Study On Microstructure And Properties Of Heat-resistant Aluminum Matrix Composites Prepared By Alloying And In-situ Reaction

In order to fully excavate the high temperature strengthening effect of the high thermostability of Al3X(X=Ni,Zr,V)aluminides,refine the aluminides size and get high volume fraction of aluminides,the traditional cast Al-Si-V alloy and Al-Mg-Mn-V alloy were selected as the matrix in this paper.Al3Ni/Al-Si-V,Al3Ni/Al-Mg-Mn-V and Al3Zr/Al-Ni-Mg-Mn-V composites were prepared by adding transition metal elements V and K2ZrF6 saltt into the matrix,using mechanical stirring and in situ reaction.The heat resistance of composites was expected to be enhanced by the formation of high temperature strengthening phase such as Al3Ni and Al3Zr.The cast microstructure and mechanical properties of the composites at room and elevated temperature were studied by means of X ray diffraction,scanning electron microscope and universal tensile test.The influence of mechanical stirring parameters on the microstructure and properties of these composites was also studied.The main results are as follows:(1)The microstructure of Al3Ni/Al-8Si-0.8V composites show that the shape of Al3Ni phase is irregular granular,needle like,small square block and circular block with the difference of Ni mass fraction.With the increase of Ni content,the tensile strength decreases and the yield strength increases,Al3Ni/Al,8Si-0.8V composite shows the best comprehensive mechanical properties with 3%Ni.Annealing is not conducive to the increase of tensile strength of the composite.The length to diameter ratio of acicular eutectic silicon can be reduced by adding Al-5Ti-1B intermediate alloy refiner,and the tensile strength of Al3Ni/Al-8Si-0.8V composite with 5%Ni is increased by 9.5%than that without refiner.After T6 heat treatment,the distribution of Al3Ni phase becomes uniform,and the mechanical properties of the composite with 5%Ni content increase.The tensile tests at 200 ? and 300 ? show that the tensile strength of the Al3Ni/Al-8Si-0.8V with 3%Ni is 143 MPa and 138 MPa respectively,the yield strength is 89 MPa and 78 MPa respectively,and the elongation is 8.30%and 5.24%respectively.Tensile fracture analysis shows that the cleavage fracture mechanism is the dominant factor at room temperature.A large number of dimples and tearing edges can be observed at high temperature,showing ductile fracture.(2)Al3Ni/Al-Mg-Mn-V composites consists mainly of a-Al phase,Al3Ni phase and Al10V phase.The Al10V phase mostly exists in the matrix with large regular blocks,which is unfavorable for the mechanical properties of composites.Al3Ni phase has high thermal stability,which can effectively improve the tensile strength at high temperature.Higher melting temperature can help to generate large amounts of Al3Ni phase,inhibiting the growth of a-Al,thus improving the strength of composites.The Al-5Ti-1B intermediate alloy for refining and strengthening is suitable for adding before casting.The tensile strength and yield strength of Al3Ni/Al-1Mg-0.8Mn-0.8V composite with 5%Ni at room temperature are 211 MPa and 107 MPa respectively.The tensile strength and yield strength of the composites at 200 ? are 210 MPa and 112 MPa respectively,and the tensile strength is only 0.5%lower than the room temperature.The tensile strength and yield strength at 300 ? are 170 MPa and 85 MPa respectively.Tensile fracture analysis shows that the tensile section at room temperature,200 ? and 300 ? is composed of ice sugar shaped intergranular fracture surface and tearing edge,showing a ductile and brittle mixed fracture;A large number of dimples are formed at the fracture surface at 350 ? which is a typical ductile fracture.(3)The effect of mechanical stirring on the microstructure and mechanical properties of in-situ Al3Zr/Al-xNi-1Mg-0.8Mn-0.8V composites was investigated.The results show that the shape of Al3Zr particles is very sensitive to the reaction temperature,and the overall performance of the material is the best when stirred at 720 ? with 600 r/min and 5 min.When the mass fraction of Al3Zr reinforced phase is 2%,the tensile strength of Al3Zr/Al-5Ni-1Mg-0.8Mn-0.8V composite is up to 189 MPa.The microstructure of Al3Zr/Al-xNi-1Mg-0.8Mn-0.8V composite exhibits diversity with different Ni content.When the mass fraction of Ni is 5.7%and 6.5%,the tensile strength of the composites at room temperature is close to 200 MPa,the tensile strength of the former at 200 ? and 300 ? is 175 MPa and 166 MPa respectively,while the tensile strength of the latter at 200 ? and 300 ? is 191 MPa and 155 MPa respectively.The micro fracture mechanism of the composites at room temperature,200 ? and 300 ? is intergranular fracture,cleavage fracture and local microporous polymerization shear fracture,while ductile fracture at 350 ?.(4)The high temperature tensile test at 350 ? show that the tensile stress-strain curves of Al3Ni/Al-Mg-Mn-V and Al3Zr/Al-Ni-Mg-Mn-V composites have the steady state creep stage and the accelerated creep stage,showing certain heat resistance and creep resistance.