Microstructure of Al-5Cu-1Li-0.6Mg-0.5Ag-0.5Mn Alloys

Abstract

Microstructural optimization of Al-Li alloys plays a key role in the adjustment of mechanical properties as well as corrosion behavior. In this work, Al-5Cu-1Li-0.6Mg-0.5Ag-0.5Mn alloy was homogenized at different temperatures and holding times, followed by aging treatment. The microstructure and composition of the homogenized alloys and aged alloys were investigated. There were Al7Cu4Li phase, Al3Li phase, and Al2CuLi phases in the homogenized alloys. The Al7Cu4Li phase was dissolved with an increase in homogenization temperature and holding time. Al2Cu phase and Al2CuLi phase coarsened during the homogenization process. The alloy homogenized at 515 °C for 20 h was subjected to a two-stage aging treatment. Peak-age alloy, which had gone through age treatment at 120 °C for 4 h and 180 °C for 6 h, was mainly composed of α-Al, Al20Cu2Mn3, Al2CuLi, Al2Cu, and Al3Li phases. Tafel polarization of the peak-age alloys revealed the corrosion potential and corrosion current density to be −779 mV and 2.979 μA/cm2, respectively. The over-age alloy had a more positive corrosion potential of −658 mV but presented a higher corrosion current of 6.929 μA/cm2.

A Quasi In-Situ Study on the Microstructural Evolution of 2195 Al-Cu-Li Alloy during Homogenization

“…When further heated to 450 °C (see Figure 15 c), the Cu-SZ disappears, the T B phases and θ phases on the grain boundary are coarsened, Mg in the S phases continuously diffuses towards the matrix, transforming the S phase into the θ phase, and the newly formed θ phase starts to dissolve, which is consistent to the XRD results (see Figure 9 ) and aforementioned diffusivity calculation. Chen et al [ 30 ] previously found that T B and θ phases coarsened during the homogenization of the Al-5Cu-1Li-0.6Mg alloy, and Yang et al [ 29 ] previously found that the S phase dissolved before the θ phase in the Al-3.52Cu-1.28Li-0.38Mg alloy. However, Chen did not distinguish the primary T B eutectic phase from the later precipitated T B phase, and neither of them found that the S phase transformed into the θ phase during homogenization.…”

“…Although Yang thought that the S phase dissolves first compared to the θ phase, he did not find that the S phase is first transformed into the θ phase and then dissolved. Chen et al [ 30 ] studied the Al-5Cu-1Li-0.6Mg alloy and observed that θ and T 1 phases are coarsened during homogenization, and T B and θ phases are dissolved at elevated temperatures. Chen noticed that the T B phase coarsened during homogenization, but he did not find that the T B phase will precipitate during homogenization and did not distinguish the primary eutectic T B phase from the T B phase that precipitated later.…”