The Influence Of Heat Treatment Process On The Precipitates And Properties Of2024Aluminum Alloy

2024aluminum alloy which belongs to the Al-Cu-Mg alloys is a kind of heat treatment strengthening aluminum alloy. It possesses comprehensive excellent performances, such as high strength, low density and good heat resistance, widely used in the aircraft industry and other fields. However the understanding about the structures and the phase transformations of the hardening precipitates in the alloy during ageing process are still poor.This thesis evaluated the alloy in terms of performance(such as hardness, strength, electrical conductivity etc.) of ageing process, by measuring the micro-hardness, tensile tests and electrical conductivity measurement. Optical microscopy, X-ray diffraction(XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and high-annular dark field (HAADF) were used to build up the relationship between the microstructure(especially the aging precipitated phase) and the properties of samples, to provide a theoretical basis for optimizing the heat treatment process and obtain a more ideal performance.The second phase particles of2024aluminum alloy included the larger size phase particle—T phase (Al20Cu2Mn3) and nano-precipitated phase—S phase (Al2CuMg).T-phase particles are very stable, which appear and grow up in the homogenization heat treatment process. Their size, structure and composition will not change, in the subsequent solution and aging heat treatment process. S-phase is one of the key strengthening precipitate phases in high-strength2024aluminum alloy.The alloy’s mechanical properties such as hardness, tensile strength and its internal microstructure are closely related to the precipitates transformation:The T6heat treatment(solid solution at495℃for1h and artificial aging at180℃), was used for the2024aluminum alloy. The micro-hardness of the alloy after solution was83HV. The hardness increased by30HV in a few minutes of initial aging. At the early stage of aging, the hardness increased because of the formation of the layered structure of atomic clusters,which is due to solute atoms (Cu and Mg) segregated from the supersaturated solid solution. After aging for5h, S phase began to nucleate and grow, and the number of S phase also increased. The hardness continuously increased until24h of aging peak. Hardness was about150HV, compare to the solid solution state, the hardness increased by nearly70HV. The hardness began to decrease significantly after aging for27h。HAADF-STEM was used to investigate the microstructure features of the T-phase particles in the2024aluminum alloy. The obtained images viewed along the [100]T direction show that there are many micro-twins in the T-phase particles. Two types of twinning microstructures exist in these particles:parallel micro-twins and spider web-like multiple micro-twins. It was observed that in the (010)T projection, a T-phase crystal consists of "eye-like" atomic structure units, and each unit has two heavy atomic columns in the core area surrounded by twelve other atomic columns. At the twin interfaces and the multiple twin junctions the arrangements of "eye-like" structure units are disordered. It is also shown that another different T-phase structure exists, whose "eye-like" structure units are distorted significantly.