Microstructure and Nitridation Mechanism of SmFeN Alloy Powders Fabricated with High Pressure Atomizing Process

Author:Song Chun Yan

Supervisor:wang shu huan gui yong liang


Degree Year:2019





SmFeN system permanent magnetic materials have been used widely in the fields of information,electronics,energy,biology,and so on,because of its excellent magnetic properties and easy forming performances.The preparation of powders is very important to obtain adhesive permanent magnetic SmFeN system materials.Traditional preparation technologies of SmFeN powders,i.e.,mechanical crushing process,have been one of the main obstacles of development of SmFeN system permanent magnetic materials,due to the defects of long time,high cost,easy to be oxided of powders,and so on.Therefore,it is necessary to develop novel fabrication technologies and investigate corresponding solidification process,microstructure and nitridation mechanisms.Gas atomization technology with advantages of high cooling rate of melt droplets and high purity of powder,was introduced into preparation of SmFeN powder in this paper.The influence of technological parameters during gas atomization process on particle size of SmFe(N)powders was revealed according to the flow field model of gas atomization process established.Microstructure evolution of rapidly quenched SmFe alloy ribbons was put forward.The nitridation mechanism of SmFe alloy and the role of nitrogen atoms in crystalline Sm2Fe17N3 was analyzed using valence electron structure theory.Experimental studies on the preparation of SmFe alloys using high-pressure gas atomization showed that the gas type has a significant influence on the morphology of the obtained powders.The powders obtained from N2 atomized SmFe alloy melt are mainly clastic rather than spherical.The particle size is small with average particle size less than50μm.While the powders obtained from Ar atomized SmFe alloy melt have higher sphericity.However,the powder size is uneven and vary from dozens to hundreds of microns.Particle size also depends on atomized gas pressure and chamber pressure.Under the constant atomization pressure conditions,the average particle diameter decreased from637.471μm to 572.911μm,resulting in decreasing degree of 10%,as the chamber pressure increased from 0.4MPa to 0.6MPa.When the atomization pressure increased from 1.5MPa to 2.0MPa with a stable chamber pressure,the average particle diameter displayed a reduction rate of 2%.With same other technical parameters,the average particle size prepared under spraying pressure of 0.6MPa and 0.8Mpa is respectively 560.827μm and558.473μm.The Sm content in SmFe alloy powders prepared with master SmFe materials of 24%Sm at normal atomization pressure is 20%,while it is 23.67%in high atomization pressure of 0.4MPa,which resulting in the corresponding loss rate is 17%and 1.38%,respectively.This phenomenon suggests that the volatilization of Sm during gas atomization process could be inhibited.During the solidification process of SmFe molten droplets,the smaller the droplets size,the finer the solidified particle size,the higher sphericity,the smoother the surface of SmFe alloy powder.Moreover,the reduction of grain size resulted in the refinement of surface microstructure.The fitted function between grain size D of SmFe alloy powders and secondary dendrite arm spacingλ2 on powder surface isλ2=0.027D-0.105.The XRD profile of Sm2Fe17 alloy powders atomized under 1atm N2 atomosphere shown that the peak of Sm2Fe17 has a small shift.It indicates that some SmFe alloy molten droplets reacted with nitrogen in the atomization process,resulting in the formation of Nirogen-bearing SmFe alloy powder and change of phase constitution.According to the investigation on effect of cooling rate on microstructure of rapidly solidified SmFe alloy,it was found that when the cooling rate increases from 1.76×105K/s to 9.29×105K/s,the coarse grains were significantly refined and microstructure evolved from coarse grain to microcrystalline to amorphous and crystal coexistence finally.When the cooling rate was up to 8.68×105K/s,obvious diffuse scattering peak were observed except crystal diffraction peaks in the X-ray diffraction patterns of the rapidly quenched SmFe alloy ribbons.This phenomenon indicates that some amorphous phases formed during rapid quenching process of SmFe alloy melt.SmFe alloy ribbons prepared at a cooling rate of9.09×105K/s were carried out nitridation treatment at 420℃.A complex compound constituted by both Sm2Fe17Nx andα-Fe crystalline phases,and amorphous nitrogen-containing phase formed by the permeating of nitrogen atoms into non-equilibrium solidified SmFe alloy ribbons,was revealed.The presence of amorphous phases increases the nitrogen solid solubility in SmFe alloy ribbons,which could be evidenced by the fact of nitrogen content of 4.155%.The valence electron structures of Sm2Fe17 and Sm2Fe17N3 crystals were established with the empirical electron theory of solids and molecules(EET theory).The calculated Curie temperature of Sm2Fe17N3 is 748.55K,which is consistent with TC=746K reported in literature.The valence electron structures of Sm2Fe17 and Sm2Fe17N3 crystals are verified by this result.Subsequently,the role of nigrogen atoms in SmFe(N)alloy was analyzed using the valence electron structure.It is revealed that nigrogen atoms entered into Sm2Fe17crystals during the atomization and quickly solidification process of SmFe alloy melting droplet,and then destroy the weaker Sm-Sm bonds and form SmN compounds or SmFeN compounds by combining with Sm-Fe bonds.Theα-Fe phase is formed in SmFe alloy powders due to the Fe-Fe bonds have the maximum bond energy and is the stablest bonds among all bonds.Therefore,the constitutional phases is Sm2Fe17Nx、SmN andα-Fe in SmFe(N)alloys.Moreover,the entering of N atoms leads to the 3%-6.9%increase of Fe-Fe theoretic bond distance.N atoms in Sm2Fe17N3 crystal is in long-distance bonding atomic pairs,which resulted in the expand of SmFe crystal.