Preparation Of Multi-scale Double-structure Al₃Ti Particle-reinforced Aluminum Matrix Composites And Their Strengthening And Toughening Mechanism

In order to improve the strength and high-temperature mechanical properties of aluminum and its alloys,ceramic particles are usually added to aluminum and its alloys or reacted in situ to form intermetallic compounds as reinforcing body particles to achieve the above purpose by forming particle-reinforced aluminum matrix composites.However,the addition of reinforcement particles will greatly weaken the plasticity of aluminum matrix composites while increasing the strength and hardness,which not only increases the difficulty of mechanical processing,but also increases the risk of catastrophic failure of the material during the process of plastic deformation,which greatly limits the industrial application of aluminum matrix composites.In order to realize the intrinsic plasticity of aluminum and its alloys as much as possible while improving the strength,a new structure of aluminum matrix composites is proposed in this paper,namely:multi-scale,dual-structured Al₃Ti particle-reinforced aluminum matrix composites,using a powder metallurgical preparation method.The multiscale is composed of nanoscale Al₃Ti particles,submicron level Al₃Ti particles with solid structure,and micron level particles with core-shell structure.The dual structure is composed of Al₃Ti reinforced particles and core-shell reinforced particles with an inner Ti"core"and an outer Al₃Ti metal"shell",which enables aluminum matrix composites to achieve high strength with almost no loss of toughness.reinforcing particles improve the work hardening and strength of aluminum matrix composites by emitting dislocations and impeding dislocation movement,while the core-shell reinforcing particles reduce the loss of plasticity by effectively impeding crack expansion.The results showed that the in situ generated Al₃Ti particles as well as core-shell structure particles gradually increased with the increase of Ti content.When the Ti content is 3wt%,the percentages of nano-level Al₃Ti reinforcing particles formed are low due to the low content,and the core-shell structure particles at the micron level are few,producing low load transfer reinforcement,and the ultimate tensile strength and elongation at break at this time are:253MPa and 15.9%,respectively.When the percentage content of Ti increases to 5wt%,the content of nano/submicron level Al₃Ti particles formed in the tissue increases significantly,and the strength of the aluminum matrix composite increases significantly while almost retaining the good plasticity of the pure aluminum matrix compared with pure aluminum,mainly because at this time the nano-level Al₃Ti particles impede the dislocation movement while some of them also act as dislocation sources to emitting continuous dislocations to alleviate the sharp reduction of plasticity by improving the work hardening of the aluminum matrix composite.In addition,the formation of a large number of micron-level core-shell structure particles,during the plastic deformation process,the crack diffusion is first restricted to the thinner"shell"and then expands to the"core".Because the"core"is relatively soft and has good toughness,the crack width becomes narrower and is hindered,and continues to diffuse into the softer aluminum matrix and eventually fades away.This allows the aluminum matrix composite to have high strength with almost no loss of fracture elongation,at which time the ultimate tensile strength and fracture elongation reach:262MPa and 21.8%,respectively.As the Ti content continues to increase to 8wt%and 10wt%,a large number of nano/submicron level Al₃Ti particles are generated in the aluminum matrix composites,which leads to severe hindrance of dislocation movement during plastic deformation,generating a large number of dislocation sinks as well as dislocation stacking,while the percent content of micron level core-shell structure particles hardly changes anymore,which makes the aluminum matrix composites produce necking fracture during the tensile The final mechanical properties are 300MPa and11.6%and 343MPa and 2.6%,respectively.Considering the optimal overall mechanical properties,it was found that the best mechanical properties were achieved with a Ti content of 5wt%.Aluminum matrix composites with Ti content fixed at Al-5wt%Ti were investigated for the effect of processing parameters on the mechanical properties of aluminum matrix composites.Given that temperature affects the reaction rate between Al and Ti,we focused on the effect of heating temperature on the organization and mechanical properties of multi-scale,dual-structured Al₃Ti particle-reinforced aluminum matrix composites.When the heating temperature was 620℃,the core-shell structure particles formed were not rounded,the"shell"layer was thin and incomplete,and the ultimate tensile strength and elongation at break were 260Mpa and 10.4%,respectively.When the heating temperature is increased to 630°C,the number of nanometer/submicron Al₃Ti particles gradually increases and the work-hardening rate increases,which means that the hindrance effect of plastic deformation process on dislocation increases.The in-situ core-shell structure has rounded particles,but the"core"is still large and the"shell"is thin,and the"shell"layer is not dense,and the mechanical properties at this time are 274.1MPa and 14.1%.The best toughening effect was achieved when the heating temperature reached640°C.Compared with the matrix,the ultimate tensile strength,yield strength and elongation at break reached 262.5MPa,223.7MPa and 21.8%,respectively,without any reduction in ductility,due to the more adequate reaction between Al and Ti,which did not completely react to generate Al₃Ti particles but generated a reasonable proportion of core-shell structure particles and Al₃Ti particles,achieving a good toughening effect.Shell structure particles and Al₃Ti particles,which achieved a good effect of increasing toughness.When the heating temperature was increased to 650℃,the reaction between Al and Ti intensified,the Al₃Ti particles formed increased,the morphology of core-shell structure particles changed,and part of the"core"was completely reacted to form solid structure Al₃Ti particles,which led to a sharp decrease in the toughness of the aluminum matrix composite,and the ultimate tensile strength The ultimate tensile strength increased to 302.6MPa and the elongation at break decreased to 14.2%.The quantitative calculations of Orowan strengthening,load transfer strengthening,solid solution strengthening,fine grain strengthening,and geometric necessary for error strengthening of aluminum matrix composites show that the contribution of load transfer strengthening to the increase of overall yield strength is 141.14MPa,followed by fine grain strengthening at 81.93MPa.The contribution values of Orowan strengthening,solid solution strengthening and geometrically necessary error strengthening were 3.22MPa,18.05MPa and 9.18MPa,respectively.That is to say,during the tensile process,the external force is transmitted from the relatively soft matrix to the higher strength reinforced particles through the interfacial shear stress or shear stress,improving the strength of the aluminum matrix composite.