Study Of The Precipitates Evolution In Al-4.0Cu-1.0Li(wt.%) Aluminium Lloy During One-step Aging

Al-Cu-Li alloys are typical age-hardening aluminum alloy. They are widely usedas structural materials in aviation and aerospace industry as a promising newgeneration materials, owing to their relatively low density, high specific strength andelastic modulus, combined with balance of fracture toughness at low temperatur e.Their ultra-high strength is attributed to a great number of nano-particles precipitatedduring ageing. Nonetheless, there are still many problems to be solved about theirmicrostructures, such as the hardening precipitates types, precipitates evolution inmorphology or structure and their distribution upon thermal ageing. Also, it is notclear about their contributions to the mechanical properties of the alloy. In addition,about the major T₁phase, and its atomic structure and structure evolution, there arestill many ambiguous conclusions that need to be clarified. In this work, we willdiscuss the relationship between the microstructures and the hardness of an Al-Cu-Lialloy, and particularly analyse the evolution of the main T₁phase.A homemade Al-4.0wt.%Cu-1.0wt.%Li alloy with small amount of Mg, Zn, Siand Zr additions was used in the present study. All samples were solution-treated andthen quenched in water to room temperature. After that, one-step aging treatmentswere carried out in an oil bath furnace under different temperatures. In order tounderstand the relationship between precipitates and the hardness of alloys,micro-Vickers hardness, optical microscopy （OM） and scanning/transmissionelectronic microscopy analysis （SEM/TEM） were carried out. The first-principleenergy calculations were used to investigate the atomic structure and evolution of theT₁phase. The obtained main results are as follows:（1） The atomic structure and structure evolution of T₁phase during ageing at165℃were studied by means of high angle annular dark-field （HAADF）-scanningtransimission electron microscopy （STEM） and the first-principle energy calculations.It is the first time to report that T₁precipitates derive from their own GPT₁zones and avariant of T₁phase exists in Li-excess area. The GPT₁zone is a hexagonal structurewith lattice parameter c=4d111Al=0.932nm, which is the same as T₁phase（c=0.935nm）.The Cu-rich layers of GPT₁zone is similar to that of T₁phase, but the atomic structurebetween the Cu-rich layers is different from T₁phase. Actually, the variant of T₁phase consists of a Li-rich layer and two T₁unit cells separated by the Li-rich layer.Furthermore, T₁precipitates grow in two ways. One is T₁phase nucleates on its GPT₁ zone, then grows regularly along its c-axis. And the other is by the variant of T₁phase. The variant of T₁phase can exist as individual, and also can nucleate on theinterface between T₁phase and the matrix. Anyway, T₁unit cell must derive fromGPT₁zone. Morever, the increasing of hardness of the alloy is attribute to the GPT₁zones during the earlier stage of ageing.（2） Based on the HAADF-STEM results, preliminary structure and models havebeen proposed for GPT₁zones and the variant of T₁phase, and then, they have beenrefined by the first-principle theory calculation. The calculation results show that theformation enthalpy of GPT₁zone and the variant of T₁phase were calculated to be-1.4kJ/mol·atom^-1and-11.1kJ/mol·atom^-1, respectively, indicating that the structureproposed is energetically reasonable.（3） Microstructure evolution of the target alloy aged at135℃,145℃and155℃,165℃, respectively, was systematically studied using TEM in combination withhardness test. It was found that the hardness of alloy aged at165℃for32h reaches amaximum hardness148HV. In the peak aged samples, there are three types ofprecipitates, including GPT₁zone, T₁phase and′/θ′composite phase.