Structural and Magnetic Study of Melt-spun CeCo5 Ribbons

　　Structural and Magnetic Study of Melt-spun CeCo5 Ribbons
　　RCo5 (R = Y, Ce, Pr, Sm)-type intermetallic compounds with CaCu5 structure have attracted considerable attention due to their large anisotropy fields HA, relatively high saturation magnetizations MS and high Curie temperatures TC. But only SmCo5 magnet has been widely used because there are some technical difficulties in fabrication of several other materials, and no easy access to high-coercivity requested by the fine micro-structure. Rapid quenching (melt-spun) possess controllable rapid cooling speed, which is easy to fabricate small and fine micro-structure. Rapied quenching (melt-spun) has been widely used in Bonded NdFeB Magnets fabrication also for its convenience and low energy consumption. Many researchers have fabricated melt-spun YCo5, PrCo5 and SmCo5 magnet, but the magnetic properties still could not satisfy the practical requirements. Recently, obvious grain orientation of melt-spun RCo5 has been reported, which makes use of melt-spun RCo5 anisotropic Ring neodymium magnet bonded magnets possible. CeCo5 posses similar inherent magnetic properties as SmCo5, especially that Ce is much cheaper than Sm in price due to its rich reseves, these make the study on anisotropic melt-spun CeCo5 permanent magnet so significant.In this work, melt-spun CeCo5 ribbons have been made, and the effect of ingredient and cooling speed on gain orientation and magnetic properties have been investigated. In addition, magnetic properties of ribbons have been improved by Cu and Fe doping and annealing treatment. At the same time, the magnetization reversal mechanism has also been discussed and analyzed. The main activities include as following:I. Structure and magnetic properties of CeCo5 binary alloy ribbons 1. melt-spun CeCo5.4 ribbonsR-rich phase has been found in arc-melt Ce-Co alloy with the ratio of 1:5 beside CeCo5 phase, which could be inhibited by increasing Co content, Ce-Co alloy with single CeCo5 phase has been obtained at the ratio of 1:5.4 finally.With surface velocities (Vr) of the Cu roll are 5 m/s, 15m/s, 25m/s and 35m/s, ribbons have been fabricated. XRD study on no-contact surface of CeCo5.4 ribbons shows that: The existence of the (111) strong peaks in 5m/s ribbons indicates that the crystal grains near no-contact surface have strong axis orientation, most crystal grains’(111) plane parallel to the surface of the ribbons, that is, [111] axis perpendicular to the surface of the ribbons, with the C-axis distributed in the [111] axis angle of about 50o of a cone. The (111) peaks of 15m/s, 25m/s and 35m/s ribbons gradually weakened, the (110) and (200) peaks gradually increased, thus, most C-axis changed to parallel to the surface of the ribbons.Magnetic properties of CeCo5.4 ribbons were tested along the ribbon length, ribbon width and perpendicular to the ribbon plane by Vibrating Sample Magnetometer (VSM), ribbon length direction is always easy magnetization direction. Thinking over the test results of XRD, we can figure out that C-axis should prefer to arrange along the ribbon length direction. http://www.chinamagnets.biz For the ribbons spun at Vr=5m/s, the difficult magnetization direction is along ribbon width, and for those spun at Vr=15m/s, 25m/s and 35m/s the difficult magnetization direction perpendicular to the ribbon plane, consistent with the crystallographic texture evident in the XRD pattern’s comparison. Using magnetic moments in three directions and Vr curve to describe the changing trends of magnetic anisotropy as Vr change, we can find out that magnetic anisotropy weakens as Vr increases.Large numbers of angle stripes were observed by scanning electron microscopy (SEM) in the sections of ribbons, which exist in most ribbons. Ares contain stripes shrink as Vr increase, and magnetic anisotropy weaken, so we can relate the stripes with magnetic anisotropy. In addition, scanning electron micrographs for the non-contact surfaces of ribbons show that grain size decrease as Vr increase. On the other hand, coercivity increase as grain size decrease, therefore, it is an effective way to increase coercivity by decreasing grain size.2. melt-spun CeCo5 and CeCo4.6ribbonsAlthough orientated textures exist in melt-spun CeCo5.4 ribbons, the coercivities are too low to satisfy the practical requirements.
　　For the reason that the magnetic anisotropy field of Rare Earth - Transition alloy increases as the rare-earth content increases, we fabricated melt-spun CeCo5 and CeCo4.6 ribbons as well. Besides crystal axis orientation and magnetic anisotropy we find out that, with Ce content increase, the magnetic anisotropy of ribbons spun at low Vr enhanced, and the trend of isotropy with Vr increased enhenced. The coercivities increased as well.II. Structure and magnetic properties of CeCo4.5Cu0.9 ternary alloy ribbons For sintered CeCo5 permanent magnet, precipitation hardening effect could be caused by dropping Cu element, which will improve coercivity. Thus, CeCo4.5Cu0.9 ribbons were fabricated. XRD and VSM test showed that for the ribbons spun at Vr=5m/s and 15m/s strong rystal-axis orientation existed; for the ribbons spun at Vr=25m/s and 35m/s tended to isotropic. Precipitation hardening effect performed good in the ribbons spun at 5m/s for coercivity improved notablely, and hardly performed in the ribbons spun at 25m/s and 35m/s for most Cu solute in Ce(Co,Cu)5, which reduce the anisotropy field and decrease the coercivity.Heat treatments were prepared to further enhance the coercivity. Large numbers of Cu separated out after 15min 400℃anneal, which was test by SEM and EDX. At the same time, hysteresis loop appeared serious Fengyao phenomenon. We conjecture that 400℃anneal process led to Ce (Co, Cu) 5→Ce2 (Co, Cu)17 + Ce5 (Co, Cu)19 + Cu decomposition reaction, the emergence of low coercivity 2:17 phase caused Fengyao phenomenon. After 15min 500℃anneal, less Cu seperated out and Fengyao phenomenon gradually disappeared. Anneal promoted the formation of CeCu5, precipitation-hardening effect caused further improvement of coercivity.III. Structure and magnetic properties of CeCo4Fe0.5Cu0.9 quaternary alloy ribbonsDropping Cu element can increase the coercivity but decrease the magnetic moment. Therefore, Fe element was dropped to increase magnetic moment without decrease coercivity. Crystal axis orientation existed in CeCo4Cu0.9Fe0.5 ribbons, (111) strong peaks existed in the ribbons spun at low Vr, and crystal-axis orientation existed. Ribbons spun at high Vr trended to isotropy. Dropping Fe element increased magnetic moment and decrease coercivity slightly. Structure and magnetic properties of CeCo4Cu0.9Fe0.5 ribbons could also be imporved by heat treatment. Excellent magnetic properties have been obtained by 500℃anneal for 15 min. On the one hand, coercivity improved significantly, especially for the ribbons spun at Vr=5m/s, the coercivity has reached 4200 Oe, which is far more than CeCo5.4 ribbons’300 Oe. On the other hand, magnetic moment decreased slightly. Furthermore, the coercivity decreased notablely and magnetic moment increased after 600℃anneal for 15 min.