【作者】 谢薇；

【导师】 王浩伟；

【作者基本信息】 上海交通大学 ， 材料学， 2011， 硕士

【摘要】 三维碳纤维增强镁基复合材料(3D-C_f/Mg)具有高比强度、高比刚度和低热膨胀系数等优点,同时,三维编织结构克服了一维和二维复合材料各向异性明显的缺点,因此在航天、航空、以及先进武器系统等领域有广阔的应用前景。但是,高温制备的3D-C_f/Mg复合材料在冷却过程中,由于基体镁合金的热膨胀系数(CTE)是碳纤维的25~30倍,两者严重不匹配,造成复合材料内部存在很大的热残余应力,严重影响了材料的力学性能,甚至会引发材料开裂。本文以调整和改善3D-C_f/Mg复合材料残余应力为目标,研究了温度形变处理对复合材料残余应力的影响。首先,采用随动硬化有限元模型,研究了3D-C_f/Mg复合材料基体热残余应力的大小、分布以及不同工艺处理对热残余应力的影响。然后,根据数值模拟的结果,设计方案对复合材料进行了高、低温处理,并对比分析了处理前后复合材料的组织结构;最后研究不同方案处理后复合材料的力学性能,分析总结了残余应力改善以后,复合材料强度、模量和硬度的变化。有限元分析的结果表明:高温制备的3D-C_f/Mg复合材料冷却后,基体内产生很大的热残余应力,平均为147.32MPa。120℃和240℃高温处理后,基体中热残余应力的大小和分布变化不明显,平均热残余应力分别为138.38MPa和138.25MPa。但经过低温处理后,复合材料基体的残余应力明显减小,且随着处理温度的降低,热残余应力呈下降趋势,经过-78℃和-196℃处理后,基体平均热残余应力分别降低至99.40MPa和34.16MPa。实验进行对材料120℃高温处理和-196℃液氮处理,利用SEM和XRD衍射对比分析处理前后复合材料的微观形貌和晶面间距发现:高温制备的3D-C_f/Mg复合材料冷却后产生很大的热残余应力,一方面,使碳纤维微丝发生了严重的扭曲和变形,截面呈现椭圆或豆瓣状;另一方面,造成镁合金基体晶面间距变大。高温(120℃)处理后,残余应力变化微小,碳纤维截面仍呈椭圆形,镁基体晶面间距略大于处理前。液氮处理后,热残余应力明显减小,一方面,碳纤维微丝的截面回复至圆形;另一方面,镁基体的晶面间距更接近AZ91D合金。最后研究了残余应力改善对复合材料性能的影响,结果表明:120℃高温处理后,复合材料仍存在较大残余应力,弹性模量、硬度与处理前相比,分别降低了5.5%和4.2%,但由于基体韧性提高,弯曲强度提高了7.0%,断口呈现韧性断裂。液氮处理后,复合材料残余应力明显减小,弹性模量提高了9.2%,但由于残余应力下降,硬化作用消失,弯曲强度和硬度明显下降,分别降低了19.9%和17.2%,断口呈现准解离断裂,且与处理前相比,小断面更多。更多还原

【Abstract】 Three-dimensional carbon fiber reinforced magnesium composites (3D-C_f/Mg) are candidate materials for aerospace structures and advanced weapon systems due to their high specific strength, high specific stiffness and low coefficient of thermal expansion (CTE). In addition, compared to unidirectional composites and laminated composites, 3D-C_f/Mg is more competitive because of its higher stiffness and strength in three orthogonal directions, especially in thickness direction. However, there is a serious mismatch of volume shrinkage and coefficient of thermal expansion (CTE) between the carbon fiber frame and magnesium alloy, resulting in a large residual stress in 3D-C_f/Mg, which will affect the mechanical properties of materials, and even lead to material cracking.In order to improve the mechanical properties, this thesis will investigate the residual stress of 3D-C_f/Mg composites after different treatments. Firstly, a nonlinear elastoplastic model is proposed and a finite element method is used to study the thermal residual stress in 3D-C_f/Mg study and its modification after different treatments. Then, an experimental investigation was realized by SEM and XRD, which indicates the influences of thermal residual stress on the microstructure of the composites. Finally, the mechanical properties of 3D-C_f/Mg composites are studied in order to analysis the influence of residual stress on the strength, modulus and hardness.The result of numerical simulation shows that high-temperature prepared 3D-C_f/Mg exhibit a large thermal residual stress, which is 147.32MPa on the average in the matrix, after cooling to room temperature. After heat-treatment at 120℃and 240℃, thermal residual stress in the matrix does not change significantly, which is 138.38MPa and 138.25MPa respectively. However, after cold-treatment, the residual stress in the matrix significantly decreases. After cold-treatments at -78℃and -196℃, the average residual stress in the matrix respectively decreases to 99.40MPa and 34.16MPa.Analyze the microstructure of the materials by SEM and XRD after heat-treatment at 120℃and cold-treatment at -196℃. The results show that high-temperature prepared 3D-Cf/Mg exhibit a large thermal residual stress, causing the deformation of cross-section of the carbon fiber and an increase of interplanar spacing of the matrix. After heat-treatment at 120℃, the residual stress changes a little, therefore, the cross-section of the carbon fiber is still oral and the interplanar spacing of the matrix is slightly larger after treatment. However, after cold-treatment at -196℃, the residual stress obviously decreases, so the cross-section of carbon fiber changed from oval to round and the interplanar spacing of the matrix return to the value of that of AZ91D.Finally, study the influence of the residual stress on the strength, modulus, and hardness the composites. The results show that after heat-treatment, the residual stress decreases slightly. And the flexural modulus and hardness decreased slightly, respectively by 5.5% and by 4.2%. However, as the plastic property of the matrix has been improved, the flexural strength increased by 7.0%. In the other hand, after cold-treatment, the residual stress in the composite significantly reduced and the flexural modulus increased by 9.2%. However, the flexural strength and hardness decreased by 19.9% and 17.2% respectively.更多还原