Brownian Motion Effects on Particle Pushing and Engulfment During Solidification in Metal-Matrix Composites
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THE primary goal of metal-matrix nanocomposite (MMNC) research is to employ nanoparticle (NP) reinforcements to produce high strength/low density materials with higher ductility than can be achieved using micron-size reinforcements. Strengthening due to the Orowan mechanism in which line defects bow around uniformly dispersed particles[1,2] is highly sought after but requires a uniform distribution throughout the matrix. Casting of an MMNC where Orowan strengthening is dominant requires the dispersion of NPs in the liquid melt followed by the engulfment of the particles within the solidifying grains. Producing homogeneous distributions of NPs without agglomeration during
J.B. FERGUSON and BENJAMIN F. SCHULTZ, Postdoctoral Research Associates, PRADEEP K. ROHATGI, Professor, and CHANG-SOO KIM, Assistant Professor, are with the Materials Science and Engineering Department, University of WisconsinMilwaukee, 3200 N. Cramer St., Milwaukee, WI 53211. Contact e-mail: [email protected] GEORGE KAPTAY, Professor, is with the Department of Nanotechnology, University of Miskolc, Egyetemvaros, Miskolc 3515, Hungary. KYU CHO, Senior Engineer, is with the U.S. Army Research Laboratory, Weapons and Materials Research Directorate, Aberdeen Proving Ground, Aberdeen, MD 21005. Manuscript submitted February 2, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS A
solidification processing (SP) is therefore essential in attaining enhanced mechanical properties by the activation of the Orowan mechanism. It is likely that MMNCs that show only grain refinement strengthening do not contain particles within the grains but rather have high concentrations at grain boundaries and/or interdendritic regions as a result of their being pushed ahead of the solidifying grains. It is, thus, vital to understand the interactions between NPs and the solidification front (SF) and the subsequent particle pushing/engulfment phenomena during solidification of MMNCs. Of the various MMNC synthesis methods, SP is expected to be not only one of the most economically viable but also one of the most challenging methods of achieving a uniform dispersion[1–6]; previous experimental evidence shows that uniform distributions are difficult to achieve in practice, and instead the NPs tend to cluster at grain boundaries.[3–6] Analyses of previous experimental data show that there is little deviation from Hall–Petch-type behavior,[7,8] which indicates that increased strength with increasing NP concentration is due to grain refinement effects (i.e., Hall–Petch strengthening effects), although there were some samples that may have been strengthened by the Orowan mechanism.[7–11] Whether particles are pushed or engulfed during solidification depends on the velocity of the particle relative to the SF. Parameters generally considered in models describing particle pushing include
differences in interfacial energies between solid, liquid, and particle, as well as temperature and composition gradients resulting from the growing solid/liquid interfaces. Several analytical models have been previo
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