The twinning in the intermetallic compound Al18Cr2Mg3 was observed and identified by transmission electron microscopy.The geometric description of the twinning in Al18Cr2Mg3 with 184 atoms per unit cell was investigated at atomic level.The classical theory of deformation twinning states that the net atomic displacements are divided into shear and shuffles during twinning.Based on the theory and the hypothesis of full mirror reflection symmetry,the displacement and angle of shuffle were calculated for each atom.The geometric calculation results show that all of the atoms in single crystal Al18Cr2Mg3 have to shuffle.Moreover,the calculation results can be divided into 22 groups,which do not show obvious regularity.
The recent developments of electron tomography（ET） based on transmission electron microscopy（TEM） and scanning transmission electron microscopy（STEM） in the field of materials science were introduced. The various types of ET based on TEM as well as STEM were described in detail, which included bright-field（BF）-TEM tomography, dark-field（DF）-TEM tomography, weak-beam dark-field（WBDF）-TEM tomography, annular dark-field（ADF）-TEM tomography, energy-filtered transmission electron microscopy（EFTEM） tomography, high-angle annular dark-field（HAADF）-STEM tomography, ADF-STEM tomography, incoherent bright field（IBF）-STEM tomography, electron energy loss spectroscopy（EELS）-STEM tomography and X-ray energy dispersive spectrometry（XEDS）-STEM tomography, and so on. The optimized tilt series such as dual-axis tilt tomography, on-axis tilt tomography, conical tilt tomography and equally-sloped tomography（EST） were reported. The advanced reconstruction algorithms, such as discrete algebraic reconstruction technique（DART）, compressed sensing（CS） algorithm and EST were overviewed. At last, the development tendency of ET in materials science was presented.
Metals heat-treated under high pressure can exhibit different properties. The heat-induced pressure on 2024 aluminum alloy during restricting expansion-deformation heat-treatment was calculated by using the ABAQUS finite element software, and the effects of the mould material properties, such as coefficient of thermal expansion （CTE）, elastic modulus and yield strength, on the pressure were discussed. The simulated results show that the relatively uniform heat-induced pressure, approximately 503 MPa at 500 ℃, appears on 2024 alloy when 42CrMo steel is as the mould material. The heat-induced pressure increases with decreasing the CTE and the increases of elastic modulus and yield strength of the mould material. The influences of the CTE and elastic modulus on the heat-induced pressure are more notable.
Electron irradiation effects on phase stability of the E （Al18Cr2Mg3） phase have been investigated by high- angle annular dark-field scanning transmission electron microscopy and high-resolution transmission electron microscopy （HRTEM）. The in situ HRTEM observations show that the Ala8Cr2Mg3 particles with different thickness undergo amorphization and dissolution under 300 keV electron irradiation at 25 ℃. The results indicate that the intermetallic compound Al18Cr2Mg3 is unstable under electron irradiation, and structural changes mainly depend on the thickness of particles. Amorphization in the thick particles is caused by a combination of chemical disordering and an increase in point defect concentration. Dissolution after amorphization in the thin particles is attributed to the diffusion of point defect towards the Al matrix.
