The Mechanical Properties And Microstructure In Al-Cu-(Sc) Alloys Processed By Combination Of Deformation And Aging

Al-Cu aluminum alloys are typical heat-treatable Al alloys possessing high strength,good plasticity and toughness,and can be easily processed into various shapes.Therefore,Al-Cu alloys are widely used as structurel materials,especially in aerospace and aeronautical industries.From a microstructural viewpoint,the properties of Al-Cu alloys are mainly determined by the crystal defects and strengthening precipitates formed during thermo-mechanical processing.The objective of this work is to investigate the mechanical properties and microstructures in Al-Cu-(Sc)alloys produced by the combination of deformation and aging,particularly to explore the effect of different processing routes and micro-alloying of Sc.The Al-Cu-(Sc)alloys processed by both conventional method(T6)and combined deformation-aging method(P-T6)are studied by hardness test,tensile test,electron back scattering diffraction method(EBSD)and transmission electron microscopy(TEM).Detailed analysis was conducted to clarify the effect of large pre-deformation and micro-alloying of Sc.The main conclusions are summarized as following:(1)The pre-treatment by solution treatment at 530℃ and natural aging for three days(NA3d)are optimized for the Al-5.46 Cu(wt.%)alloy processed by the combination of deformation and aging.Compared to the conventional T6 treatment,room-temperature rolling of Al-Cu alloy with a thickness reduction of 80%plus subsequent artificial aging gives rise to a strength improvement of about 130MPa without loss of ductility.(2)After post-aging,Al-5.46 Cu(wt.%)alloy still retains a high number density of dislocations accumulated during large pre-deformation.Though dense dislocation walls and dislocation tangles are formed in the cold-rolled Al-Cu alloy,the plate-shaped precipitates formed on {001}Al planes upon post-aging keep a homogeneous distribution.The precipitates(θ’-approximant)in the alloy post-aged at 150℃ have a crystal structure similar with 0’ phase but with the body-centered position not fully occupied by Cu,and is much finer than θ’ precipitates in T6 peak-aged alloy.Rough estimation of the precipitation hardening in Al-Cu alloy from current strengthening model suggests θ’-approximant phase should be shearable during deformation,unlike shear-resistant 0’ phase.The deformation-mediated precipitation in Al-Cu alloy is inferred to play an important role in the strength-ductility balance.(3)The age hardening ability and thermal stability of Al-5.0 Cu-0.3 Sc(wt.%)alloy are better than that of Al-5.0 Cu(wt.%)alloy produced by combination of deformation and aging process.The age hardening behavior is similar in the two alloys aged at the same temperature.For the low temperature aging at 120℃ and 150℃,the hardness firstly rises to the peak then decreases slowly down with the extending of aging time.The difference value between the intial hardness and that obtained after a long duration is smaller in the Al-Cu-Sc alloy.When the aging temperature is elevated to 180℃,the hardness of the alloy decreases with the aging time and reaches a constant value after a certain period,then it fluctuates slightly with the prolonged aging up to 300h.It should be noted that the hardness reduction in Al-Cu-Sc alloy is much smaller.The observations by EBSD and TEM show that the addition of trace Sc can significantly refine the grain size of the solution treated sample,and the precipiates have a higher number density and smaller size in Al-Cu-Sc alloy.This may explain the enhancement of the microhardness and the thermal stability in the alloy containing minor Sc.The mechanism of the influence of minor Sc addition in the alloy processed by P-T6 is also discussed.