Study On The Nucleation,evolution And Structure Of T₁ Phase In Al-Cu-Li Alloy During Aging

Al-Cu-Li alloys have justifiably attracted much attention as new promising structural materials in the aircraft and aerospace industries such as large carrier rocket and human spaceflight program from cryogenic propellant tank to large aircraft shell in recent years,due to their high stiffness-to-density ratios,perfect precipitation hardening responses,good heat resistance and corrosion resistance.In the latest generation of Al-Cu-Li alloys,the main contribution to strengthening is provided by the precipitation of the T₁precipitate,which forms as platelets on the{111}_Al planes.Understanding the nucleation,evolution and crystal structure of the T₁ precipitate in the heat treatment process can guide composition design and optimize heat treatment process,and is of great significance to improve the comprehensive mechanical properties of the alloy.This research mainly focuses on the hot-rolled Al-Cu-Li alloy which undergoes solid solution treatment,pre-deformation and artificial aging treatment at 160 ^oC with different aging time.Then this research uses an advanced equipment of transmission electron microscopy（TEM）,high-angle annular dark-field scanning transmission electron microscopy（HADDF-STEM）,annular bright-field scanning transmission electron microscopy（ABF-STEM）,electron energy loss spectroscopy（EELS）,energy dispersive spectrometer（EDS）,differential scanning calorimetry（DSC）and atom probe tomography（APT）combined with first-principles calculations to systematically and deeply study the nucleation and crystal structure of the main strengthening phase（T₁ precipitate）of Al-Cu-Li alloy.（1）Advanced equipment such as TEM,TEM,HADDF-STEM,EELS,EDS and DSC were used to study the effect of different pre-deformation levels on precipitation temperature,aging hardness and mechanical properties of the aged Al-Cu-Li alloy.It is found that the nucleation of the T₁ phase is usually at the sub-grain boundaries and dislocations,because the sub-grain boundaries and dislocations provide fast diffusion channels for Cu and Li elements.Moreover,T₁ precipitate heterogeneously nucleates at the precursors that are designated as GP_T1-I and GP_T1-II zones.In the 0%pre-deformed alloy,,the number density of T₁ precipitates is relatively low and the distribution of T₁precipitates is heterogeneous.T₁ precipitates are mainly distributed at the sub-grain boundaries and easy to be coarsened,and a small amount of T₁ precipitates are distributed in the defect-free area of the matrix.The heterogeneous distribution of T₁ precipitates leads to low ultimate tensile strength,yield strength and elongation.In the 4%pre-deformed alloy,although the T₁ precipitates also preferentially nucleate at the sub-boundaries,as the aging time increases,the T₁ precipitates will also nucleate homogeneously at defects such as dislocations.Therefore,the overall distribution of the T₁ precipitates is relatively homogeneous,resulting in 4%pre-deformed Al-Cu-Li alloy has high ultimate tensile strength,yield strength and elongation.The peak-aged 0%and4%pre-stretched alloys have ultimate tensile strength,yield strength and elongation of454 MPa,365 MPa and 9.5%,and 484 MPa,420 MPa and 12.3%,respectively.With increasing prestrain,the comprehensive mechanical properties of the Al-Cu-Li alloy is improved.（2）Using advanced technologies such as HADDF-STEM and APT combined with first-principles calculations,the type,composition and structure of precursors are studied.The GP_T1-I zone is characterized during aging process,and its morphology is plate-shaped with the habit plane of{111}_Al,and is completely coherent with the aluminum matrix.The GP_T1-I zone is mainly composed of three copper-rich layers,in which two interface layers are Al-Cu layers,and the central layer is Cu-Li layer.First-principles calculations show that the GP_T1-I zone is more stable when Li elements are in the center layer.The GP_T1-II zone is composed of two Al-Cu layers at the interface and one Al-Li layer in the center.When viewed along the<110>_Al zone axis,the brightness of the Al-Cu atomic columns is more uniform,but from the<112>_Al direction,the Al-Cu atomic columns are periodically arranged with one higher intensity column and two lower intensity columns.The aluminum-copper layers provide the atomic skeleton for the transition process of T₁ precipitates,which is formed during the formation of GP_T1-I zone,and provides the support for the diffusion of Al,Cu and Li elements.Its structure always maintains inversion symmetry in the GP_T1-I and GP_T1-II zones.But when GP_T1-I and GP_T1-II zones transform into T₁ precipitates,the aluminum-copper layers have hexagonal symmetry.The precipitation process of T₁ precipitate is expressed as:SSS→GP_T1-I→GP_T1-II→T₁.（3）Using advanced atomic resolution HADDF-STEM,ABF-STEM and EELS characterization combined with first-principles calculations,the atomic structure of the T₁ precipitate in the Al-Li-Cu alloy was systematically studied.The distribution of Li elements in the T₁ precipitate is revealed.The outermost interfaces in all thicknesses of the T₁ precipitate are a corrugated Al（2）-Li（2）layers but the shared interface layers between two adjacent repeating units of the T₁ precipitate are coplanar Al（2）and Li（2）layers,which goes against the process of precipitation phase thickening,which is a repeated process of unit cells.Moreover,this thin corrugated shell（corrugated Al（2）-Li（2）layer）does not exist in the single crystal T₁ precipitate.The thin corrugated shell can modulate the crystal structure of T₁ precipitate,leading to the change in the length of the c-axis and the composition with the change in the thickness of the T₁ precipitate.In addition,this study proposes a new orderly arrangement of Al and Cu atoms in the Al-Cu（1）layer.According to the experimentally observed Al-Li layer and the newly proposed Al-Cu layer,the crystal structure of the T₁ precipitate was refined,and a new T₁atomic model was proposed,and the first-principles calculations were used to verify that the new T₁ atomic model is very stable.The T₁ precipitate is an orthogonal structure,the space group is Cmmm,and the orientation relationship with the Al matrix is[100]_T1|[112]_Al,[010]_T1||[110]_Al and（001）_T1|（111）_Al.