چكيده به لاتين
Aluminum based metal matrix nanocomposites belong to metal matrix composite materials in which used a nano-sized reinforcement or a nanostructured matrix, and are expected to have more and newer applications in the future due to their higher mechanical properties. Their superior mechanical properties over common metal matrix composites have led to the production of these nanocomposites and the evaluation of their properties to be considered as a new subject of research in the field of materials Science and engineering. One of the simple and cheap methods that researchers are interested in, is the modified stir casting method. Accordingly, in this dissertation, various aspects of this method for making A356-SiCP cast nanocomposite have been investigated. In this regard, the effect of reinforcment type, master powder type, melt temperature, melt stirring speed, nanoparticle concentration, and cooling rate on the microstructure and mechanical properties of cast nanocomposites were investigated. The microstructure was studied by different techniques included OM, SEM, S/TEM, HRTEM, XRD, XPS, and SKPFM (Scanning Kelvin probe atomic force microscopy). In addition, solidification parameters were obtained by different thermal analysis techniques including differential thermal analysis (DTA) and computer aided-cooling curve analysis (CA-CCA). Mechanical properties were measured by Hardness, tensile and compression tests. The results showed that the remarkable improvement in mechanical properties and nanoparticles distribution is obtained via a combination of modified stir casting method and solidification under high cooling rate. This combination brings up compressive yield strength of 300 MPa and compressive strength of 1500 MPa for A356-1.0SiC cast nanocomposite. These properties are approximately two times and four times higher than similar properties of cast A356 alloy, repectively. According to the S/TEM and HRTEM results, the reasons for these unique properties come from that the interface between SiC nanoparticles and Al is semicoherent, clean, defectless, and has good strength. The analytical modeling activities include (1) modeling of SiC nanoparticles dispersion in liquid A356 alloy and extraction of a formula for prediction of the minimum stirring speed and (2) suggesting a new mechanism for the nanoparticle capturing by the solidification front called the engulfment of a nanoparticle wet cluster by the solidification front and the extraction of formulas for critical solidification velocity and critical diameter of a wet cluster. The results obtained from the modeling showed that the model proposed for the distribution of nanoparticles in the melt predicts accurately (The adaptation of the model to the experimental results is more than 99%) the distribution of SiC nanoparticles in the A356 alloy melt. In addition, the comparison of the critical solidification velocities calculated according to the new formula with the velocities measured in this dissertation and the values reported by other researchers showed that the new formula predicts the critical solidification velocity more accurately as compared with the current formulas.
