The Microstructure And Structural Evolution Of T Phase And ?' Phase In Aluminum Alloys

As lightweight structural materials, aluminum alloys are increasingly applied in the field of transport facilities, such as automobiles and aircraft. The properties of aluminum alloys are closely related to the microstructure, for example, the type, morphology, structure and structural evolution of the second phases can influence the mechanical properties deeply. In this thesis, the structure of the common dispersoid T(Al2oCu2Mn3) and the structural evolution of ?' in aluminum alloys have been studied systematically by means of advanced electron microscopy techniques in spherical aberration-corrected transmission electron microscopy, such as atomic-resolution high-angle annular dark-field (HAADF) and chemical mapping using energy dispersive X-ray spectroscopy (EDS-mapping).T phase is one type of orthorhombic approximants to the decagonal quasicrystal in Al-Cu-Mn alloys with complex structure. Combining the atomic-resolution high angle annular dark field results and first-principles approach, the structure of T phase was investigated basing on the isostructural Mn11Ni4Al60. The site occupation of 156 atoms in the unit cell was determined. The T phase is formed by parallel tessellation of flattened hexagons along b axis, and each unit cell contains two hexagonal subunits. The Cu and Mn atoms occupied the positions at the center (n, o) and perimeter (p, q, r) of the hexagons respectively. And the lattice parameters of the fully relaxed structure is a= 23.98 A, b= 12.54 A, c= 7.66 A.The three type of line defects in the T phase were analysed by means of the Penrose rhombus tiles and the atomic decaration of the three defect configarations was proposed. And the presence of line defects is one cause of the compositon range in the T phase. Besides, the twinning structure was also studied. On the twin boundaries, hexagons alternate between two different orientations at 144°with each other. According to the atomic matching rules and the symmetry, the twinning mode of the T phase was suggested to exhibit translation-rotation symmetry with the translation vector being 1/4<101> and the twofold rotation axis as the twinning direction<101>. In addition, using the high-resolution HAADF image and fast-fourier transformation (FFT), the orientation relationship between T phase and the a-Al matrix was proposed as follows:[010]T//<010>Al,{703}T//{002}Al; [010]T //<010>Al,{1101}T//{002Al; [010]T//<010>Al,{004}T//{002}Al, with the habit planes as {200}T and {101} T.The formation mechanism of the precipitate phase ?' in an Al-5.7Cu alloy is investigated by means of high-angle annular dark field (HAADF) method. The results show that in the presence of ?",?'phase is formed by in-situ transformation from ?" phase by rearranging atoms within the precipitate. The nucleation of ?' takes place randomly at different sites within the ?" precipitate. With transformation proceeding, the preferentially formed ?' sections grow and form a whole ?' precipitate. According to the interface structure between ?" and ?', the atomic rearrangement mechanism within the precipitate was discussed. In addition, the origin of antiphase domain boundaries (APDBs) in 0'was determined.