Preparation Of Bio-Inspired Al/B₄C And Al/Al₂O₃ Composites By Directional Freezing And Pressure Infiltration

In the 21st century,a major challenge is to develop new lightweight and high-strength structural materials for diverse applications,such as aerospace,construction and transportation.Metal/ceramic composites are ideal materials due to the combination of the toughness of metals and the high stiffness and strength of ceramics.However,due to many factors such as material,structure,and processing difficulties,the following problems need to be solved for the preparation of high-performance composites.Firstly,conventional metal matrix composites are often characterized by homogeneous structures,which has proven to be unfavorable for the coordination and multi-mechanism coupling between components.Secondly,there are the limitations in traditional manufacturing techniques,which lead to difficulties in material preparation.Thirdly,there is a lack of universal and precise preparation methods,which vary with raw materials.How to solve these problems has been a challenge in composite science.An effective way to solve these problems is to learn from nature.This thesis focuses on this point and takes three biological materials as a guide.We analyzed the relationship among the composition,structure,and properties of biological materials such as nacre,horn and bone,and extracted their structural units as design templates for the preparation of high-performance bio-inspired metal/ceramic composites.We reviewed the research progress of the bio-inspired materials from the perspective of preparation methods,focusing on the current research of directional freezing and its advantages in controlling material's structure.Combining directional freezing with the melt infiltration and using the ideas of wettability improvement,emulsion templating,and gradient distribution,we developed some new preparation methods.Two typical material systems,i.e.Al-B₄C and Al-Al₂O₃,were adopted for the preparation of bio-inspired composites and key processing factors were investigated.The main results are as follows:(1)Inspired by the lamellar structure and the strengthening/toughening mechanisms of nacre,we developed a universal directional freezing-reactive sintering-pressure infiltration method to prepare lightweight and high-performance Al/B₄C lamellar composites.The combination of directional freezing and reactive sintering solves the weakness and collapse problems of B₄C scaffolds via an in-situ reaction.The formation of Ti B₂ improved the wettability of Al with B₄C,and the lamellar Al/B₄C composites were prepared at a low temperature(850 ^oC)under a small pressure(2 MPa).The elimination of free carbon(from raw B₄C and dispersant carbonization)in the scaffolds avoided the Al₄C₃ phase in the composites;the generation of Ti B₂ alleviated the reaction of Al with B₄C.The effects of the phase composition and microstructure of the composites on mechanical properties were studied.The Al/B₄C composites with 20 wt.%initial Ti O₂ achieved maximum strength and toughness owing to the reduction in the brittle reaction products and the continuity of the lamellar structure.The crack propagation paths and the fracture morphologies were observed.The excellent fracture toughness of the Al/B₄C lamellar composites was primarily attributed to the intrinsic toughening of the ductile Al layers and the extrinsic toughening of crack deflection and metal bridging.(2)Inspired by the lamellar/tubular structure and energy-absorbent/toughening mechanisms of horns,we developed a scalable emulsion directional freezing and pressure infiltration method to prepare lamellar/tubular Al/Al₂O₃ composites.For the first time,directional freezing combined with emulsion solvent templating and pressure infiltration breaks the constraint that only one structure can be prepared by directional freezing.The effects of solvent components(water and cyclohexane)on the microstructures of the scaffold were studied.The larger the addition of cyclohexane,the higher the viscosity of the slurry,resulting in structural transition of the scaffold from lamellae to homogeneity.At a cyclohexane:water volume ratio of 50:50,the lamellar/tubular structure is the closest to that of natural horns.The relationships between the structure and mechanical properties of the horn-inspired lamellar/tubular Al/Al₂O₃ composites were clarified.The synergistic deformation mechanisms of the lamellar/tubular structure enhanced the energy-absorbent capacity and endowed them excellent mechanical properties.The specific energy absorption per unit volume,compressive strength,bending strength and fracture toughness(K_Ic)reached 107±11 MJ/m³,188±9 MPa,262±9 MPa and 8.1±0.3 MPa×m^1/2,respectively,which were higher than those of the natural horns.To a large extent,we have achieved the goal of learning from nature while surpassing it in one aspect.(3)Inspired by the lamellar/gradient structure and lightweight/strong features of bones,we proposed a novel sedimentation or centrifugation-directional freezing-pressure infiltration method to prepare lamellar/gradient metal/ceramic composites.In the sedimentation-directional freezing,mullite fibers and Al₂O₃ particles were used as model materials,where gravity rapidly caused a gradient distribution,and then directional freezing locked and introduced the lamellar structure.The effects of ceramic distribution and geometry on the microstructure of the scaffolds were studied.Ice crystals can push Al₂O₃ particles,but difficult to push the mullite fibers.In centrifugation-directional freezing,the alumina particles were distributed in a gradient under centrifugal force.The effects of centrifugal rotation rate and rotation time on the structure of scaffold were studied.The results showed that increasing the rotation rate or the rotation time could make the scaffold's gradient feature obvious.Then,the composites with bone-like structure and function were prepared by pressure infiltration Al or Al alloy into ceramic scaffolds.The response relationships among the component distribution and microstructure of the composites,and their mechanical properties was revealed.The gradual increase in strength,hardness and wear resistance of the composites was due to the increase in ceramic phase content,while the increase in fracture toughness was attributed to the increase in metal phase content and the lamellar toughening behaviors such as crack deflection,metal bridging and lamellar pulling.In short,this thesis explored the preparation of bio-inspired MMCs using directional freezing and pressure infiltration.We have achieved the goal of bio-inspired synthesis of metal/ceramic composites to a certain extent,and hope that it can provide some references for the development of high-performance composites.