Phase transformation of nonstoichiometric cubic tungsten carbide on the surface of carbon nanotubes during high-temperature annealing of aluminum matrix composites

In this work, metal matrix composites based on 5049 aluminum alloy reinforced with multi-walled carbon nanotubes (CNTs) coated with nonstoichiometric cubic tungsten carbide were obtained by powder metallurgy. For the first time for this system, high-temperature annealing of the synthesized composites at 500-600 degrees C for 0.5 h was carried out. Effect of the annealing on the evolution of the structural-phase composition and the change in the microhardness and Young's modulus of the bulk composite was studied. Characterization of the structure shows that despite the growth of the matrix grains, the structural heterogeneity inherent for composites in the as -synthesized state is retained after heat treatment. Along with coarse grains, fine grains still remain, which in-dicates an increased resistance to recrystallization of the composite even at a temperature of -0.9Tm. XRD analysis shows that annealing at temperatures above 525 degrees C leads to a solid-state interfacial reaction of the ceramic coating on the CNTs surface with the matrix material, resulting in the in-situ formation of the WAl12 intermetallic compound around the reinforcing particles and Al4C3 nanorods. At the same time, the structural integrity of the CNTs is preserved. An increase in the annealing temperature contributed to an increase in the intensity of phase transformations and an increase in the fraction of the in-situ phases. The microhardness and Young's modulus of the composites decreased by-20% and-27%, respectively. Nevertheless, despite the in-crease in the grain size, the level of these properties remained quite high and equal to 141 HV and 80 GPa, correspondingly, due to the formation of a larger fraction of the in-situ WAl12 and Al4C3 phases. The obtained results are applicable for varying the mechanical properties of the composite by controlling the degree of in-situ reaction between the matrix alloy and the ceramic coating on the CNTs.