Fabrication And Mechanical Properties Of Nano-SiC Reinforced Aluminum Matrix Composites By Powder Metallurgy Method

SiC particles reinforced aluminium matrix(SiCp/Al) composites possess low density, high specific modulus, high specific strength, low expansion, high thermal conductivity, high wear resistance, high damping and low cost, so they have great application potential in military and civil fields, such as national defense, traffic and electronic fields. However, the disadvantages of the micro-sized SiC particle reinforced aluminium matrix composites are that the ductility decreases dramatically when the strength increases, and the improvement of the high temperature mechanical properties are not satisfactory. These are bottleneck problems of particle reinforced aluminium matrix composites. Some researchers have reported that these problems can be resolved by nano-sized SiC paiticulates. While, there are many issues which should be addressed, for example:(i) because of the high specific surface energy of nano SiC, they are easily agglomerate in aluminium matrix, which leads to the difficulty of homogeneous dispersion in aluminium matrix;(ii) the weak bonding strength between SiC and aluminium matrix limited the full play of strengthing effect. To solve the two main problems, the solvent-assistant dispersion and mechanical ball milling methods were attempt to disperse the nano SiC and the nano SiCp/2014 Al composites were successfully fabricated by powder metallurgy and hot extrusion techniques. The effect of nano SiC on the the microstructure、mechanical properties and ageing behavior of the composites were investigated by tensile and compressive test、 micro-hardness test as well as wear and corrosion tests. The strengthening mechanism of nano SiC/2014 Al composites was also discussed by SEM and TEM. The results have certain reference meanings to the fabrication of nano paiticles reinforced aluminium matrix composites.The main results are as follows:1) An approach that combines solvent-assistant and mechanical ball milling process was proposed to improve the homodisperse of nano SiC in 2014 Al matrix; the influence of nano SiC content on the mechanical properties of composites at room temperature and high temperature was revealed. The optimal tensile properties(the yield strength 378 MPa、ultimate tensile strength 573 MPa and elongation 9.0% at room temperature; the yield strength 303 MPa、ultimate tensile strength 409 MPa and elongation 12.5% at 473 K) were observed in 0.5 vol.% SiC/2014 Al composite which indistinctively sacrificed the ductility at room temperature.2) The investigation of the interface between nano SiC and 2014 Al found that the interface was clean and the nano SiC bonded well with the matrix. Due to the low temperature(520 ℃) and short time(50 min), no obvious interfacial product(Al4C3) was found. These are helpful to interface control of nano particles reinforced aluminium composites. The strengthening mechanisms of nano SiC/2014 Al composites are load transfer strengthening, Orowan strengthening, dislocation strengthening and grain refinement strengthening. The load transfer strengthening and Orowan strengthening play a leading role.3) Besides two kinds of main precipitates — disk-shaped Al5Cu2Mn3 and needle-shaped θ’(Al2Cu), there are three kinds of minor precipitates — hexagonal-shaped plate-like W(Al2Cu)、cuboid-shaped T(Al20Cu2Mn3) and cubic-shaped s(Al5Cu6Mg2) in nano SiC/2014 Al composites. Al5Cu2Mn3 phase emerges earlier than θ’(Al2Cu), they lead to the two ageing peaks. The minor precipitates are beneficial to the high temperature properties of the composites.4) It is revealed that the nano SiC accelerated the ageing kinetics of precipitates in the composite(12 h for 2014 Al alloy and 10 h for SiCp/2014 Al composite) and refined significantly Al5Cu2Mn3 and θ’ phases(210 nm and 37 nm for 2014 Al alloy and 35 nm and 33 nm for SiCp/2014 Al composite at peak ageing). The mechanisms are as follows:(?)large number of interface caused by grain refinement can provide both the nucleation energy for precipitation and the spread channels for the solute atoms.(??)The difference of thermal expansion coefficient between SiC and 2014 Al produced a high density of dislocations which could provide nucleation position for θ’ and simutaneously caused the relative scarcity of solute atoms and then limited the growing up of precipitates.5) The optimal corrosion and wear properties were observed in two layers functionally graded material with the composition of 3 vol.% SiC/2014 Al composite in outer layer and 2014 Al alloy in inner layer. The wear rate at 15 N was 0.61 mm3/m and the corrosion current density was 1.75 × 10-6 A/cm2.