Z.Q. Feng

HRTEM and HAADF-STEM tomography investigation of the heterogeneously formed S (Al2CuMg) precipitates in Al–Cu–Mg alloy

The distribution of variants and three-dimensional (3D) configurations of the heterogeneously formed S (Al2CuMg) precipitates at dislocations, grain boundaries and the Al20Cu2Mn3 dispersoid/Al interfaces were studied in this research. By means of high resolution transmission electron microscopy, we systematically investigated the orientation relationships (ORs) between these heterogeneously formed S precipitates and the Al matrix, and further unraveled that the preferred orientation of S variants at grain boundaries and at dispersoid/Al interfaces are respectively associated with the OR between the precipitate habit plane and the grain boundary plane, and the OR between the precipitate habit plane and the interface plane. The inherent characteristic of the crystal structure of the S phase, i.e. the symmetry of the pentagonal subunit, was considered to be the fundamental factor determining the preference of the variant pair. By using high angle annular dark field scanning transmission electron microscopy tomography, we successively obtained the 3D reconstruction of the S precipitates at these defects. Both the morphology of an individual S precipitate and the overall configuration of the S precipitates nucleated at these defects can be clearly observed without misunderstandings induced by the overlap and projection effects of the conventional two-dimensional methods.

Precipitation sequence of η phase along low-angle grain boundaries in Al-Zn-Mg-Cu alloy during artificial aging

The precipitation sequence of η(MgZn2) phase along low-angle grain boundaries in Al-Zn-Mg-Cu alloy was investigated by examining samples aged at 135 °C for various times from 5 min to 6 h. High resolution transmission electron microscopy (HRTEM) observations and energy dispersive X-ray spectroscopy (EDX) analysis indicate that the precipitation sequence of η phase along low-angle grain boundaries should be supersaturated solid solution (SSS)→vacancy-rich clusters (VRC)→GP II zones→η′→η. Based on the theory of non-equilibrium grain boundary segregation (NGS) and non-equilibrium grain boundary co-segregation (NGCS), the excessive solute elements gradually segregate to the grain boundaries by the diffusion of the solute-vacancy complex during aging treatment. The grain boundary segregation plays an important role in the nucleation and growth of VRC, GP II zones, η′ phase as well as η phase.

Precession electron diffraction assisted orientation mapping of gradient nanostructure in a Ni-based superalloy

Surface mechanical grinding of a Ni-based superalloy can introduce a gradient microstructure in the surface layer with a grain size from nanoscale to microscale. In-depth investigation of the crystal orientation distribution of the surface nanostructured layer is more often, however, not an easy work by using the scanning electron microscope (SEM) based electron backscatter diffraction (EBSD) method due to its sensitivity to lattice distortions and spatial resolution limitation. Here we use a newly developed precession electron diffraction (PED) technique coupled with transmission electron microscopy (TEM) to investigate the microstructural and crystallographic characteristics of the surface gradient nanostructure, with particular emphasis on the topmost nanocrystalline layer. A strong shear texture and a minor Copper texture were identified according to orientation analyses of the 1.6 pm thick near-surface nanocrystalline layer. The PED technique is proved to be practical for two dimensional orientation mapping of severely deformed microstructures at the nanoscale.

Atomic Scale and 3D Characterization of the Heterogeneously Formed S (Al2CuMg) Precipitates at Dislocations in Al-Cu-Mg Alloy

The variant distribution and three-dimensional (3D) configurations of the heterogeneously formed S (Al2CuMg) precipitates at dislocations were studied by means of high resolution transmission electron microscopy (HRTEM) and high angle annular dark field scanning transmission electron microscopy (HAADF-STEM) tomography. The preferred S variant pair along dislocation was proved to be S1 & S4 or its counterparts, and the inherent characteristic of the crystal structure of the S phase, i.e. the symmetry of pentagonal subunit, was considered to be the fundamental factor determining the preference of variant pair. The obtained 3D reconstructions of the S precipitates formed at helical dislocations can clearly reveal both the morphology of individual S precipitates and the overall configuration of the S precipitates nucleated at these dislocations.