MMCs reinforced with CMAs
Complex metallic alloys (CMAs) have recently attracted much attention ranging from scientific curiosity about their complex structure, physical and mechanical properties to technological aspects of preparation and potential applications. In particular, CMAs display several attractive properties for reinforcement applications, such as high strength to weight ratio, good oxidation resistance and high-temperature strength.
Among the different CMAs, the ß-Al3Mg2 phase (1168 atoms per unit cell) has been extensively investigated with particular attention to its structure as well as to its physical and mechanical properties and, therefore, it represents the ideal complex intermetallic compound to analyze the effectiveness of CMAs as reinforcing agents in metal matrix composites. In addition, the ß-Al3Mg2 phase displays interesting properties, such as low density (about 2.25 g/cm3) and high-temperature strength (~300 MPa at 573 K), which further makes this material an attractive candidate as reinforcement in MMCs.
Accordingly, to test the effect of the ß-Al3Mg2 phase on the properties of the composites, we have produced Al-based metal matrix composites containing high-strength ß-Al3Mg2 CMA particles. The work was focused on two specific aspects : evaluation of the mechanical properties through room temperature compression tests and modeling of the resulting properties.
Room temperature compression tests reveal that the addition of the ß-Al3Mg2 reinforcement remarkably improves the mechanical properties of pure Al. In particular, the composites with 20 and 40 vol.% of reinforcement display yield and compressive strengths exceeding that of pure Al by a factor of 2-3, while retaining appreciable plastic deformation ranging between 45 and 15 % (Figure a). The strength of the material is further increased for the samples with 60 and 80 vol.% of ß-Al3Mg2 phase, however, the composites show negligible plastic deformation. Furthermore, the addition of low-density β-Al3Mg2 particles decreases the density of the material below that of pure Al, considerably increasing the specific strength of the composites (Figure b).
The mechanical properties of the composites have been modeled by taking into account the combined effect of load bearing, dislocation strengthening and matrix ligament size effect (Figure c). The calculations are in very good agreement with the experimental results and reveal that the reduction of the matrix ligament size, which results in a similar strengthening effect as that observed for grain refinement, is the main strengthening mechanism in the current composites.
(a) Room temperature compression stress-true strain curves for pure Al (V = 0) and composites with 20, 40, 60 and 80 vol.% of ß-Al3Mg2 particles; (b) specific strength of the composites as a function of the amount of CMA reinforcement. For comparison purposes, the values for Al-based MMCs reinforced with SiCp and Al2O3 are also shown; (c) Normalized yield strength of the composites as a function of the volume fraction of reinforcement: experimental data (points) and calculated values (lines) from f1 (load bearing effect), fd (dislocation strengthening) and fs (matrix ligament size).
 S. Scudino, G. Liu, M. Sakaliyska, K.B. Surreddi, J. Eckert, Acta Mater. 57 (2009) 4529.