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Anomalous Eutectic and Nanolaminated Composites of Mg-Al Alloys: Microstructure and In-Situ Compression Test
Eutectic alloys demonstrate excellent casting behavior and composite properties. Their morphologies are characterized by the simultaneous growth of at least two solid phases from the liquid phase. Besides, the solidification condition can affect the final morphology. Under a non-equilibrium condition, a transition from the regular to anomalous eutectic can occur. However, the formation mechanisms of the anomalous eutectic are still debated. In this study, melt spinning was applied to rapidly solidify Mg-33.3%Al eutectic alloy at different wheel speeds from 5 to 50 m/s. Four distinct anomalous eutectic microstructures were identified, and their corresponding formation mechanisms were determined using scanning electron microscopy (SEM), scanning/transmission electron microscopy (S/TEM), and high resolution TEM (HRTEM) analyses. Owing to their low density and high strength-to-weight ratio, Mg alloys are one of the most potential alloys to replace aluminum, zinc, and steel for automotive and aerospace applications. However, the current use of magnesium in the industry is limited because of low strength and poor formability at ambient temperatures. These limitations are due to the insufficient independent deformation modes in Mg. To achieve a good combination of strength and ductility, microstructural engineering was applied and Mg-Al in-situ composites were fabricated by melt spinning considering that the nanolaminated composites exhibit outstanding mechanical properties depending on the layer thickness and interface structure. Two different morphologies of Mg-Al nanocomposites were examined by the in-situ SEM microcompression test; the layered and fibrous eutectics. It was shown that the combination of mechanical properties of nanolaminated composite was superior to that of fibrous eutectic and pure Mg. The reason was attributed to the high interfacial area and the effect of α layers in restricting the shear propagation. The investigation of orientation relationship between the layers demonstrated weak interphase interfaces. The dislocation entrapment by the weak interfaces improved the strength of nanolaminated composites. Moreover, the enhanced activity of dislocations in the β phase and suppressing the propagation of deformation instabilities by the α phase resulted in the increase of ductility.
Azadehranjbar, Soodabeh, "Anomalous Eutectic and Nanolaminated Composites of Mg-Al Alloys: Microstructure and In-Situ Compression Test" (2020). ETD collection for University of Nebraska-Lincoln. AAI28031586.