Search

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.

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.

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.

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.