Effects of ZrB₂ Nanoparticles on the Microstructures and Tensile Properties of a Hot Extruded In Situ AA6111 Composite

Particle-reinforced aluminum matrix composites (PRAMCs) always show a low forming stability because the particles are easily separated from the matrix, leaving voids and forming cracks. Extruded in situ PRAMCs are gradually replacing ex situ PRAMCs due to the higher interface bonding between the in situ reinforcements and matrix. In this study, a hot-extruded in situ AA6111 composite with ZrB₂ nanoparticles was obtained from an Al-Zr-B system. The effects of the ZrB₂ content on the geometrically necessary dislocation (GND) density, grain size, grain boundaries, textures and tensile properties of AA6111 composites were researched. The results showed that as the volume fraction of ZrB₂ increased, the mean GND dislocation density increased from 1.53 × 10¹⁵ m⁻² to 3.23 × 10¹⁵ m⁻², and most dislocations were located around the ZrB₂ nanoparticle clusters. In addition, an increase in the ZrB₂ content decreased the frequency of high-angle grain boundaries (HAGBs) from 79.28 to 69.45% and increased the frequency of low-angle grain boundaries (LAGBs). ZrB₂ nanoparticle clusters which were located along the grain boundaries promoted continuous dynamic recrystallization (CDRX) and remarkably refined the grains, with the mean size decreasing from 48.2 to 1.8 μm. However, dispersed ZrB₂ nanoparticles inside the grains suppressed CDRX. Tensile tests showed that the properties rose dramatically with an increasing volume fraction of ZrB₂, and the optimum ultimate tensile strength (UTS), yield strength (YS) and elongation (El) of the 2 vol% ZrB₂/AA6111 composite were 365 MPa, 280 MPa, and 25.6%, respectively. The mechanisms for CDRX, strengthening and plasticity enhancement were determined and discussed.