The Research On Regulating Microstructure Heterogeneity Of 2024 Al Fabricated By Powder Thixoforming And Corresponding Strengthening And Toughening Mechanisms

Synergistic enhancement of strength and ductility of metal materials has always been a long-term pursuit for the material researchers in the field of metal structure materials.In recent years,the design concept of heterostructured materials has received extensive attention in the field of metal materials.That is,the introduction of soft/hard domains with distinct strength or hardness differences in metallic materials promotes the accumulation and interaction of dislocations during the deformation process,producing high hetero-deformation induced（HDI）strengthening and hardening,and thus,enabling materials to achieve excellent strength-ductility combination.The heterogeneity level（the differences in microhardness and volume fraction between soft and hard domains）of materials has a decisive effect on their mechanical properties.In this work,based on powder thixoforming technology,the 2024Al alloy with a continuously adjustable heterogeneity level within a certain range and good mechanical properties is achieved via the introduction of fine Al₃Ti particles and tailoring the process parameters of thixoforming（such as remelting temperature,remelting time,and mold temperature）.In-depth discussions were conducted on the effect of the heterogeneity level on HDI strengthening and hardening mechanism.In addition,considering the inherent heat-treatable properties of the 2024Al alloy and to further improve the mechanical properties,an investigation was conducted to study the effects of T6 heat treatment on both its microstructure and mechanical properties.The main results indicate that:（1）After 8 hours of low-speed ball milling,the optimal tensile properties of the 2024Al alloy were achieved.The yield strength,ultimate tensile strength and elongation to failure are 275 MPa,430 MPa and 8.03%,showing an enhancement of 36.1%,41.9%,and 10%respectively,compared with those of the 2024Al matrix alloy.The main reason is that the introduction of Al₃Ti particles increases the heterogeneity level of 2024Al alloy（the difference in microhardness of 1.17 GPa）,resulting in high HDI strengthening and hardening,consequently improving both strength and ductility.In addition,the uniform distribution of Al₃Ti particles helps to avoid severe stress concentration,further enhancing the mechanical properties of 2024Al alloy.（2）The heterogeneity level gradually increases as the remelting temperature increases.However,the remelting time and mold temperature have opposite effects on the heterogeneity level compared to the remelting temperature.The remelting temperature has the most significant influence on the heterogeneity level.The factor in determining the heterogeneity level includes the solid solubility of solute atoms,amount of liquid phase,content of eutectic compound,the size of primaryα-Al grains and Al₃Ti particles.After optimizing the process parameters of thixoforming,i.e.,when heated at 655℃for 80 min with the mold temperature of 250°C,the 2024Al alloy achieved a moderate heterogeneity level（the differences in microhardness and volume fraction of 1.23 GPa and 2 vol.%）.As a result,the 2024Al alloy exhibits enhanced mechanical properties with an ultimate tensile strength of 455 MPa,yield strength of 291 MPa,uniform elongation of 7.95%,elongation to failure of 8.65%,and elastic modulus of 73.34 GPa.（3）The heterostructured 2024Al alloy are composed of soft and hard domains.The soft domains consist of coarse grains（primaryα-Al grains）,while the hard domains consist of fine grains（secondary primaryα-Al grains and dynamic recrystallized grains）,Al₃Ti particles,and eutectic compounds（Al₂Cu and Al₂Cu Mg）.Furthermore,the deformation process of 2024Al alloy occurs in a progressive order,starting from coarse grains to fine grains,then followed by eutectic compounds,and finally Al₃Ti particles,i.e.,the soft domains begin to deform first and then hard domains.At the early deformation stage,HDI hardening is primary work hardening mechanism,which corresponds to the pile-up of geometrically necessary dislocations（GNDs）against the interfaces of soft/hard domains.At the later deformation stage,the HDI hardening becomes weaker while the dislocation hardening is more predominant due to the formation of dislocation walls and cells.Strengthening and toughening mechanisms are attributed to the moderate heterogeneity level,which enhances the coordination deformation ability between the soft and hard domains.This,in turn,promotes the accumulation of more GNDs at the interface of the soft/hard domains,resulting in higher HDI strengthening and hardening.Additionally,the submicro-Al₃Ti particles can serve as dislocations source to emit the dislocations,further improving the strain strengthening and hardening.（4）After T6 heat treatment,the 2024Al alloy achieved a moderately improved heterogeneity level（the differences in microhardness of 1.50 GPa）,resulting in a prominent combination of ultimate tensile strength,yield strength and elongation to failure of 539.8 MPa,342.7 MPa,and 11.14%,presenting an enhancement of 18.6%,17.8%,and 28.8%compared with those of as-fabricated2024Al alloy,respectively.These excellent tensile properties are attributed to the further enhancement in heterogeneity level.As a result,a large number of GNDs accumulate at the interface of the soft/hard domains,which contributes to a higher HDI strengthening and hardening.In addition,the formation of Guinier Preston（GP）zones and fine needle-shaped S"phase during the heat treatment process impedes the movement of dislocations,further improving the strength.At yield,the dislocations continuously cut through the GP zones and move forward,thereby promoting ductility.