Composition Design, Process Optimization And Magnetic Characteristics Of TbCu₇-type Sm-Co Based High Temperature Permanent Magnetic Alloys

TbCu7-type Sm-Co based high temperature permanent magnetic alloys have attractedmuch attention with the rapid development of aerospace and other industries. In this work,rapidly quenched TbCu7-type Sm-Co hard magnetic alloys with outstanding combination ofproperties such as high coercivity, high oxidation resistance, high corrosion resistance andhigh elevated temperature stability have been obtained based on composition design andprocess optimization. The magnetic characteristics and underlying physics have beeninvestigated.Firstly, Sm(Co,Zr)7alloys with high coercivity and stable structure have been preparedby melt-spinning. TbCu7-type SmCo7phase is stabilized by adding Zr element. The Zraddition also improves the coercivity effectively by enhancing the anistropic magnetic field.For SmCo6.7Zr0.3alloy, the highest remanence Jr=0.6T and maximum energy product(BH)max=64.5kJ/m3have been obtained after annealing at400oC, and the highest coercivityHc=1560kA/m is obtained after annealing at650oC. The coercivity of SmCo6.6Zr0.4alloy isvery high as218kA/m at400oC.Secondly, TbCu7-type Sm(Co,Si,Zr)7alloys with high coercivity, high oxidationresistance and high corrosion resistance have been obtained by adding Si element intoSmCo6.7Zr0.3alloy. Si and Zr co-doping enhances the oxidation resistance of the alloys. Theoxidation layer thicknesses are157,230and106μm for the as-cast SmCo6.7Si0.3,SmCo6.7Zr0.3and SmCo6.4Zr0.3Si0.3alloys, respectively, oxidizied at500oC in ambient atmosphere for90h.The coercivity of SmCo6.4Zr0.3Si0.3alloy ribbon oxidized at300oC for10min is as high as884kA/m. The Si addition decreases the self-corrosion current and increases self-corrosionpotential of the alloy, and thus improves the corrosion resistance of the alloy. Zr and Sico-substitution has combined the advantages from both individual Zr and Si element additionsin enhancing the coercivity, oxidation resistance and corrosion resistance.Thirdly, to optimize the microstructure and improve the intrinsic properties,SmCo6.4Zr0.3Si0.3Cxand Sm0.8RE0.2Co6.4Zr0.3Si0.3alloys have been designed by carbon additionand rare earth (RE) element substitutions, respectively. Carbon addition can significantlyrefine the grains and effectively improve the hard magnetic properties and thermal stability. The SmCo6.4Si0.3Zr0.3C0.2alloy with grain size close to critical single domain size has shownexcellent hard magnetic properties with Hc=1577kA/m, Jr=0.53T, and (BH)max=52.1kJ/m3. The values of remanence temperature coefficient α are-0.025and-0.081%/oC, and thecoercivity temperature coefficient β are-0.268and-0.215%/oC, in temperature ranges of27~200oC and27~400oC, respectively. Heavy rare earth elements of Gd and Ho substitutionsfor20at.%of Sm can improve the coercivity and thermal stability of SmCo6.4Zr0.3Si0.3alloy,especially at high temperatures around400oC. Between27and200oC, the alloy with Gdsubstitution has the best thermal stability with α=-0.032%/oC and β=-0.269%/oC. TheSm0.8Gd0.2Co6.4Si0.3Zr0.3alloy shows the best combination of hard magnetic properties with Hc=658kA/m, Jr=0.43T, and (BH)max=31.7kJ/m3at200oC. Between27and400oC, thealloy with Ho substitution obtains the best thermal stability with α=-0.064%/oC and β=-0.19%/oC. While at400oC, Sm0.8Ho0.2Co6.4Si0.3Zr0.3alloy has shown the best hard magneticproperties of Hc=331kA/m, Jr=0.34T and (BH)max=11.0kJ/m3.In addition, nanocrystalline Sm0.8RE0.2Co6.4Zr0.3Si0.3C0.2alloy with grain size ofapproximate20nm has been prepared by melt spinning with carbon and RE elementco-doping. Because of the existing of Co phase and amorphous phase, a shoulder appears onthe demagnetization curve and limits the coercivity and remanence of the as-spun alloys.After annealing at600oC for1h, the alloy has crystallized well and the grains have grown upto100nm, which lead to the disappearance of the shoulder and improves the coercivity andremanence. The nanocrystalline Sm0.8Er0.2Co6.4Si0.3Zr0.3C0.2alloy has high (BH)maxof58.8kJ/m3resulting from high coercivity and high remanence of0.59T.Finally, an abnormal initial magnetization curve has been found in arc-melted Sm-Coalloys in this work, which has been never reported before. In arc-melted SmCo7-xSixandSmCo7-xSixZr0.2alloys with Si content x≤0.45, the initial magnetization curves locatedoutside the magnetization hysteresis loop in the first quadrant at285-380K around roomtemperature. This abnormal initial magnetization phenomenon has been investigated by phaseidentification, thermal analysis and magnetic characterization. The results indicate that theappearance of this phenomenon is closely related with the composition, temperature andapplied magnetic field. The available data indicate that an irreversible second order phasetransition from antiferromagnetism to ferromagnetism (AFM-FM) induced by a relatively high magnetic field is responsible for this abnormal phenomenon. This AFM-FM phasetransition may result from the interaction between the Sm and Co layers in the Sm-Co crystallattice and also be related to the spin-spin coupling of Sm and Co (and/or Si) atoms on3gsites. The transition temperatures found in Sm-Co based alloys are at the vicinity of roomtemperature, much higher than those appeared in other compounds. This finding of theabnormal initial magnetization phenomenon is of significance for the magnetic physicsresearch and the future applications.