Heterogeneous precipitation evolution and dislocation accumulation in CNT/2024Al composites with dual heterostructure

Abstract

Heterogeneous structure can simultaneously improve the strength and ductility of composites. However, the inherent structural and compositional differences pose a significant challenge for heat treatment. In this work, a dual-heterostructured CNT/2024Al composite with non-uniformly distributed reinforcements and heterogeneous grain structure was fabricated by accumulative extrusion bonding. Meanwhile, the heterogeneous precipitation evolution and dislocation accumulation in the composite were systematically investigated. Compared with conventional aging (180°C × 12h), pre-stretching combined with low-temperature aging (100°C × 60h) can refine precipitates in both the soft and hard zones, thereby improving the yield strength and ultimate tensile strength by 41% and 17%, respectively. Soft and hard zones exhibit distinct precipitation behaviors, and the added reinforcements, such as CNTs and Al₄C₃, serve as nucleation sites for precipitation in the hard zone, promoting the formation of precipitate-free zones and interfacial phases. Moreover, a high density of mobile dislocations is induced by pre-stretching, thereby suppressing the formation of Lüders bands. As two-beam diffraction and stereo-pair analyses results show, the [110]$\left(\overline{1}11\right)$ slip dislocations nucleate at heterogeneous interfaces and slip into the soft zone, and the shear stress experienced by dislocations decreases with increasing distance from the interface. The slip systems of three dislocation segments in the hexagonal dislocation network are $\left[01\overline{1}\right]$(011), [110](001), and [101]$\left(\overline{1}11\right)$. This work offers new insights for improving the mechanical properties of heterogeneous composites.