Microstructure and Properties of In-Situ ZrB2 np/AA6111 Composites Synthesized Under an Electromagnetic Field

6xxx alloys have become of particular interest in automotive structural applications as replacements for low carbon steels, mainly because of the increasing demand for the utilization of lighter materials in the automotive industry. However, the strength and formability of the 6xxx alloy are inferior to those of fully annealed low carbon steels, which is partially due to the different crystallographic textures of these two materials. In-situ nanoparticle-reinforced composites have always been extensively used due to their high modulus, high strength, specific stiffness and excellent comprehensive properties. However, traditional in-situ methods require long reaction time and high reaction temperatures, leading to further growth or agglomeration of the reinforcement particles and decreasing the mechanical properties. In this work, in-situ ZrB2/AA6111 composites were successfully prepared via an in-situ melt reaction with the assistance of an electromagnetic field. The effect of electromagnetic field on distribution, size and morphology of in-situ particles, interface structure between particles and matrix, and dislocation morphology in composites were characterized by XRD, OM, SEM and TEM. The action mechanism of electromagnetic field and the effect of microstructure on tensile strength were analyzed. The results indicated that with the assistance of electromagnetic field during in-situ reaction, the large particle clusters were broken into smaller clusters that were uniformly distributed in the matrix, the distribution of ZrB2 nanoparticles was diffused and homogeneous with the size decreasing to 50 similar to 100 nm, and the corners of the nanoparticles clearly became obtuse. In addition, the interface between the particles and the matrix was well bonded without any impurities. The uniformity of the ZrB2 nanoparticle distribution improved, resulting in dislocation propagation and entanglement. When electromagnetic frequency was 10 Hz, the optimal ultimate tensile strength (UTS), yield strength (YS) and elongation (EI) of the composites prepared under the electromagnetic field were 362 MPa, 253 MPa and 25%, respectively, correspondingly increasing 38.7%, 68.6% and 28.7% over the respective properties of the ZrB2 (np)/AA6111 composite. These improvements were due to the Orowan strengthening, load transmitting strengthening, grain refinement strengthening, and dislocation strengthening caused by the nano-sized ZrB2 particles synthesized under the coupled electromagnetic and ultrasonic field. In addition, the Orowan strengthening contributed most to the improvement of properties.