Materials Science Forum Vols. 546-549

Abstract: The evolution of second phases and its effect on tensile mechanical property in artificial ageing and RRA process of super-high strength aluminum alloy is investigated.The result of tensile test shows that the samples of Al-Zn-Mg-Cu-Ag alloy aged at lower temperature(100°C) not only have higher tensile strength up to 753Mpa but also relatively higher tensile elongation above 9% than normal temperature(120°C) in artificial ageing. The sample of Al-Zn-Mg-Cu alloy has the highest tension strength upto 788Mpa when aged at 100°C for 48 hours in single step of artificial ageing. Further more in RRA process samples of Al-Zn-Mg-Cu alloy preaged at 100°C for 24 hours retrogressed at 200°C for 7min and reaged at 100°Cfor 24 hours present the best tensile strength of 795Mpa than others. The tensile strength of Al-Zn-Mg-Cu alloy after RRA treated decreases with prolonging of retrogression time and reageing time starting from 7mins and 24 houres respectively. SEM observation shows that crack of the samples in tensile test is created at large particles in the fracture while there are more particles of undissolved phase in Al-Zn-Mg-Cu alloy containing Ag. TEM observation shows that the dominant strengthening particle corresponding to the peak strength of Al-Zn-Mg-Cu alloy containing Ag when aged at 120°C for 8 hours is η’ phase while dominant strengthening particle is G.P zone when aged at 100°C for 80 hours. However, η’ phase as the dominant strengthening particles corresponds to the peak strength of Al-Zn-Mg-Cu alloy without content of Ag when aged at 100°C for 48hours. TEM observation also shows that G.P zone as strengthening particle is dominant in the samples of Al-Zn-Mg-Cu alloy reaged at 100°C for 24 hours, and strengthening particles is coarsened when the sample is retrogressed at 200°C and reaged for a longer time. It is suggested that whether at the presence of coarse particles of undissolved phases or when G.P zone and η’ particle grow up in the retrogression ,sample needs deformable G.P zone instead of undeformable η’ in subsequent artificial ageing and reageing as dominant strengthening particle, in order to present a larger freedom spacing for dislocation to slip and let the sample not to behave too brittle to display high resistance to imposed plastic deformation or high tensile strength.

Abstract: The effect of Sc and Zr on the fatigue property of Al-6.2Mg-0.4Mn alloy was investigated by control experiment; the fatigue lives of Al-6.2Mg-0.4Mn alloy with and without Sc and Zr at different loading stress amplitudes were measured. The relationships between their fatigue properties and microstructures were studied by means of optical microscope, scanning electronic microscope and transmission electron microscope. The addition of Sc and Zr improved the fatigue lives and fatigue strength of Al-6.2Mg-0.4Mn alloy. It is difficult for crack to initiate and propagate in the Al-6.2Mg-0.4Mn-Sc-Zr alloy. The fine and pancake-like grains of this alloy lead to the increase of fatigue life and fatigue strength. The high densities of dislocation and grain boundary in the Al-6.2Mg-0.4Mn-Sc-Zr alloy can prevent microcrack from propagating effectively. Additionally the dispersion precipitation of Al3Sc and Al3(Sc1-xZrx) particles enhance the toughness of Al-6.2Mg-0.4Mn-Sc-Zr alloy.

Abstract: The influence of aging treatment on microstructure, mechanical and electrical properties of 7A52 alloy was investigated by mechanical property test, electrical conductivity measurement, XRD and TEM analysis. The results showed that the alloy was a typical aging hardening alloy. During aging supersaturated solution decomposed, G.P. zones and η'(MgZn2) precipitated. The precipitates of the alloy aging at 100°C were mainly G.P. zones with the size of about 5nm. During aging at 120°C, η’ precipitates nucleated from the G.P. zones. With increasing aging temperature and time, the size and amount of η' increased. As aged at 120°C for 24h the tensile strength, yield strength, elongation and electrical conductivity of the alloy were 496 MPa, 445 MPa, 9.5% and 26 %IACS, respectively. The G.P. zones and η' dispersion strengthening are the major strengthening mechanisms for the studied alloy at the single-stage peak aging.

Abstract: Six kinds of Al-Zn-Mg-Cu alloys, modified with nickel and zirconium, have been produced by rapid solidification using spray deposition(the Osprey process). The effect of nickel on the structures, mechanical properties of ultrahigh strength aluminium alloy is studied, and the probable maximum of mechanical properties is predicted. There are three nickel-rich phases, Al3Ni2, Al7Cu4Ni and MgNi2, forming in 1%Ni alloy, its ultimate tensile strength(UTS) increased with increasing extrusion ratios significantly while maintaining high levels of ductility, Futhermore, the extruded bars show enhancing more clearly than extruded plates in UTS and ductility. The content of Ni should decreased with increasing of Zn and Zr, and the highest properties(UTS=832MPa, Elongation=7.5%)are attained in 0.20Zr＋0.30Ni (wt%)alloy. In addition, the size, the shape and the homogeneous distribution of zirconium-rich phase produced in solidification is the key to effecting the mechanical properties of materials when Zr content is 0.2~0.5％.

Abstract: The influence of one- and two- stage solution heat treatment on the microstructure and mechanical properties of aluminum alloy 7055 was investigated. The results showed that in the case of one-stage solution heat treatment in the range of 450 °C to 470 °C, the strength increased while the ductility decreased with temperature. Further increasing temperature led to more dissolution of soluble constituents but a large amount of recrystallization, therefore, the strength decreased while ductility increased. The two-stage solution heat treatment, which included first stage of low-temperature-long-duration and second stage of high-temperature-short-duration heating was suggested for improvement of mechanical properties.

Abstract: The Vickers hardness time-temperature-properties (TTP) curve of aluminum alloy 7055 was evaluated by interrupted quench technique. The critical temperature range of TTP curve was determined from 210 °C to 420 °C with the nose temperature of 355 °C. The hardness decreased rapidly with isothermal duration in the critical temperature range. The hardness of aluminum alloy 7055 was predicted by quench factor analysis using the TTP curve and cooling curves. The predicted hardness agreed reasonably well with the measured results. The influence of time step and summation temperature range on the predicted results was studied. The cooling rate through the critical temperature range had dominant influence on the final hardness of the alloy.

Abstract: A novel shear-deformation technique, named ‘shear pressing’ (SP), was developed for fabrication of plate-shaped fine grained metallic materials. The principle of SP is that a material is subjected to shear deformation by utilizing pressing with inclined plane dies. A micrometer order grain structure was obtained in an Al-Mg-Li alloy at strain of ε = -2.3 by utilizing this technique. The grain refinement sequences during pressing were examined by electron backscatter diffraction. The enhancement of grain refinement to the Al-Mg-Li alloy was compared with plane strain compression (PSC) at similar strains. The effect of the shear strain on the accelerated grain refining during compressing has been discussed.

Abstract: Grain refinement of 01420 Al-Li alloy through particle stimulated nucleation(PSN) of recrystallization is reported. The results showed that the rolling in the overaged 01420 Al-Li alloy resulted in the formation of the deformation zones associated with the second phase particles larger than 0.80 μm which can act as the nucleation sites for recrystallized grains. The precipitates larger than 0.80 μm are sticked shaped S-phase(Al2MgLi) and globular β-phase(Mg2Al3), and the density of β-phase particles is approximately as two to three times as the S-phase particles. The S-phase particles can’t be as PSN sites since they were broken to small dispersoid particles during rolling. The average grain size of 01420 Al-Li alloy solutioned at 470°C for 2h, aged at 300 °C for 48h, 81% rolled at 300 °C and finally recrystallized at 500 °C for 10min is approximately 10 μm.

Abstract: Void growth in aluminum single crystals is simulated using the finite element method, to illustrate the effect of grain orientation on void growth, a rate dependent crystal plasticity constitutive theory is implemented as a user-defined plasticity subroutine. A three-dimension unit cell including a sphere void was employed using three-dimensional 12 active slip systems. The computed results for several grain orientations are compared, which have shown that crystallographic orientation has significant influence on growth behavior of void. And the void growth direction and shape significantly depend on the crystallographic orientation. Due to plastic flow localization and anisotropic behavior, void which has an initial sphere shape, develops an irregular shape and some corners.

Abstract: The behaviors of trace element Er on binary Al-Mg and ternary Al-Mg-Mn alloys have been investigated through optical microscopy, x-ray diffraction, scanning electron microscopy with energy dispersive spectroscopy and transmission electron microscope. It was found that Er additions more than 0.4% (wt%) produced a remarkable refinement in grain size of Al-Mg and Al-Mg-Mn alloys castings. Er can enhance the tensile strength of the two group experimental alloys significantly but not greatly decrease the elongation due to the formation of many fine like spherical and dispersive primary and precipitation Al3Er particles during casting and heating process. Al3Er particles have L12 crystal structure (space group Pm3m) with a lattice parameter of 0.42119nm, and have a coherent relationship with the matrix phase α-Al (mismatch only 4.1%), which can be acted as the heterogeneous nucleus during solidification to increase the rate of nucleation, and the other hands it can strongly pin up dislocations and subgrain boundaries and retard the recrystallization of alloys.