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Title: Mechanical and thermal characteristics of HIPed A1-TiB2 MMCs
Other Titles: Er peng hua tai wei ke li qiang hua lü ji fu he cai liao de re deng jing ya zhi bei ji qi ji xie yu re xing neng yan jiu
Authors: Tam, Kam Fai (譚錦輝)
Department: Dept. of Physics and Materials Science
Degree: Doctor of Philosophy
Issue Date: 2005
Publisher: City University of Hong Kong
Subjects: Aluminum
Metallic composites
Notes: CityU Call Number: TA481.T35 2005
Includes bibliographical references (leaves 187-211)
Thesis (Ph.D.)--City University of Hong Kong, 2005
xxi, 211 leaves : ill. ; 30 cm.
Type: Thesis
Abstract: Al-based metal matrix composites (MMCs) reinforced with ceramic particulates have attracted considerable attention of materials scientists over the past two decades. This is because particulates-reinforced metal matrix composites (PMMCs) exhibit superior mechanical properties and thermal behaviors. The machinability of PMMCs is relatively poor due to their high hardness and modulus. Therefore, capsule–free hot isostatic pressing (HIP) provides a one step method to fabricate the net-shaped Al-based MMCs with uniform distribution of the ceramic particles. In this regard, the mechanical and thermal properties of the composites can be improved accordingly. Al-based PMMCs are widespread used in aerospace and automobile industry. The severe temperature fluctuation and undesirable vibration induce the strength degradation of the Al-based PMMCs. For the safe use of Al-based PMMCs in industrial sectors, a comprehensive understanding of their tensile and thermal strain behavior under the cycling loading environment, e.g. temperature changes and dynamic stresses, is needed. The present work has been undertaken to demonstrate the feasibility of the fabrication of capsule-free HIPed PMMCs. Furthermore, the thermal cycling behavior and damping capacity of the product materials were also investigated. In this study, Al-based MMCs reinforced with TiB2 particulates were fabricated by powder metallurgy (PM) process followed by consolidation via HIPing. The microstructure, mechanical and thermal characteristics of such PMMCs are investigated by means of the differential scanning calorimetry (DSC), tensile test, scanning electron microscopy (SEM), thermomechanical analysis (TMA) and dynamic thermomechanical analysis (DMA). The structure–property relationship of these materials is determined accordingly. The results showed that the microstructure of Al-TiB2p MMCs depends greatly on the blending processing routes and the relative particle size (RPS) ratio between the Al and TiB2p adopted to produce the materials. Moreover, HIPing parameters also play an important role for densification of the net-shaped Al-TiB2p MMCs with good mechanical properties. Tensile measurements showed that the Young’s modulus and ultimate tensile strength (UTS) of Al-TiB2p MMCs tend to increase with increasing TiB2p volume content at the expense of their tensile ductility. The experimental Young’s modulus of Al-TiB2p MMCs HIPed at 640 °C generally in good agreement with the data predicted from the Eshelby concept for randomly oriented inclusion. SEM fractography revealed that the tensile failure of Al-TiB2p MMC is mainly initiated from the particle fracture and particle clustering, particularly for PMMC with high reinforcement content. The nucleation and growth of voids from fragmented TiB2 particles leads to the formation of microcracks. MMCs are excellent candidate materials for structural and aerospace industries due to the high specific modulus and strength. However, such structural components are often subjected to severe thermal and mechanical loads in service. In this aspect, the thermal strain response and dynamic thermomechanical behavior of Al-TiB2p MMCs must be fully understood. After cooling from the HIPing temperature, large coefficient of thermal expansion (CTE) difference between the reinforcement and matrix would induce residual tensile stress in the matrix and compressive stress in the reinforcement of the Al-TiB2p MMCs. From TMA measurements, there exists a complicated competition between the tensile residual and compressive thermal stress acting on the Al matrix of composite. There is no simple relationship between the CTE and temperature. The experimental variations of CTE with particle volume content agree reasonably with those predicted from Kerner’s model and Eshelby concept for randomly oriented inclusion. Furthermore, the onset temperatures of permanent plastic deformation for the Al-TiB2p MMCs with 15 – 20 vol% TiB2p are in good agreement with those predicted from Daehn’s model. As mentioned above, tensile residual stress is induced in the metal matrix, while the compressive residual stress is generated in the reinforcements upon cooling from fabrication temperature. This thermal residual stress would affect the tensile and compressive yield strength of the Al-TiB2p MMC dramatically. Thermal cycling the Al-TiB2p MMCs between temperature 85 – 500 °C with the heating rate of 30 °C min-1 and cooling rate of 10 °C min-1 does not deteriorate but even increase the tensile strain of the Al-TiB2p MMCs significantly. Dynamic thermomechanical analysis is a potential technique to characterize the internal fraction and damping capacity of the MMC specimens. The storage modulus of Al-TiB2p MMCs increases with the addition of TiB2 particles as expected. For the damping capacity, the loss tangent of Al-TiB2p MMCs also increases with TiB2p volume content. The enhancement of damping capacity is mainly attributed to the matrix dislocation damping due to the large difference of CTE between the reinforcement and matrix, and to the grain boundary and interface damping at relatively low temperature. At elevated temperature, the concentration of the dislocation near the interfaces decreases and the metallic matrix become relative soft. In this regards, the damping behavior is dominated by the grain boundary slipping and interfacial slipping at high temperature. The internal friction of Al is significantly improved with addition of TiB2p, especially under low frequency (0.1 Hz) and/or high temperature (> 250 °C). The addition of BNp significantly improves the internal friction of Al and Al-TiB2p MMCs at the expense of storage modulus. This is because BN particles having hexagonal and layered structure similar to that of graphite (Gr). It is considered that ceramic BN particles are potential candidate materials for high damping capacity. However, TiB2p addition restores or enhances the storage modulus of Al-BNp MMCs. The improved damping capacity of the Al-BNp and Al-(BNp+TiB2p) MMCs are attributed to the intrinsic damping of BNp volume content, especially under low temperature (30 – 230 °C) and/or low strain amplitude (< 0.01). The net-shaped HIPed Al-TiB2p MMCs are successfully fabricated and exhibit good mechanical strength, thermal stability and damping capacity in the present investigation. Capsule-free HIPing via PM route provides a practical way to fabricate net-shaped PMMCs for industry applications. Several processing parameters must be taken into considerations. These include blending technique, HIPing temperature, RPS ratio, compact mould design for stress distribution and shrinkage.
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