The Microstructures And Mechanical Properties Of Al2014 Composites Reinforced With Ti₅Si₃-coated SiC_P

The powder metallurgy and stir casting are the most common ways to fabricate the Al alloy-based composites reinforced with SiC_P.The powder metallurgy method possess some advantages,such as well-dispersed of SiC particles,Near-Net-Shape formability and avoid of harmful interface reaction between SiC_P and Al matrix because of low fabrication temperature.However,there are always some micropores between SiC_P and Al alloy matrix,which indicates the poor interfacial bonding between reinforcement and Al matrix,and may be acted as the crack initiation,thus,it would limit the reliability and stability of composite materials during their service.Moreover,stirring casting is another widely used method to fabricate the Al alloy-based composites reinforced with SiC_P.Compared with powder metallurgy,stirring casting method is low in cost,uncomplicated equipment and technology and flexible to prepare workpieces with complex shape and larger size.However,the poor dispersion of SiC_P in the composites and generation of harmful Al4C3 phase have limited the further development and application of SiC_P/Al composites.Researchers at home and abroad try to solve these problems by optimizing the process parameters,matrix alloying and surface modification of SiC_P,and have made some headway.However,the dispersion of SiC_P,the control of the interfacial reaction and the interfacial bonding between the reinforcements and the matrix in the composites prepared by powder metallurgy is still a difficult problem and a research hotpot in the field of metal matrix composites reinforced with ceramic particles.This work has explored the effects and mechnism of different SiC_P surface modifications on wetting of Al2014 melt on SiC,the interfacial reaction and the interfacial bonding between SiC and ?-Al,which was aimed to find a solution to the problems of poor-dispersion of SiC_P,harmful interfacial reaction and poor interfacial booding in tranditional SiC_P/Al composites.It was found that the salt-bath plated Ti₅Si₃ coating layer was the best surface modification for the SiC particles.The Al2014 composites reinforced with Ti₅Si₃-coated SiC_P exhibited the well-dispersion of SiC_P,improved interfacial bonding,excellent mechanical properties and wear resistance.The capture mechanism of Ti₅Si₃-coated SiC_P into ?-Al during the solidification and mechanism of improved mechanical properties have been revealed.The main results of this paper are as follows:1)The reaction mechanism of salt-bath plated Ti₅Si₃ coating layer and effects of different SiC_P surface modifications on the compressive property of SiC_P/Al2014 composites have been revaled.i)The reaction mechanism of salt-bath plated Ti₅Si₃ coating layer: the C atom would firstly meet the Ti atom to generate Ti C because C atom exhibited higher diffusion speed than Si atom,Ti atoms would meet the Si atoms on the surface of the SiC to prepare Ti₅Si₃ coating layer,and its thickness would increase with the increasing sintering time.The diffusion difficulty of C atoms through the coating layer would increase with the increasing thickness of coating layer,thus,C atoms would gather in the SiC/Ti₅Si₃ interface and reacte with Ti and Si atoms to generate the compound of Ti3 SiC2 when the C atom concentration was above a certain value.ii)The improved order of different reinforcement surface modifications on strength and plasticity might be concluded through a transition chain: Salt bath plated Ti₅Si₃> Salt bath plated Ni2 Si and Cr3 Si > Uncoated and pre-oxidized SiC_P.The mechanism of improved strength and plasticity for surface-modified SiC_P/Al2014 composites was attributed to the enhanced interfacial and relative density for the composites.The Al2014 composites reinforced with Ti₅Si₃ exhibited the best compressive strength and fracture strain,which reached 806 MPa and 23.2 %,respectively.iii)Optimum parameters of salt-bath plated Ti₅Si₃-coated SiC_P: Ti content in the mixer of Ti+SiC_P is 10 wt.%,and the sintering temperature and time were 1173 K and 1.5 h,respectively.2)It was revaled that the density,elastic modulus,strength and ductility of Al2014 composites reinforced with Ti₅Si₃-coated SiC_P prepared by powder metallurgy was significant higher than that of uncoated one:i)The process of the interface evolution: SiC/Ti₅Si₃/?-Al ? SiC/Ti5?Si,Al?3/?-Al ? SiC/Ti2?Al,Si?5/?-Al ? SiC/Ti?Al,Si?3/?-Al ? SiC/Ti Al3/?-Al.The mechanism of interface reaction: The diffusion of Al atoms into Ti₅Si₃ phase to generate the solidsolution of Ti5?Si,Al?3,which will cause the volume expansion and energy enhancement for the solidsolution,thus the lattice transformation would be happened to decrease the lattice distortion,and the solidsolution of Ti5?Si,Al?3 was gradully transmited into Ti2?Al,Si?5 and Ti?Al,Si?3.ii)The improved density mechanism of Ti₅Si₃-coated SiC_P/Al2014 composites: Ti₅Si₃ coating layer significantly decreased the contact angle between SiC and molten Al2014 alloy,which is beneficial for molten alloy to spread on the SiC_P surface,reducing the micropores between particles and matrix.iii)The elastic modulus,compressive strength,fracture strain of Ti₅Si₃-coated SiC_P/Al2014 composites were 104.8 GPa,756 MPa and 16.6% when the particle volume fraction of 15%,respectively,which were 7.9 GPa,109 MPa and 5.4 % higher than those of uncoated one.The mechanism of improved elastic modulus,strength and ductility for Ti₅Si₃-coated SiC_P/Al2014 composites was mainly due to the enhanced interfacial bonding and improved relative density,which improved the derformation constraint for the matrix and decreased the possibility of interfacial debonding between SiC_P and Al2014 marix.3)It was found that the Al2014 composites reinforced with Ti₅Si₃-coated SiC_P fabricated by powder metallurgy exhibited a higher wear resistance than uncoated ones:i)When the volume fraction of SiC_P increased from 5% to 10%,15% and 20%,the abrasive wear resistances of Ti₅Si₃-coated SiC_P/Al2014 composites were 13.6%,15.9%,22.5% and 32.1 % than that of uncoated ones.ii)It was revealed that the mechanism of improved wear resistance: Ti₅Si₃ coating layer improved the hardness of composites and the interfacial bonding between SiC_P and Al2014 matrix,which could prevent the abrsive particles penetrating too deep into Al2014 matrix and reduce the possibility of interfacial debonding between particles and matrix,thus the Ti₅Si₃-coated SiC_P would be more effective to be reinforcement for the abrasive wear resistance of the composites.4)It was revaled the mechanism that the Ti₅Si₃-coated SiC_P/Al2014 composites fabricated by stir casting method exhibited better dispersion of SiC_P and higher mechanical properties than uncoated ones with the same SiC_P content.i)It was revealed that the evolution process of Ti₅Si₃-coated SiC/?-Al: SiC/Ti₅Si₃/?-Al ? SiC/Ti Al3/?-Al ? SiC/Ti3 Al C2/?-Al.ii)The elastic modulus,tensile strength and fracture strain of 4 vol.% Ti₅Si₃-coated SiC_P/Al2014 composites were 1.69 GPa,34 MPa and 2.9% higher than those of uncoated one,respectively.The mechanism of enhanced elastic modulus,strength and ductility was mainly attributed to the improved the uniform dispersion of SiC_P and enhanced the interfacial bonding,which could reduce the interfacial debonding between particles and matrix and be beneficial to uniformity and harmonization for the plastic deformation.iii)This work analyzed the interactions between particles,particle and molten alloy or liquid/solid interface in the three stages of the casting process,and proposed the mechanism for the effect of the improved wettability on the uniform dispersion of particles in the matrix: a)The improved wettability is favorable for the spreading of molten alloy on the surface of reinforcements during addition of particles;b)The improved wettability increased the energy barrier which could make the ceramic particles self-stabilized in the melt during the stirring process;c)During the solidification,the resistance of the particles was reduced or changed as dragging force owe to the improved wettability,and reduced the total energy of the solidification system when the particles were captured into solid phase.iv)For the first time,the aluminum matrix composites with most of SiC_P captured into ?-Al interior were fabricated under the conventional solidification condition.It was revaled the capture mechanism of Ti₅Si₃-coated SiC_P into ?-Al: The enhanced wetting of molten Al2014 alloy on the surface of Ti₅Si₃-coated SiC_P could improve the efficiency of capture of SiC_P into Al2014 melt during the addition process of reinforcement;The enhanced the wettability could increase the energy barrier needed to cross for the particle agglomeration and improved the self-stabilization for Ti₅Si₃-coated SiC_P;It also decreased the repulsive force between solid/liquid interface and Ti₅Si₃-coated SiC_P,which was of benefit to the capture of Ti₅Si₃-coated SiC_P into ?-Al.This paper developed a new method of Ti₅Si₃ coating layer on the surface of SiC_P by salt-bath plating,which could effectively improve the wetting of molten Al2014 alloy on the surface of SiC,the uniform dispersion of particles,mechanical performance and abrasive wear resistance for the composites.In addition,the coating layer also could avoid the harmfull interfacial reaction between SiC_P and Al2014 matrix.The mechanism for the effects of Ti₅Si₃ coating layer on the enhanced dispersion of SiC_P,higher strength,higher plasticity and the improved wear resistance for the compositeswas revealed.The results obtained in this paper provided some technical and theoretical basis for the further development and application of the Al alloy-based composites reinforced with SiC_P.