Effect of melt spinning on grain size and texture in Ni-Mo alloys
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INTRODUCTION
RAPID solidification of metallic alloys is known to result in refined microstructure, extended solute solubility, reduced microsegregation, and formation of metastable phases. Chill block melt spinning (CBMS) has been extensively used to produce such rapidly solidified thin ribbons of many alloys. It is important to study the microstructure of these alloys because it controls the mechanical properties. CBMS is inherently a very complex process with many variables, including the alloy composition, wheel material, speed and surface finish, atmosphere, melt temperature, ejection pressure, melt flow rate, angle of melt jet impingement, etc., determining the physical and microstructural properties of the ribbon. A range of microstructures, plane front solidified, cellular, dendritic, and equiaxed dendritic, are observed across the melt spun crystalline ribbons due to the varying local solidification conditions. Rapid solidification is known to result in a large reduction in grain size. Boswell and Chadwick t~] assumed a homogeneous nucleation and isotropic growth behavior to explain the observed cooling rate dependence of aluminum grain size during splat quenching, t2'31 These assumptions were justified because no pronounced texturing and grain elongation was observed during splat quenching. Melt spinning is, however, known to result in elongated grains and texture in the ribbons. No systematic study has been reported for the dependence of the observed grain size on the processing variables for melt spun alloys. In an alloy cast against a chill surface (in a metallic mold) three zones of solidification behavior have been generally observed, equiaxed grains (at the chill surface), columnar grains with a cellular/dendritic microstructure, and equiaxed dendritic grains (at the mold center). At the mold/metal interface the cooling rate is highest, and many small grains having random orientations are nucleated. These grains rapidly become dendritic, and develop side arms which grow along preferred crystallographic direction ((100) for cubic crystals). Grains with their primary dendrites aligned parallel to the heat flow direction grow the fastest, dominate the liquid-solid interface, and slowly eliminate the other grains thus establishing the characteristic columnar zone observed in castings, with a (100) texture. Growth at the liquid-solid interface in alloy melts is controlled by kinetics of atom attachment at the interface, capillarity, S.N. TEWARI is Associate Professor, Department of Chemical Engineering, Cleveland State University, Cleveland, OH 44115. Manuscript submitted August 17, 1987. METALLURGICALTRANSACTIONS A
and diffusion of mass and heat. Any processing condition affecting these variables would be expected to affect the growth morphology and texture. Melt spinning has been observed to result in preferred textures and grains oriented at an angle to the quench wheel surface normal, ta'Sj Huang e t a / . t41 observed that grain growth axis and (100) crystal direction are at a 15 deg inclinatio
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