Aluminum matrix composites reinforced with high-entropy oxides and in situ formed Al2O3 and intermetallic compounds through aluminothermic reactions during spark plasma sintering

Abstract

This work proposes a new strategy for producing aluminum matrix composites with strong interfaces by forming additional reinforcing particles in situ through an aluminothermic reaction between the system components. Sol-gel synthesized high-entropy spinel-type (Cr0.23Mn0.22Fe0.22Co0.19Ni0.13)3O4 oxides (HEO) were added to Al nanopowder and the resulting mixture was subjected to high-energy ball milling (HEBM) and then spark plasma sintering (SPS). As a result of the aluminothermic reaction during SPS, HEO are partially reduced, providing excess oxygen for the formation of reinforcing Al2O3 nanoparticles, and the reduced metals react with Al with the formation of various intermetallic compounds (Al9Me2, Al5Me2, and Al2Me), which, in addition to HEO, serve as secondary strengthening phases. A new hexagonal AlMex phase with lattice parameters of approximately a = c = 1.76 nm has been well documented. HEBM has been shown to be an important step not only for uniform HEO distribution, but also for the aluminothermic reaction to occur at a relatively low temperature by improving the interaction between activated Al and HEO nanopowders. The maximum increase in hardness with the addition of HEO is 198 %, tensile strength 90 % (25°C) and 97 % (500°C), and compressive strength 220 % (25°C) and 250 % (500°C). Excellent mechanical properties are achieved at 500°C: the ultimate tensile and compressive strength are 258 MPa and 410 MPa, respectively. The introduction of HEO also leads to a significant reduction in the coefficient of friction (from 0.7 (0 %) to 0.2 (3 %HEO)) and a noticeable reduction in dynamic wear (5.9–6.3 (3 %HEO) and 10.6–14.3 (5 %HEO) times depending on applied load). Thus, our research opens up exciting new possibilities for creating lightweight and high strength composites for use in the mid-temperature range.