Microstructure Control And Nitridation Mechanism Of Sm-Fe-N Alloy

Sm-Fe-N permanent magnetic materials have been widely studied for excellent magnetic properties and high temperature resistance.But the magnetic properties are limited by the inhomogeneity of composition and microstructure.Microstructure reconstruction can improve the heterogeneity.The first step is to prepare the Sm-Fe amorphous alloy.The "free volume defect" in amorphous was fully utilized to study amorphous nitrogen enrichment.Finally,an appropriate crystallization process was selected to achieve the purpose of increasing nitrogen and regulating the microstructure.Firstly,the ability of amorphous formation of Sm-Fe?Zr,Nb,Cu,B?alloy system was determined by using Greer chaos criterion and Inoue's empirical law.The atomic size difference and mixing enthalpy of Zr,Nb,B and element Fe are conducive to the formation of amorphous.The addition of B element has the most obvious promoting effect.The difference of atomic size and mixing enthalpy of Fe-Cu do not meet the judgment conditions,but it can effectively inhibit the formation of ?-Fe soft magnetic phase,stabilize the 1:9 metastable phase,and refine the microstructure.Therefore,the optimal composition ratio is Sm₂Fe₁₇ ZrNb_0.4Cu_0.2B_0.2 series alloy,and the completely amorphous ribbons can be obtained at the rapid quenching speed of 40 m/s.Then,the effects of different rapid quenching speed and ball milling time on the amorphous formation behavior were studied.The rapid quenching speed has no significant effect on the formation of amorphous of Sm-Fe binary alloy,but it has significantly facilitation for Sm-Fe-Zr-Nb and Sm-Fe-Zr-Nb-Cu-B alloys.For Sm-Fe?Zr,Nb?alloy which cannot be prepared by rapid quenching directly,the amorphous alloy can be prepared by the combination of rapid quenching and ball milling.The Sm₂Fe₁₇ alloy powder with excellent completely amorphous structure could be prepared by ball milling for 3?5 h.The milling time of Sm₂Fe₁₇ ZrNb_0.4 alloy prepared at 36 m/s was only 2 h,which also confirmed that Zr and Nb elements promoted the formation of amorphous.For Sm₂Fe₁₇ ZrNb_0.4Cu_0.2B_0.2 alloys,a completely amorphous ribbon was obtained at 40 m/s.Although the aerosolization method could not achieve the amorphous of the alloy,but the method has a large research space in the preparation of Sm-Fe powder.Nitriding experiments on Sm-Fe?Zr,Nb?alloys show that,the nitrogen contenttends to increase with the increase of amorphous contents,for the increasing of "free volume defect".The nitrogen content of Sm₂Fe₁₇ ZrNb_0.4 alloy ribbons was 2.54%,and increased to 2.82% when the amorphous content reached 84.9%.Compared with crystal nitriding,amorphous nitriding shortened nitriding time and improved nitriding efficiency.The uniform microstructure can be obtained after isothermal crystallization annealing,but the precipitation and growth of the ?-Fe phase are obvious.However,low temperature pre-annealing treatment can increase the number of ordered atomic clusters in short/medium range,which acts as the nucleus in the crystallization process,and inhibit the precipitation and growth of ?-Fe phase,reduce the crystallization temperature.The Sm₂Fe₁₇ ZrNb_0.4Cu_0.2B_0.2 alloy obtained at 40 m/s was treated with the methods of isothermal crystallization + nitriding and nitriding + step-step crystallization.The coercivity was 2.18 k Oe after isothermal crystallization + nitriding.After nitriding +isothermal crystallization,the nitriding products were almost all decomposed for the high temperature,so the coercivity disappeared.As the number of ordered atomic clusters in short/medium range is increased by pre-annealing,the crystallization process acts as an explosive nucleation,which can inhibit the precipitation and growth of ?-Fe phase and reduce the crystallization temperature.Therefore,when the pre-annealing treatment was combined with the amorphous nitriding process,the uniform microstructure with grain size of ?35 nm was obtained after crystallization at 580 ? for 10 min.The corresponding coercivity was 1.28 k Oe,higher than that of isothermal crystallization + nitrification.Therefore,the step-step crystallization including pre-annealing not only improves the homogeneity and magnetic properties of the microstructure,but also makes the experimental operation of the amorphous alloy after nitridation possible.The above work is only for the preparation.The crystallization of amorphous of microeconomic regulation and mechanism of nitrogen amorphous are detailed described.Amorphous crystallization after nitriding treatment needs to be given in-depth research.In the subsequent study,should be further coordination and optimization of amorphous nitriding process and crystallization annealing process,overcome nonuniformity and nitride inadequate effects on magnetic materials Figure 106;Table 25;Reference 169...