Preparation,Microstructure And Properties Of CNTs/Mg Micro-nano Layered Magnesium Matrix Composites

The unique“brick-mud”structure of nacre achieves simultaneous improvement of strength and toughness,which is expected to solve the problem of mismatch between the strength and toughness of the magnesium matrix composite.In this paper,50?m magnesium foil and CNTs are used as raw materials to simulate the shell structure of shell nacre“brick-mud”layer structure.Firstly,CNTs were uniformly deposited on the surface of magnesium foil by electrophoretic deposition.The CNTs/Mg laminated composites were obtained by vacuum hot-pressing sintering.The microstructure and properties of the laminated composites were controlled by rolling and extrusion deformation.The microstructure and mechanical properties of the laminated composites were studied,and the local strain distribution analysis of the in-situ stretching process was analyzed by digital image correlation technique to reveal the strengthening and toughening mechanism of the biomimetic laminated structure on the magnesium matrix.First,CNTs/Mg layered element was successfully prepared by electrophoretic deposition.On the basis of pre-dispersion of CNTs,Al³⁺is adsorbed on the surface of CNTs by introducing Al?NO₃?₃ electrolyte,and electric field force pushes CNTs to deposit on the surface of magnesium foil.The optimized process parameters of CNTs and electrolyte were 0.05g/L and 0.1g/L,respectively.The electrophoretic deposition voltage was 30V.The uniform distribution of CNTs and the controllable content of CNTs can be achieved by controlling the deposition time?3min,4min,5min?.The layered elements are stacked in layers,and CNTs/Mg laminated composites are prepared by vacuum hot-pressing sintering.The rolling and extrusion deformation process can improve the interface between the composite layers and regulate the layer structure parameters.In the rolling deformation research,it is found that the single-pass40%large deformation rolling provides greater deformation force to facilitate the interfacial interface bonding and improve the strength and elongation of the composite.The total rolling amount is increased by multi-pass rolling,and the mechanical properties of the composite material are gradually improved as the total rolling deformation increases.The total rolling deformation was increased by 40%,60%and 80%.The yield strength of composites deposited with 4min CNTs was gradually increased by 122MPa,130MPa and 139MPa,and the elongation was gradually increased by 3.3%,4.6%and4.8%.It is found that the extrusion deformation treatment with 400°C and 16:1 extrusion ratio can effectively improve the interlayer bonding of sintered composites.Among them,the extruded composites with 4 min CNTs depstited have the best comprehensive performance,with a yield strength of 119 MPa and a tensile strength of 203 MPa.At the same time,it has an elongation of 14.2%,which exceeds the elongation of the layered structure of pure magnesium compared to the substrate by 12%.The theoretical calculation of the main strengthening mechanism of CNTs and layered structure for composites is fine grain strengthening and thermal mismatch strengthening.With the increase of CNTs content,there is also a back stress strengthening mechanism.The results of local strain analysis under combined analysis OM and SEM in-situ tensile found that the toughening mechanism of CNTs/Mg layered composites was carried out by both early strain homogenization and late suppression crack propagation.The layered structure facilitates uniform deformation of the composite material as a whole and relieves stress concentration.In the later layered structure,there is crack insensitivity,and it is difficult to expand the crack in the layered structure.The combination of two toughening mechanisms effectively increases the elongation of the composite.