Study On The Evolution Of Precipitates And Mechanical Property Of Al-Cu-Li Alloy During Thermomechanical Treatment

In this research,by means of transmission electron microscopy(TEM),high-angle annular dark field scanning transmission electron microscopy(HADDF-STEM),and tensile test,we focus on an Al-Cu-Li alloy and systematically investigate the effects of different pre-strain modes,including pre-stretch and large plastic rolling deformation,on the microstructure and mechanical property of the aged alloy,and different ageing temperatures on the evolution of precipitates.In addition,we also study the structure and distribution of precipitates at the atomic scale.Based on these characterizations,a possible formation mechanism of T1 phase is given and discussed.The main conclusions obtained are reached as follows:(1)T1 and ?' phases with large dimension are dominant in the aged Al-Cu-Li alloy without pre-strain prior to ageing.While in the aged alloy subjected to the pre-strain,the dimension of the precipitates becomes obviously smaller.The amount of ?' phases becomes smaller and the amount of the T1 phase is larger compared to those in the aged alloy without pre-strain.T1 phase is primary precipitate in the aged alloy with pre-strain.With increasing the pre-strain,although the dimension of the precipitates becomes smaller,the area fraction of those is almost the same.The results of tensile test indicate that when larger pre-strain was applied,the alloy showed higher strength and lower plasticity.The measures of the yield stress corresponding to the samples at the peak ageing indicate that with decreasing the pre-strain,the strengthening effect caused by T1 phase becomes smaller,which also agrees with the suggestion by Dorin.(2)The yield stress of the aged samples with large plastic rolling deformation at liquid nitrogen temperature is obviously higher,and the plasticity is almost the same compared to those of the samples with pre-stretch at room temperature.During the rolling deformation at liquid nitrogen temperature,the recovery process is inhibited.As a consequence,lots of dislocations would exist in the matrix,resulting in a lager dislocation strength effect.At the same time,lots of dislocation would provide more nucleation sites of T1,leading to a higher density of T1 phase which could cause a larger precipitation strengthen effect.While,when the aged sample are subjected to the large plastic rolling deformation at the room temperature,there is no obvious advance in terms of the corresponding yield stress and plasticity compared to those of the samples with prestretch at room temperature.This may be because during the rolling deformation at room temperature,the density of the dislocations in the matrix would be decrease and also a larger number of sub-grains are formed accompanying the recovery process.Consequently,the subsequent T1 phases are mainly generated at the vicinity of the boundaries of sub-grains and the area fraction of T1 phases is relatively small.(3)The precipitates of Al-Cu-Li alloy at different ageing temperatures(120,160 and 200 oC)have been investigated in detail,and the evolution of precipitates at different ageing stages has also been discussed.The results indicate that four kinds of precipitates in Al-Cu-Li alloy always exist without respect to the ageing temperature,including T1 phase,GP zone/?' phase,?' phase and ? phase(Al5Cu6Li2).However,the precipitation sequence,amount and distribution of these phases are depended on the ageing temperature.For instance,at the early ageing stage at 120 oC,a large number of fine and dispersed ?' phases can be uniformly formed in the matrix,while,only a few thick ?' phases but some T1 phases near the dislocations can be found in the matrix at the same ageing stage when ageing temperature are 160 and 200 oC.In addition,at the peak-ageing stage,the main precipitations are GP zone/?' phase,?' phase and ? phase for the samples aged at 120 oC,whereas T1 phases are dominant for the samples aged at 160 and 200 oC.Besides,there exists over-aging stage when samples are aged at 200 oC,showing that the thickness of precipitates becomes obviously larger.This ageing stage is not obvious under other ageing conditions(120 and 160 oC).There is difference between the evolutions of precipitates corresponding to different ageing temperatures(120,160 and 200 oC).(4)The detailed investigations of the structures of precipitates in the aged Al-Cu-Li alloy by means of HAADF-STEM indicate that: 1)?' phase can flank the straight ?' phase.Regarding to the corresponding interfacial structures,there exist two kinds of relationships,including in-phase and anti-phase,which of them is dominant depending on the thickness of ?' phase.2)T1 phase with its habit plane nearly paralleling to the grain boundary can be formed easily in the vicinity of grain boundary.3)T1 phase has a precursor.According to the HAADF-STEM observations,the corresponding structure model of this precursor is established,and its stability is also confirmed by first-principles calculations,indicating that it is reasonable that this precursor can transform into T1 phase.