Characterization Of Grain Microstructures And Properties Of Formed Phases In Rare Earth Nitrocarburized Layer Of A Nanocrystallized 18NI Alloy

18Ni alloy was first nanocrystallized and then rare earth (RE) nitrocarburized at low temperature to obtain the nitrocarburized layer with high strength and excellent ductility and decrease the deformation of components in this thesis. The optical microscope (OM), X-ray diffractometer (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), microhardness and nanoindentation testers were used to study the effects of process parameters, RE addition and its flow rate on the structures and properties of nitrocarburized layer. At the same time, the change of grains, grain boundary character and texture in nanocrystallized (NC) 18Ni alloy after nitrocarburizing as well as the effect of them on the structures and properties of nitrocarburized layer were also study by electron backscattering diffraction (EBSD) technique. At last, the properties of nitrocarburized phases, the effects of the alloy elements on the properties of nitrides were calculated using first-principles calculation software. The strengthening and toughening mechanisms of nitrocarburized layer were also discussed based on the experimental and calculated results.18Ni alloy can be whole nanocrystallized by a compolex thermomechanical treatment, including solid solution, high and low temperature complex deformations and fast heating recrystalliztion. The average grain size of NC 18Ni alloy is about 70nm.The treated temperature and time have great effects on the thickness of nitrocarburized layer of NC 18Ni alloy. The accelerated and modified effects of RE are not obvious at low temperature and short time, and it even impedes nitrocarburized process at some time. The ratio of N₂ to H₂ also has certain effects on the thickness and hardness of nitrocarburized layer. The surface phases of nitrocarburized layer obtained at low temperature are mainly composed ofα-Fe solid solution with N, C and a small amount ofγ'-Fe₄N. The low nitrogen compound FeN_x (x=0.0324-0.0950) is formed at 430°C, and the highest content ofγ'-Fe₄N in nitrocarburized surface is obtained at 460°C, while reversed austenite A_R is produced at 500°C. The addition of RE in gas atmosphere is benefical for the formation ofγ'-Fe₄N and FeN_x, and suppressing the inverse transformation ofα'-Fe to A_R. The enhancement of N₂/H₂ ratio also can increase the content ofγ'-Fe₄N in the surface of nitrocarburized layer. The relative intensity ofγ'-Fe₄N (200) is highest, and A_R possesses (200) preferred orientation. The appropriate RE flow rates corresponding to the nitrocarburized layer with higher thickness and hardness obtained at 400, 430, 460 and 500°C are 0.025, 0.050, 0.100 and 0.150L/min, respectively. The enhancements of temperature and RE flow rate can increase the content of RE in nitrocarburized layer.The dislocation density in nitrocarburized layer is reduced after the NC 18Ni alloy nitrocarburized at 430°C. The grain boundaries absorbing dislocations as well as the Ni, Mo elements aggregating at grain boundaries and precipitating Ni₃Mo and Ni₃Ti compounds lead to the thickening of grain boundaries. Phasesγ'-Fe₄N and FeN_x with a size of several nanometers precipitates from grain boundaries and intracrystallines. It grows perpendicularly to grain boundary whenγ'-Fe₄N precipitates from grain boundaries. FeN_x with a size of several nanometers also finds in intracrystallines. Dislocation network configurations are formed at grain boundaries of martensite in the radical direction of nitrocarburized layer obtained at 460°C, which leads to the more decrease of dislocation density in intracrystallines. Ni and Mo elements aggregate at intracrystallines, leading to the transformation of martensite to austenite with high Ni and Mo contents in aggregative areas. The (111) plane ofγ'-Fe₃NiN is parallel to the (101) plane of FeN_x and (110) plane ofα'-Fe. The precipitate of nitrides leads to the distortion of martrix grains, and the maximum distortion angle is about 15°. Many thin martensite bands are found in axial direction of the nitrocarburized layer. Many FeN_x,FeNi₃,Ni₃Ti and MoN phases with sizes no more than 5nm distribute dispersively in the martensite bands. The dislocation density is reduced greatly and grain boundaries become clear in the nitrocarburized layer produced at 500°C, thus, the restoration of grains is basically finished. There are more N, C elements solid solution into martensite and austenite when the N₂/H₂ ratio enhances from 1:9 to 1:3, resulting in the high dislocation density in the nitrocarburized layer.The shape ofα-Fe andγ'-Fe₄N grains in NC 18Ni alloy and its nitrocarburized layer is mainly columnar. The nitrocarburization treatment does not lead to the coarsening ofα-Fe grains, and the grain size ofγ'-Fe₄N is thinner and thinner along nitrocarburized layer. NC 18Ni alloy possesses high fraction of general large angle boundaries and a certain fraction of small angle and coincidence site lattice (CSL) boundaries. The content of CSL decreases and that of general large angle boundries increases after nitrocarburizing. The CSL ofα-Fe in NC 18Ni alloy is mainly composed ofΣ13b,Σ3 andΣ11, and the RE addition reduces the decrease of them. The CSL ofγ'-Fe₄N mainly consists ofΣ3,Σ9 andΣ17b.The typical cold-drawn texture <110> exsits in NC 18Ni alloy and its intensity is respectively enhanced and lowered in the axial and radical directions of nitrocarburized layer. Theγ'-Fe₄N grains in the axial direction of nitrocarburized layer have high intensity of <100> texture which is very weak in the radical direction, and the RE addition weakens the texture ofγ'-Fe₄N. The Taylor factors for each slip system ofα-Fe grains are all decreased and the resistance to plastic deformation is inceased after nitrocarburizing, and the RE addition results in the higher resistance to plastic deformation ofα-Fe grains. The resistance to plastic deformation and wear of substrate are also enhanced after nitrocarburizing.The nitrocarburized phases are all easy to form and stable, andγ'-Fe₄N is easiest to form whileα'-Fe is most stable. Ni₃Mo phase is bonded by metallic bonds and its toughness is most excellent. Other nitrocarburized phases are bonded by a mixed bond with covalent, ionic and metallic characters. Phases MoN and supersaturated austenite possess excellent elastic properties and their toughness is poor. Phaseγ'-Fe₄N is easy to produce elastic deformation.γ'-(Fe_1-xNi_x)₄N is easy to form whileγ'-(Fe_1-xCo_x)₄N andγ'-(Fe_1-xMo_x)₄N is hard to form, and the alloy ability and stability ofγ'-(Fe_1-xNi_x)₄N are decreased with the increase of Ni content. The subsitition of Ni and Co for Fe atoms leads to the decrease of lattice parameters ofγ'-(Fe_1-xM_x)₄N, while the bigger Mo atom makes the lattice paramenters ofγ'-(Fe_1-xMo_x)₄N enlarge along substituted direction and shorten along unsubstituted direction. The Fe2-N bond is stronger than Ni2-N bond inγ'-(Fe_1-xNi_x)₄N. The substitution of Ni weakens Fe2-N bond, but the strength of Fe2-N bond and Ni2-N bond increase with Ni content. The modulus ofγ'-(Fe_1-xNi_x)₄N are better than those ofγ'-Fe₄N, andγ'-(Fe_1-xNi_x)₄N presents good toughness. The hardness value ofγ'-(Fe_1-xNi_x)₄N is decreased with Ni content.