Study On Microstructure And Mechanical Properties Of Ti@Al₃Ti_p/2024Al Matrix Composites Prepared By Powder Thixoforming

The traditional particles reinforced aluminum matrix composites always have a relatively poor elongation due to ceramic reinforcing particles,The existing investigation indicated that core-shell structured reinforcing particles not only have strengthening role,but also have good toughening role.Thus a new in situ core-shell structured Ti@Al₃Ti particle reinforced composites was proposed,and simultaneously,a new processing technology,powder thixoforming,was also proposed by combining the merits of powder metallurgy and thixoforming.The effects of processing parameters on the primary particles,secondarily solidified structures,the structure of the core-shell structured Ti@Al₃Ti reinforcing particles and the fracture behavior of the resulting composite were investigated.The results indicated that the Ti@Al₃Ti_p/2024 Al based composites thixoformed under the reheating at 645 ? for 60 min and the mold temperature of 200 ? had spheroidal primary particles compact secondarily solidified structures,uniform and compact reaction Al₃Ti shell for the reinforcements,and thus had the best tensile properties,UTS of 361 MPa and elongation of 13.7 %.That is,the composit not only had high strength,but also had excellent elongation.As the reheating temperature increased from 620 ? to 645 ?,the microstructure compactness was improved,the Al₃Ti shell of the reinforcing particles became thickened and uniform,the fracture mechanism of the composites transformed from defect induced fracture to normal ductile fracture,The ultimate tenslile strength?UTS?and elongation of the composite gradually increased.However,when the reheating temperature exceeded 645 ?,the primary particles coarsened seriously and their sphericity decreased.In addition,the Al₃Ti shells begin fracturing the secondarily solidified structures and the primary particles became more and more agglomerated.All of these resulted in the mircostructure inhomogeneity and poor deformation coordination,and thus the decreased tensile properties and increased brittle fracture.When the reheating time was 45 min,the microstructure was not compact due to the poor shringkage feed ability from the less liquid fraction and the strengthening role of the reinforcements was poor due to the inhomogeneous Al₃Ti shell.So the resulting composite had poor tensile properties.The fracture regime was a mixture of transgranular and intergranular regimes.As the reheating time was prolonged,the microstructure became more and more compact,the thickness of Al₃Ti thickened,and so the tensile properties were improved.But when the reheating time exceeded 150 min,the tensile properties were decreased because of the fracture and pelling off of the Al₃Ti shells and the coarsening of the primary particles correspondingly,the fracture regime was mainly intergranular regimes.When the mold temperature was at 100 ?,the solidification was rapid and shrinkage feed ability was poor,so porosity then formed and the tensile properties of the composite were low.As the mold temperature raised,the microstructure became compact and the tensile properties increased.The fracture regime was mainly transgranular regimes.By in situ tensile test it was found that the propagation direction of cracks would bypass the reinforcing particles.In addition,the crack size was relatively small if cracks generated within the reinforcements,and the two tips of the cracks encountered the soft Al matrix and Ti core respectively,and their propagation was delayed by crack tip blunting.