等离子体渗氮过程中钢表面纳米晶层形成机理研究

发布时间：2018-03-26 02:00

  本文选题：等离子体渗氮　切入点：热力学　出处：《哈尔滨工业大学》2017年硕士论文

【摘要】：在过去十年时间,本课题组开展了对不同Cr当量淬火钢进行低温等离子体氮化或氮碳共渗处理实验研究,如17-4PH、M50、2Cr13、304、316、M50NiL、38CrMoAl、40CrNi和30CrMnSi等,并且已经成功实现了表面层的纳米化,但是现有的纳米化的机制无法解释纳米化后出现低氮化合物的原因。本文研究了 Fe-Cr-N体系的调幅分解反应,旨在揭示等离子体渗氮后钢表面发生纳米化的机制,基于“伪二元”和双亚点阵两种热力学模型计算了 Fe-Cr-N体系的吉布斯自由能。在“伪二元”模型得到Fe-Cr-N体系的吉布斯自由能-成分曲线出现了双势阱,说明Fe-Cr-N体系在一定条件下可以发生调幅分解。双亚点阵模型计算了不同Cr当量钢中含氮马氏体的吉布斯自由能与含氮量的关系,得到了不同Cr当量钢渗氮调幅分解的温度区间和最佳氮势。结合多组实验的TEM分析,本文提出了Cr当量钢渗氮纳米化的机制:在一定条件下,随着N含量的增加,α'N发生调幅分解反应,生成纳米尺寸的低氮化合物FeNz和高氮马氏体α"N,从而使渗层组织细化。通过第一性原理对不同Cr当量含氮马氏体和低氮化合物的结构进行了模拟,发现含氮马氏体和低氮化合物结构的总能都小于0,说明在O K下这些结构都可以存在。并且Cr当量为1/11和1/7的含氮马氏体结合能和形成能随含氮量的变化出现了双势阱趋势,进一步提出渗氮纳米化组织可能是调幅分解导致的微观机制。由于Fe-3d、Cr-3d和N-2p的杂化,在不同Cr当量含氮马氏体的费米能级附近处,都出现了赝能隙,从电子结构角度说明这些结构在0K下都是相对稳定的,为调幅分解在渗氮温度下发生提供了条件。低氮化合物的第一性原理计算从能量和电子结构两个方面都说明低氮化合物面心立方比体心立方结构更稳定一些,解释了在实验过程中检测到的低氮化合物都为面心立方结构的原因。
[Abstract]:In the past ten years, our group has carried out experimental research on nitridation or nitrocarburizing treatment of different Cr-equivalent quenched steels at low temperature, such as 17-4PHM50 / 2Cr13304316M50 / NiL338CrMoAl4Cr40CrNi and 30CrMnSi, and has successfully realized the nanocrystallization of the surface layer. However, the existing mechanism of nanocrystalline can not explain the reason of the formation of low nitrogen compounds after nanocrystalline. In this paper, the amplitude modulation decomposition of Fe-Cr-N system is studied to reveal the mechanism of nanocrystalline formation of steel surface after plasma nitriding. The Gibbs free energy of Fe-Cr-N system is calculated based on the two thermodynamic models of "pseudo-binary" and double-sub-lattice. The Gibbs free energy-component curve of Fe-Cr-N system is obtained by using "pseudo-binary" model. The results show that amplitude modulation decomposition can occur in Fe-Cr-N system under certain conditions. The relation between Gibbs free energy and nitrogen content of nitrogenous martensite in different Cr equivalent steels is calculated by double sub-lattice model. The temperature range and optimum nitrogen potential for nitriding amplitude modulation decomposition of different Cr equivalent steels were obtained. The mechanism of nitriding nanocrystalline nitriding of Cr-equivalent steel was put forward by TEM analysis under certain conditions. With the increase of N content, the amplitude modulation decomposition reaction of 伪 -N occurs. Nano-sized low nitrogen compounds FeNz and high nitrogen martensite 伪 "N were formed to refine the microstructure of the layer. The structures of different Cr equivalent nitrogenous martensite and low nitrogen compounds were simulated by first principles. It is found that the total energy of both nitrogen-containing martensite and low-nitrogen compounds is less than 0, which indicates that these structures can exist at O K. and the binding energy and formation energy of nitrogen-containing martensite with Cr equivalent of 1 / 11 and 1 / 7 have a double potential well trend with the change of nitrogen content. It is further suggested that the nitriding nanocrystalline structure may be the microcosmic mechanism of amplitude modulation decomposition. Due to the hybrid of Fe-3d Cr-3d and N-2p, pseudo-energy gaps appear near the Fermi energy levels of different Cr equivalent nitrogenous martensite. From the point of view of electronic structure, these structures are relatively stable at 0K. The first principle calculation of low nitrogen compounds shows that low nitrogen compounds are more stable than bulk centered cubic structures in terms of energy and electronic structure. The reason why all the low nitrogen compounds detected in the experiment are face-centered cubic structures is explained.
【学位授予单位】：哈尔滨工业大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TG156.82
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本文编号：1665835