Fabrication Of Nacre-inspired Al Alloy/SiC Lamellar Composites And Their Microstructures And Properties

Billions of years of evolution have produced extremely efficient b iomaterials, whose microstructure, composition, morphology and function were optimized. Nacre, the one of the excellent representative of the biomaterials, developed unique layered "brick and mortar" structure at nanoscale by alternative arrangement of soft and hard phases, and thus exhibited high strength, excellent fracture toughness and good wear resistance. It has been the source of inspiration for people to design and construct of high-performance hybrid materials.Silicon carbide particles reinforced aluminum matrix composites（Al/SiCp） possess high specific strength, high specific modulus, superior wear properties, anti-radiation, high heat conductivity, low thermal expansion coefficient and good electrical conductivity, and thus are widely used in many fields, such as aerospace, automotive, electronics and sports, and so on. However, the disadvantages of Al/SiCp are that the fracture toughness and fracture energy decreases with the increase in brittle ceramic content for the composites, therefore Al/SiCp cannot keep a high safety in application. Meanwhile, high preparation cost and secondary processing difficulty of Al/SiCp limit their wide application. These are also the unsolved bottleneck problems of particles reinforced metal matrix composites（PMMC） for many years. In order to improve the strength and toughness of the Al/SiCp with medium/high volume fraction of SiC particle, mimicking the lamellar structure of nacre has been a hot topic of composites research. However, at present the reports of nacre-inspired lamellar composites are relatively rare. Thus, the fabrication of nacre–inspired lamellar composites and influence mechanism of lamellar structures on the properties need to be further explored and revealed.Therefore, in this study ZL205 A alloy, Al–12Si–10Mg alloy and micron–sized SiC particles were selected as the matrix alloy and main reinforcement, respectively. First of all,the environmentally friendly water-based SiC slurries were prepared, and then lamellar porous SiC scaffolds were produced by freeze casting. Finally, the nacre–inspired ZL205A/SiC and Al–Si–Mg/SiC composites with lamellar structure were fabricated by infiltrating ZL205 A and Al–Si–Mg alloy into the porous SiC scaffolds, respectively. In this process, the microstructures of the composites were effectively controlled by adjusting the parameters of freeze-casting technique. The infiltration dynamics of the molten alloy as well as its reaction mechanism with reinforcement were revealed. The influence mechanism and internal relationship on the technological parameters, microstructure, macroscopic mechanical and wear properties were studied. The effective mechanism of the composite on improving properties was present.The main results are as follows:1) Lamellar porous SiC scaffolds were successfully fabricated by freeze castingi) Homogeneous porous SiC scaffolds were prepared by quick- freezing of water-based SiC slurries; furthermore, lamellar porous SiC scaffolds with gradient structure were fabricated by unidirectional freezing method. The wavelengths and wall thickness of the porous SiC scaffolds increased with the distance away from the cold finger along the freezing direction. Both the wavelength and wall thickness of the porous scaffolds decreased with lowering the freezing temperature, but it increased with the increase in initial solid load of the slurries. The wavelength and wall thickness of the porous scaffolds with initial solid loads of 20⁴0 vol% are within the range of 10¹00 μ m and 15¹40 μm, respectively.ii) During sintering in air, the surface of SiC was covered by a continuous SiO₂ layer,which provided low sintered strength for pure SiC scaffolds. The silicate-bonded porous SiC scaffolds with lamellar structure were prepared by freeze casting and liquid-phase sintering. The silicate phase was produced by adding some amount of Al2O3 and Mg O powder（AM） into water-based slurries as sintering aid. As compared with pure SiC scaffolds, the compression strength of silicate-bonded porous SiC scaffolds was improved.2) The mechanical properties of lamellar-interpenetrated ZL205A/SiC composites fabricated by freeze casting and vacuum-gas pressure infiltration.i) Both the uniformly distributed and multi-scale layered ZL205A/SiC composites were fabricated by infiltrating molten ZL205 A alloy into the porous SiC scaffolds using 2 MPa at 850 oC. The resultant composites have dense structure and good interfacial bonding, and no harmful Al4C3 was obtained. The lamellae thickness of the lamellar composites is controlled in below 100 μm.ii) Mechanics performance test shows that the flexural strength and fracture toughness of the lamellar-interpenetrated ZL205A/SiC composites are higher than that of the uniformly distributed ZL205A/SiC composites, especially the crack-growth toughness（KJC） and work of fracture increased by 2 and 4 times, respectively. Moreover, the lamellar ZL205A/SiC composites exhibited higher flexural strength and fracture toughness in the longitudinal direction（parallel to the lamellae） than that in the transverse direction（perpendicular to the lamellae）, and showed obvious damage-tolerance anisotropy. With the increase in initial ceramic content, the strength and toughness of the composite decreased, but its elastic modulus and hardness increased. The maximum flexural strength of the lamellar composites is increased by 25% and 148% as compared with the matrix alloy and SiC ceramic, respectively. The highest crack- initiation toughness（KIC） of the composites is close to that of the matrix alloy, and both the crack-growth toughness（KJC） and work of fracture of the composites are higher than that of pure SiC about one order of magnitude.iii) It is revealed that the good damage-tolerance of lamellar- interpenetrated ZL205A/SiC composites could be attributed to multiple toughening mechanisms such as plastic deformation in the matrix alloy, crack deflection, crack branching and uncracked-ligament bridging of the ductile al oy layers.3) The mechanical properties of lamellar- interpenetrated Al–Si–Mg/SiC composites fabricated by freeze casting and pressureless infiltration.i) The Al–Si–Mg/SiC composites with lamellar- interpenetrated structure were successfully fabricated by freeze casting and pressureless infiltration. The Al–12Si–10Mg alloy spontaneously infiltrated into lamellar porous SiC scaffold at 950 oC in N2 atmosphere.ii) It is found that the compressive strength of lamellar- interpenetrated Al–Si–Mg/SiC composites first increased then decreased with increasing initial ceramic content. However, the elastic modulus and hardness of lamellar- interpenetrated Al–Si–Mg/SiC composites increased with the increase in initial ceramic content. The highest compressive strength was achieved in the samples with 30 vol.% initial SiC load in a longitudinal direction. As the AM content increased, the compressive strength of the composite increased. The maximum compressive strength（952±24 MPa） is 2.6 time of that of the matrix al oy.iii) It is put forward that the strengthening mechanism of lamellar- interpenetrated Al–Si–Mg/SiC composites. The composites have strong interfacial bonds, and the interface plays the role of load transfer. When the composite was loaded along the lamellae direction, the SiC lamellae acted as the main supporting matter and it restrained the plastic deformation of the alloy layer. The composites failed in a catastrophic brittle manner, due to both the agglomeration of some brittle Mg2 Si crystals in the matrix and unfilled defect.4) The dry sliding wear properties of lamellar- interpenetrated ZL205A/SiC compositesi) The dry sliding wear test show that the wear resistance of the lamellar- interpenetrated composites in the transverse section（sliding direction is perpendicular to the freezing direction） is better than that in the longitudinal section（sliding direction is parallel to the freezing direction）. In the transverse section, ceramic layers played a supporting and restriction affect as a framework.ii) It is found that the wear resistance of lamellar- interpenetrated composites is better than the matrix alloy. The wear resistance increases with increasing ceramic content in the composites. The larger the external load, the higher the wear rate, which shows a linear relationship between them. The wear resistance of ZL205A/S i C composites with 40 vol.% initial ceramic loading was increased about 88% compared with the ZL205 A al oy under applied load of 30 N.iii) It is put forward that the abrasion mechanism of lamellar- interpenetrated ZL205A/SiC composites. First, the mild wear（i.e., the abrasive and adhesive wear） is dominant; with the increase in ceramic content, adhesive wear translate to abrasive wear. Second, a mechanical mixed layer（MML） is formed, and the oxidation wear are the main mechanisms. Third, the MML is peeled off and the wear is mainly controlled by delaminating.In a word, the mechanism of damage-tolerance and abrasion of nacre- inspired Al alloy/SiC composites with lamellar structure is proposed. The necessary experimental data and technological basis for development of lightweight, damage-tolerance and wear-resistance lamellar Al/SiC composites are presented. At the same time, the two different and complementary processing techniques given by the research will provide the technical approach and referencable ideas for nacre- inspired lamellar composites which are more prevailed internationally.