Microstructure And Mechanism Of Nitrogen-Quenched Sm₂Fe₁₇ Alloy

Compared with Co-based permanent magnets,Sm₂Fe₁₇N_x compounds have excellent properties and lower cost.A lot of research work has been done on the preparation process of Sm₂Fe₁₇N_x compounds.Most of these studies are based on the change of experimental conditions,which affect the reaction against samarium-iron alloy with nitrogen,and do not go deep into the electronic level to study the micro-mechanism of nitrogen atom and Sm,Fe atom.Based on empirical electron theory of solids and molecules?EET theory?,the valence electron structures,theoretical bond energies and binding energies of Sm₂Fe₁₇ crystals were calculated using the bond length difference?BLD? method.Samarium-iron alloy rapidly solidified samples were prepared by copper mold spray casting technology.The cool rate of samarium-iron alloy in copper mold was calculated by using the unsteady thermal conduction process of thin material.The microstructure changes of Sm-Fe alloy during rapid solidification and the phase structure changes after nitrogen spray casting were studied by means of optical microscope,scanning electron microscope,energy dispersive X-ray diffractometer and X-ray photoelectron spectrometer.Sm and Fe atom state hybridization parameters,19 experimental bond distance values obtained from the crystal structure data of Sm₂Fe₁₇ were input into a program compiled according to the key distance difference method,and the output results are run and screened to obtain the valence electron structure of Sm₂Fe₁₇.The valence electron distribution of Sm2Fe17 indicates that the Sm atom in Sm2Fe17 unit cell is in the first order for B-type hybridization and the number of lattice electrons is 1;the Fe atom is in the ninth order for C-type hybridization and the number of lattice electrons is 0.0322.Totally number of covalent electrons is 165.000.The results of bond energy calculation show that the Fe-Fe bond energy of?=1 in Sm₂Fe₁₇ crystal is the largest,which is the main bond to be destroyed when Sm₂Fe₁₇ crystals melt.The bond energies of Fe-Fe and Sm-Sm with ?=18 and ?=19 are smaller,which is the main reason of Sm oxidation and volatilization in SmFe alloy at high temperature.It is explained electronically that Sm₂Fe₁₇7 in molten SmFe alloy can interact with nitrogen to destroy the original bonding mode and form new chemical bonds,Thus,a new interstitial compound Sm₂Fe₁₇N_x is formed,which is the reaction mechanism between nitrogen and 2:17 type SmFe alloy in non-equilibrium solidification state.The cool rate of SmFe alloy in copper mould is about 16.8 K/s after being sprayed into copper mould by nitrogen gas.The results of metallographic and electron microscopic observation show that the dendrite spacing decreases with the increasing cool rate,which promotes the grain refinement and restrains the inter diffusion between phases,and the composition of the alloy is more homogeneous.The fitting results of XRD pattern of the spray-cast sample after powder preparation were compared with the scanning results of XPS full spectrum elements on the surface of the spray-cast sample.The results of nitrogen content analysis by oxygen-nitrogen-hydrogen analyzer show that the gas-liquid contact time of molten samarium ferroalloy is shorter of nitrogen quenching.The nitriding reaction is inadequate,and the nitriding reaction only takes place on the surface of the sample,and the product is mainly composed of Sm₂Fe₁₇N_x and nitrogen hydroxide,in which about34.36% of the nitrogen atoms exist in the form of Sm₂Fe₁₇N_x.Figure 24;Table 15;Reference 66.