Effect Of Pre-strain/Ageing Treatment On Microstructure And Macro-Yield And Micro-Yield Strength Of Al-Cu-Li Alloy

Aluminum alloys exhibit poor dimensional stability due to their low micro-yield strength,which limits their application in high precision instruments.Aim at this problem,a high-strength lightweight Al-Cu-Li alloy alloyed with Mg and Sc that developed by our research group was choosed as the research object in this study.The solution-treated alloys were pre-deformed to various strains and then treated with single-stage ageing and double-stage ageing.The effects of pre-strain and ageing treatment on the microstructure and macro-yield strength and micro-yield strength of alloys were studied.The following research results have been achieved:1.Pre-strain increases the dislocation density in solution treated Al-Cu-Li alloys and promotes the precipitation of T1 precipitates in aged Al-Cu-Li alloys.With the increase of pre-strain,the high density dislocation in the solution-treated alloy caused with pre-strain increases the heterogeneous nucleation rate of the T1 precipitates and speeds the diffusion of Cu and Li atoms during aging.Therefore,the diameter of T1 precipitates gradually decreases,the number density gradually increases,and the distribution gradually becomes uniform.Double stage ageing promotes the precipitation of T1 precipitates and causes the coarsening of T1 precipitates.2.The pre-strain prior to ageing can simultaneously improve the macro-yield strength and micro-yield strength of pre-strained Al-Cu-Li alloys after single-stage aged at 150?×24h.However,excessive pre-strains are not beneficial to the micro-yield strength of the Al-Cu-Li alloys.2% pre-strained Al-Cu-Li alloy has the highest micro-yield strength(491.3MPa),which is about 15.1% higher than that of reference alloy;8% pre-strained Al-Cu-Li alloy has the highest macro-yield strength(722.3MPa),which is about 12.5% higher than that of reference alloys.3.The 2% pre-strained Al-Cu-Li alloy treated with the double stage ageing at150?×12h+190?×4h has the micro-yield strength of 516.2MPa,which increased by20.8% and 5.1% compared with that of the reference alloy and 2% pre-strained and single-stage aged alloy respectively.However,continuing to prolong the secondary ageing time will result in a decrease in the micro-yield strength.Contrary to the micro-yield strength,secondary ageing reduces the macro-yield strength of Al-Cu-Li alloys.Combining the aforementioned investigation results,the solution treatment followed by 2% pre-strain and double stage aging at 150°C for 12h+190°C for 4h is is a promising method for improving the dimensional stability of Al-Cu-Li alloys.4.The strain strengthening and the precipitation strengthening are the main strengthening mechanisms of pre-strained Al-Cu-Li alloys.Pre-strain enhances strain strengthening but reduces precipitation strengthening.The increase in the strength of the Al-Cu-Li alloys caused by the pre-strain is mainly due to the increase in strain strengthening.The macro-yield strength and micro-yield strength of pre-strained Al-Cu-Li alloys can be better estimated by a simplified strength model when considering the difference in the strengthening mechanism of the T1 precipitates for different ageing processes.However,there is a large error between the estimated and measured micro-yield strength for the large pre-strained Al-Cu-Li alloys(6-8%pre-strains).The reason is that the effect of the movable dislocation density on the micro-yield strength of the alloy is not considered in the simplified strength model.