Microstructural Evolution And Properties In Microalloyed Aluminium Alloys

With the development of aerospace industry, there is an increasing demand for the aluminium alloys with high specific strength, specific modulus, damage tolerance, heat resistance and corrosion resistance. Over the past several years the remarkable influence which microalloying or so-called trace element additions can have on the microstructure and properties of aluminium alloys continues to attract attention. Despite their commercial success and the fact that considerable research has already been done, many details of the underlying microalloying mechanisms are unclear. The present work is to investigate the effect of small addition of Ag, Sc and Zr on the ageing characteristics and microstructure evolution of Al-Ag, Al-Li and a wide range of Al-Cu-Mg alloys. The major results can be summarized as follows:1.The investigation of microstructure evolution in the Al-0.2Sc and Al-0.2Sc-0.12Zr alloys shows that there are Al3Sc and Al3(Sc1-xZrx) precipitates in as-rolled Al-0.2Sc and Al-0.2Sc-0.12Zr alloy respectively, and their orientation is(001) a // (001) Al3Sc or AI3(Sc1 -xZrx)> [010] a // [010] AI3Sc or A13(Sc1-xZrx)The Al3Sc particles in as-rolled Al-0.2Sc alloy are from hot rolling process, while the larger Al3(Sc1-xZrx) particles in as-rolled Al-0.2Sc-0.12Zr alloy are from the solidification, and the small particles are from hot rolling process. There is segregation of Sc and Zr in the Al3(Sc1-xZrx) precipitation, and Sc is rich in the outside shell while Zr is rich in the core of the particles. The grain and subgrain boundaries can be pinned by the A13Sc particles when annealing after cold-rolled, and this may lead to restricting the recrystallization of the Sc-containing alloys. And it's not until the dissolution of Al3Sc that the recrystallization can happen in this kind of alloys.2.The microstructure of the Al-4Cu-0.3Mg-0.2Sc alloy is dominated by the high density of dislocation loops, which are not present in equivalent alloys containing trace additions of Ag, suggesting that Ag can prevent the agglomeration and condensation of vacancies during theincubation period. The hardening response of Al-4Cu-0.3Mg-0.2Sc alloy can be further enhanced by Small addition of Ag. Microstructure studies show that small Sc retards the dominant precipitation process: Super saturated solid solution(SSSS) , while stimulate another two processes: SSSS-GPB I -S and SSSS-. The presence of Ag stimulates SSSS- process.Silver can segregate to the other precipitates, such as , , whereas only a very small amount of Sc associated with these precipitates. Sc is detected in association with GPB besides mainly existing in form of Al3Sc.3.Small additions of Sc may enhance the age hardening response of the Al-15Ag and Al-2.5Cu-l.5Mg alloys.The size and distribution of precipitates have been affected by the small additions of Sc in Al-15Ag and Al-2.5Cu-l.5Mg alloys, the addition of Sc does result in a refinement of the distribution of r' phase in the Al-Ag alloys and GPB precipitates in the Al-2.5Cu-l.5Mg alloys respectively. Scandium can segregate to the interface of r'/ a and GP /a while it is mainly present within the Al3Sc precipitates in the Al-15Ag-0.2Sc alloy. The mechanism by which trace Sc affects the microstructure of the Al-15Ag alloy is to reduce the ledge density of y ' precipitates.4.The ageing behavior and microstructures of the AI-4Mg-1.5Cu alloys containing silver and Lithium have been studied. The results show that the effect of Li is to increase the rate of hardening, however, Lithium additions in the range 0 to 2.0% does not progressively change age hardening and precipitation process in the alloy Al-4.0Mg-l.5Cu-0.4Ag-0.12Zr aged at 200 C,and significant age hardening and strengthening response only occur with the addition of 2.0wt%Li. TEM investigation results show that the remarkable increase of age hardness and strength attributes to the finely dispersion of ',Z phase and spherical Al3Zr/Al3Li compound particles in the 2.0wt%Li-containing alloy.5.The microstructural evolution and ageing behavio...