Study On The Enhancement Of Hydrogen Absorption Capacity Of Sm₂Co₁₇ Casting-strips And Preparation Of Magnets By Hydrogenation Of Strips

Sm₂Co₁₇ permanent magnets exhibit excellent magnetic properties,high Curie temperature,good temperature stability,and strong corrosion and oxidation resistance,making them widely applicable in defense,aerospace,high-precision instruments,high-end motors and many other fields.To meet the development demands for device miniaturization and precision,high-performance Sm₂Co₁₇ magnets are currently a research hotspot.Increasing the content of high-magnetic-moment Fe elements in magnets is the primary way to enhance the maximum magnetic energy product of the magnets.However,when the Fe content exceeds 20 wt%,significant composition segregation and many impure phases can be observed in the Sm₂Co₁₇ ingot.These phases are difficult to eliminate during subsequent heat treatment processes,leading to deterioration in the coercivity of the magnets.To suppress the precipitation of impure phases during the alloy solidification process and increase the coercivity,strip-casting and hydrogen decrepitation process can be introduced into the preparation of Sm₂Co₁₇ magnets.However,the lower hydrogen absorption capacity and higher hydrogen absorption pressure limit the application of the hydrogen decrepitation process in the preparation of Sm₂Co₁₇ magnets.In this paper,the hydrogen absorption characteristics of high-iron content Sm₂Co₁₇ ingots and casting strips were investigated,and the correlation between the microstructure of Sm₂Co₁₇ alloys and their hydrogen absorption capabilities was revealed.Through microstructure optimization,the hydrogen absorption capacity of strips was enhanced.The effects of hydrogenation treatments of ingots and strips on the microstructure and magnetic properties of magnets were studied.Finally,the hydrogen decrepitation behavior of Sm₂Co₁₇ magnets was analyzed,and the hydrogen resistance of magnets was improved through oxidation treatment and surface coating.A brief summary of main research result are below:（1）The ingots and strips with nominal composition Sm_26.2Co_balFe_xCu_4.0Zr_2.3（x=23,25,27,30 wt%）were prepared.The hydrogen absorption characteristics of samarium cobalt ingots and strips were studied,and the influence of internal defect morphology of ingots and strips on hydrogen absorption capacity（HAC）was discussed.With the increase of Fe content in samarium cobalt ingots,the grain size of2:17 phase becomes smaller,the distribution of 1:5 phase becomes more diffuse,the hydrogen absorption rate of ingots becomes faster and the HAC increases,but the average size of ingot particles after hydrogen absorption and crushing becomes larger.The HAC of the as-prepared strips is close to zero;after the pre-breaking treatment,the HAC of the strips particles with reduced size increases.Meanwhile,there is no crushing behavior observed in the pre-crushed Sm₂Co₁₇ strips after hydrogen absorption.Therefore,the main reason for the low macroscopic HAC of the strips is that the strips do not undergo a crushing behavior in the hydrogen absorbing process.Micro-computed tomography shows that the defects in the ingot with Fe content exceed 23 wt%are mainly small-volume spherical holes(volume<2×10^-5 mm³)and large-volume lamellar micro cracks(volume>2×10^-5 mm³),and the defects in the strips are mainly elliptical holes with a volume of less than 2×10^-5 mm³.ABAQUS simulation results show that the two-dimensional alloy produces larger stress（28.2MPa）at both ends of the lamellar crack after stretching.After the hydrogen absorption and expansion process of samarium cobalt ingot,the two ends of the lamellar crack produce a large stress concentration,and the ingot breaks up,which is conducive to promote the ingot to continue to absorb hydrogen,and the ingot shows a strong hydrogen absorption capacity.（2）The hydrogen-absorbing capacity of Sm₂Co₁₇ strips is enhanced by the growth of grain size of the strips,the enlargement of the internal pores and the increase of the surface roughness.Compare to Sm_26.2Co_balFe_xCu_4.0Zr_2.3（x=23,27wt%）strips（Fe23 and Fe27 strips）,heat-treated strips（H-Fe23 and H-Fe27 strips）and corrosion+heat-treated strips（HC-Fe23 and HC-Fe27 strips）have a higher HAC.After hydrogen absorption,crushing behavior occurs in HC-Fe23（HAC of 0.26 wt%）,H-Fe27（HAC of 0.24 wt%）and HC-Fe27（HAC of 0.32 wt%）strips.The grain sizes of the Fe23 and Fe27 strips are less than 1μm,and distinct river patterns and disintegration steps（with an average disintegration step width of 1μm）can be observed in the fracture morphology.For the H-Fe23 and H-Fe27 strips,the fracture morphology is flat,and the deconstructed steps basically disappear.After heat treatment,the size of the holes inside the strips increases,the average volume of holes in the H-Fe27 strips is 46.8 times that of the Fe27 strips.The surface roughness of Fe23 and Fe27 strips are 0.33μm and 0.80μm,respectively,while the surface roughness of H-Fe23 and H-Fe27 strips increase to 0.59μm and 1.11μm.The maximum profile valley depth of the HC-Fe23 strips（13.34μm）is 2.5 times greater than that of the H-Fe23 strips（5.43μm）.Heat and corrosion treatments have the effect of increasing the surface roughness of strips.As the grain size growth,the fine crystal strengthening effect disappears;the larger internal holes produce greater internal stress in the strips;as the surface roughness increases,stress concentration occurs in the surface of the strips.By reducing the structural stability of samarium cobalt strips,the crushing behavior occur in the strips and the HAC is improved.（3）The introduction of hydrogenation treatment in the preparation of Sm₂Co₁₇magnets can increase the magnet grain size,reduce the size of the cell structure,enhance the pinning effect during the demagnetization process and improve the coercivity and squareness of the magnets.Compared to the magnets prepared from unhydrogenated ingots,the average grain size of magnets prepared from ingot hydrogenation treatment increased by 37.4%,the average size of the cell structure decreased by 11.1%,and the coercivity increased by 10.6%.After hydrogenation treatment,the strong hydrogen-absorbing region in the strips has higher energy and is more prone to crushing behavior,thus the average particle size of the particles after ball milling of the hydrogenated strips is 89.8%of that of the unhydrogenated strips after ball milling.At the same time,hydrogenated ball milling powder has a stronger trend of grain growth during heat treatment.Therefore,under the same heat treatment conditions,the grain size of magnets prepared by hydrogenated alloys is larger than that of magnets prepared by unhydrogenated alloys.Because the particle size after ball milling of strips（more than 3μm）is much larger than the width of columnar crystals（less than 1μm）,all the strip-prepared magnets are poorly oriented and the magnetic properties are low.After heat treatment（1150℃×10 min）,the grain size in the strips and the magnet orientation increases,and the magnet coercivity exceeds26 k Oe.（4）The effect of surface roughness on the hydrogen absorption rate of Sm₂Co₁₇magnets was studied,and the hydrogen decrepitation behavior of Sm₂Co₁₇magnets was analyzed.The hydrogen embrittlement resistance of the magnets was improved through two methods:magnet oxidation treatment and surface coating.The hydrogen absorption activation time of the magnet polished with 2000 grit sandpaper is 1.83times longer than that of the magnet polished with 400 grit sandpaper.During the hydrogen absorption process of the magnet,linear or Z-shaped initial cracks first form on the outer surface of the magnet.The cracks propagate parallel to the surface of the magnet,causing transgranular fracture of the magnet and ultimately breaking into flake like particles.The oxide layer or coating on the surface of the magnet can act as a hydrogen barrier,improving the magnet’s resistance to hydrogen embrittlement.