Research On Strength And Plasticity Co-regulation And Deformation Behavior Of Ti₂AlC-Al Composites

Metal matrix composites play an important role in the aerospace field due to their excellent properties such as light weight,high strength and specific stiffness.However,there has always been a bottleneck problem of high strength and poor plasticity,which greatly limits their application.The hard and high-strength ceramic reinforcement used in traditional metal matrix composites cannot coordinate with matrix to deform,which greatly reduces the plasticity while providing the strength of composite.Researchers have tried to solve this problem by designing the structure of composite and reducing the size of reinforcement,but the results are not significant.Therefore,in order to solve the problem of the inversion of strength and plasticity,this paper uses deformable MAX phase Ti2AlC as reinforcement and pure Al and Al-Mg-Si alloy as matrix,and uses spark plasma sintering combined with thermal deformation process to fabricate Ti2AlC/Al composites.The microstructure,mechanical properties and strengthening mechanism were systematically explored.And the plastic deformation mechanism of deformable Ti2AlC particles in composites was also revealed by in-situ tensile test,and the strongplastic mechanism was comprehensively analyzed.In addition,in order to further improve the strength of composite,Si was added to the Ti2AlC-Al composite system by SPS method,and Ti2(Al,Si)C solid solution with high hardness and modulus was in-situ fabricated.The solution strengthening design of Ti2AlC was achieved.Finally,the Ti(Al,Si)C/Al-Mg-Si composite was successfully designed and fabricated by using high strength Al-Mg-Si alloy as the matrix and adjusting the composition of composite.The Ti2AlC/Al composites were designed and prepared,and their microstructure was adjusted.The mechanical behavior of Ti2AlC/Al composites was simulated by finite element analysis,and the feasibility of coordinated deformation of deformable Ti2AlC particles reinforced Al matrix was confirmed.According to the deformable properties of Ti2AlC,the shear hysteresis model is modified,and the mechanical model of Ti2AlC/Al composites based on load transfer strengthening is obtained.The microstructure analysis of Ti2AlC/Al composite shows that the relationship between the crystallographic orientations of Ti2AlC and Al is:Al[011]//Ti2AlC[1210],Ti2AlC(0001)//Al(111).While the(112)crystal plane of the interface reaction phase TiAl3 is along the Ti2AlC(1012)crystal plane grows in the 95° direction and also exhibits good interfacial bonding.The microstructure and mechanical properties of Ti2AlC/Al extruded composites were studied.Combined with the hot deformation process,the maximum pole density of Ti2AlC {0001} basal plane along the deformation direction was increased by 315%,showing preferred orientation.As the addition of Ti2AlC increased from 10 vol.%to 25 vol.%,the yield strength and tensile strength of the composites gradually increased.When the addition content of Ti2AlC was 20 vol.%,the composite exhibits the best matching of strength and plasticity.After further hot isostatic pressing densification,the tensile strength of the composite can reach 305 MPa,with strain of 21.2%,showing high strength and high plasticity at the same time.The strengthening mechanism of Ti2AlC/Al composites was analyzed and discussed.The yield strength strengthening mechanisms are mainly dislocation strengthening,grain refinement strengthening and load transfer strengthening.In the load transfer strengthening mechanism,since the interfacial reactant TiAl3 grows in-situ along Ti2AlC,so both Ti2AlC and TiAl3 play the main role in bearing the load,and with the content of Ti2AlC increasing,the strengthening effect of TiAl3 gradually increases.The plastic deformation mechanism of Ti2AlC in aluminum matrix composites was revealed.The plastic deformation behavior of Ti2AlC/Al composites were analyzed by in-situ tensile test and DIC technology.The results showed that Ti2AlC particles elongated along the tensile direction and contracted in the vertical tensile direction during the deformation process,which coordinated with Al matrix.Among them,Ti2AlC has two deformation mechanisms including crystal torsion and interlayer slip.Ti2AlC twists in favor of slip with tensile deformation,accompanied by coordinated rotation of Al grains.Combined with the tensile fracture morphology,it can be inferred that Ti2AlC had interlayer sliding,sliding along the Al atomic layer and left a thin layer of TiC,and finally extended out of the fracture surface to show a unique wing shape of cicada.Ti2(Al,Si)C/Al-Mg-Si composites were successfully designed and prepared.Ti2(Al,Si)C solid solution with high hardness and high modulus was in-situ prepared in the composite by using the spontaneous solution of Si in Ti2AlC.Based on the formation rule of Ti2(Al,Si)C solid solution,the high strength Al-Mg-Si alloy was used as the matrix.By adjusting the composition of the composite,the Ti2(Al,Si)C/Al-Mg-Si composite material was finally designed and prepared,with a yield strength of 357 MPa and a fracture strain of 4%,showing good match of strength and plasticity.This paper provides a new solution to the problem of the inversion of strength and plasticity of metal matrix composites,and provides experimental evidence for revealing the strengthening mechanism and deformation mechanism of Ti2AlC on aluminum matrix composites,and opens up a new way for the preparation of metal matrix composites with high strength and high plasticity.