Search

Unidirectional freeze casting followed by sintering and infiltration treatments has been proved to be a promising method to produce lamellar composites with good strength and toughness. However, it remains a great challenge to tailor the microstructure, particularly the lamellar orientation at the centimeter scale and the architectural features over multiple scales. Herein, we prepared Al/(Al 2 O 3 –TiC) composites with a long-range ordered lamellar architecture via bidirectional freeze-casting and melt-infiltration techniques. The incorporation of a certain amount of TiC (∼3 μm in diameter) into the Al 2 O 3 slurry increased the solidification velocity and decreased the ceramic layer thickness. Furthermore, it greatly facilitated the infiltration of liquid Al into the interlamellar channels and in-layer cavities, thus improving the mechanical properties of the composites. The maximum flexural strength (474 ± 8 MPa) and crack-growth toughness (42.7 ± 2.7 MPa m1/2) appeared in the composite containing ∼27 vol% ceramics with Al 2 O 3 :TiC = 5:5. Moreover, the composites exhibited anisotropic mechanical properties. The flexural strength loaded in the direction parallel to the ceramic layers was higher than that perpendicular to them, while the toughness exhibited an opposite trend. The main toughening mechanisms included crack blunting, crack deflection, plastic deformation of the metal layers and multiple cracking. This work offers a cost-effective and scalable method for the fabrication of laminated composites with exceptional damage tolerance. [Display omitted] • A new strategy is proposed to construct 3D-aligned lamellar hierarchical structure. • The presence of TiC facilitates the infiltration of Al into the ceramic lamellae. • Increasing TiC:Al 2 O 3 ratio reduces layer thickness and promotes multiple cracking. • Enhanced strength and toughness stem from the optimized structure and composition. [ABSTRACT FROM AUTHOR]

We constructed a nacre-inspired laminate-reticular hierarchical architecture in Cu/(TiC−Cr 3 C 2) composites by tuning the wettability between loose freeze-cast ceramic lamellae and Cu melt. This unique architecture takes full advantage of superb intrinsic strength of the ceramic particles and toughness of the metal, stimulating multiple extrinsic toughening mechanisms such as synergistic deformation of the metal and ceramic layers, ductile-ligament bridging and multi-cracking mode. As a consequence, the resultant composites containing ~30 vol% ceramics exhibited exceptional damage-tolerant properties with a fracture toughness of 62.6 ± 1.0 MPa⋅m1/2 and a bending strength of 639 ± 27 MPa. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Nacre-inspired laminated composites have been proven to possess a unique combination of strength and toughness. In this study, we fabricated nacre-mimetic Cu/TiC composites via unidirectional freezing of aqueous TiC slurries containing different amounts of NiO additives, followed by ice sublimation, carbothermal reduction of NiO to Ni during sintering and then gas-pressure infiltration of the Cu melt. The introduction of Ni greatly facilitated the densification of ceramic lamellae and enhanced the interfacial bonding between Cu and TiC. The resultant composites displayed outstanding damage tolerance and anisotropic electrical conductivities. Specifically, for an ∼31 vol% TiC–Cu composite containing 24 wt% Ni in the ceramic lamellae (based on the TiC content), a fracture toughness (K Jc) of 72.5 ± 1.0 MPa·m1/2, work of fracture of 53.4 ± 3.5 kJ/m2, bending strength of 725 ± 11 MPa and longitudinal electrical conductivity of 22.7 MS/m (∼60% of the Cu matrix) were achieved, which were approx. 81%, 536%, 122% and 97% higher than those of the Ni-free composite, respectively. Noticeable toughening was demonstrated to be a consequence of multiple cracking, plastic deformation and uncracked-ligament bridging of the metal layers, as well as crack deflection and blunting. On the other hand, significant strengthening resulted from tailoring the microstructures in the ceramic layers and at the Cu/TiC interface as a result of Ni doping. We believe that the facile strategy adopted herein provides an effective way to solve the problems of wetting and bonding related to metal infiltration and can be readily extended to the preparation of other nacre-inspired metal−ceramic composites. [ABSTRACT FROM AUTHOR]

We developed a novel and simple bidirectional freeze-casting technique to prepare porous ceramics with long-range ordered lamellar structures in three dimensions (3D). A side copper or stainless-steel plate was used as the cold source to generate dual temperature gradients for the control of the nucleation and growth of ice crystals. The Al 2 O 3 architectures prepared with the side copper plate exhibited well-assembled lamellar structures with abundant cross connections of the ceramic lamellae. The compressive strength of the resultant Al 2 O 3 architectures with ∼ 50% porosity reached 158 ± 3 MPa in the primary freezing direction and 42 ± 6 MPa in the secondary freezing direction, ∼ 32% and ∼ 130% higher than that of the counterparts prepared by unidirectional freeze casting. In contrast, the Al 2 O 3 architectures prepared with a side stainless-steel plate exhibited thicker ceramic lamellae and wider pore channels as well as decreased cross connections, leading to much weaker compressive strength. [ABSTRACT FROM AUTHOR]

We prepared a large-scale aligned structure in Al 2 O 3 porous ceramics by a simple and novel bidirectional freeze-casting technique. This technique uses a side metal plate to achieve cold-surface cooling from the bottom to generate dual temperature gradients to control the nucleation and growth of ice crystals. By recording the slurry temperature in situ, we obtained various temperature gradients for the samples prepared by this technique using 3 mm thick copper, aluminium and stainless-steel plates as the cold surfaces. The architectures of the scaffolds produced under these freezing conditions were investigated to determine the effect of the temperature gradients on the morphology of the microstructure. Finally, a model was proposed to account for the temperature gradient regulatory mechanism of the transition from a dendritic structure to a parallel structure. [ABSTRACT FROM AUTHOR]

[Display omitted] • A simple method is proposed to construct a nacre-inspired "brick–mortar" structure. • The feature of freeze-cast scaffolds depends notably on ceramic particle size. • In-situ reaction increases the hard-phase content in composites up to 86 vol%. • The presence of "mortars" promotes multiple cracking and improves toughness. The construction of a hierarchically ordered "brick-and-mortar" structure with a high content of "brick" phase is a key challenge in the preparation of nacre-mimetic composites. Herein, inspired by the transverse cracks in the ceramic lamellae after freeze-drying caused by the secondary nucleation of ice crystals during directional freezing of water-based ceramic slurry, we propose a new idea to use these structural defects and an in-situ reaction to construct composites with a nacre-like "brick-and-mortar" structure and high hard-phase content. This uses ice-templating to first prepare lamellar TiO 2 scaffolds with transverse cracks that separated the ceramic lamellae into "brick" whose size was controlled by adjusting the initial ceramic particle size (50, 300, and 700 nm). After the reactive infiltration, these cracks were filled with molten Al, leading to the formation of a "brick-and-mortar" structure with a high hard-phase content (86 vol%). The increasing ceramic particle size reduced the hard-phase content but significantly increased the aspect ratio (height/width) of the ceramic "brick" and the fracture toughness. The excellent damage tolerance was attributed to multiple toughening mechanisms, i.e. , multiple cracking, crack deflection, and uncracked-ligament bridging of the metal layers. The existence of "mortar" increased the probability of the formation of multiple cracking. [ABSTRACT FROM AUTHOR]

A nacre-like magnesium matrix composite with high carbon nanotube (CNT) contents (up to 4.40 wt%) was successfully prepared under the combined effects of bidirectional freeze casting, pressureless infiltration, and hot pressing. The bidirectional freeze casting was carried out to fabricate large-scale parallelly layered CNT scaffolds. The spontaneous infiltration of an AZ91 magnesium alloy was achieved using nano-silica as the wetting promoter, and MgO and Mg 2 Si (reaction products) provided the strength of the as-synthesized composites. Furthermore, the hot pressing increased the average thickness ratio of reinforcing phase/alloy layers in the AZ91/CNT composites from 0.89 (before hot pressing) to 1.46 (after hot pressing) and also significantly improved the specific strength from 142.3 MPa/(g⋅cm−3) to 158.7 MPa/(g⋅cm−3) without sacrificing the specific toughness ((17–18) MPa⋅m1/2/(g⋅cm−3)). The resultant composites were mainly strengthened by CNTs, reaction products, and the strong interfacial bonding between CNTs and the Mg matrix. The key toughening mechanisms for the as-prepared composites were crack deflection, branching, and CNT pull-out. This study provides a new idea for the economical and efficient preparation of lightweight and damage-tolerant composites with high CNT contents. Image 1091852 [ABSTRACT FROM AUTHOR]

We prepared Al/TiC composites with different ceramic volume fractions (15, 25 and 35 vol%) using ice-templating and pressure infiltration. The thickness of the lamellar layer and the porosity in the ceramic layer of the TiC scaffolds were controlled by varying the slurry concentration. The Al/15 vol%TiC composite had a thick metal layer and a low-density ceramic layer, which effectively dissipated the stress at the crack tip and fractured in a multiple-crack-propagation mode, giving bending strength of 355±3 MPa and fracture toughness of 81±2 MPa m 1/2 . However, the Al/25 vol%TiC and Al/35 vol%TiC composites had much higher bending strength (417−500 MPa) but lower fracture toughness (46−33 MPa m 1/2 ) as compared to the Al/15 vol%TiC composite, and they fractured in a single-crack-propagation mode. In addition, an increase in the brittle TiAl 3 phase with increasing ceramic volume at the fracture surface greatly deteriorated the toughness of the Al/TiC composites. Finally, the relationship between cracking mode and structure features in the laminated composites was discussed to account for the toughening mechanism. [ABSTRACT FROM AUTHOR]

Using freeze casting and pressureless infiltration methods, we prepared lamellar Al−Si−Mg/Al 2 O 3 −ZrO 2 composites with initial ceramic loading of 30 vol% and different Al 2 O 3 :ZrO 2 weight ratios (Al 2 O 3 :ZrO 2 =1:9, 3:7, 5:5, 7:3 and 9:1). The resultant composites inherited the lamellar structure of the Al 2 O 3 −ZrO 2 scaffolds, and the thickness of both metal and ceramic layers showed a trend of first increase and then decrease with increasing Al 2 O 3 content. During pressureless infiltration, multiple chemical reactions took place between ZrO 2 and the Al−12Si−10Mg alloy and the main reaction products were (Al 1−m , Si m ) 3 Zr, Al 2 O 3 and ZrSi 2 phases. The degree of the reaction depended on the ZrO 2 content in the ceramic composition. In general, the compressive strength of the composites decreased with increasing Al 2 O 3 content, but three-point bending strength showed a first decrease and then increase. When Al 2 O 3 :ZrO 2 =1:9, the compressive and bending strength of the composites reached about 997±60 MPa and 426±10 MPa, respectively. A simple model was proposed to illustrate the fracture mode and toughening mechanism of the composites. [ABSTRACT FROM AUTHOR]