Directional solidification and annealing of Mn 55 Al 45 alloys
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vibrating sample magnetometer was used to measure room temperature magnetization as a function of angle with respect to solidification direction and applied fields up to 15 kOe at Grumman's Corporate Research Center. As shown in Figure 1, the microstructure of a directionally solidified (V = 0.8 cm/h and GL--55 ~
RONALD G. PIRICH Control of natural, gravitationally induced convection during directional solidification processing can result in preferred crystallographic orientation, microstructure, and enhanced magnetic properties of ferromagnetic materials. J It is known that the fen'imagnetic, metastable r-phase in the AI-Mn system ( - 5 5 at. pct Mn) has a uniaxial magnetocrystalline anisotropy which is characterized by a high anisotropy constant2 and gives this phase great potential as a permanent magnet material. 3 However, difficulty in achieving aligned microstructure necessitates mechanical deformation, usually with a carbon addition to enhance workability and z-phase stability, to achieve uniaxial anisotropy. 4 The intent of this work was to investigate the applicability of directional solidification to produce aligned or desired magnetic microstructures and to establish the effect of moderate solidification rates on z-phase formation and transformation. Directional solidification was performed using the Bridgman-Stockbarger method in a high temperature, three zone furnace assembly which translated about a stationary, evacuated pyrolyzed boron nitride (BN) ampoule. The furnace assembly/ampoule arrangement was thermally characterized by monitoring Pt-Rh thermocouples incorporated into solid materials (Ti, graphite, and BN) of various thermal conductivities, within evacuated pyrolyzed BN ampoules, over the range of temperatures (900 to 1500 ~ anticipated for the A1-Mn compositions of interest..The range of achievable GL/V (thermal gradient in the liquid at the liquid-solid interface, GJfurnace (solidification) velocity, V), a measure of morphological stability, was deduced to vary from - 5 x 103 ~ 2 (1 ~ 2= 104 K-s/m 2) at V = 40 cm/h (1 cm/h = 2.8 • 10-6 m / s ) to ~2 x 105 ~ at V = 0.8 cm/h. Directionally solidified samples were prepared for metallographic examination using standard abrasive polishing techniques. After polishing, a dilute HC1 acid etch was applied to enhance phase contrast. Micrographs both transverse and longitudinal to the solidification direction were obtained using conventional optical and Kerr effect (polarized light) microscopy. Selected microscopic regions (-hundreds of microns in size) were further analyzed by energy dispersive X-ray analysis on an AMR-1000 scanning electron microscope to determine composition of various phases. Magnetization of cylindrically shaped samples was measured parallel to the solidification direction at 290 K (room temperature) and 77 K in applied fields up to 200 kOe ( 1 0 e = 79.577 A/m) using a low frequency vibrating sample magnetometer at the Francis Bitter National Magnet Laboratory. A Princeton Applied Research high frequency RONALD G. P1RICH is Hea
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