The influence of grain size on the mechanical properties of nanocrystalline aluminium

Preparation of Ti-46Al-8Nb Alloy Ingots beyond Laboratory Scale Based on BaZrO3 Refractory Crucible

The high Nb-containing TiAl-based alloy ingot beyond laboratory scale with a composition of Ti-46Al-8Nb (at.%) was prepared by a vacuum induction melting process based on a BaZrO3 refractory crucible. A round bar ingot with a diameter of 85 mm and a length of 430 mm was finally obtained, and the chemical composition, solidification pathway, microstructure and tensile properties of the ingot were investigated. The results show that the deviations of Al and Nb content along a 430 mm long central part of the ingot are approximately ±0.39 at.% and ±0.14 at.%, and the oxygen content in the ingot can be controlled at around 1000 ppm. The structure of the alloy ingot is a full lamellar structure composed of γ and α2 phases, and the thickness of the lamellae is approximately 0.53 μm. In case of the α2 phase, the surface content of the ingot is higher than the middle region and the centrical region; also, it indicated a decreasing trend. During cooling, the alloy solidified from a peritectic reaction (L + β→α) rather than the solidified via β phase (β→α). In addition to Al segregation and Nb segregation, β-phase particles associated with γ phase at the triple junction of the colonies were observed. Moreover, the tensile properties of the longitudinal-cut sample in the ingot is significantly better than those of the transverse-cut sample, with a tensile strength of up to as high as 700 MPa and a corresponding fracture elongation of 1.1%. However, the tensile strength of the transverse-cut sample is only 375 MPa, and the fracture elongation is 0.52%.

Nanostructure Characteristics of Al3Sc1-xZrx Nanoparticles and Their Effects on Mechanical Property and SCC Behavior of Al-Zn-Mg Alloys

The Nanostructure characteristics of Al₃Sc_1−xZr_x nanoparticles and their effects on the mechanical properties and stress corrosion cracking (SCC) behavior of Al–Zn–Mg alloys were investigated by 3D atom probe analyses, high-angle annular-dark-field scanning transmission electron microscopy methods, electron back scattered diffraction techniques, electrochemical measurements, slow strain rate tests and quantitative calculations. The results show that adding small amounts of scandium (0.10 percent by weight) and zirconium into Al–Zn–Mg extrusion bars can precipitate Al₃Sc_1−xZr_x nanoparticles with a number density of (7.80 ± 3.83) × 10²¹ per cubic meter. Those particles, with a low lattice misfit with matrix (1.14 ± 0.03 percent) and stable core-shell L12-nanostructure in aged Al–Zn–Mg alloys, can increase the yield strength by 161 ± 7 MPa via strong Orowan strengthening (the theoretical calculated value is 159 MPa) and weak Hall-Petch strengthening (the theoretical calculated value is 6 MPa). Moreover, Al₃Sc_1−xZr_x nanoparticles can change the fracture mechanism of alloys in 3.5% NaCl solution from intergranular cracks to transgranular failure, and decrease the proportion of high-angle grain boundaries from 87% to 31%, thus reducing the microchemistry differences around the grain boundaries and anodic dissolution kinetics, and improving intergranular SCC resistance and ductility. This study offers a new approach to the simultaneous improvement in mechanical property and corrosion performance of high strength alloys.

Deformation Mechanism of Nanocrystalline Al-Fe Alloys by Analysis from Ab-Initio Calculations

Recently nanocrystalline Al-Fe alloys produced by a vapor quench method have been reported. These alloys are supersaturated solid solution and exhibit high strength with good ductility. It is postulated that the high strength of the Al-Fe alloys could be achieved by both the nano-grained structures and the solid solution strengthening. The contribution to the yield strength due to both the grain size strengthening and the solid solution strengthening were analyzed from the experimental data. Then the contribution to the yield strength due to the solid solution strengthening was estimated from the misfit strain calculated from the first principles in order to compare with analytical results estimated from the experimental data.