Directional solidification and characterization of near eutectic Sm 2 Co 17 /Co alloys
- PDF / 1,675,564 Bytes
- 7 Pages / 594 x 774 pts Page_size
- 8 Downloads / 247 Views
I.
INTRODUCTION
EXTENSIVE effort has been carried out over the last several years involving rare-earth/transition metal 1'2'3 and transition/normal metal based alloys. 4 The primary objective to these studies has been to produce new permanent magnets exhibiting improved magnetic properties. This effort has led to commercial production of permanent magnets with energy products on the order of 20 to 30 MG-Oe 5 (1 G-Oe = 79.577 + 10-4 Tesla-A/m), i.e., about 70 pct of the theoretical maximum for this class of materials. 6 Directional solidification processing has been shown to produce composite morphologies, for low volume fraction, ferromagnetic eutectic and off-eutectic forming alloys, which exhibit aligned fibrous and crystallographic characteristics advantageous for enhanced magnetic performance. 7'8 For the Co-Sm system, Sahm and Hofer 9 have demonstrated that aligned, rod eutectic microstructures can be produced using induction heated, directional solidification. However, the directionally solidified Sm2ColT/Co eutectic exhibited rather poor intrinsic coercivities, ,J-/c, and magnetic remanence, Br, e v e n for the smallest Co rod diameters, d, observed, i.e., mHc = 490 Oe (1 Oe = 79.577 A/m) and Br = 1600 gauss (1 gauss = 10-4 Tesla) at d = 0.2 izm. Later Glardon and Kurz ~~ directionally solidified monovariant eutectic C072~Smj25Cu~50, C072.sSm7.sLas.0Culs.0, and C06osFe~20Sm~25Cu~50 alloys which were characterized by a small volume fraction of Co dendrites in a crystallographically aligned, two phase (Co, Cu)x-Sm or (Co, Cu)x-Sm-La matrix. Crystal growth was dendritic rather than cooperative but resulted in significantly improved mechanical properties. The magnetic properties RONALD G. PIRICH is Head, Solid State Physics and Chemistry Laboratory, Corporate Research Center, Grumman Corporation, Bethpage, NY 11714. Manuscript submitted February 4, 1985. METALLURGICAL TRANSACTIONS A
were poorer than sintered Co-Sm magnets but much higher than the directionally solidified, eutectic Sm2Col7/Co. In the eutectic Sm2COl7/Co case, the magnetic hardness appears limited by the Co fiber shape anisotropy and the size and magnetic anisotropy of the Sm2COl7 matrix. In the CoSm-Cu and other monovariant eutectics case, the precipitation hardening of the Co-Sm matrix by Cu seems to result in enhanced magnetic hardness even though the dimensions of the Co dendrites are larger than those encountered for Co rods in the eutectic case. The scope of this effort was to investigate further the directional solidification of selected compositions which form via eutectic and off-eutectic liquid-solid reactions near the Sm2Col7/Co eutectic. The intent was to determine the degree of off-eutectic macrosegregation, quantify the dendritic and eutectic morphologies observed, and determine the resultant differences in magnetic properties due to Co phase shape and size change, i.e., dendritic for off-eutectic vs rod for eutectic.
II.
EXPERIMENTAL PROCEDURES
A. Sample Preparation Starting material was prepared from commercially pure Co (99.
Data Loading...
