AA2024 reinforced with varying amounts of titanium boride (TiB 2) was developed using stir rheocasting to study the effect of solution heat treatment on aging behavior and mechanical properties. The developed specimens were subjected to solution heat treatment at 490 °C for 1, 2.5, and 5 h, followed by artificial aging at 190 °C for an aging duration of 1-12 h. The metallurgical characterization was done by x-ray diffraction, optical microscopy, and field emission scanning electron microscope and mechanical characterization by Vickers hardness tester and universal testing machine. Microstructural observation showed the formation of globular grains and fairly uniform particle distribution. Up on peak aging, the AA2024/2 wt% TiB 2 composite has shown better improvement in yield strength (YS = 230 MPa), tensile strength (UTS = 342 MPa), and Vickers hardness (VH = 147) amongst other developed specimens. The improvement in mechanical properties of AA2024/2 wt% TiB 2 composite up on peak aging was nearly 83%, 77%, and 20%, respectively, over their as-cast states. However, the percent elongation has decreased by 51% for the AA2024/2 wt% TiB 2 composite against its as-cast state. The optimum solution heat treatment time was found to be 2.5 h that led to the fastest aging kinetics and peak hardness. Due to incomplete dissolution of precipitates at a solution treating time less than 2.5 h, the hardness decreased, and the aging kinetics decelerated. The aging kinetics further accelerated with increasing solution heat treating time beyond 2.5 h, but the hardness decreased due to grain growth of the matrix.
In this research, microstructure and mechanical properties of stir rheocast AA2024/TiB2 metal matrix composite have been investigated. The working temperature was 640℃, which was the selected semisolid temperature that corresponds to 40% of the solid fraction. Two weight percentage, 4 wt%, and 6 wt% of the TiB2 reinforcements were added to the matrix. The field emission scanning electron microscope micrographs of the developed composites showed a uniform distribution of the particles in the case of the 2 wt% and 4 wt% of the reinforcements. However, the particles agglomerated as the weight percentages of the reinforcement increases to 6%. The optical microscope of the liquid cast sample showed the dendritic structure, whereas the rheocast samples showed a globular structure. The X-ray diffraction analysis confirmed the distribution of the reinforcements in the matrix and the formation of some intermetallic compounds. Mechanical properties significantly improved by the addition of the reinforcements in the matrix. An increase in tensile strength of 13.3%, 40%, 28%, and 5% was achieved for the unreinforced rheocast sample, 2 wt%, 4 wt%, and 6 wt% reinforced rheocast samples respectively, compared to the liquid cast sample. An increase in 20% of hardness was attained for the composite with 2 wt% TiB2 compared to the liquid cast sample. According to the fractography analysis, small dimples were observed on the fractured surface of the unreinforced rheocast sample, whereas small and large voids were dominant on the fractured surface of the 2 wt% composite, which shows the ductile fracture mode.
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