A Study of the Processing and Compositional Variables on the Microstructural Development and Oxidation Characteristics o
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A STUDY OF THE PROCESSING AND COMPOSITIONAL VARIABLES ON THE MICROSTRUCTURAL DEVELOPMENT AND OXIDATION CHARACTERISTICS OF RAPIDLY SOLIDIFIED NiAICo ALLOYS M.L STEWART AND P.B. ASWATH University of Texas at Arlington, Department of Mechanical and Aerospace Engineering, Materials Science and Engineering Program, Arlington, Tx. ABSTRACT Oxidation tests were performed isothermally at 10000 C on chill block melt spun metallic ribbons with alloy compositions consisting of Ni(46-50 at%), A1(31-33 at%), Co(17-20 at%) and Hf(0-1 at%). The oxidation rates were calculated and the microstructure was examined. Results indicated the difference in microstructural and oxidation characteristics could largely be attributed to the differences in the superheat of the melt. There was no indication that hafnium retarded grain growth. The oxidation behavior of two of the ribbons exhibited a logarithmic rate law. INTRODUCTION Ordered intermetallic nickel aluminides are currently receiving a great deal of attention for high temperature structural applications due to several outstanding properties including an increase in strength with increasing temperature, good oxidation resistance and a density about 10% less than nickel based superalloys.[1,2,3,4] However, the inability to process these materials due to their low ductility and low fracture toughness had, until recently, discouraged their development for structural applications.[3,4] Processing ordered intermetallic nickel aluminides by rapid solidification techniques has the potential to improve ductility by increasing homogeneity, producing very fine grain sizes and creating novel microstructures.[5,6] However, the characteristics of rapid solidified materials which lead to the enhanced properties can be lost during high temperature consolidation. The primary objective of the research contained herein is to study the microstructural changes and oxidation behavior of six rapidly solidified intermetallic nickel aluminide alloys exposed to air at 10000C for 4 hours and 68 hours. EXPERIMENTAL PROCEDURE Five slightly different NiA1CoHf alloy compositions were rapidly solidified using the chill block melt spinning process (CBMS) in order to produce metallic ribbons, by the NASA Lewis Research Center in Cleveland Ohio. Alloy compositions are listed in Table I and amount of melt super heat in Table II. After CBMS processing, each of the six types of metallic ribbons were examined for variations in the thickness, width, and length dimensions as well as the differences porosity and surface texture. An SEM analysis was made of the ribbons in the as-received state in two orientations: (a) horizontally mounted so as to expose the edge of the ribbon, through the thickness, parallel to the wheel direction and (b) vertically mounted and then broken, perpendicular to the wheel direction in order to observe fracture surfaces. Individual and multiple samples were prepared for oxidation testing. Individual ribbon samples were approximately 2mm in width and 3mm in length. Each multi-piece sample contained
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