The influence of convection during solidification on fragmentation of the mushy zone of a model alloy
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INTRODUCTION
IT has long been recognized that refining the microstructure of an alloy casting improves its low-temperature strength, toughness, heat-treatment characteristics, and hot tearing tendency. Solidification microstructures are, in reality, frozen remnants of a two-phase region, commonly called the ‘‘mushy zone,’’ which has a transitory existence during the progressive freezing of any alloy. It is within the mushy zone that chemical microsegregation and the resultant phase relationships are established for the fully frozen or as-cast material. Although the initial dendrite microstructure may be altered substantially by heat treatment and mechanical working subsequent to solidification, the initial dendritic patterns within a material are, as a practical matter, never fully eliminated. Furthermore, competitive processing methods increasingly rely upon near-netshape casting, rheocasting, and as-cast materials, which increases the need for microstructural control during the solidification process. It is usually desirable to control the number and dispersion of crystallites, or grain density, since the resultant cast microstructure is metallurgically most useful when a fine, C.J. PARADIES, formerly Graduate Research Assistant, Rensselaer Polytechnic Institute, is Postdoctoral Fellow, Laboratory of Metallurgy and Physics, Ecole Polytechnique Federal de Lausanne, CH-1015 Lausanne, Switzerland. R.N. SMITH, Professor, Department of Mechanical Engineering, Aeronautical Engineering, and Mechanics, and M.E. GLICKSMAN, John Tod Horton Professor of Materials Engineering, Department of Materials Science and Engineering, are with the Rensselaer Polytechnic Institute, Troy, NY 12180-3590. Manuscript submitted February 27, 1995. METALLURGICAL AND MATERIALS TRANSACTIONS A
uniform, random distribution of equiaxed grains occurs. Typical commercial practice in nonferrous foundries involves adding, just prior to casting, ‘‘grain refiners,’’ which are proprietary mixtures of catalytically active materials that stimulate the nucleation process within an alloy melt by reducing its free energy barrier to crystallite formation. However, this practice does not always work well, particularly for certain aerostructural alloys containing strong carbide formers like zirconium. An alternative procedure is to induce grain refinement mechanically rather than chemically by stirring the molten metal with sufficient vigor, so that a portion of the fine, columnar, dendritic crystals may break free from the leading edge of the mushy zone and be carried away by the surrounding melt stream. These ‘‘seeds,’’ or grain fragments, become the sites for new grains, which in turn might provide new seeds if the fluid mechanical, thermal, and chemical conditions are favorable. The comminution, detachment, dispersion, and regrowth of grain fragments constitutes a significant but complex grain refinement process that is totally independent of nascent nucleation events and the action of grain refiners. In fact, grain refiners and nucleation events operate as
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