Study On The Solidification Process Of Nano-sized Particles Reinforced Magnesium Matrix Composites

Magnesium matrix nano-composites (Mg-MMNCs) with low density can provideexciting mechanical properties for numerous applications in various industries, such aselectronics, communication, automotive, aerospace, military hardware, medicalapparatus and instruments. A tremendous amount of study was conducted to thefabrication of Mg-MMNCs. However, it is extremely challenging to dispersenano-elements (less than100nm) in metal melt and further capture them uniformly inthe metal grains since the nanoparticles readily form agglomerations due to their highsurface energy. Heterogeneous dispersion of nano-elements results in limitedstrengthening effect of MMNCs. It is thus very important to understand the interactionof nano-particles and solidification front, and to provide enabling pathways for effectivenanoparticle capture during solidification process. This paper employed the approachesof theoretical analysis, directional solidification experiment and squeeze castingexperiment to establish a theoretical framework for the fundamental understanding ofnanoparticle capture during solidification process, and to investigate the solidificationconditions favoring the nano-particle capture.Van der Waals force (Fvdw), viscous force (Fv), and force due to interfacial energy(Fi) played important roles in the particle capture/push transition during thesolidification process. It was predicted that the capturing of nanoparticles could beachieved via two processes. The prerequisite was that the combination of Fvdwand Fvdetermined particles at a distance of about0.4~50nm would be dragged nearer thesolidification front (about to0.2~0.4nm away). A positive Hamaker constant of“particle-metal-melt” system, high cooling velocity, high temperature gradient, highsolidification stress and high viscosity of the liquid metal would favor the capture andevenly distribution of nano-particles. Then Fidetermined whether or not the particleswould be captured. Good wettability between the nano-reinforcement and the metalwould favor the capture process of nano-particles. Besides, the interface morphologyalso played an important role in the particle capture/push process. Compared with planeinterface, a concave interface would favor the capture of nano-particle and a convexinterface would adverse to the capture process.Liquid metal cooling directional solidification experiments were conducted in2.0%nano-sized SiC particle-reinforced AZ91D (2.0%n-SiCp/AZ91D) composites toobserve the distribution of nanoparticles in different solidification microstructures undervaried drawing velocities, wherein the cooling rates varied from0.0038K/s to0.19K/s.It was found that when solidified with plane, SiC nanoparticles were pushed by thesolidification front and aggregated in the grain boundaries. When solidified withcolumnar or equiaxial grains, SiC nanoparticles were captured by the solidification frontbut distributed uniformly only in the grain boundaries. The pushing phenomenon of SiCparticles by the solidification front of α-Mg during directional solidification experiments were due to the low cooling rate.Squeeze casting experiments were also conducted in2.0%n-SiCp/AZ91Dcomposites to observe the distribution of nanoparticles in different solidificationmicrostructures under varied solidification stresses, wherein the cooling velocitiesvaried from100K/s to300K/s. It was found that the solidification structures wereequiaxial grains and SiC nanoparticles distributed uniformly in the α-Mg grains. Thecapture of SiC particles by the solidification front of α-Mg during squeeze castingexperiments were due to the high cooling rates.Qualitatively, the analytical predictions and the solidification experiments resultsshowed that reasonable solidification process and good wettability between thenano-reinforcement and the metal would result in the capture of nano-sized particles.