Composite Materials Research Progress

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294 S.C. Tjong Apart from forming ultrafine grains in composites, ECAP treatment can also consolidate ultrafine raw powders to produce fully dense (> 98%) bulk composite materials. Alexandrov et al. used the HPT technique to consolidate the Al powder (50 μm) and Al2O3 nanoparticle (50 nm) to form the Al/5 vol.% Al2O3 nanocomposite under a pressure of 1.5 GPa at room temperature. The powder mixture of nanocomposite was ball-milled for 30 min to ensure a uniform distribution of ceramic particles [21]. Fig. 20 shows the TEM micrograph of the HPT consolidated Al/5 vol.% Al2O3 nanocomposite. The nanocomposite exhibits an UFG structure having an average gain size of 120 nm. Room temperature tensile tests showed that the Al/5 vol.% Al2O3 nanocomposite have limited ductility of 1 to 2%. At 300 ºC, the nanocomposite tested at a strain rate of 10 -3 s -1 had a plastic flow stress of ~ 66 MPa and a tensile ductility of ~ 20 % (Fig. 21). In contrast, pure Al had a flow stress of ~ 60 MPa and a tensile ductility of ~ 40 % tested at the same strain rate. However, the Al/5 vol.% Al2O3 nanocomposite showed a high strain-rate sensitivity of flow stress at 400 K; the strain-rate sensitivity (m) was 0.35. Strain rate sensitivity defined as the slope of logarithmic plot of the flow stress vs. strain rate. It is an inverse of stress exponent (n) and an important parameter in superplasticity. The Al/5 vol.% Al2O3 nanocomposite exhibited a low flow stress of 20 MPa but a high tensile ductility of ~ 200 %. The enhanced tensile ductility observed in the HPT consolidated nanocomposite with a total elongation of ~ 200 % indicating the occurrence of superplastic-like flow behavior. According to the literature, high strain rate super-plasticity can be achieved in ECAP processed aluminum alloys with UFG structures [59]. High strain rate superplasticity in the sub-micron metals is often characterized by very high flow stresses or pronounced strengthening. Grain boundary sliding is considered to be the dominant deformation mode for superplasticity in the sub-micron and nanocrystalline metals [60]. Future challenges for materials scientists are to elucidate the underlying creep and superplastic deformation mechanisms of aluminum based nanocomposites having UFG and nanocrystalline matrices. Conclusions The development of aluminum nanocomposites is still in embryonic stage and there are many challenges in this field in the years ahead. Considerable progress has been made in the fabrication, microstructural and mechanical characterization of novel aluminum-based metal matrix nanocomposites in recent years. The nanocomposites can be simply prepared by incorporating very low volume contents of ceramic nanoparticles into aluminum matrix via PM or ingot casting. The nanocomposites thus prepared exhibit excellent mechanical properties including high yield strength and superior creep resistance. However, agglomeration of nanoparticles occurs readily during the composite fabrication, leading to poorer mechanical performance of composites with higher filler content. This problem can be eliminated in cast nanocomposites by using high-intensity ultrasonic waves to disperse the nanoparticles in molten aluminum. In the case of PM nanocomposites, cryomilling and severe plastic deformation processes have emerged as the two major processes to produce aluminum based composites having ultrafine grained matrix structures and homogeneous dispersion of reinforcing particles within the matrices.
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COMPOSITE MATERIALS RESEARCH PROGRE
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Copyright © 2008 by Nova Science P
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vi Contents Chapter 9 Recent Advanc
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viii Lucas P. Durand scale transiti
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x Lucas P. Durand machine. In paral
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In: Composite Materials Research Pr
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Multi-scale Analysis of Fiber-Reinf
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T300 fibers (Soden et al., 1998; Ag
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1 I L = L : r v Multi-scale Analysi
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I E ⎡0 ⎢ ⎢0 ⎢ ⎢ ⎢ ⎢0
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Applied macroscopic load Correspond
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Optimization of Laminated Composite
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⎧σ x ⎫ ⎡ mE ⎪ ⎪ ⎢ x
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and γ 2 γ 0 Optimization of Lamin
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110 W.H. Zhong, R.G. Maguire, S.S.
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112 Reinforcement: Advanced materia
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130 Yuanxin Zhou, Hassan Mahfuz, Vi
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140 Heat Flow (W/g) Heat Flow (W/g)
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144 Material Yuanxin Zhou, Hassan M
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146 SEM Analysis Yuanxin Zhou, Hass
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150 Governing Equation For the fibe
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152 ⎧ UU = ⎨ ⎩ ⎧ UD = ⎨
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156 Stress (MPa) 120 80 40 0 Yuanxi
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166 Giangiacomo Minak and Andrea Zu
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170 ⎟ ⎞ ⎠ s ⎜ ⎛ ⎝ = a E
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188 A B E (MPa) D 250000 200000 150
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204 Conclusion Giangiacomo Minak an
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Research Directions in the Fatigue
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Bending force [N] Research Directio
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Damage Variables in Impact Testing
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Damage Variables in Impact Testing
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