Composite Materials Research Progress

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288 S.C. Tjong threshold stress resulting from the interaction between moving dislocations and alumina oxide nanoparticles (Fig. 13). Such dislocation-particle interaction would impede lattice dislocation movement, thereby reducing creep rate of the composite. By incorporating the threshold stress into analysis, plots of creep rate versus effective stress yield straight lines with different slopes, i.e. n = 3 for low stress region and 5 for high stress regime (Fig. 14). Hence, the creep behavior of nanocomposite is consistent with the behavior of Al-Cu solid solution alloy (2014 Al) that exhibits a transition from viscous glide (n = 3) to the high-stress region (n = 5) where dislocations break away from the solute atom atmosphere. They indicated that the true creep characteristics of PM 2024 Al/Al2O3 nanocomposite are consistent with those reported for aluminum solid-solution alloys [43]. Therefore, the creep deformation of nanocomposite with a matrix containing solutes is controlled by a viscous glide slip mechanism. Ultrafine Grained Matrices As mentioned above, conventional PM blending method yields Al nanocomposites with inhomogeneous distribution of reinforcing particles within the metal matrix. Cryomilling can provide a homogeneous dispersion of reinforcing particles in submicrometer or nanocrystalline matrix. The subsequent hot consolidation of cryomilled nanopowders into final bulk products causes the composites to have an UFG structure as a result of grain growth of the matrix. Schoenung and coworkers investigated the microstructure and tensile behavior of bulk nanostructures 5083 Al/5 vol.% SiC (25 nm) composite prepared by cryomilling followed by hot isostatic pressing and hot rolling [48]. They reported that the hot rolled composite consists of regions dispersed with SiC nanoparticles (100 -200 nm) and regions free of SiC nanoparticles (~ 700 nm). Fig. 15 shows the TEM micrograph of the SiCdispersed region in which SiC nanoparticles are distributed homogeneously within the ultrafine grains of the 5053 al matrix. The tensile properties of such composite from room temperature to 573 K are shown in Fig. 16. The composite exhibits very high tensile strength at room temperature but extremely low ductility. The strength decreases but the ductility increases with increasing test temperatures. In a nanocomposite with an UFG matrix, the dislocation movement in the matrix is restricted by the high density of grain boundaries. Consequently, the composite exhibits high tensile strength but very low tensile ductility. It is well known that nanocrystalline (NC) materials exhibit very low tensile ductility and toughness due to the lack of strain hardening [9]. The presence of coarser grains within the nanocrystalline matrix can enhance the ductility of nanostructured materials at the expense of mechanical strength [49-51]. Different toughening approaches have been proposed to enhance the ductility of NC materials either via thermomechanical treatment or cryomilling. Recently, Lavernia and coworkers reported that the UFG Al-Mg alloys with a bimodal microstructure exhibit a combination of high strength and good ductility [52- 55]. Such alloys were synthesized by consolidation of a mixture of cryomilled Al-Mg and unmilled powders. Consequently, strain hardening is regained in CG regions while maintaining high strength in NC regions. The CG grains can provide more dislocation activity than the NC grains. Ductilephase toughening in bi-modal structured Al-Mg alloys is attributed to the occurrence of crack bridging as well as delamination between UFG and CG regions during plastic deformation [51].
Recent Advances in Discontinuously Reinforced Aluminum… 289 Figure 15. Bright field TEM micrograph of the hot rolled 5053 Al/5vol.% SiC composite showing the dispersion of SiC nanoparticles in ultrafine grains [48]. Figure 16. Tensile stress-strain curves for the hot rolled 5053 Al/5 vol.% SiC composite at various temperatures [48]. Based on this approach, Schoenung and coworkers prepared bimodal 5083Al/10 wt%B4C composite by blending cryomilled composite powders with an equal amount of CG 5083 Al followed by CIP and extrusion [15]. Figs. 17(a) -17(b) show the microstructure of bimodal composite consisting of UFG (NC) Al and CG Al. The B4C are uniformly distributed in the
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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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4 x 10 5 4 3 2 1 Compliance (Nmm) 0
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3.5 3 2.5 2 1.5 1 Optimization of L
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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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190 D D 1.0 0.9 0.8 0.7 0.6 0.5 0.4
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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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Composite Materials Research Progress

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