Effect of processing and Microalloying Elements on the Thermal Stability of Cr-Cr 3 Si and NiAl-Mo eutectic alloys
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0980-II05-36
Effect of Processing and Microalloying Elements on the Thermal Stability of Cr-Cr3Si and NiAl-Mo Eutectic Alloys A. Gali1, H. Bei1,2, and E. P. George1,2 1 Materials Science and Engineering, University of Tennessee-Knoxville, Knoxville, TN, 37996 2 Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN, 37831
ABSTRACT The thermal stability of multiphase intermetallics at temperatures to 1400°C was investigated by studying two model eutectic systems: Cr-Cr3Si having a lamellar microstructure and NiAl-Mo having a fibrous microstructure. In drop cast Cr-Cr3Si, coarsening was found to be interface controlled. The coarsening rate could be reduced by microalloying with Ce and Re, two elements which were chosen because they were expected to segregate to the Cr-Cr3Si interfaces and decrease their energies. Similarly, directional solidification, which is also expected to lower the Cr-Cr3Si interfacial energy, was found to dramatically decrease the coarsening rate. In the case of NiAl-Mo, coarsening was found to occur by fault migration and annihilation. Microalloying with B was found to significantly decrease the coarsening rate. The fiber density in the B-doped alloy was smaller than in the undoped alloy, suggesting that B affects the coarsening rate by lowering the fault density. INTRODUCTION Cr-Cr3Si and NiAl-Mo eutectic alloys are potential materials for high temperature applications because of their high melting points (1705°C and ~1638°C respectively). Directionally solidified Cr-Cr3Si exhibits a well-aligned lamellar microstructure [1] while NiAlMo [2] exhibits a fibrous microstructure. Since mechanical properties depend on microstructure, it is important that the composite microstructure not degrade significantly at elevated temperatures. Microstructural degradation in eutectic alloys is manifested as a size and or a shape change of its constituent phases. In the absence of stresses (applied or residual), the driving force for microstructural degradation is the minimization of total interfacial free energy per unit volume, which can be written as: G = ΣσiAi
(1)
where G is the total interfacial energy per unit volume, Ai is the interfacial area per unit volume, and σi is the interfacial energy per unit area of surface i. Thermal stability can be increased by decreasing G which can be achieved by decreasing the total interfacial area or by decreasing σ. The total interfacial area can be decreased by increasing the average lamellar or fiber spacing, which can be achieved by decreasing the solidification rate. Interfacial energy per unit area can be decreased by segregation to the interface or by forming low energy interfaces by directional solidification. In this paper we study the effect of processing and microalloying additions on the
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