores, being originated from the changes in crystallinity of ALZ under different temperatures. The significant crystallization of La_0.28Zr_0.72O_1.86 sintered at 875 ℃ impeded the viscous flow of amorphous powder. Concurrently, the phase transition from La_0.28Zr_0.72O_1.86 to La₂(Zr₂O₇) with lower crystallinity occurred at 875-900 ℃, which facilitated the viscous flow of amorphous powder. However, a further increase in crystallinity at 927 ℃ and 1 000 ℃ was not conducive to the viscous flow of amorphous powder.