Study On The Effect Of Li Containing Precipitates On The Microstructures And Textures Of AA2099 During Thermomechanical Processing And Its Mechanisms

In this thesis,an extrusion bar of AA2099 Al-Li alloy in H112 state was selected as the initial material.Firstly,the microstructure evolution of AA2099 during uniaxial compression was investigated.The optimum hot deformation parameter of AA2099 was also determined based on the stress-strain curves of AA2099 uniaxial compression test.Subsequently,initial sheets were cut from the extrusion bar and pre-heat-treatments were performed before rolling in order to obtain different kinds of precipitates?such as Al₃Li,Al₂CuLi,Al₆CuLi₃,etc?in the matrix.The effects of these precipitates on the microstructure and texture of these sheets after rolling were systematically investigated.Additionally,these rolling sheets were annealed to investigate the effects of these precipitates on the recrystallization.And the mechanisms of these effects were discussed.Conclusions and important finds were listed below:?1?The dynamic restoration mechanism of AA2099 in H112 state deformed under the regime of 480°C-520°C and 0.001s^-1-0.1s^-1 is discontinuous dynamic recrystallization,while under other regimes the dynamic restoration mechanism is dynamic recovery.Combining the processing map and the microstructures after compression,the optimum hot deformation regimes for AA2099 in H112 state are at 420°C-460°C and 0.09s^-1-0.3s^-1 and 480°C-520°C and 0.001s^-1-0.1s^-1.?2?The nucleation of dynamic recrystallization during compression is closely related to the evolution of fiber texture.During the compression tests,the textures were changed from<111>//ED into<110>//ED.At the same time,the strain incompatibility between grain boundaries and interiors leads to recrystallization nucleation.The thermal stability of Al₃Zr particles is strongly affected by boundary migration during the slower hot deformation(0.001 s^-1),resulting in significant decrement of Zener-pinning force.?3?When there are no precipitates in the matrix,the morphology of GNBs in Copper grains of AA2099 cold rolled sheets is similar to that of pure Al,which is consist of two set of GNBs paralleling to the trace of co-planar slips and co-directional slips,respectively.When there are Al₃Li and Al₂CuLi in the matrix,significant orientation division is found and the misorientation relationship is 50°⁶0°<111>.The GNBs parallel to the trace of co-planar slips?4?The Li-containing precipitates remarkably affect the intensity of?fiber texture and it position in euler space in as-rolled AA2099 sheets.The microstructures of as-rolled sheets containing different kinds of Li-containing precipitates are obviously distinct.Shear bands can be easily found in the specimen containing Al₃Li due to planar slip.The dislocations slip homogenously in the specimen containing Al₂CuLi.Localized deformation zone are found in the specimen containing Al₆CuLi₃ due to Orowan looping near Al₆CuLi₃ particles.?5?The orientations of recrystallization grains are mostly 30°ND Cube orientation and P orientation.The Al₂CuLi precipitates impede the formation of localized deformation zone leading to a suppression of particle stimulated nucleation and orientation growth,while the suppression will be removed when precipitate free zones appear.