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Mg-xAl alloys (x = 3–12 wt%) were prepared by high pressure solidification. The microstructure of the alloys was studied using Scanning Electron Microscope (SEM) and Electron Backscatter Diffraction (EBSD), and the strengthening mechanism was analyzed in combination with the compressive properties of the experimental alloys. Furthermore, the grain boundary segregation of Al solute was investigated by experiments and first-principles calculations. For the Mg-xAl alloys solidified under atmospheric pressure, the microstructure will be a single solid solution when the Al content of the alloy is less than 5 wt%. However, for the Mg-xAl alloys solidified under 4 GPa pressure, the microstructure will always ba e single solid solution when the Al content of the alloy is less than 12 wt%. Moreover, the segregation of Al solute with a high concentration gradient can be observed at plenty of grain boundaries (area fraction is as high as 46%), because α-Mg dendrites are significantly refined under high pressure. In addition, many Al atoms are dissolved in the α-Mg matrix owing to high pressure. On this basis, it is found that grain boundary segregation of Al solute can decrease the grain boundary energy, and increase the bonding strength of grain boundaries. As a result, the Mg–Al alloys solidified under high pressure have superior compression properties. The compression strength of the Mg–9%Al alloy and the Mg–12%Al alloy solidified under 4 GPa is as high as 458 MPa and 490 MPa respectively, the alloys retain superior plasticity simultaneously. [ABSTRACT FROM AUTHOR]

Abstract: Variant selection of intergranular bcc-Cr precipitates in a Ni–43mass% Cr alloy was studied by electron backscattering diffraction (EBSD) in a scanning electron microscope equipped with a field emission gun (FESEM) and Kikuchi line analysis in transmission electron microscope (TEM). A single variant was invariably selected at the underlying grain boundaries (GB) when GB-precipitates were KS-related with respect to both the adjacent matrix grains. Meanwhile multiple variants were formed at the GB when GB-precipitates were KS-related to one of the adjacent matrix grains. Theses variant selections were examined with the tilt angle to underlying GB of low-energy interphase boundaries and the orientation relationship with respect to both the adjacent matrix grains. The underlying mechanism was discussed from the viewpoint of activation energy for nucleation of two-dimensional nuclei. [Copyright &y& Elsevier]

The mechanical properties of Al-Zn-Mg alloys via multi-stage aging treatments were studied to explore the strength-toughness trade-off. The microstructures of aged alloys were characterized by scanning electron microscope (SEM) and transmission electron microscope (TEM). The results indicated that large numbers of GP zones and some part of η′ phases were formed during the low-temperature (90 °C) aging at the third-stage aging, which consumed considerable solid solubility. This prevents fast growth and coarsening of precipitates at the fourth-stage aging (150 °C). Finally, the mixed precipitation state of mainly η′ phases and some η phases formed. The grain boundary phase grew insignificantly, which is beneficial to decrease the strength gap between intragranular and grain-boundary. The intragranular equilibrium η phase is beneficial to pin the movement of dislocation, which avoids fracture caused by accumulation of dislocations around the large second-phase and on the grain boundaries. Although the strength of materials decreased slightly, the impact toughness of materials significantly increased due to the positive effect of the intragranular equilibrium η phase. Compared with the conventional retrogression and re-aging (RRA) treatment (120 °C/24 h + 185 °C/105min + 120 °C/12 h), the designed four-stage ageing treatment (FSA) (120 °C/24 h + 185 °C/105min + 90 °C/12 h + 150 °C/12 h) resulted in the decrease of tensile strength and yield strength by only 3% and 4%, respectively, but contributed to the increases of elongation and impact toughness by 15% and 17%, respectively. [ABSTRACT FROM AUTHOR]

In this work, the effects of shear-resistant plate-shaped θ′ (Al 2 Cu) precipitates on mechanical performance and fracture behavior of a heat-treatable Al–Cu alloy were studied. The precipitates, dislocations and micro-voids around grain boundaries were observed using transmission electron microscope and scanning electron microscope equipped with electron backscatter diffraction. The Al–Cu alloy was overaged to ensure that the θ′ phase was the only precipitate in the material. The results show that concentration of local strain and pile-up of dislocations occurred along grain boundaries during tensile deformation. A method was developed to calculate the total dislocation density nearby the grain boundaries in the deformed alloy, from which a quantitative relation was established between precipitate number density and void areal density on intergranular fracture surface. The effect of precipitates on the mechanical properties of Al–Cu alloy was also discussed qualitatively. The precipitate number density of the over-aged Al–Cu alloy acted as the dominating factor that influenced the dislocation density and void density on grain boundaries, which thus determined the mechanical properties and fracture behaviors of the alloy. [ABSTRACT FROM AUTHOR]