Effect Of Cu/Mg Ratio On Strength And Toughness Of Al-Cu-Mg Alloy

As an age-strengthened alloy,content of Cu and Mg elements and the state of heat treatment determine type and number of precipitation phases in the alloy,which in turn affect strength and toughness of the Al-Cu-Mg.In this paper,Al-5.3Cu-0.8Mg and Al-5.3Cu-0.5Mg with Cu/Mg of 6.625 and 10.6 respectively were prepared and homogenized and solution aging treatment was carried out.The Al-Cu-Mg alloy was precipitated by aging The theoretical modeling and calculation of the phase volume fraction,the XRD quantitative analysis and determination of the volume fraction of the two alloy phases,the calculation of the interface valence electron structure between the alloy matrix and the aging precipitation phase and the theoretical modeling and calculation of the tensile strength,Cu/Mg The effect of the ratio on the strength and toughness of Al-Cu-Mg alloy.Based on empirical electron theory（EET）,volume fraction calculation model of S phase andθ′phase of aging precipitation phase of Al-Cu-Mg is established in this paper,and aging precipitation volume fraction of Al-Cu-Mg under the ideal peak aging state is calculated.The calculation results show that as Cu/Mg increases,volume fraction of S phase gradually decreases,and volume fraction ofθ′phase gradually increases.The calculation results of valence electron structure also show that during the aging process of Al-Cu-Mg alloy with Cu/Mg>2.7,S phase will be precipitated first.Higher Cu/Mg,fewer Mg atoms and faster S phase is completely precipitated.Quantitative XRD analysis shows that with the extension of aging time,Al-5.3Cu-0.8Mg has a three-peak hardening curve with a similar hardness value and a three-peak S phase volume fraction change curve;Al-5.3Cu-0.5Mg alloy has a distinct single Peak age hardening curve and bimodal S phase volume fraction change curve.The fundamental reason why Al-Cu-Mg alloys have different age hardening behaviors is that the Cu/Mg ratio of the alloys is different,which leads to different precipitation phase volume fractions and different evolution processes.In Al-Cu-Mg alloys with medium and high Cu/Mg ratio,the degree of S phase competing withθ′phase for precipitation is reduced,and the retardation effect of S relative toθ′phase precipitation is weakened.In Al-5.3Cu-0.8Mg,the precipitation of large volume fraction of S phase significantly delays the aging precipitation process ofθ′phase.The precursor phase of S phase,θ′phase andθ′phase together form alloy strengthening,forming the three peaks of the alloy Hardening characteristics.In the Al-5.3Cu-0.5Mg,the small volume fraction of the S phase precipitates faster,resulting in the subsequent aging process of the alloy closer to the aging process of the Al-5.3Cu.Theθ′phase,which accounts for the absolute volume fraction,is the alloy strengthening main body.The effects of aging precipitation S phase andθ′phase on alloy toughness are different.The calculation results of valence electron structure of interface between the Al-Cu-Mg matrix and strengthened phase show that:（001）_Al//S interface has high stability and low stress,and the cracks are not easy to nucleate and propagate,leading to the strength and toughness of the alloy Strengthened;and（001）_Al//θ′interface stress is large,cracks are easy to nucleate,which makes the alloy less tough,Because the strengthening effect ofθ′relative to the alloy is greater than that of the S phase,the larger the Cu/Mg,the greater the phase volume fraction of theθ′phase and the higher the tensile strength of the alloy.The study of the effect of Cu/Mg on the strength and toughness of Al-Cu-Mg has a certain theoretical and practical significance for design and improvement of composition.