| TC4 particle(TC4p)(15 and 35μm) reinforced AZ91 matrix composites with contents from 10 vol.% to 20 vol.% were fabricated by the optimization of process parameters. The effect of size, shape and volume fraction of TC4 p on microstructures and properties was studied. The interface between TC4 p and the matrix was analyzed to clarify the strengthening mechanism. Systematic research was made to indicate the evolution of microstructures, properties and textures of as-extruded composites of different composition(size and shape of particles and volume fraction). The effect of controlling of interface interactions on microstructures and properties was also studied, which revealed the effect of Al-Ti interface phase on composites further.The results showed that, TC4 p distributed uniformly in the matrix, and particle-sedimentation phenomenon did not take place obviously. Compared with that of AZ91 alloy, the grains of composites were refined clearly, and the mechanical properties were improved significantly. The more volume fraction of TC4 p, the more fine grains of the composites was observed, and the strength and rigidity were increased significantly, however, the elongation decreased. With the decrease of the TC4 p size, the effect on grain refinement of the composites became more obviously, and its strength and elongation were further improved. For the spherical TC4p/AZ91 matrix composites, due to interface reactions between the spherical TC4 p and the matrix, the coarse granular reaction products led to the deterioration of mechanical properties.Compared with the as-cast TC4p/AZ91 matrix composites, the TC4 p distribution in the composites was improved significantly after hot extrusion deformation, and the TC4 p segregation areas were reduced or disappeared. In addition, the TC4 p arranged along the extrusion direction, thereby eliminating the type of chain distribution in the as-cast composites. When the content of TC4 p was too high, some particles were fractured in the squeezed composites. After hot extrusion deformation, DRX in the alloy and composites was basically completed,therefore, the grains were refined significantly. The DRX grains in the composites were smaller than that of the alloy. With the increase of TC4 p volume fraction, thesize of grains in the squeezed composites followed the trend of decreasing to increasing. In addition, with the decrease of the TC4 p size, the size of composites grains decreased further. For spherical TC4p/AZ91 composites, there was a small amount of non-recrystallization zone appeared, and the grain size was similar with the random TC4p/AZ91 composites. Compared with the as-cast AZ91 alloy and TC4p/AZ91 composites, the strength and rigidity were improved greatly after hot extrusion. When the content of TC4 p increased gradually, the strength of the composites followed the trend of increasing to decreasing, caused by particle broken in the matrix. In addition, when the size of TC4 p reduced, the tensile strength and elongation of the composites were improve obviously. Extruded alloy and composites are characterized by the typical base texture, namely(0002) plane was parallel to the extrusion direction, and(10-10) cylinder was perpendicular to the extrusion direcrion. TC4 p had not changed the texture type of the matrix alloy.But, TC4 p weakened the strength of the composites texture. And with the increase of TC4 p volume fraction, the(0002) plane and(10-10) cylindrical texture intensity of the composites was reduced gradually. Moreover, the decrease of the size can weaken of the strength texture more seriously.There are three main kinds of interface structure between TC4 p and AZ91: the first is smooth interface; the second is nano-Mg O appearing at the interface; the third type is Al-Ti intermediate phase precipitated at the interface. Wherein, Al-Ti intermediate phase include: small rod Al3 Ti, coarse particulate Al2 Ti and Al Ti3 grown in the interior of the TC4 p. Besides, the rod Al3 Ti was the majority, the precipitation of the granular Al2 Ti and the dendritic Al Ti3 was little. In addition to the thermal mismatch strengthening mechanism, fine-grain strengthening mechanism and the load transfer function, there is also Orowan strengthening mechanism of the nanoscle second phase formed in the interface. Owing to ignoring the Orowan reinforcement contribution to yield strength of the composites contribute to the considerable difference between the theoretical value and experimental value. When Al content was low in the matrix, the rough granular Al2 Ti generated in the interface deteriorated the mechanical properties of the composites. The increase of Al content in the matrix can promote the formation of the club-shaped Al3 Ti in the interface between TC4 p and matrix, and the composites strength increment was increased accordingly. |
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