| To improve the toughness of particulate reinforced A1 matrix composites and overcome the problems related to fabrication techniques, a new in situ core-shell structured Ti@(Al-Si-Ti) particle reinforced A356 composites(Ti@(Al-Si-Ti)p/Al) was synthesized by powder thixoforming.The effects of reheating duration, reheating temperature and mold temperature on the microstructure, especially the microstructure of the Ti@(Al-Si-Ti)p, and the tensile properties of the resulting composites were investigated to find the optimal processing parameters. In addition, in situ tensile testing was also conducted to observe crack generation and propagation and verify the strengthening and toughening mechanisms. A modified micromechanical modeling considering indirect strengthening mechanisms and the intermetallic shell structure was proposed to predict the YS of the Ti@(Al-Si-Ti)p/A356 composites.The experimental results indicate that as the reheating duration increased at the semisolid temperature of 600℃ , the tensile properties of the resulting composite were first enhanced due to the increased compact shell thickness of the Ti@(Al-Si-Ti)p,and the improved microstructure compactness. Consequently, the fracture changed from an intergranular regime to a mixture of transgranular and intergranular regimes. However,when the reheating duration exceeded 50 min, the tensile properties then decreased because of the generation of cracks within the shells, the subsequent fracture and peeling off of the intermetallic shells, the transformation from τ1 phase to the (Al,Si)3Ti phase and the coarsening of the primary particles. The UTS and YS were improved again when the reheating exceeded 90 min due to the enhanced strengthening role of the increased and more uniformly distributed small (Al,Si)3Ti particles originated from the peeling off. But the elongation still continuously decreased due to the increase of the intermetallic amount and the coarsening of the primary particles.The Ti@(Al-Si-Ti)_p formed after reheating for 50 min can blunt the sharp tips of cracks by plastic deformation of the Ti core and A1 matrix. The subsequent crack propagation path within the Ti core was not straight, but would change for several times based on the distribution of the small cracks in the Ti core. That is, the firstly formed cracks were only limited within the shells and the subsequent propagation was delayed, resulting in the elongation improvement of the composite. The MSL model modified by considering the grain refinement, solid solution strengthening, thermal mismatch strain, geometrically necessary dislocations and the shell thickness of Ti @ (Al-Si-Ti)p can accurately predict the YS of the Ti@(Al-Si-Ti)p/A356 matrix composites thixoformed from 35 min to 50 min.As the reheating temperature increased,the tensile properties of the resulting composite were first enhanced due to the improved feeding ability to the original pore and solidification shrinkage of the increased liquid phase during thixoforming and the thickened compact shell of the Ti@(Al-Si-Ti)p. However, when the reheating temperature exceeded 600℃, the solidification behavior during thixoforming was close to the solidification behavior of complete liquid melt because of the further increase of liquid phase, resulting in the decreased microstructure compactness. In addition, the strengthening role of the reinforcing particles weakened due to their fracture and peeling off. All of this impaired the tensile properties, especially the elongation.When the mold temperature was low, porosity existed in secondarily solidified structures due to fast solidification rate during thixoforming. The applied load could not be effectively transferred to the core-shell structured reinforcing particles, resulting in the low tensile properties. As the mold temperature increased,the tensile properties of the resulting composite were greatly enhanced with the improved homogeneity and compactness of microstructure. However, when the mold temperature exceeded 250℃ , the primary particles gradually formed solid skeleton due to the adhered growth of the second primary phase and the individual liquid pools could not be gotten enough feeding during solidification process. Thus shrinkage porosity formed, resulting in the decreased tensile properties.The Ti@(Al-Si-Ti)_p/A356 matrix composite thixoformed at reheating for 50 min 600℃ and mold temperature of 200℃ has the best comprehensive tensile properties, UTS of 373 MPa, YS of 268 MPa and elongation of 8.3%. The UTS and YS were only decreased by 2.1% and 3.5%respectively, but the elongation was increased by 167.8% compared with those of the (Al,Si)3Tip/A356 matrix composite thixoformed at 180 min. |
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