Directional solidification and characterization of near eutectic Sm₂Co₁₇/Co alloys

Abstract

The effects of directional solidification processing on the microstructural, compositional, and magnetic properties of near eutectic Co-Sm alloys (∼9 at. pct Sm) have been studied. Because these sytems have high melting temperatures (T _m > 1000 °C) and are quite reactive to oxidizing environments, special containment techniques during solidification were developed. Initial investigations have been performed at modest thermal gradients in the liquid,G _L ≤ 60 °C/cm (1 °C/cm = 10⁻² K/m), and over a range of furnace (solidification) velocities, 0.8 ≤V ≤ 45.4 cm/h (1 cm/h = 2.8 × 10⁻⁶m/s). Since the range ofG _L/V values, a measure of the degree of interfacial morphological stability, was rather low, aligned dendritic morphologies, macrosegregation, and transition to rod eutectic growth were encountered. The primary dendrite spacing for near eutectic Sm₂Co₁₇/Co scaled withV ^−1/2 and varied from ∼50 μm for V ≥20 cm/h to hundreds of microns forV < 10 cm/h while the rod eutectic diameter and interred spacing were an order of magnitude smaller. For both dendritic and cooperative growth, the associated permanent magnet properties were rather poor,e.g., remanence less than 4 kG (1 gauss = 10⁻⁴ Tesla) and coercive force less than 1 kOe (1 Oe = 79.577 A/m) for the smallest dendrite and rod diameter dimensions encountered, although the magnetic hardness for the rod eutectic was larger than for the dendritic microstructure. Magnetization as a function of sample orientation indicated that the easy axis of magnetization was primarily along the direction of solidification for both ferromagnetic phases.