| Super-high strength Al-Zn-Cu-Mg-Zr alloy containing Sc was prepared by using active flux and water chilling copper mould, The ingot was homogenized, hot rolled, annealled and cold rolled into a sheet with thickness of 2.3mm。Hot deformation of alloy over the strain rate range 10-3s-1 to 10s-1 and the temperature range 380℃to 470℃has been studied by thermal simulation test at Gleeble 1500 thermal-mechanical simulator。The tensile mechanical properties, microstructure and it's evolution of the studied alloy on different heat treatment was investigated using Differential Scanning Calorimetry analysis, hardness and electric conductivity measurement, room temperature tensile test, optical microscopy (OM), electronic microstructure analysis (SEM, TEM), and then, the results were analysed and discussed theoretically 0 The experimental results show that:(1) The flow stress of alloy increases with the rising strain rate crazily at first and tends to be constant after a peak value。The flow stress increases with strain rate increasing and decreases with deformation temperature increasing。(2) When alloy deforming at high temperature, the deformation process of this alloy is a thermal activation process similar to high-temperature creep process。Thus, the high temperature deformation material constant of this alloy are: n=7.16,α=0.013, A=4.71×1010, Q=162.1KJ/mol。The alloy under thermal deformation conditions meet the following relationship:(3) On the same strain rate, when the deformation temperature is lower than 410℃, the hot deformation is mainly dynamic recovery, dynamic recrystallize occurred in the alloy until 440℃; On the same Deform temperature, dynamic recrystallizate increases when deform temperature increases。Deform temperature increases or strain rate decreases, sub grain size increases, dynamic recovery change to dynamic recrystallization, the main soften mechanism of the alloy is dynamic recovery and recrystallization。The favorable hot processing temperature range is 380-410℃。(4) The proper single solution treatment of alloy is 475℃/40min。after 120℃/24h aging treatment, at this condition, tensile strength, yield strength, elongation were 676MPa,623MPaå'Œ8.1%, respectively。(5) The proper two-step solution treatment is 465℃/40min+490℃/30min。Two-step solution treatment can decrease the remain second phase effectively, and increase the super-saturation degree of solution matrix after quenching, then enchance the effect of final ageing strengthening。Compared with single solution treatment, at the same ageing condition, two-step solution treatment can let the tensile strength and yield strength increase by 18MPa and 28 Mpa, respectively, with remaining elongation of about 8%。(6) The fracture of alloy is in transgranular/ intergranular mixed mode。By two-step solutionizing, the intergranular rupture increases and the transgranular dimpled rupture initiated from remaining constituents decreases, which can avoid fracture early, the deformation of granular become more homogenous。(7) After two-step solution treatment, The proper single ageing treatment of alloy is 120℃/24ho at this condition, tensile strength, yield strength, elongation, hardness and electrical conductivity were 694MPa,651MPa,7.9%,199HV and 24.5%IACS, respectively; The proper two-step ageing treatment is 120℃/8h+ 160℃/16h。at this condition, tensile strength, yield strength, elongation, hardness and electrical conductivity were 628MPa,571MPa,8.9%,176Hv and 34.5%IACS, respectively。Compared with single ageing, strength was loss, but electrical conductivity was increased 10%IACS, which reflects the distinct improvement of SCCR (Resistance to stress corrosion crack)。(8) RRA treatment (Retrogression and Re-ageing), Retrogression temperature and time have greatly influence on the properties of studied alloy。Suitable RRA process is pre-ageing at 120℃for 24h, retrogression at 180℃for 30min, and then re-ageing at 120℃ for 24h, at this condition, the tensile strength, yield strength, elongation and electric conductivity is 687MPa,648MPa,7.4% and 30.2%IACS respectively。Compared with single ageing, the mechanical properties of studied alloy were remained, electric conductivity SCCR were increased。(9) Alloy after single ageing treatment, the intracrystalline are dispersive and homogenousη' and the grain boundary precipitate is continuous; the precipitates in grain interior and boundary have grown up and coarsened after two-step ageing treatment, the discontinuous distribution of precipitate and the precipitated-free-zones (PFZ ) appear on the grain boundary; after RRA treatment, the intracrystalline areη' and a littleη, the large coarsening grain boundary precipitate grow up obviously, compare with single ageing, PFZ become wide, the intracrystalline microstructure is similar to that of single ageing and the grain boundary precipitate is similar to that of two-step ageing condition。... |
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