Correlated Evolution Of Grainprecipitates And The Performance Response In 2A97 Aluminum Alloy

The correlated evolution of grain/precipitates and its influence on the performances in the third-generation Al-Li alloy were studied,along with the hot working-heat treatment process in 2A97 Al alloy thick plate.The microstructure evolution was characterized using SEM,EBSD and TEM and the performances were evaluated via hardness testing,tensile testing,and immersion testing.The grain/precipitates correlated evolution and the performance response were thereafter systematically analyzed.The influence from coarsened T₁ particles on the high-temperature deformation was revealed and that the mechanism of particles stimulated nucleation（PSN）and the related recrystallization texture was elucidated.The effects of grain structure on the precipitation behavior of T₁ phase were revealed,the discrepant precipitation strengthening effect and the influence on through-thickness strength gradient of thick plate were thereafter revealed.Also,the effect of micro-chemistry along grain boundaries on intergranular corrosion was clarified.It was found that the dynamic recovery dominated the softening process at 340-490^oC during the hot compression in 2A97 Al alloy of solid solution condition（2A97-SS）,while the dynamic recrystallization occurs at 340-460 ^oC in the as-homogenized2A97 Al alloy（2A97-AH）containing coarsened T₁ phases.Compared with that in2A97-SS alloy,the flow stresses and hot deformation activation energy were lower in the case in 2A97-AH alloy,indicating better workability.The coarsened T₁ phase reduced the solid solution strengthening in 2A97-AH alloy,while the dynamic precipitation of T₁ phase andδ’phase induced increased deforming resistance.The presence of coarsened T₁ phase promoted the dynamic recrystallization and further induced the<001>//ND recrystallization texture via the PSN effect.The grid-like distribution of plate-like T₁ phase lying on{111}_Al planes divided the matrix into units and therefrom limited the migration of dislocations and grain boundaries as well in the matrix units.The matrix units were found to rotate progressively to<001>orientation and recrystallized grains were formed during the rotation.When a strain of 0.8 was applied during the compression at 380 ^oC and 0.01 s^-1,the recrystallization fraction reached at 61%and the average grain size was 2.7μm.The grain size and preferred orientation of the recrystallized structure did not change significantly after exposed to solution treatment at 520 ^oC.The study on the quench precipitation behavior showed that T₁ phases andδ’phases precipitated in the matrix when 2A97 Al alloy was quenched in air from solution treatment temperature.Precipitates formed in recrystallized grains（RGs）were much less than that in sub-grains（SGs）when the specimen was quenched as a whole.Sub-grains presented significantly higher sensitivity of quench-induced precipitation（QIP）and a cooling rate higher than 3^oC/s was required to avoid the QIP.When solution-quenched 2A97 Al alloy was aged at 155^oC,the precipitation sequence was SS→θ’+δ’→θ’+δ’+T₁→T₁+θ’+δ’→T₁+θ’+δ’+δ+σ.T₁ phase in RGs had larger size and lower number density in comparison with those in SGs in the specimens of T6condition.Small zones within whichθ’phases precipitated intensively while the precipitation of T₁ was significantly suppressed,shaped up adjacent to the sub-grain boundary（SGB）.When a pre-stretch of 3%was introduced before the aging,the precipitation sequence was alerted to SS→θ’+δ’+T₁→T₁+θ’+δ’.Moreover,the discrepancy between the T₁ phase’s precipitation in RGs and that in SGs was significantly reduced.The introducing of pre-stretch promoted the precipitation of T₁phase within the grains and inhibited that on SGBs/GBs,which avoided the formation ofθ’phase gathering zone.The difference in dislocation density contributed to the discrepancy between precipitation in the recrystallized grain and that in sub-grains.The higher density of dislocations in sub-grains provided more cites for T₁ phase to nucleate,resulted in fine and uniformly distributed T₁ phase with smaller sizes.The high density of dislocations in the sub-grains also served as diffusion channels for solutes and thereafter promoted the growth of T₁ phases.The grain/precipitation correlation strongly influenced the properties of the alloy.The through-thickness strength gradient in 2A97 Al alloy thick plate increased gradually as aging proceeded.The yield strength difference between the center layer and the surface layer of the plate reached 88 MPa when the alloys was aged to T6condition.The yield strength difference reduced to 50 MPa by introducing a pre-stretching of 3%.The yield strength gradient of thick plates was attributed to the difference in precipitation strengthening of T₁ phase that in SGs and that in RGs.The fraction of sub-grains was highest in the central layer of the plate,contributing to highest precipitation strengthening.The intergranular corrosion resistance and exfoliation corrosion resistance in 2A97 Al alloy was as follows:under-aged>Peak-aged state>Over-aged state.SGBs and GBs were the main propagation channel of corrosion and SGBs were more prone to corrosion than GBs.Electrochemistry analysis indicated that sub-grains had lower surface potential,lower impedance and higher corrosion susceptibility relative to that of recrystallized grains.The precipitation free zone near GB had lower surface potential than the adjacent GB and matrix due to the lower content of Cu solute.The preferential dissolution of the precipitation free zone was the main reason for the intergranular corrosion in 2A97Al alloy.The introduction of a pre-stretching resulted in significantly reduced heterogeneous distribution of elements at SGB and lower corrosion susceptibility of SGB which contributed to remarkably improved corrosion resistance of 2A97 Al alloy.