| 7A56 alloy,which is a newhigh alloying 7xxx aluminum alloy with high strength,high toughness and corrosion resistance,has great potentialapplication in the field of aerospace.This novel alloy was chosen as the experimental material in this thesis.Some analytical methods,such as,optical microscopy(OM),scanning electron microscopy(SEM),transmission electron microscopy(TEM)were used to observe the microstructure of the alloy.Some properties,such as,hardness,conductivity,tensile strength,fracture toughness,exfoliation corrosion and stress corrosion were tested for the alloy.According to the result of the microstructure and properties of 7A56 alloy during the solution and aging treatment,the correlations of aging precipitates and properties at the typical state,such as,strength,fracture toughness and corrosion resistance had be gained,which could provide experimental basis and technical support for theindustrial production and application.There were large number of AIZnMgCu quaternary phase and rich-iron phase in the as-rolled alloy with thickness of 40mm and 60mm.And the melting initiation temperature of the low melting AlZnMgCu phase was about 474℃.Compared with the 60mm thick alloy sheet,the size of grains in 40mm alloy was smaller.Soluted at 470℃ for 4h,The AIZnMgCu phase had been dissoluted in the matrix.But the size of the gain turned to increase.The conductivity of the quenched sample was 15.6～15.7MS/m.Correspondingly,the hardness for the peak-aged sample was 204～206HV.Aging behaviors of the 7A56 alloy with the thickness of 40mm after one-step,two-step and three-step aging treatment were investigated.After one-step aging treatment,the main precipitates were η’ phase and GPⅡ zone inside grains.The size of precipitates uniformlydistributed in the matrix was vabout 1～9nm.The grain boundary precipitates along the boundary was first continuous,at last discontinuous.After two-step aging treatment,the size of precipitates in the matrix grew to 20nm.And η phases had appeared.The grain boundary precipitates distributed discontinuously and became coarser,and the width of the precipitate-free zone(PFZ)increased.After two-step aging treatment,the slight precipitate dispersed in the matrix also mainly were the η’ phase and GPⅡ zone,Similar with the one-step aging treatment.,While,the grain boundary precipitates distributed discontinuously and the PFZ was broad,Similar with the two-step aging treatment.with the degree of aging(temperature,time)increasing,the hardness and strength of the alloy showed the trend of increasing first and then decreasing.While,the conductivity of the alloy gradually increased.The correlations of aging precipitates between the strength,fracture toughness and corrosion resistance had been analyzed.In the early aging stage,the fine dispersion of precipitates in the matrix were mainly η’ phase and GPⅡ zone and the size was small,which was beneficial to improve the strength of the alloy.The precipitates in the matrix turned to grow up.which lead to reducing the alloy strength.Continuous grain boundary precipitation easily became a favorable way of crack propagation,and also easily became a continuous anode corrosion channel,reducing the fracture toughness and corrosion resistance of the alloy.However,the coarsening and intermittent grain boundary precipitates were beneficial for improving the fracture toughness and corrosion resistance of the alloy.After the treatment of 120℃/8h+175℃/140min+120℃/24h,the precipitates in the matrix mainly consisted of η’ phase andGPII zone.The size was about 2～11nm.The grain boundary precipitation phasesshowed intermittent distribution and coarsening significantly.All of theset hadlead to a good overall performance.That is,the tensile strength and yield strength were 637MPa and 617MPa.The elongation was 11.6%.The conductivity was 22.5MS/m.The value of Kic was 40.1 MPa·m1/2.The exfoliation corrosion grade was EB.The stress corrosion threshold stress intensity factor(Kiscc)was 44.2MPa·m1/2... |
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