Effect Of Ti,Si Doping On Growth And Morphologies Of In-situ TiC And SiC In Aluminum Melt

The aluminum matrix composites reinforced by in-situ particles not only have the excellent properties belonging to the aluminum matrix of low density,high plasticity,and the easier machinability and formation,but also possess the characteristics of high strength,high hardness and wear resistance of the in-situ reinforcing phases.Therefore,they receive widely concern.In particular,the microstructure of the in-situ aluminum matrix composites reinforced by TiC and SiC can be artificially adjusted,improved and controlled by the compositional design at the early stage of the preparation process.So they have large industrial application value.In this paper,the thermodynamics reactions of two in-situ systems?Ti-C-Al and C-Si-Ti-Al doped with small amount of Ti?were studied by theoretical calculation and differential scanning calorimeter?DSC?.The sintering for the quaternary C-Si-Ti-Al system at high temperature were carried out.In-situ TiC/Al and in-situ?TiC-SiC?/Al matrix composites were prepared by contact reaction method.The effects of the compositions and the structure of in-situ systems and the preparation processes on the morphological evolution,dimensional variation and distribution of particle-shaped in the in-situ reinforcements were studied through optical metallographic microscopy?OM?,X-ray diffraction analysis?XRD?,scanning electron microscopy?SEM?,transmission electron microscopy?TEM?and Image-Pro Plus 6.0 software.The main research conclusions are as followed:The Al-12Si-4Cu matrix composites can be prepared by the contact reaction method through Ti-C-Al system.The in-situ reinforcing phase is mainly TiC,but a little TiAl₃ phase exists.Most of the TiC particles present spheroidal-shaped,while others are short rod-like.With the addition of 0.35 wt.%La-Ce mixed rare earth to the Al-12Si-4Cu composites,the in-situ spheroidal-shaped TiC particles decrease from 2 to 0.5?m in diameter,while the short rod-like TiC particles reduce from 7 to 1?m in length.And the length of the TiAl₃ also decreases from 100 to 20?m.The effect of various ultrasonic treatment power on the morphologies of in-situ particles in the Al-12Si-4Cu matrix composites is remarkable.The agglomeration equivalent diameter?ED?of the TiC particles without the ultrasonic treatment is approximately 70?m,and reduce to 45?m when the ultrasonic treated at 1500 W.With the ultrasonic treatment power is increasing to 3000 W,the TiC particles are uniformly dispersed in the matrix without the agglomeration phenomenon.The length of the long rod-like TiAl₃ reduces to 50-150?m,and the plate-like TiAl₃ particles are transformed into independent small-size blocks.The effect of Ti-C-Al system with different treatment processes on the morphologies of the in-situ particles is also remarkable.When the powder system of Ti-C-Al is simply mixed,the TiC particles are relatively coarse in size and seriously aggregated;their equal circle diameters are 4-6?m.When the Ti-C-Al powder system is grinded manually,the TiC particles are 2-3?m in diameter.When the same systems are ball milling with high energy mixture,the TiC particles reach the nanometer level in diameter.When the Al-3C master alloy added Ti powder is used as the in-situ system,the TiC particles in the composites are bright white flocculent.But there are a small amount of black Al₄C₃ particles around the TiC particles.The elements of Si in Al-Si matrix alloys has the great effect on the microstructure of the composites prepared with Ti-C-Al powder system and Al-3C master alloy dopped with Ti powder system.When using the former system and the content of Si?7 wt.%,the TiC particles are spherical and discernible independently,but only local aggregation occurs.The TiC particles transform into small polygonal blocks,and their size increases from 2?m to 6?m with 7 wt.%<Si<24 wt.%.The morphologies of TiC particles present as polygonal block with Si=24 wt.%.In contrast,when using the latter system,the TiC particles are mainly flocculent and a little Al₄C₃ remains under the content of Si less than 7 wt.%.The flocculent TiC particles transform into long rod-like,and the size has obviously increased to 5-8?m under the content of Si between 7 wt.%-24 wt.%.The TiC particles present long rod shape and the Al₄C₃ particles are thoroughly absent from the microstructure under the content of Si reaches 24 wt.%.The mixed powder type and the layered powder type were designed to achieve the in-situ synthesis of SiC particles based on the preparation of in-situ TiC particles by Ti-C-Al powder system.The results show that the SiC phase can be in situ produced with the two system sintered at obtained at a high temperature of 1100?.When using layered powder system,the sintering reaction is complete.There are no any other by-products except for TiC phase and SiC phase.In-situ?TiC-SiC?/Al composites were prepared by contact reaction method.When the melt temperatures change?950?,1100?and 1250??,the in-situ TiC particles and SiC particles can be obtained.When preparation occurs in the 1100?,the in-situ TiC and SiC particles are fine and distributed uniformly.Especially when the start powders are processed by high energy ball milling,the in-situ particles have reached nano-scaled in size,and are uniformly distributed.