Preparation And Properties Of Hot Deformed NdFeB Doped With Nano-TiC

Nd-Fe-B alloy is the high-performance rare earth permanent magnet material,which is often used in the fields of motor,electronic communication,aerospace,transportation and so on.These magnets can be fabricated through sintering,bonding or hot-deformation process,in which the last is an effective method to obtain highly textured and density magnet.Such hot-deformed magnets usually consist of misligned coarse grains and inhomogeneous stripe-shaped Nd-rich phase.This is not conducive to the coercivity and corrosion resistance of the magnet,which limits the application of NdFeB.To solve this problem,hot deformed NdFeB magnets were prepared by doping nano-TiC based on the pinning theory of hard phase grain boundary.The relationship between the magnetic properties,corrosion resistance and the amount/size of nano-TiC,hot deformation pressure and holding time of hot deformed NdFeB were systematically studied,and the structure-property relationship was analyzed.Results show that:The magnet has high corrosion resistance and high magnetism when the deformation pressure was 100 MPa and the holding time was 30 min.In addition,for the 0.25-1 wt%nano-TiC doped magnet,H_cj(kOe)+(BH)_max(MGOe)was 55?56,which was 3.3%?6.1%higher than that of TiC-free magnet.The(BH)_maxincreases first and then decreases with the addition of nano-TiC.The maximum value of 16.61 kOe can be obtained at 0.25 wt%nano-TiC,which was 2.5%higher than the value of coercivity for TiC-free magnet.Besides,the maximum of comprehensive magnetic properties of 50nm sized TiC added magnet after tempering treatment was 59.57(MGOe+kOe),which was 3.6%higher than the corresponding value of undoped magnets.The coercivity of 40nm sized TiC added magnet was 15.24 kOe,which was 2.3%higher than the corresponding value of TiC-free magnet.Additionaly,doping nano-TiC improved the corrosion resistance of the magnet in acidic and neutral medium containing,and the best corrosion resistance was obtained at 0.5 wt%nano-TiC.The corrosion current density for the surface parallel to pressure direction of magnet in 0.005 M H₂SO₄and 0.6 M Na Cl aqueous solution decreased from 7.039mA/cm²and 3.569mA/cm²to 4.5mA/cm²and 0.906mA/cm²,respectively reduced the value of 36.1%and 74.6%.Based on the structure-property relationship analysis of NdFeB,the magnetic properties are closely related to the morphology and distribution of Nd-rich phase,the later is related to the key parameters of hot deformation and the number/size of nano-TiC.Magnets prepared at a deformation pressure of 100 MPa and a holding time of 30 min have uniform Nd-rich phase distribution and small average grain size(175nm),resulting in excellent coercivity and squareness.The distribution and content of the stripe-shaped Nd-rich phase were improved by adding appropriate amount of nano-TiC.The main phase nano-crystalline particles were uniformly encapsulated by the intergranular Nd-rich phase,and the magnetic hardening effect was enhanced.During hot pressing and hot deformation,the nano-TiC with high hardness and stability flowed into the magnet along with the liquid Nd-rich phase.At the grain boundary the nano-TiC has the effect of preventing the grain growth,which was beneficial to improve the magnetic performance.In addition,the corrosion resistance of magnets is connected with the morphology,distribution and chemical stability of Nd-rich phase,which are affected by the amount/size of nano-TiC.The Nd-rich phase of 0.5 wt%(50nm)TiC added magnet was found to be evenly distribute,which can effectively prevent the diffusion of corrosive ions such as SO₄^2-and Cl^-from surface to inside.At the same time,the chemical stability of the Nd-rich phase was improved,thus reduced the potential difference between the Nd-rich phase and the main phase.It improved corrosion resistance of the hot deformed magnet due to the reduced corrosion power.