Incorporated SiC nanoparticles are demonstrated to influence the solidification of magnesiumzinc alloys resulting in strong, ductile, and castable materials. By ultrasonically dispersing a small amount (less than 2 vol pct) of SiC nanoparticles, both the strength and ductility exhibit marked enhancement in the final casting. This unusual ductility enhancement is the result of the nanoparticles altering the selection of intermetallic phases. Using transmission electron microscopy (TEM), the MgZn 2 phase was discovered among SiC nanoparticle clusters in hypoeutectic compositions. Differential thermal analysis showed that the MgZn 2 formation resulted in elimination of other intermetallics in the Mg-4Zn nanocomposite and reduced their formation in Mg-6Zn and Mg-8Zn nanocomposites.
Semi-solid casting (SSC) techniques have proven useful in the mass production of high integrity castings for the automotive and other industries. Recent research has shown metal matrix nanocomposite (MMNC) materials to have greatly improved properties in comparison to their base metals. However, current methods of MMNC production are costly and time consuming. Thus development of a process that combines the integrity and cost effectiveness of semi-solid casting with the property improvement of MMNCs would have the potential to greatly improve cast part quality available to engineers in a wide variety of industries. This paper presents a method of combining SSC with MMNC in a way that benefits from MMNCs’ tendency to naturally form the globular microstructure necessary for SSC. This method uses ultrasonically dispersed nanoparticles as nucleating agents to achieve globular primary grains such that fluidity is maintained even at high solid fractions. Once particle dispersion is achieved, the material needs no further processing to become a semi-solid slurry of globular primary grains as it cools. This quiescent method of slurry production, while still imposing some constraints on cooling rates, has a large process window making this process capable of industrial rates of throughput. It was found that the key factor to achieving globular microstructure is a sufficiently slow cooling rate at the onset of solidification such that particle-induced nucleation can occur. Once nucleation occurs, continued cooling is virtually unconstrained, with globular microstructure evident in quenched samples as well as samples cooled at rates as slow as 1 °C/min. This method was demonstrated in several material systems using zinc (Zn), aluminum (Al), and magnesium (Mg) alloys and nanoparticles of aluminum oxide (Al2O3), silicon carbide (SiC), and titanium oxide (TiO2). Additionally, several nucleation models are examined for applicability to nanoscale composites.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.