| SiC/Al composite has light-weight high elastic modulus,micro yield strength and moderate thermal expansion coefficient,which is an ideal material for manufacturing high-precision inertial instruments.In the past 20 years,aluminum matrix composites have made great progress in the fields of high-quality preparation,dimensional stability design and dimensional stabilization technology and have shown good application prospects in the field of inertial navigation and guidance.The micro-plastic deformation studied in this paper refers to the irreversible deformation of 10-4 or even lower order of magnitude of materials under environmental loads.Various service environments of inertial instruments（hot standby,multiple start stop,accelerated flight）will cause the components to be subjected to constant temperature,thermal shock or external force environmental loads,which will cause micro plastic deformation and damage the accuracy of the instrument.At present,the micro plastic deformation behavior of SiC/Al composites under environmental loading has many unexplained points,such as internal stress,micro defects and precipitates can affect the trend and order of dimensional change respectively;Whether the dominant factors of dimensional change are the same under different composites and different environmental loads;Whether the stacking faults found in SiC/Al composites can improve the micro plastic deformation resistance,etc.,these problems that go deep into the micro mechanism level restrict the design and development of inertial instrument materials with high dimensional stability.In this paper,the effects of internal stress,micro defects and precipitation behavior on the micro plastic deformation behavior in SiC/Al composites are systematically studied by using high-precision thermal dilatometer,transmission electron microscope,X-ray diffractometer and universal electronic testing machine,which provide ideas and scientific basis for the design and development of high dimensional stable instrument materials.The dimensioanl change behaviors of quenched and annealed SiC/1199Al composites and SiC/2024Al composites at 190℃were studied.Research showed that the dimensional increase of quenched SiC/1199Al composites at 190℃for 3500 minutes was+1.5×10-4,while the constant temperature dimensional change of the annealed composites remained unchanged.Comparing the changes of matrix lattice constant and dislocation structure between the two materials,it was found that the average matrix lattice constant of quenched composite increases by+5.93×10-5,and higher dislocation density and more entangled dislocation configuration at room temperature(The average lattice constant change of matrix in Chapter 3 and Chapter 4 is estimated according to（311）Al diffraction peak.);However,the average lattice constant of the annealed composite did not increase significantly,and the dislocation density was low at room temperature,showing a linear shape.It is believed that the high-density dislocation and entangled dislocation configuration increased the elastic strain stored in the matrix and cause greater stress relaxation after entering the constant temperature stage,leading to a greater dimensional change.In addition,the dimensional change behavior of SiC/2024Al composite during constant temperature preservation at 190℃was studied.The dimensional change of SiC/2024Al composite during constant temperature preservation at 190℃for 3500 minutes was+4.5×10-4.The lattice of precipitates was characterized in detail,and it was found that the precipitates in the composite reduce the dimensional change.In addition,the change of lattice constant and dislocation structure were also characterized.It was found that the average lattice constant change of quenched SiC/2024Al composite was+3.6×10-4,significantly higher than that of SiC/1199Al composites under the same conditions.This showed that the addition of alloy elements increases the stored elastic strain of the matrix,and caused the strengthening of the original dimensional change mechanism and became the dominant mechanism.The dimensional change and mechanism of SiC/Al composites under 500 times thermal shock at cyclic temperatures of-196℃～190℃was studied.The dimensional change of SiC/1199Al composites increased by+3.3×10-3 after 500 times of thermal shock,and the dimensional change is not significantly stable within 500 times of thermal shock.It was found that the dimensional increase was due to the accumulation of internal stress and the change of dislocation morphology.After thermal shock,the average lattice constant of the matrix was estimated to increase by about+1.8×10-3.The dislocation density in Al matrix was significantly increased and entangled dislocations were widely distributed in the matrix.The dimensional change of SiC/2024Al composite changed by-6.1×10-4 after 500 times of thermal shock,and the dimensional change was significantly stable after 50 times of thermal shock.It was found that the dimensional decrease was due to the decrease of lattice constants of matrix and precipitates,secondary precipitation and the change of dislocation structure.After thermal shock,the estimated average lattice constant of the matrix decreased by about-6.4×10-4,the number density of precipitates increased by an order of magnitude.In addition,the increase of dislocation density in the matrix resulted in more precipitates nucleating unevenly along the dislocation line,which plays a role in pinning dislocations.The micro-plastic deformation behavior of SiC/1199Al composites with stacking faults under external force was studied.The nano-scaled stacking fault structure was found in the original SiC/1199Al composite,and the proportion limitσp5andσp100 of the SiC/Al composite containing stacking fault was increased by 8%and 22%respectively compared with the prediction value of the empirical formula of micro-deformation.It was speculated that there are two reasons for the formation of stacking faults:on the one hand,severe lattice distortion inhibited the nucleation and movement of thermal mismatch dislocations;On the other hand,the serious lattice distortion led to the reduction of the effective stacking fault energy,which leads to the decomposition of full dislocations.In-situ TEM observation of composites containing stacking faults showed that the stacking faults extended in the early plastic deformation stage.In addition,the starting tendency between different stacking faults was not completely determined by Schmid law,but also affected by stress concentration.The increase of stacking faults in SiC/1199Al composites’proportional limit resulted from the following three aspects:1)Formation of immovable rod dislocations;2)Elastic interaction caused by stacking fault propagation;3)Intersection of stacking fault network and dislocation array.The multiple strengthening effects of stacking faults at the stage of micro plastic deformation will gradually increase with the progress of micro plastic deformation. |
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