Strengthening controlled by carbon nanotube morphology and size in aluminum matrix composites: a three-dimensional discrete dislocation dynamics study

Abstract Carbon nanotubes (CNTs) are promising reinforcements for aluminum matrix composites; however, the dislocation-level mechanism by which CNT morphology, size, and spatial arrangement govern strengthening remains insufficiently understood. In this work, three-dimensional discrete dislocation dynamics (3D-DDD) simulations are employed to systematically investigate how CNT morphology, volume fraction, and geometric size affect dislocation pinning, Orowan bypass, and the resulting strengthening behavior in CNT-reinforced Al matrix composites. A topology-based dislocation-CNT interaction model is introduced to describe dislocation pinning, bowing, and bypass around impenetrable CNT obstacles, and the model is benchmarked against the Bacon-Kocks-Scattergood critical shear stress model. The effects of CNT morphology, volume fraction, and geometric parameters are then examined by considering straight and curved CNTs, CNT volume fractions of 0.5%, 1.0%, and 2.0%, CNT lengths of 250-750 nm, and CNT radii of 6.6-20 nm. The simulations show that curved CNTs provide stronger resistance to dislocation motion than straight CNTs because their curved geometry introduces multiple dislocation-CNT contact sites and promotes the accumulation of pinned dislocations. Increasing the CNT volume fraction enhances the critical shear stress by reducing the effective obstacle spacing, thereby increasing the stress required for Orowan bypass. In contrast, variation in CNT length has only a weak effect on strengthening, whereas reducing the CNT radius leads to a pronounced increase in critical shear stress. These results demonstrate that CNT radius, rather than CNT length, is the dominant geometric parameter controlling the strengthening efficiency of CNT/Al composites, because at a fixed volume fraction the radius governs the effective obstacle spacing. This work provides a dislocation-based understanding of CNT morphology- and size-controlled strengthening and offers guidance for the microstructural design of high-performance CNT-reinforced aluminum matrix composites.