The Effects Of Precipitates On Mechanical Property And Deformation Mechanism Of Aluminum Alloys Using Quasi-2D Multicrystal Samples

Due to the strong demand for fuel efficiency and weight-reduction,the use of aluminum in automotive industry increases a lot.As the most important precipitates of aging Al-Mg-Si and Al-Si-Mg alloys,?” and ?' phase strengthen the alloys.But the decrease of ductility when cast aluminum to get high strength after aging makes it a limit use in automotive structural components.In order to get rid of the influence of non-surface grains,other second phases and difference grain orientation between different samples,directional solidification were used to smelt Al-1Mg-0.3Si alloys,quasi-2D multicrystal samples were got from the ingot.The strength,slip and precipitates in alloys within different aging time or temperature were investigated through optical microscope(OM),transmission electron microscope(TEM),electron backscatter diffraction(EBSD),slip trace analysis,and quasi in-situ tensile test.And the passage tries to understand the inter-relationships between precipitates,mechanical property and deformation mechanism.The results after tensile test and aging test shows the ductility and strength of alloys can increase in the same time: 175°C-12h(peak age)state's yield strength is 138.26 MPa,elongation is 18.84%;175°C-9h(under age)state's yield strength is 124.41 MPa,elongation is 24.2%;225°C-1h(peak age)state's yield strength is 114.79 MPa,elongation is 12.53%.175°C-12 h state comparing 225°C-1h state,the yield strength increased by 20.5% and the elongation increased by 50.4%;175°C-9h(under age)state comparing 225°C-1h(peak age)state,the yield strength increased by 8.4% and the elongation increased by 92.8%.And the follow-up is testing the three alloys condition's microstructure and deformation characteristics.Combines TEM and tensile test,and the results shows: 175°C-9h state have 50 nm length coherent needle-like ?” precipitates and the inter-precipitate spacing is 33nm;175°C-12 h state have 63 nm length semi-coherent rob-like ?' precipitates and inter-precipitate spacing is 25nm;225°C-1h state have 150 nm length semi-coherent ?' and plate-like B' precipitates,and inter-precipitate spacing is 50 nm.The precipitates changing from ?” to ?'/B' when the aging time go through under-age to peak-age.Through slip trace analysis,quasi in-situ tensile test and OM,and the macroscopic results shows: 1)175°C-12 h and 225°C-1h state's have heterogeneous deformation,some part of grains' slip intensity are high;otherwise the solid solution state and 175°C-9h state have homogeneous deformation and the slip on sample's surface uniformed distribution.2)the solid solution state and 175°C-9h state which have better ductility,there are multiple slips in most of grains;while there are few grains have multiple slips in 175°C-12 h and 225°C-1h state's.Combine TEM,and the microcosmic results shows: 3)there are sheared precipitates on the matrix of deformed 175°C-9h and 225°C-1h state's.However,deformed 175°C-9h state featured by dislocation loops and multiple slips without sheared precipitates.Above all,the distribution of sheared precipitates contribute to inhomogeneous deformation in alloy and decrease the elongation.While non-sheared precipitates make alloy deformed homogeneous and increase the elongation.The non-sheared precipitates and small inter-precipitate spacing lead to a high strength of the alloy.