Correlation between unsteady-state solidification conditions, dendrite spacings, and mechanical properties of Al-Cu allo
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I. INTRODUCTION
AFTER casting, metallic alloys are generally used in one of four conditions: as-cast, heat treated after casting, mechanically worked after casting, and worked and heat treated. In all cases, the casting process has a significant influence upon the mechanical properties, especially in the former two cases. These properties are governed mainly by such factors as porosity, presence of a second phase, grain size, and dendrite spacings. While pouring molten metal from a ladle into a mold, a substantial amount of atmospheric air is entrained. The oxygen, nitrogen, and moisture in the entrained air react with molten metal and with mold coatings to form inclusions and porosity. These defects substantially lower the desired properties of cast metal, due primarily to stress concentration at the pores, which depends on pore shape, and to reduction in load-bearing area.[1–3] It is generally found that the strength of a metallic material increases as the grain size decreases. The well-known Hall– Petch equation shows that the yield strength is proportional to the reciprocal of the square root of the grain diameter.[4,5] The improved properties of fine-grain-sized castings are due to the finer distribution of microporosity and second-phase particles. On the other hand, within each grain, there is a dendritic network, typified by a solute-poor region along the central dendrite axis. Moving away from this central area, the solute content will increase and microsegregation JOSE´ M.V. QUARESMA, formerly Graduate Research Assisant, Department of Materials Engineering, State University of Campinas, UNICAMP, is Associate Professor, Department of Mechanical Engineering, Federal University of Para, Para, Brazil. CARLOS A. SANTOS, Research Assistant, and AMAURI GARCIA, Professor, are with the Department of Materials Engineering, State University of Campinas, UNICAMP, 13-083-970, Campinas-SP, Brazil. Manuscript submitted May 1, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS A
will occur between dendrite arms. In some cases, the interstices of the dendrites are sufficiently rich in solute that areas of nonequilibrium second phase or eutectic will form in a normally single-phase alloy. The outer boundary of this dendritic network will be the grain boundary, which will also be a preferred site for porosity, eutectic, or secondphase formations. For the as-cast, as well as the heat-treated or mechanically worked conditions, the fineness of the casting structure is recognized to yield superior mechanical properties to coarser ones, notably with respect to tensile strength and ductility. Figure 1 shows a schematic representation of a typical as-cast microstructure, with a grain boundary, a dendritic array, a second phase, and a porosity distribution. The dendrite fineness can be of even more importance in the prediction of mechanical properties than in the prediction of grain size. Consequently, to control the properties of cast alloys, it is necessary to understand the mechanism and characterization of primary and secondary den
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