Investigations On Mechanical Properties,Strengthening And Toughening Mechanisms Of Copper And Copper Alloys

The investigations of the interrelationships between the structures,fabrication,properties,and performances of materials have been the main theme of materials science for long time.For instance,the strengths and ductilities of metallic structural materials are two important properties considered for application.Strength and ductility of the materials represent the threshold value and safety factor of the acceptable load,respectively.However,the strengths and ductilities of metallic materials are mutually exclusive and plotted as trade-off relationship.Therefore,the simultaneous improvement of strength and ductility of metallic materials is a hot issue in the research field of structural materials.In this thesis,copper alloys were selected to explore the mechanisms for simultaneous improvement of strength and ductility of metallic materials.The effects of temperature,chemical composition,uniformity of microstructure and plastic deformation method on the mechanical properties of copper alloys were investigated systematically.Based on the mechanisms of dislocation multiplication and annihilation the exponential hardening model was established to depict the stress-strain relationship of strain hardening process.In order to understand the mechanisms for simultaneous improvement of strength and ductility of metal in further;the mechanical properties were related with the microscopic deformation mechanism.The knowledge of dislocation activities was applied to solve practical engineering problems of elastic copper alloys.The main contents and the experimental results of this dissertation are listed as follows:1.Effect of temperature on mechanical properties of partially recrystallized pure copper.The mechanical properties and microstructure evolutions of partially recrystallized pure copper were investigated by tensile tests at 293 K and 77 K and microstructural characterization techniques.As temperature decreases,the dislocation slip mode of pure copper is transferred from wavy mode to planar mode.Meanwhile,the strengths and ductilities of pure copper are improved simultaneously.The microstructures consisted of the rolled lamellar matrix and the recrystallized grains are in parallel structure,so the relationship between mechanical properties and recrystallization percentage is approximately linear at 293 K and 77 K.It is revealed that there is no deformation twins and yield points during tensile deformation of pure copper at 293 K and 77 K.This is explained by the effects of grain sizes and temperatures on dislocation slip.An exponential hardening model which describes work hardening behaviors quantitatively is established.The physical meanings of different parameters in the model are discussed.The saturation stress represents the upper limit of the dislocation density and the hardening exponent represents the effective annihilation distance of the dislocations.The simultaneous improvement of strength and ductility caused by cryogenic temperature is analyzed.For pure copper,decreasing temperature can not only reduce the hardening exponent but also enhance the dislocation saturation stress.2.Influence of composition and temperature on mechanical properties of partially recrystallized Cu-Al alloys.The mechanical properties and microstructure evolutions of partially recrystallized Cu-5Al alloys were investigated by tensile tests at 293 K and 77 K and microstructural characterization techniques.It is found that the deformation mechanisms of Cu-5Al alloy transfer from wavy dislocation slip to planar dislocation slip and deformation twinning as temperature decreases.The simultaneous improvement of strength and ductility is achieved at 77 K.The relationship between mechanical properties and the recrystallization percentage at 293 K and 77 K has been revealed.According to the coupling effect of microstructures and deformation mechanisms,the different linear relationship between uniform elongation and recrystallization percentage at 293 K and 77 K are discussed.The effects of composition and temperature on simultaneous improvement of strength and ductility were analyzed from the microscopic mechanism.Decreasing temperature and aluminum addition both can reduce the hardening index and increase the saturation stress because the low stacking fault energy inhibits dynamic recovery based on cross-slip.3.The comparative effects of cold-rolling and pre-strain on the mechanical properties of Cu-Al alloys.The plastic deformation was imposed on Cu-Al alloys by cold-rolling and tensile tests,respectively.The mechanical properties and micro-structure evolution of deformed Cu-Al alloy were investigated by tensile tests and microstructure characterization techniques.The effects of Al content on the work hardening behaviors and grain refinement mechanisms of Cu-Al alloy were analyzed.The yield strengths and uniform elongations of the rolled Cu-2.5Al alloy,Cu-5Al alloy,Cu-8Al alloy are larger than the pre-tensile specimens.The yield strengths of rolled Cu-11Al alloys are close to the pre-tensile specimens,but the uniform elongations of the rolled specimens are smaller.According to the microstructure merits and mechanical properties,two principles for simultaneous improvement of strength and ductility are proposed:1)Composition optimization is aim to reduce the effective annihilation distance and the hardening exponent via decreasing stacking fault energy;2)Microstructure optimization is aim to increase the proportion of low energy strengthening phase such as grain boundary and twin boundary.4.Relationship between stress relaxation resistance and mechanical properties of copper alloy.Three high strength copper alloys,Cu-Co-Be,Cu-Ni-Si and Cu-Ni-Sn alloys,were processed by cold-drawing and aging treatment.The microstructure and mechanical properties of the copper alloys were investigated by microstructure characterization technique and compression tests.The principal strengthening mechanisms of three copper alloys are work-hardening and precipitation strengthening.Among three copper alloys,Cu-Ni-Sn alloy exhibits the highest hardness and strength.Then,the bending stresses of copper alloys in stress relaxation experiment were measured via a self-made experimental apparatus.It was revealed that the stress relaxation resistances of different copper alloys are similar at room temperature.However,at 450? and-196? the copper alloys with higher hardness exhibit the better stress relaxation resistance.According to the interaction mechanism between precipitates and dislocations,a simple criterion for stress relaxation resistance of the precipitation reinforced copper alloy is proposed.