2019 Volume 60 Issue 5 Pages 704-707
Thixomolding technology was successfully used to produce Mg–5 mass% Zn matrix composites reinforced with nano-SiC. The composites containing 5 mass% of SiC nano-particles were fabricated under pure Ar or Ar+CO2 atmosphere. Microstructure characterization has been carried out by scanning and transmission electron microscopy methods. It was found that regardless of the gas used, the composites consisted of unmelted globular α(Mg) grains surrounded by small areas of a magnesium solid solution formed directly from liquid and a mixture of SiC and MgO. Reaction of CO2 with liquid alloy during process leads to the in-situ formation of MgO nano-particles, which results in an increase in the amount of oxide particles and refinement of the microstructure compared to the composite produced in pure Ar. These microstructure changes increased the hardness to 70 HV.
Fig. 3 (a) TEM bright-field microstructure and (b) corresponding electron diffraction pattern obtained for Mg5Zn/SiC–Ar composite.
Magnesium, as the lightest metallic structural material has great potential to reduce weight and energy consumption in automotive, aerospace, military and other industries. Magnesium alloys, characterized by excellent castability and machinability, high damping capacity, low elastic modulus and ductility have limited use in engineering applications because of poor creep and abrasion resistance and high corrosion rate.1,2) These disadvantages can be overcome by addition of suitable alloying elements and reinforcements to form new magnesium alloys and composites, e.g. Refs. 3, 4). It was shown that micron-size reinforcements lead to reduction in the ductility due to particle cracking and void formation at particle-matrix interface. Addition of nano-reinforcements (oxides, nitrides, borides, carbon nanotubes, graphene) improves the strength and ductility of magnesium.5) The nanocomposites have been produced by different methods, but the most often casting processing was used. The most common problem for the production of composites is a uniform distribution of the nanoparticles. For this purpose various techniques were used to disperse the reinforcements, as mechanical stirring6) or ultrasonic waves.7)
Semi-solid metal processing (SSM) seems to be promising technique in nano composites fabrication as the alternative method to conventional casting.8) It is an advanced technology which uses thixotropic behaviour of alloys or metal matrix composites in the solid state. In particular, thixomolding is a semi-solid route, which involves reheating material to solidus liquidus temperature range and then forming it to near net shape. Additional advantages is high shearing and mixing in the semi-solid state by the rotation of screw and the transport of suspension through slits, which induce the refinement and homogenous distribution of particles.9)
The aim of this study was the characterization of magnesium alloy base composites containing silicon carbide nano-particles, which have been produced by semi-solid processing technology. Special attention was paid to the influence of the atmosphere in which the thixomolding process proceeded.
The Mg–5 mass% Zn alloy as a matrix and spherical nano-particles of silicon carbide (55 nm in average) as an reinforcement were used for producing the composites by thixomolding technology. This basic binary alloy (Zn is one of the most important alloying elements in many commercial magnesium alloys) was chosen to understand processes that occur during preparation of the composite. Additionally, the DSC measurements showed that this alloy characterized by wide solid-liquid range (about 95 K) which is necessary for the tixoforming process. Mixture of alloy granules and SiC particles with the amount corresponding to 5 mass% of the composite were introduced into cylinder, where the slurry were sheared and injected by a rotating screw to the die (detailed description of the method is presented in Ref. 10)). During the process granules of the alloy were partially melted after preheating to 625°C (semi-solid temperature range was determined by DSC measurement). The average cooling rate to the temperature of the die was estimated to be around 100°C/s. The process in the melting zone inside the thixomolding equipment was performed in an inert gas. To compare the effect of protective and reactive atmosphere on the microstructure and properties of obtained composites the samples were produced in two variants: under pure argon (Mg5Zn/SiC–Ar) and in the mixture of argon and carbon dioxide in the ratio 1:10 (Mg5Zn/SiC–CO2).
The longitudinal cross-section of the samples was subjected to the microstructural characterization after polishing and etching in Nital. The observations were carried out using a Leica DM IRM metallographic microscope and FEI SEM XL30 scanning electron microscope (SEM). The chemical composition of micro areas was performed using the Energy Dispersive X-ray spectroscopy (EDS) technique with an EDAX Apollo XP spectrometer. Detailed microstructural examination was made by transmission electron microscopy (TEM) using FEI Tecnai G2 operating at 200 keV equipped with high-angle annular dark field detector (HAADF-STEM) and energy dispersive X-ray (EDX) EDAX microanalysis. Thin foils were prepared by electropolishing in an electrolyte containing nitric acid and methanol (1:3) at the temperature of 243 K and voltage of 15 V and by ion milling using Leica EM RES 101. The Vickers hardness was measured using a Zwick/ZHU 250 tester under the load of 5 kg in accordance with ASTM E92.
The microstructures of the cross-section of the Mg5Zn/SiC–Ar and Mg5Zn/SiC–CO2 composites are presented in Fig. 1. For both samples the globular grains of the magnesium solid solution with the sizes up to 100 µm are well visible in Fig. 1(a) and 1(b). The black dots visible in the spherical grains (Fig. 1(a)) formed during the sample preparation and are not related to the gas used. The results of EDX microanalysis showed that these grains contained about 99 at% of magnesium and small amount of zinc (below 1 at%). Based on the presented micrographs, the volume fraction of the solid phase, which remained unmelted during the process, was estimated to be 15%. This value is in the range presented in the previous papers, where the content of solid fraction around 10–20% is typical for the thixomolded magnesium alloys and allows to obtain complete filling of the die cavity.11) In the presented micrographs the unmelted grains are surrounded by the mixture of magnesium solid solution, visible as small areas with irregular shape and the other phases located between them. The small grains of Mg solidified during thixomolding process from the liquid phase and then the SiC together with magnesium oxide are moved to the regions of the outer zone of these grains.
Light micrographs (a), (b) and SEM microstructures (c), (d) of the Mg5Zn/SiC–Ar (a), (c) and Mg5Zn/SiC–CO2 (b), (d) composites.
In the Mg5Zn/SiC–Ar composite (Fig. 1(c)) the phase with the brightest contrast containing about 50 at% of Zn was identified by electron diffraction patterns (not shown here) as Mg21Zn25 with trigonal structure and lattice parameters, a = 2.5518 nm and c = 0.8713 nm,12) similarly as in the cast alloy. This phase, presented in the newest phase diagram,13) solidified as an divorce eutectic from the liquid part of the alloy. The light grey areas occurring in both composites are enriched in silicon, carbide and oxygen. The presence of the CO2 during processing lead to the increase in the amount of phases located between the magnesium solid solution areas and also refined the microstructure compared to the alloy produced in pure Ar atmosphere. In the case of the Mg5Zn/SiC–CO2 composite the Mg21Zn25 eutectic phase was not observed.
TEM examinations showed that the SiC nanoparticles were homogenously distributed in the intergranular regions in both investigated composites. These particles were clearly visible in the images of the elemental maps; the example obtained for the Mg5Zn/SiC–CO2 composite is presented in the Fig. 2. It could be noticed that spherical particles with different diameters enriched in Si and C are decorated by the particles containing oxygen. Zinc is uniformly distributed in the observed area, which means that Zn atoms were dissolved in the magnesium rich matrix and did not form additional phases.
STEM-HAADF image and distributions of the elements in the Mg5Zn/SiC–CO2 composite.
Identification of the phases in the SiC containing regions was made by analysing selected area diffraction patterns (SADP). The example of TEM bright-field image of Mg5Zn/SiC–Ar composite showing the mixture of nano crystallites is presented in Fig. 3(a). The corresponding SADP (Fig. 3(b)) contains the reflections lying along the rings which could be indexed as cubic silicon carbide and cubic magnesium oxide. The continuous rings reveal that SiC and MgO particles are in the nanocrystalline range and that they are randomly oriented.
(a) TEM bright-field microstructure and (b) corresponding electron diffraction pattern obtained for Mg5Zn/SiC–Ar composite.
The presence of the MgO nano-particles of rectangular shape typical for the magnesium oxide was confirm by high resolution (HREM) observations (Fig. 4). Fast Fourier transform obtained for the presented nano-particles corresponded to the [001] zone axis of MgO.
High resolution image of the magnesium oxide and fast Fourier transform from the marked area obtained for Mg5Zn/SiC–Ar composite.
The magnesium oxide observed in both investigated composites was probably introduced during thixomolding process from the surface covering the magnesium alloy granules. The oxide films were broken down, fragmented and mixed together with added SiC particles and found in the liquid part of the alloy, as evidenced by the fact that SiC particles are always co-exist with MgO particles. This suggestion is consistent with the results of Fan et al.,14) who observed uniformly dispersed MgO particles formed from an oxide film and then being precursors of heterogeneous nucleation of a magnesium solid solution. In the present study, MgO nanoparticles may additionally play a role of a strengthening phase, but the assessment of their contribution to the reinforcement of the composite in comparison with the added SiC particles requires further studies. In the case of the Mg5Zn/SiC–CO2 composite, MgO nanoparticles could form in-situ by the reaction of liquid magnesium and reactive CO2. Because of that the amount of MgO particles increased and simultaneously refined the microstructure of the composite.
The Vickers hardness was measured on the cross section of both composites. The mean values calculated for ten measurements are 54 ± 0.7 HV and 70 ± 1.4 HV for Mg5Zn/SiC–Ar and Mg5Zn/SiC–CO2, respectively. For the sample produced in CO2 atmosphere hardness increased about 30% compared to the composite fabricate in pure Ar. It could be connected with refinement of the microstructure and greater amount of MgO nano particles which form in-situ during tixomolding process.
The work was carried out within the statutory research (Z-5) of the IMIM PAS. The studies were performed in the Accredited Testing Laboratories at the Institute of IMIM PAS.
