To study the effect of Sc and Sr additions on modifying eutectic silicon particles and mechanical properties for Al-Si-Mg casting alloy, they were added with different amounts in F357 alloy without beryllium addition in the present work. It was found that (0.4 wt.% Sc and 0.04 wt.% Sr)-modified F357 alloy presented the optimal tensile properties when compared with the individual addition of Sc or Sr. This was mainly attributed to the synergic modification of eutectic Si in F357 alloys due to the combined additions of Sc and Sr. The silicon modification mechanisms via Sc and Sr were emphasized to be examined in this paper, and the fracture mechanism of the obtained alloys was also discussed.

Recently, mechanical properties improvement through microstructure design has attracted worldwide attention. In the present study, a “bottom to up” route including mechanical milling and spark plasma sintering was employed for fabricating ultrafine lamellar structured Al2024 alloy. Microstructure observation revealed that flake shaped powder with a mean grain size of about 60 nm was obtained by mechanical milling for 20 h. The flake shaped powder self-assembled during sintering, forming a bulk sample with periodic lamellar structure and ultrafine grain size (1.14 µm). Tensile test revealed that the ultrafine lamellar structured alloy exhibited significantly enhanced strength (yield strength 375 MPa, tensile strength 456 MPa) compared to the conventional O-state counterpart, but the tensile ductility was reduced (tensile elongation 5%). Strategies for further optimizing the mechanical properties of the bulk samples were discussed.

Laminated Ti–Al composite sheets with different mesostructures have been fabricated through hot pressing. The influence of mesostructure on mechanical properties of the composite is investigated. The results indicate that with the increase of sintering temperature, different mesostructures of composite are obtained, that is, laminated Ti/Ti–Al composite, laminated Ti3Al/TiAl composite, and monolithic Ti3Al/TiAl composite. The mechanical properties tests reveal that laminated Ti/Ti–Al composite exhibits better comprehensive mechanical properties, including flexural strength, fracture toughness, and microhardness, than those of laminated Ti3Al/TiAl composite and monolithic Ti3Al/TiAl composite. The fracture analysis shows that the propagation route of crack is zigzag for Ti/Ti–Al composite, curving for laminated Ti3Al/TiAl composite, and approximately a straight line for monolithic Ti3Al/TiAl composite. The relevant strengthening and toughening mechanism of the composites is discussed.

•The Ti/Ti-Al composites were fabricated from pure Ti and Al foils by hot press sintering.•The composites contain α-Ti layer and Ti-Al intermetallic layer with Ti3Al, TiAl, TiAl2 and TiAl3 phases.•The α-Ti volume fraction has significant effect on the strength and fracture toughness.•The sample with 53 vol.% α-Ti exhibit much higher mechanical property than TiAl alloy and Ti/Ti3Al laminates reported.•The strengthening mechanism and toughening mechanism were discussed.Ti/Ti-Al multilayered composites with different volume fractions of α-Ti layer were fabricated by hot press sintering. Microstructural observation indicates that the phase compositions in all the composites are α-Ti, Ti3Al, TiAl, TiAl2 and TiAl3. The tensile strength and fracture toughness of Ti/Ti-Al multilayered composites are influenced by the volume fraction of α-Ti layer. The composite with 53 vol% Ti presents the highest tensile strength and suitable ductility at room temperature, and its fracture toughness can reach 47.6 MPam, which is much higher than that of TiAl intermetallic alloy and Ti/Al3Ti composites. Fracture analysis reveals that a certain volume fraction of Ti layers can effectively prevent the crack propagation when the crack is extended to the interface, resulting in high tensile strength and acceptable ductility at the same time. The improvement of toughness is associated with crack deflection and crack bridging provided by the appropriate thickness of remaining α-Ti layer.

Graphene is considered as an excellent reinforcement due to its excellent physical and mechanical properties. In this work, milling balls of small diameter were used to reduce the collision energy during the ball milling process, and graphene nanoplates (GNP) reinforced Al5083 alloy matrix composites were successfully fabricated by ball milling, hot pressing and hot extrusion. The microstructures of the ball-milled powders and GNP/Al5083 composites were characterized by X-ray diffraction, Raman spectroscopy, scanning electron microscopy and transmission electron microscopy. The results showed that the GNP could remain after ball milling. However, the Al4C3 phase was found in bulk GNP/Al5083 composites, suggesting that some of the GNP reacted with Al and formed Al4C3 during the consolidation process. Tensile tests revealed that both the yield strength and ultimate tensile strength of the 1.0 wt.%GNP/Al5083 composite were increased by 50% compared with pure Al5083 fabricated under the same procedure. The relevant strengthening mechanisms of the composites were discussed.

The Al-5Ti-0.2C-based grain refiners with different contents of rare earth (RE) were successfully prepared via powder metallurgy and vacuum casting. The microstructural evolution has been studied by X-ray diffraction (XRD), differential scanning calorimetry (DSC), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results showed that the RE addition resulted in the formation of TiAl3/Ti2Al20RE core-shell structured primary particles, and the size of TiAl3 core decreased, while the thickness of Ti2Al20RE increased with increase of RE contents. As compared to Al-5Ti-0.2C grain refiner, the grain refining efficiency was gradually improved with increase of RE contents, which was mainly attributed to the TiAl3/Ti2Al20RE core-shell structured primary particles and insoluble TiC nuclei formed in a-Al matrix. The formation mechanism of core-shell structure was further investigated based on Ginstling-Brounstein model.SEM images and line scan elemental profiles across the primary particle of Al-5Ti-0.2C-3.0RE (a, b)

In this study, Al–Ni–Ce–Fe–Cu alloy powders with composition of Al87Ni8.5Ce3Fe1Cu0.5 (at%) were atomized and then subjected to mechanical milling for different hours. The consolidation of the mechanical milled powders was performed by using spark plasma sintering followed by hot extrusion. Microstructure analysis and mechanical properties test were employed to the powders and the bulk samples, respectively. XRD patterns and DSC curves indicated that amorphous and nanocrystalline microstructure was obtained after milling for 170 h. Room temperature compression test showed that the as-extruded bulk sample made from 170 h milled Al–Ni–Ce–Fe–Cu powders had fracture strength of about 780 MPa, however, without any visible fracture deformation. In addition, the ductility can be significantly optimized by introducing 20 wt% pure Al powder. The strengthening mechanisms of the bulk samples were discussed.

•Al-2024/FMG composites have been prepared by powder metallurgy method.•Mechanical milling resulted in significant grain refinement of the Al matrix.•The high strength is attributed to the refined microstructure and FMG particles.Fe-based metallic glass (FMG) particles reinforced Al-2024 matrix composites were fabricated by using the powder metallurgy method successfully. Mechanical alloying result in nanostructured Al-2024 matrix with a grain size of about 30 nm together with a good distribution of the FMG particles in the Al matrix. The consolidation of the composites was performed at a temperature in the super-cooled liquid region of the FMG particles, where the FMG particles act as a soft liquid-like binder, resulting in composites with low or zero porosity. The microstructure and mechanical properties of the composites were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and compression test. The yield and fracture strength of the composites are 403 MPa and 660 MPa, respectively, while retaining a considerable fracture deformation of about 12%. The strengthening mechanism is associated with the grain refinement of the matrix and uniform distribution of the FMG particles.

In the present paper, hybrid structured B4C particles reinforced Al2024 matrix composites were prepared by the powder metallurgy method. The composites made from 100% mechanical milled composite powders have fracture strength of 670 MPa. With the addition of un-milled Al2024 powder increased from 10 vol% to 40 vol%, the room temperature compression strength decreased from 1115 MPa to 739 MPa, without any visible plastic deformation. However, when the fraction of un-milled Al2024 powder increased to 50 vol%, the compression strength was decreased to 580 MPa, while retaining a remarkable fracture strain up to 10%. The microstructures of the composites with different composition were examined by a scanning electron microscope and a transmission electron microscope. The contribution from different strengthening mechanisms was discussed. The hybrid structures are proved to account for the dramatic change of the fracture mechanism of the composites.

•Al-2024/B4C composites have been prepared by powder metallurgy method.•Mechanical milling resulted in significant grain refinement of the Al matrix.•The compression strength of Al-2024/20 wt.% B4C are 950 MPa.•The high strength is attributed to the refined microstructure and B4C particles.In this work, we have evaluated the effect of high volume fraction of B4C particles (>20 vol.%) on the microstructure and mechanical behavior of Al-2024 matrix composites. The composites were fabricated by mechanical milling of the matrix and the reinforcements together followed by hot extrusion. Mechanical milling resulted in significant grain refinement of the Al-2024 matrix phase (∼35.4 nm), while hot extrusion provided the combined advantage of homogenous dispersion of the reinforcement in the matrix as well as clean and strong interface between the two phases. After consolidation at 823 K, the grain size of the Al matrix has grown up partially, while the grains around B4C particles are still at nano-scale because of the large difference in the coefficients of thermal expansion (CTE) between the matrix and the reinforcement. The microhardness and compression strength of the composites reinforced by the B4C particles are 260 and 950 MPa, respectively, which are almost twice as high as that of the matrix. The superior mechanical properties are attributed to the refined microstructure of the matrix and the homogeneous dispersion of the B4C particles.

This paper proposed a new method for evaluating the structural stability of bulk metallic glasses (BMGs) based on dilatometric measurements. During heating in the dilatometric experiments, the BMGs expanded continuously with increasing temperature. When the temperature reached the glass transition temperature (Tg), viscous shrinkage occurred due to the viscosity of material becoming lower. Since the inhomogeneous nature of the metallic glasses at atomic level, the processes of rigid expansion and the viscous shrinkage co-exist in a certain temperature region. The expansion stopped completely at a temperature (named Tp here) beyond Tg. The values of the temperature region, ΔTgp = Tp − Tg, and the corresponding time interval (Δtgp) and the activation energy (Ep) corresponding to the expansion processes, are the reflection of the structural stability of BMGs. Investigating the co-existing processes kinetically and thermodynamically, we can make an insight into the structural stability of metallic glasses. Based on this idea, the thermal expansion behaviors of Mg-, Pd-, Zr-, Ti- and Fe-based BMG were studied, and their structural stability was evaluated by the parameters of ΔTgp, Δtgp and Ep.

•Large amounts of nano-scaled Si crystals were achieved by high pressure solution treatment and aging treatment.•Ledge growth of Si precipitates was directly observed by TEM in Al(Si) solid solution.•Relationship between the Gibbs free energy and ledge density on Al/Si interfaces was quantitatively developed.A dense uniform distribution of nano-scaled Si precipitates has been achieved in Al-7Si alloy by high pressure solution treatment (HPST) and aging treatment. Precipitation behavior of Si phase was investigated using transmission electron microscopy (TEM). The results reveal that Si clusters initially precipitate from the Al(Si) solid solution, providing preferable nucleation sites for the equilibrium Si phase. The Si crystals exhibit spherical shape at first, and then grow rapidly parallel to {111}Si planes. The shortage of growth ledges on {111} interfaces leads to the inhibition of growth perpendicular to these planes, which results in Si triangles and platelets with high aspect ratio. An equation about the relationship between the relative free energy and the equilibrium ledge densities on Al-Si interfaces was proposed.Download high-res image (772KB)Download full-size image