Talk about the strengthening mechanism of in-situ aluminum matrix composites

The mechanical properties of in-situ aluminum matrix composites were studied by material scientists and the in-situ tensile results showed that the room temperature strength of the composites increased significantly with the increase of the volume fraction of the endogenous reinforcement particles.In situ tensile test, points out that the expansion of the crack in composite material is selective, and to enhance particle distribution is very uneven, when met particle dense group of main crack propagation front end, the direction will deflect, poor to grain area, around the particles concentration areas, along the interface between the particle concentration areas and poor areas to extended forward.Prefacethe strengthening mechanism of (A13Zr+A12O3)/A356 composite material is as follows: Orowan strengthening, dispersion strengthening, fine crystal strengthening, solid melt strengthening

● Orovan reinforcement

Orovan reinforcement is the reinforcement that occurs when a dislocation passes close by a small hard particle and is blocked by the particle.In the inborn A13Zr and A12O3 particle reinforced aluminum matrix composites, due to the small particles and high particle volume fraction, most of them are distributed in grain boundaries. According to the Orowan dislocation obstruction theory, the smaller the particle spacing is, the larger the curvature around the particles of the dislocation line will be, so the dislocation movement resistance will rise and the material strength will increase.

A356 alloy (mass % : Si6.5 ~ 7.5, Fe0.6, Cu0.25, MNo.35, Mg0.20 ~ 0.40, Zn0.35, Ti0.25, other total 0.15, the rest A1) is a kind of aging reinforced alloy for (A13Zr+A12O3)/A356 composite material.Therefore, the effect of enhanced A13Zr and A12O3 particles on the distribution and size of precipitated Phase Mg2Si of A356 alloy must be considered.In the composite material (A13Zr+A12O3)/A356 synthesized in the pulsed magnetic field, the matrix internal dislocation is dense and distributed in a network, and it is connected with the aging precipitated phase Mg2Si.This is because the introduction of in-situ reinforced particles increases the dislocation density of the matrix significantly, and the dislocation promotes the generation of aging precipitated phase, and makes precipitated phase fine and dispersed.Therefore, the reinforcement particles in the composites are of great benefit to the strengthening of the precipitated phase.

● Fine crystal strengthening

The grain structure of matrix has great influence on the mechanical and physical properties of as-cast metal matrix composites.When the intra grain dislocation moves to the vicinity of grain boundary, it will be blocked by grain boundary, and to continue the deformation, the external force must be increased. Moreover, the finer the grain is, the greater the proportion of grain boundary is, and the more the dislocation movement obstacles are.In addition, due to the adjacent grains exist difference, each grain slip orientation is different also, when a grain plastic deformation occurs, is led by neighboring grain resistance and obstacles, must coordinate with each other in elastic deformation, the elastic deformation becomes continue to plastic deformation resistance, polycrystal plasticity deformation resistance increase, in addition, the grain boundary of impurity atoms is more, is unfavorable to plastic deformation, strengthened the absence made.Grain refinement can also improve the plasticity and toughness of the metal, because the finer the grain is, there will be more grains in a certain volume, and the more grains are borne by the same deformation amount, the more uniform the deformation will be, and there will be no serious stress concentration, so greater deformation can be produced without fracture, showing good plasticity.Moreover, the winding grain boundary of fine grains is not conducive to crack propagation and expansion, and the crack crossing the grain boundary needs to consume more energy, so the toughness increases.

There are two grain refinement mechanisms of aluminum matrix composites: enhancing heterogeneous nucleation on the surface of the body and heat exchange between the reinforcement and the matrix alloy.Since the enhanced phase is formed from the inside of the melt by chemical reaction, A13Zr and A12O3 can be used as the heterogeneous nucleating basement of -Al and Si phases, respectively, resulting in thinning.As a result of the formation of A13Zr and A12O3 particles in matrix under the action of pulsed magnetic field, the matrix morphology crystallized into fine equiaxed crystal, and the eutectic changed from long needle to short stick or granular.Therefore, the fine grain strengthening mechanism of in situ (A13Zr+A12O3)/A356 composites is the heterogeneous nucleation mechanism of the matrix on the enhanced body surface.

● Solid solution strengthening

After the foreign atom solid dissolves in the matrix, on the one hand can hinder the dislocation movement, on the other hand because of the foreign atom and the matrix metal atom diameter is different, will cause the lattice distortion, will produce the strain field, and will have the interaction with the dislocation.Solute atoms as hinder dislocation motion, improve the plastic resistance, which is caused by two reasons: one is the solute atom lattice distortion, increase the dislocation density, the solute atoms of lattice distortion degree and solubility due to the difference of solute and solvent atoms and dissolution of different and different, solute atoms dissolved, the more the lattice distortion, the more serious, the greater the strengthening effect.On the other hand, the interaction between solute atoms and dislocation can make the dislocation in a stable state, and most solute atoms tend to distribute around the dislocation.In the case of replacement solid solutions, solute atoms smaller than solvent atoms tend to diffuse to the tensile site below the blade dislocation, forming "Cottaell air masses".When the gas is formed, the dislocation energy decreases and is in a stable state, binding the dislocation.As the dislocation struggles to move away from the Cordran air mass, it must exert greater force, which means that the metal's deformation resistance rises.

● Dislocation reinforcement

The dislocation density in metal always increases with the decrease of particle size. With the increase of particle volume fraction, the dislocation density increases and the strength increases.In the (A13Zr+A12O3)/A356 composite material, due to the small size and high volume fraction of the particle reinforcer, the thermal expansion coefficient of the reinforcer A13Zr and A12O3 is different from that of the matrix A356 alloy, which will cause a large number of additional dislocation around the particles, so the dislocation density of the composite material increases greatly, thus improving the strength of the composite material.

There's always something wrong in a metallic crystal.Both the crystal integrity and the theoretical strength calculation show that the strength can be improved only by eliminating the dislocation. On the contrary, the metal crystal defect theory holds that the strength can be improved effectively by increasing the dislocation density.In the metal matrix composites reinforced by particles, the thermal expansion coefficient between the reinforcement and the matrix is very different, which leads to huge thermal stress, plastic deformation, and dislocation density in the composites.

Scientists' calculations show that the four strengthening mechanisms of in-situ aluminum matrix composites contribute differently to strength, with dislocation contributing the most and solid solution the least.Various strengthening mechanisms improve the strength of this kind of complex material as follows: бo＝21.26MPa，бg=18.45MPa，бd=24.85MPa，бs=16MPa，After a total б = 80.56 MMPa。