| Silcock had found that trace addition (～0.05 wt.%) of In, Cd or Sn in Al-Cu alloys could dramatically accelerate the precipitation kinetics, and increase the peak strength.There were also some researches about Indium effecting on Al-Cu-Mg, Al-Cu-Li, yet no unified mechanism was established up to date.Thus, systematically and comprehensively works on the microalloying effects of In to 2xxx series Aluminums are aspired to a reliable mechanism, and instructional significance for alloy design.Three groups of Al-Cu, Al-Cu-Mg and Al-Cu-Li alloys with-or-without In, and another Al-Cu-Li-Mg-In alloy were prepared. Micro Vickers Hardness, ambient tensile properties measurements were used to study the effects on mechanical properties.Microstructure evolution, precipitate identification were carried out by Differential Scanning Calorimetry (DSC), Transmission Electron Microscope (TEM and high resolution TEM) and three dimensional atom probe (3DAP). Finally this paper comes to the following conclusions:(1)Indium additions speed aging response rate comparing to In-free Al-Cu alloy, the time needed to peak hardness is even less than 1/3 of base alloy aging at 175℃;peak hardness is 20HV higher; theθ' precipitates coarse much slower at the late stage of aging. In'were also observed at one corner of platelets having their broad faces in the plane of view other than the traditional heads of edge-on platelets positions, proving that almost all In'could act as the heterogeneous nucleation sites forθ'.(2)The base alloy precipitates process:SSSS→GP. zones→θ"+θ'→θ'→θ, has transformed to:SSSS→In'→In'+θ'with 0.5 wt.% Indium additions. Most quenched-in vacancies were seized by Indium atoms due to the strong interaction between Indium atoms and vacancies, thus depressed the formation of GP. zones; the vacancies-seized In' could move freely, the moving In'-V clusters would act as the heterogeneous nucleation sites for ;θ'once meeting to Cu atoms. In general, Indium affects Al-Cu alloy mainly through vacancies. (3) T1 and 0'phase dominate the Al-Cu-Li microstructure in T6 temper, an earth-shaking change take place after adding 0.5% Indium:In place of the{111}αplate T1 phase, the significant occurrence of a novel disperse cubic phase was detected, meantimeθ'was also found a little increasing. Inversely, the cubic phase was then substituted by T1 in T8 temper (6% predeformation+155℃).(4)Further study of the cubic phase shows that it belongs to the cubic system, lattice constant a≈8.4A, following the orientation with the aluminum matrix:{001}p//{001}α,<100>p//<100>α.3DAP and Simulation for HREM analysis indicate it to be Li-richγ1(Al4Cu9) orχ(Al5Cu6Li2).(5)The cubic phase tend to nucleated in the matrix homogenously, while plate-like T1 nucleated easier in dislocations heterogeneously because of it's big volumetric energy. Just like in Al-Cu-In alloy, most quenched-in vacancies, which would collapsed into loops, were seized by Indium atoms. T1 phase precipitation become hard without these loops acting as heterogeneous nucleation sites.The cubic phases then precipitate fluently without T1 competitive for Cu and Li atoms.(6) The tensile strength and yield strength of Al-Cu-Mg alloy is 345 MPa and 258 MPa, respectively, withθ'and S'dominating the microstructure, and a little amount ofΩ.6% prior aging deformation give 40MPa and 74MPa rise to US and YS, respectively, there are large amount ofΩphase,θ'become finer and a little more, while S'decrease. There comes to a conclusion:dislocations could act as the heterogeneous nucleation sites in both Al-Cu-Mg and Al-Cu-Mg-Ag alloy. The depressing phenomenon ofΩin the latter one is just because deformation disturbed the superior sites:Mg-Ag co-clusters and the acceleration can't compensate it's lose.While in the former one, the heterogeneous sites rise from none to some dislocations, thus it shows a promotion ofΩ.(7) 0.5% Indium addition increase the US 25MPa and YS 50MPa, suppress GPB zones formation, like GP zones depressed in Al-Cu alloy, and make S'andθ'in a greater amount. In quinary Al-Cu-Li-Mg-In alloy, T1 phase again substitute cubic phase, the strength is 80MPa higher. Mg atoms will nucleate T1 just like dislocations do, consuming Cu, Li atoms which are required by cubic phase.S'andθ'are still increased.(8) The microalloying effects of Indium in 2xxx series could be summarised as:accelerateθ'and S'for the strong interaction between Indium and Cu and Mg; on the other hand, reduce heteregeneous nucleation sites of T1 phase, making cubic phase precipitate swimmingly. |
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