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To optimize the mechanical properties of the Mg matrix composites, the carbon nanotubes (CNTs) and in-situ micro-Mg2Sim and nano-Mg2Sin hybrid reinforced composites were prepared by dispersion of nano-Sin and carboxylated CNTs on both surface of AZ91D and hydroxylated micro-Sim through hydrogen bonding and ball milling, and chemical reaction between the Mg matrix and Si particles occurred during spark plasma sintering (SPS). The multi-scale in-situ generated Mg2Si and nano-MgO formed at the CNTs/Mg interface enhance the interfacial bonding. As the ratio of CNTs/Mg2Si increases from 0.1 to 0.5vol.%, the mechanical properties of CNTs-Mg2Si/AZ91D composites gradually improve and then decrease. The 0.3CNTs-2.7Mg2Si/AZ91D composite has the best hardness (92.7 Hv), compressive ultimate strength (408.2 MPa), compressive yield strength (238.3 MPa), and elongation (24.5%) that are increased by 15.6%, 53.2%, 50.4%, and 2.9%, respectively compared with the AZ91D alloy, which is ascribed to the strong interface bonding between the matrix and multi-scale hybrid reinforcements that optimizes the synergetic strengthening of load transfer, thermal mismatch and Orowan mechanisms. An effective method is proposed to achieve the excellent performance of the Mg matrix composites.

The evolution of semisolid microstructure during partial remelting of (CNTs + Sip)/AZ91D powder compacts prepared by cold pressing was studied. The results indicate that rapid grain coarsening is driven by the dissolution of eutectic β phase material during the initial heating period of 0–10 min, so the AZ91D powders with fine equiaxed grains surrounded by intergranular eutectic phases evolve into compact particles. As the heating time proceeds, α-Mg particles were gradually separated by liquid due to the phase transformations of α-Mg + β → L and α-Mg → L. The primary particles coarsened rather slowly and the as-received Mg powder evolved into nearly spheroidal particles surrounded by liquid phase after partial remelting. The in situ synthesized Mg2Sip were distributed homogeneously around the CNTs with maintained structural integrity during partial remelting. Moreover, this microstructural evolution was accompanied by densification through pore filling. An ideal semisolid ingot suitable for thixoforming can therefore be obtained by partially remelting a (CNTs + Sip)/Mg powder compact.

The strengthening effect of composites is rather limited in comparison with the excellent properties of graphene due to difficulty in acquiring strong interfacial bonding. To enhance the interfacial bonding and reduce the interface mismatch between the matrix and reduced graphene oxide (rGO), a novel strategy in this study is proposed through generating hybrid layered double oxides (LDO) nanoparticles on rGO (LDO@rGO). The 2024Al composites with heterogeneous structure were constructed by ball milling and spark plasma sintering (SPS), which was reinforced by flake-like LDO@rGO-rich zones contained LDO@rGO in the Al matrix with fine grain size of ~ 1μm. The yield strength, elongation and fracture energy of 1 vol.% LDO@rGO/Al composite with heterogeneous microstructure were 69.6%, 63.9% and 140.5% higher than those of the composite reinforced by uniformly distributed 0.67 vol.% graphene oxide (GO), respectively, achieving an improvement in the strength-ductility synergy of the fabricated LDO@rGO/Al composite. The rationally spatial arrays of LDO@rGO-rich and LDO@rGO-free zones are beneficial for promoting the synergistic strengthening of Orowan, solid solution, thermal mismatch and load transfer and simultaneously toughening the composite through enhanced crack deflection and bridging effects. The proposed method offers a promising route for fabricating composite with optimized and improved material properties by coupling interface and heterogeneous structure.

In this study, SiC particle reinforced Al2024 matrix composites were fabricated by powder thixoforming (PT). Meanwhile, 2024 alloys were fabricated by permanent mold cast (PMC) and PT, respectively, to reveal superiorities of PT technology over the traditional processing technologies and the resulting composite over the matrix alloy. The microstructures and mechanical properties of the three materials were comparatively investigated. The results indicated that both the PT materials possessed finer spheroidal primary particles and smaller eutectic concentration, but the PMC alloy comprised large equiaxed grains, continuously net-shaped eutectic structures, and many porosities. The mechanical properties of the PT alloy were significantly higher than those of the PMC alloy because of the enhanced compactness and work hardening, decreased eutectic concentration, and fine primary particles. The incorporation of SiCp to the PT alloy further brought improvements, the ultimate tensile strength (UTS), yield strength (YS), and hardness were increased by 29.3% (UTS = 388 MPa), 35% (YS = 297 MPa), and 46.8% (hardness = 122.6 HV), respectively. A strengthening model considering different strengthening mechanisms and SiCp failure was proposed and YS of composite could be exactly predicted.

In this study, the effects of mold temperature on the microstructure and tensile properties of 2024 Al-based composites reinforced with SiC particles (SiCp; 10 vol.%) and fabricated via powder thixoforming were investigated. The tensile properties of the composite were dependent upon the mold temperature because it affected the secondary solidification behavior, compactness of the secondary solidified structures, dislocation density introduced by the plastic deformation that occurs during thixoforming, and recrystallization behavior. The tensile properties of the composite that was thixoformed at 350 °C exhibited the largest improvements, with the ultimate tensile strength (UTS) increasing by 235.4% (UTS = 379 MPa) and elongation decreasing by 31.0% (elongation = 4.0%) compared to those of the as-cast 2024 alloy. The increased UTS was ascribed to the thermal-mismatch and load-transfer strengthening mechanisms, enhanced compactness, increased dislocation density, decreased grain size, and lower concentration of the deleterious θ-phase. The fracturing of the composites was caused by the cracking and debonding of the SiCp as the mold temperature increased, which in turn led to the total matrix failure. Keywords: Powder thixoforming, Al-based composite, Tensile properties, Microstructure

SiCp/2024 composites were fabricated by powder thixoforming, and the effects of pressure on the microstructure and mechanical properties were studied. The results indicate that the pressure applied during thixoforming affected the secondary solidification behavior by altering the solidification rate, microstructure compactness, plastic deformation, loading capacity of SiCp, and thus the fracture regimes and the mechanical properties. The tensile strengths increased as the pressure increased from 128 to 224 MPa because of the improved compactness, enhanced work hardening and loading capacity of SiCp, and increased concentration of the θ-phase and then decreased owing to the serious stress concentration and θ-phase harmfulness. The composite thixoformed under 224 MPa exhibited the largest improvements, with an ultimate tensile strength of 388 MPa, a 0.2 % offset yield strength (YS) of 297 MPa, and an elongation of 3.8 %, which were increased by 29.3 and 35 % and decreased by 63.5 %, respectively, compared with those of the 2024 alloy. The increment in the tensile strength was due to the synergetic contributions resulting from the strengthening mechanisms of load transfer, thermal mismatch, geometrically necessary dislocations, and grain refinement.

A new technology for preparing and forming ceramic particle-reinforced metal matrix composites, powder thixoforming, has been proposed. The effect of the SiCp volume fraction on the microstructure and tensile properties of thixoforged SiCp/2024 Al-based composites was studied. The results indicated that the volume fraction affected the effective liquid fraction, primary particle size and shape, and microstructure compactness. A composite with 10 vol% SiCp had the best comprehensive tensile properties, an ultimate tensile strength of 388 MPa, a yield strength of 295 MPa, and an elongation of 3.8%, representing increases of 29.3 and 33.5%, and a decrease of 63.5%, respectively, compared with the values for the thixoforged 2024 Al matrix alloy. During tensile testing, cracks were initiated in the secondary solidified structures, the debonded SiC/Al interface, and the cracked SiCp. For composites containing over 10 vol% SiCp, agglomerated SiCp acted as additional zones of crack initiation.

Aggregation and weak interfacial bonding limit the superior strengthening potential of the carbon nanotubes (CNTs) in the metal matrix composites. To overcome these challenges, in situ Mg2Si nanoparticles (Mg2Sip) have been hybridized with surface modified CNTs through chemical oxidization. The synergistic strengthening was enhanced by tailoring the volume ratios of CNTs to Mg2Sip in the Mg matrix through powder thixoforming technology. Herein, when 0.3CNTs-1.2Mg2Sip was added, a yield strength, ultimate tensile strength, and elongation of 213 MPa, 271 MPa, and 6.3%, respectively, were obtained, which were 18.3%, 16.8%, and 3.3% higher than those of the 0.75CNTs-0.75Mg2Sip/Mg composites, respectively. The strengthening effects of the CNTs-Mg2Sip reinforcements were more effective than those of the individual 1.5CNTs and 1.5Mg2Sip owing to the effective thermal mismatch and load transfer strengthening mechanisms. In this study, we propose an effective approach to harness the superior performances of the hybrid reinforcements for enhancing the composites.

Graphene as one of the best reinforcements for improving the comprehensive properties of the composites has attracted extensive attention. However, the severe aggregation of graphene in the matrix obviously weakened the strengthening efficiency. A novel method, flake powder thixoforming (FPT) that combines both the advantages of flake powder metallurgy and thixoforming, was proposed to construct the FPT-0.4 wt%RGO/Al composite with hierarchical microstructure that reduced graphene oxide (RGO) only uniformly distributed into the local regions of secondary solidified structures (SSSs). The ultimate tensile strength (UTS, 439 MPa), yield strength (YS, 294 MPa) and elongation (8.5%) of FPT-0.4 wt%RGO/Al composite fabricated by FPT were increased by 45.8% and 44.8% and decreased by 44.1% compared with the 2024Al alloy, respectively. Moreover, a 34.9% enhancement in YS and a 54.5% improvement in elongation were achieved when compared with the HEM-0.4 wt%RGO/Al composite prepared by high energy milling (HEM). The RGO-rich zones in the FPT-0.4 wt%RGO/Al composite behaved as reinforcing units that can sustain the tensile stress and thus strengthened the matrix. Simultaneously, crack deflection and bridging contributed by the RGO-free zones effectively toughened the composite. FPT is promising for strengthening and toughening the RGO/Al composites by tailoring the spatial arrangement of RGO to form hierarchical microstructure.

The thixoforging technology has been proved as an effective method to fabricate the in situ Mg2Sip/AM60B composite with excellent performances. The effects of reheating temperature on microstructure and tensile properties have been investigated. The results indicate that the liquid amount, the solubility of Al in α-Mg particles, and the coarsening of the α-Mg particles are changed as the reheating temperature changes, and thus the subsequent solidification behavior and plastic deformation are thereby changed. The morphology of the Mg2Si particle also varies as the reheating temperature rises owing to partial remelting operating in the edges and corners of the particles. The best ultimate tensile strength and elongation of 209 MPa and 11.9% of the thixoforged composite, which are 93 and 138% higher than the traditional permanent mold casting respectively, are obtained under the reheating temperature of 600 °C.

The microstructure and tensile properties of the thixoforged in situ Mg2Sip/AM60B composite were characterized in comparison with the as-cast composite and thixoforged AM60B. The results indicate that the morphology of α-Mg phases, the distribution and amount of β phases and the distribution and morphology of Mg2Si particles in thixoforged composite are completely different from those in as-cast composite. The Mg2Si particles block heat transfer and prevent the α-Mg particles from rotation or migration during reheating. Both the thixoforged composite and thixoforged AM60B alloy exhibit virtually no porosity in the microstructure. The thixoforged composite has the highest comprehensive tensile properties (ultimate tensile strength (UTS)) of 209 MPa and an elongation of 10.2%. The strengthening mechanism of the Mg2Si particle is the additive or synergetic effect of combining the load transfer mechanism, the Orowan looping mechanism and the dislocation strengthening mechanism. Among them, the load transfer mechanism is the main mechanism, and the latter two are minor. The particle splitting and interfacial debonding are the main damage patterns of the composite.

In the present work, the tensile properties and fracture behavior of a 2024Al composite reinforced with 10 vol % SiCp and fabricated via powder thixoforming (PT) were studied at temperatures ranging from 25 °C to 300 °C with a strain rate of 0.05 s−1, as well as the PT 2024 alloy. The results indicated that the tensile strengths of both the PT materials were all decreased with increasing the temperature, but the decrease rate of the composite was smaller than that of the 2024 alloy, and the composite exhibited higher tensile strength than that of the 2024 alloy at all of the employed testing temperatures due to the strengthening role of SiCp. Increasing temperature was beneficial for enhancing the ductility of materials, and the maximum elongation was reached at 250 °C. The elongation decrease over 250 °C was attributed to the cavity formation due to the debonding of the SiCp/Al interface and the fracturing of the matrix between SiCp. The fracture of the composite at room temperature initiated from the fracture of SiCp and the debonding of the SiCp/Al interface, but that at high temperatures was dominated by void nucleation and growth in the matrix besides the interface debonding.

AZ91D Mg-based composites containing carbon nanotubes (CNTs), magnesium silicide (Mg2Sip) nanoparticles, or CNTs-Mg2Sip hybrid reinforcements were synthesized via powder thixoforming using blending and pressing procedures in powder metallurgy, followed by partial remelting and thixoforming technology. The thixoformed microstructure of the CNTs-Mg2Sip hybrid reinforced Mg composite consisted of spheroidal primary α-Mg particles, intergranular secondary solidified structures (SSSs), and hybrid reinforcements homogeneously dispersed within the SSSs. A yield strength of 180 MPa was achieved in the Mg matrix composite reinforced by 0.75CNTs-0.75Mg2Sip hybrids, which was 26% and 12% higher than those of composites reinforced with individual 1.5CNTs and 1.5Mg2Sip, respectively (143 and 161 MPa, respectively). This was attributed to the in situ-synthesized Mg2Sip around the CNTs, which not only restricted the aggregation and pulling out of CNTs but also facilitated the synergistic strengthening effect of the CNTs. This work presents a promising strategy for the synthesis of metal matrix composites with impressive mechanical properties by employing hybrids of CNTs and Mg2Sip as reinforcements.

A new method, powder thixoforming, has been proposed to fabricate an in situ Al3Tip/2024Al composite. During partial remelting, the microstructural evolution of the bulk alloy prepared by cold pressing of the Ti, Al, 2024Al powder mixture was investigated, and the formation mechanism of the Al3Ti particles produced by the reaction between the Ti powder and the Al alloy melt is also discussed in detail. The results indicate that the microstructural evolution of the 2024 alloy matrix can be divided into three stages: a rapid coarsening of the powder grains; a formation of primary α-Al particles surrounded with a continuous liquid film; and a slight coarsening of the primary α-Al particles. Simultaneously, a reaction layer of Al3Ti can be formed on the Ti powder surface when the bulk is heated for 10 min at 640 °C The thickness (X) of the reaction layer increases with the time according to the parabolic law of \(X = -0.43t^{2} + 4.21t + 0.17\). The stress generated in the reaction layer due to the volume dilatation can be calculated by using the equationσ \(\sigma_{Al_{3}Ti} = -\frac{ E_{Al_{3}Ti} }{6(1-v{Al_{3}Ti})} \frac{ t^{3}_{Al_{3}Ti} }{t_{Ti}} \left(\frac{1}{R} - \frac{1}{R_{0}} \right) \). Comparing the obtained data with the results of the drip experiment, the reaction rate for the Ti powder and Al powder mixture is greater than that for the Ti plate and Al alloy mixture, respectively.

A novel method, powder thixoforming, for net-shape forming of the particle-reinforced Aluminum matrix composites in semi-solid state has been proposed based on powder metallurgy combining with thixoforming technology. The microstructural evolution and phase transformations have been investigated during partial remelting of the 2024 bulk alloy, prepared by cold pressing of atomized alloy powders to clarify the mechanisms of how the consolidated powders evolve into small and spheroidal primary particles available for thixoforming. The effect of heating temperature on the resulting semisolid microstructure has also been discussed. The results indicate that the microstructural evolution includes three stages—the initial rapid coarsening of the fine grains within the powders, the formation of continuous liquid layer on the primary particle surface (the original powder), and the final coarsening—that result from the phase transformations of θ→α, α→L, and α→L and L→α, respectively. The coarsening rate of the primary particles is low, and one original powder always evolves into one spheroidal particle with a continuous liquid layer surface. Properly raising the heating temperature is beneficial for obtaining an ideal semisolid microstructure.