【作者】 黄陆军；

【导师】 耿林； Hua-Xin Peng；

【作者基本信息】 哈尔滨工业大学 ， 材料学， 2010， 博士

【摘要】 本文以提高钛基复合材料室温塑性和高温强度为目标,设计出一种增强体呈准连续网状分布的钛基复合材料,并利用原位反应自生技术结合粉末冶金的方法,采用机械混粉加热压烧结技术,基于大尺寸钛合金粉与细小TiB2粉成功制备出增强体准连续网状分布的TiB晶须增强钛基复合材料(TiBw/Ti)。网状分布的增强相有效地提高了增强体的增强效果及基体的韧化效果,因此复合材料表现出优异的室温及高温综合力学性能。采用扫描电镜(SEM)对复合材料中增强体准连续网状分布结构特征、增强体形态与分布、基体组织及裂纹扩展行为进行了研究。采用室温拉伸、室温压缩、超声共振技术及高温拉伸方法对不同状态复合材料强度、塑性、弹性模量、泊松比及高温性能进行了测试及评价,分析了增强体网状分布复合材料的增强机制;并研究了后续热挤压、热轧制、热处理等强化手段对其组织与性能的影响。采用相同的原料及烧结工艺、不同的球磨工艺,成功制备出了增强体分别呈均匀分布与准连续网状分布的TiBw/Ti复合材料。室温拉伸性能测试表明,增强体呈准连续网状分布较增强体呈均匀分布的烧结态TiBw/Ti复合材料显示出了更优异的强度及塑性。增强体体积分数为8.5%的TiBw/Ti复合材料抗拉强度较纯Ti提高了71%,室温延伸率保持到11.5%。采用相同工艺制备了系列不同增强体含量的准连续网状结构TiBw/TC4(200μm)复合材料,优化了网状结构参数(网的尺寸及网中局部增强相含量)。其中网的尺寸取决于原始基体颗粒尺寸;网中局部增强相含量取决于原始基体颗粒尺寸和复合材料平均增强相含量。室温拉伸性能及SEM组织分析表明,由于特殊的网状结构,存在最佳的增强体含量及基体颗粒尺寸。其中对于直径为200μm的TC4原始基体颗粒,当复合材料平均增强相含量为5vol.%时,得到最佳的网状结构和综合性能。在网状结构中,除了观察到大量的棒状晶须,还发现了树枝状、自焊接、机械锁、爪子状等分叉晶须,进一步增加了增强相晶须的增强效果和裂纹阻碍作用。在复合材料冷却过程中,由于β→α相变的体积收缩受到网状结构的限制,促进了网内等轴α相的形成,提高了基体塑性。原始颗粒尺寸和复合材料平均增强相含量对复合材料拉伸性能有很大影响。当原始颗粒尺寸一定时,随平均增强相含量的提高,复合材料的强度提高,塑性下降;当平均增强相含量一定时,随原始颗粒尺寸的增加,复合材料的强度提高,塑性下降。当原始TC4颗粒尺寸为110μm,复合材料平均增强相含量为8.5vol.%时,烧结态TiBw/TC4复合材料抗拉强度与延伸率分别达到1288MPa和2.6%;当原始TC4颗粒尺寸为200μm,复合材料平均增强相含量为3.5vol.%时,TiBw/TC4复合材料抗拉强度与延伸率分别达到1035MPa和6.5%。网状结构的高效承载能力和梯度界面导致高的界面结合强度是复合材料强化的主要原因;网内基体合金塑性的充分发挥和网中晶须对裂纹的阻碍作用是复合材料塑性提高的主要原因。断裂研究表明,在复合材料整体断裂之前,网中部分晶须发生折断,并伴随着微裂纹的产生,裂纹在网中扩展过程中受到晶须阻碍作用,裂纹发生分叉,提高了断裂消耗的能量,改善了复合材料的强韧性。由于网状结构中,局部增强相含量较高,使网状结构具有较高的高温承载能力,同时网中晶须的梯度分布使网与基体的界面结合强度提高,形成高温“晶界强化”效应,导致网状结构TiBw/TC4复合材料表现出更高的高温强度。高温拉伸性能测试表明,以相同的拉伸强度为判据,网状结构TiBw/TC4复合材料的使用温度比TC4合金可以提高150~200℃。热变形和热处理可以进一步改善复合材料的力学性能。热挤压变形导致基体形变及热处理强化、晶须定向排列和局部增强相含量降低,有效地提高了复合材料的强度和塑性。热处理强化了基体合金,提高了复合材料的强度,但降低了塑性。烧结态TiBw/TC4复合材料经强化热处理后最高抗拉强度可以达到1423MPa。更多还原

【Abstract】 The aim of the present work is to increase the room temperature ductility and high temperature strength of titanium matrxi composites (TMCs). TiB whiskers (TiBw) reinforced titanium matrix (TiBw/Ti) composites with a novel quasi-continuous network reinforcement distribution were designed and successfully fabricated by in situ technique, powder metallurgy (PM) method and mechanical blending and hot-press sintering process based on the system of large and spherical Ti powders and fine TiB2 powders. The prepared composites exhibit a superior combination of mechanical properties: high strength at room and high temperatures due to the strenghening effect of reinforcement and high tougheness by tailoring the novel network reinforcement distribution. The characteristic of the reinforcement quasi-continuous network structures, morphology and distribution, the matrix microstructure and the crack propagation behavior are studied by a scanning electron microscopy (SEM). To evaluate the strength, ductility, elastic modulus, possion’s ratio and high temperature properties of TMCs with different treated conditions, tensile and compressive testing, elastic modulus testing at room temperature and tensile testing at high temperatures are carried out. And then, the strengthening mechanism of the network distributed reinforcement is analyzed. The effects of subsequent hot-extrusion, hot-rolling and heat treatment on the microstructure and mechanical properties of TMCs with a network reinforcement distribution are also studied in the present work.Using the same raw materials and sintering parameters but different ball milling parameters, TiBw/Ti composites with different reinforcement distributions (homogeneous and quasi-continuous network) have been successfully fabricated. The results of tensile testing and SEM microstructure analysis show that TiBw/Ti composites with a quasi-continuous network reinforcement distribution exhibit much superior strength and ductility over those with a homogeneous reinforcement distribution. For the 8.5vol.%TiBw/Ti composite with a network microstructure, the elongation is 11.5%, and the ultimate tensile strength increases by 71% compared with that of pure Ti fabricated by the identical process.A range of TiBw/TC4(200μm) composites with different volume fractions of reinforcement have been fabricated by the same process and parameters, and the network parameters are optimized by comparison. The network structure parameters mean the network size and the local reinforcement fraction. The network size is related to the matrix particle size of raw materials while the local reinforcement fraction to the matrix particle size and the average reinforcement fraction of the composite. The results of tensile testing and SEM microstructure analysis show that the optimal size of matrix particle and volume fraction of reinforcement must be considered due to the special quasi-continuous network microstructure. The optimal combination of properties of the composite with a network microstructure can be obtained by coordinating 5vol.% of the average reinforcement fraction and 200μm of the matrix particle size. TiB whiskers with multi-branching, self-joining, mechanical locking and claw-like shapes are observed besides the rod-like whisker. These unexpected pretty structures of whiskers are believed to be beneficial to the mechanical properties of TiBw/TC4 composites with a network microstructure by increasing the strengthening effect of reinforcement and the blocking effect to crack propagation. In the matrix of the prepared composite with a network structure, equiaxedαmicrostructure instead of the widmanst?tten microstructure is formed because the equiaxed network structure effectively constrains the shrinkage of matrix during the cooling process of the sintered composites, which improves the ductility of matrix.The tensile properties are obviously affected by the matrix particle size and the average reinforcement fraction. For the same matrix particle size of different composites with a network microstructure, the strength of the composites increases but the ductility decreases with increasing the average reinforcement fraction. For the same average reinforcement fraction, the strength of the composites increases but the ductility decreases with increasing the matrix particle size. When 110μm of the matrix particle size coordinates with 8.5vol.% of the average reinforcement fraction, the ultimate strength and elongation of the as-sintered TiBw/TC4 composite with a network microstructure reach to 1288MPa and 2.6%, respectively. When 200μm of the matrix particle size coordinates with 3.5vol.% of the average reinforcement fraction, the ultimate strength and elongation of the composite reach to 1035MPa and 6.5%, respectively. The main reasons for the superior strength of the composites with a quasi-continuous network microstructure are that the network structure can effectively bear load and the the interface bonding strength can be effectively increased by the gradient distribution of TiB whiskers in the network structure. The main reasons for the superior ductility are that the ductility of matrix can be effectively exploited since crack propagation is effectively blocked by the TiB whiskers in the network structure.The analysis of fractographs and crack propagation path show that partial TiB whisker breaks in the network structure, which leads to the generation of micro-crack before the overall fracture of the composite. The toughness of the composite is improved due to the increasing fracture energy by crack branching generated by the effective blocking effect of TiB whiskers in the network structure.The superior ability of the network structure to bear load due to the much higher local reinforcement fraction in the network structure, and the superior interface bonding strength due to the gradient distribution of TiB whiskers can achieve the“intercrystalline strengthening effect”at high temperatures, which makes TiBw/TC4 composites with a network microstructure exhibit superior mechanical properties at high temperatures. The tensile tests at high temperature show that on the basis of the judgment criteria of the same tensile strength, the service temperature can be increased by an increment of 150~200oC compared with that of the monolithic TC4 alloy.The mechanical properties of TiBw/TC4 composites with a quasi-continuous network microstructure can be further improved by subsequent hot deformation and heat treament. The strength and the ductility of the composite can be effectively increased by hot extrusion which leads to working hardening and heat treatment strengthening effect of matrix, orientation arrangement of TiB whisker along the extrusion direction and decrease of reinforcement local fraction. The strength of the composite can be effectively increased but the ductility is decreased by heat treatment which mainly leads to heat treatment strengthening effect of matrix. The ultimate tensile strength of the as-sintered TiBw/TC4 composite can be increased to 1423MPa by heat treatment.更多还原