ROLE OF TIN ADDITIONS ON PROPERTIES AND PHASES OF SmCo 5 ALLOYS

Abstract

The samarium cobalt (SmCo 5 ) magnets provide higher magnetic force density, unlike the Alnico and other previously used magnets. The small sized magnets embedded in the properly designed appliance provide the forces necessary in dental, biomechanical treatments, removable partial dentures and implant prosthodontics. Cast SmCo 5-x Sn x (x=0.01, 0.06, and 0.12) alloys were prepared by arc melting with subsequent thermal homogenization. Microstructural and x-ray diffraction studies revealed that the alloys contain three phases i.e. SmCo 5 , Sm 2 Co 7 and Sm 2 Co 17 . The addition of Sn also caused grain refinement of the alloys. It was found that Sn addition had promoted nucleation of Sm 2 Co 17 , segregation of solute atoms and increase in unit cell volume, consequently, significant augmentation in remanence to maximum magnetization ratios were achieved. The nano-phase structures and magnetic properties of as milled powders were investigated. The Sn additions resulted in development of nanocrystalline structures producing exchange-coupled magnets with better remanence magnetization to maximum magnetization ratios (M r /M max ), typically 0.92 at 9.9 kOe coercivity. In addition, it was observed that the Sn concentrations lead to higher M r /M max ratios and maximum magnetization accompanying lower coercivity. X-ray diffraction revealed formation of 2:17 and 2:7 phases in 1:5 matrix, which were found to be dependent on Sn percentage. It appeared that higher Sn concentrations promoted 2:17 phase and helped in the formation of nano-sized phases. The investigation of sintered bulk SmCo 5-x Sn x alloys elucidated the presence of exchange coupling phenomena. Addition of Sn in the alloys developed nanocrystalline structure that in turn appeared to be responsible for producing exchange coupled magnets. The exchange coupling mechanism was evaluated by switching field distribution (SFD), dc—demagnetization M r (H) and magnetization M(H) curves as function reverse applied field (H r ). Sn additions resulted in increase in maximum magnetization and enhanced exchange coupling phenomena. The energy product (BH max ) of 7.4 MGOe iv(59.2 kJ/m 3 ), remanence magnetization to maximum magnetization ratios (M r /M max ) of 0.97 and remanence coercivity to intrinsic coercivity ratios (H r /H ci ) of 1.75 were achieved for sintered alloy containing 0.01 % Sn. Exchanged coupling and magnetic properties of SmCo 5 alloys containing Sn were investigated in sintered magnets with and without magnetic field annealing. It was found that applying magnetic field along the alignment direction of the samples was favorable for enhancing the reversibility in comparison to annealing without magnetic field. The remanence coercivity to intrinsic coercivity ratio increased from 1.75 to 2.0 with magnetic field annealing. The effect of Sn additions on electrochemical corrosion properties of sintered nanocrystalline SmCo 5-x Sn x alloys in comparison to conventional SmCo 5 binary alloy was investigated with the dc cyclic polarization technique in simulated marine environment. It was observed that Sn additions significantly reduced the corrosion rate up to 84% with only 0.01% Sn and it was further decreased to 90% with 0.12% Sn compared to the binary SmCo 5 alloy. The results showed that the corrosion rate was reduced with decrease in grain size. Scanning electron microscopy revealed pitting mechanism for both the binary and ternary alloys. Improved corrosion resistance of the alloys with Sn alloying is attributed to the interaction of Sn with grain boundary phases altering potential of the electrochemically more active samarium rich phases around the grain boundaries. This work has shown that Sn additions can be beneficial to the magnetic properties and enhanced the corrosion resistance of the alloys.