α-Fe磁力学效应的第一性原理计算

发布时间：2019-02-24 12:59

【摘要】：钢铁等铁磁性金属材料广泛的应用于各行各业中,尤其是在石油化工、航天航空、铁路建筑等领域中。金属构件在长期服役过程中,由于受到疲劳载荷的作用,其金属材料会经历弹性形变、屈服、塑性形变这一过程,最终导致裂纹缺陷的产生,造成铁磁性金属构件失效。塑性形变作为裂纹缺陷产生的早期形式,对于构件的塑性形变区进行有效的检测,可避免大型突发性事故的发生。金属磁记忆检测技术作为21世纪最有应用前景的新型无损检测技术之一,可对金属构件的应力和形变集中区进行快速、有效的检测。目前,此技术已成功应用于管道、锅炉及铁轨应力检测等领域。但由于磁力学效应影响复杂,磁记忆信号极易受到金属构件本身的参数、外界环境等因素的干扰,因此在应力集中达到临界屈服点前后时的磁记忆信号特征尚不明确,进而无法准确评估及预测铁磁性金属构件的使用寿命。本文从固体能带理论出发,建立了铁磁晶体的磁力学耦合模型,从微观上研究了铁磁晶体在应力集中达到临界屈服点前后时的晶体结构与磁记忆信号特征,并系统分析了磁记忆现象机理。采用基于密度泛函理论的第一性原理方法计算了体系的晶格结构、原子磁矩、电子自旋态密度、差分电荷密度、能带结构等随应力的变化情况,并计算得到Fe晶体发生从bcc结构向hcp结构转变的临界应力值。理论研究结果表明:晶体结构稳定性受内部原子间的结合力影响,当应力不断增加时,原子间结合力减弱,晶体结构稳定性变弱;当应力集中达到一定程度时,Fe晶体将发生相变,发生相变的临界应力值约为10.5GPa,晶体结构发生改变,体系磁特性将发生突变。为验证理论研究结果,本文设计了钢管打压试验,利用高精度磁通门传感器对磁记忆信号进行采集,测量了在不同压力作用下、强磁磁化前后及不同提离值状态下的磁记忆信号的变化情况;并进行了系统的实验研究,定量分析了构件屈服前后的磁记忆信号变化特征。试验结果表明:磁记忆信号与应力具有一一对应关系;外界强磁场可以覆盖磁记忆信号,但不会破坏磁记忆信号;提离值在一定范围内不会影响检测结果;当应力集中程度达到材料的屈服强度时,磁记忆信号曲线出现拐点,曲线斜率发生改变。本文的理论研究结果与试验结果具有很好的一致性。
[Abstract]:Ferromagnetic metal materials such as iron and steel are widely used in various industries, especially in petrochemical, aerospace, railway construction and other fields. During the long service of metal members, due to the effect of fatigue load, the metal materials will undergo the process of elastic deformation, yield and plastic deformation, which will eventually lead to the production of crack defects, resulting in the failure of ferromagnetic metal members. Plastic deformation, as an early form of crack defects, can effectively detect the plastic deformation zone of members and avoid the occurrence of large sudden accidents. As one of the most promising new nondestructive testing techniques in the 21st century, metal magnetic memory testing technology can be used to detect stress and deformation concentration areas of metal components quickly and effectively. At present, this technology has been successfully applied in the fields of pipeline, boiler and rail stress detection. However, because of the complex influence of magnetic force effect, the magnetic memory signal is easily disturbed by the parameters of the metal component itself and the external environment, so the characteristics of the magnetic memory signal before and after the stress concentration reach the critical yield point are not clear. Furthermore, the service life of ferromagnetic metal components can not be accurately evaluated and predicted. Based on the theory of solid energy band, the magnetic coupling model of ferromagnetic crystals is established. The crystal structure and magnetic memory signal characteristics of ferromagnetic crystals before and after the stress concentration reaches the critical yield point are studied microscopically. The mechanism of magnetic memory phenomenon is analyzed systematically. The changes of lattice structure, atomic magnetic moment, electron spin density, differential charge density and energy band structure with stress are calculated by using the first-principle method based on density functional theory. The critical stress values of Fe crystal transition from bcc structure to hcp structure are calculated. The theoretical results show that the stability of crystal structure is affected by the interatomic binding force. When the stress increases, the interatomic adhesion decreases and the stability of crystal structure weakens. When the stress concentration reaches a certain degree, the phase transition will take place in Fe crystal, the critical stress of phase transition is about 10.5 GPA, the crystal structure will change and the magnetic properties of the system will change. In order to verify the results of theoretical research, this paper designs the test of steel tube compression, uses high precision flux gate sensor to collect the magnetic memory signal, and measures the magnetic memory signal under different pressure. The changes of magnetic memory signals before and after magnetization and in different lifting states; A systematic experimental study was carried out to quantitatively analyze the variation characteristics of magnetic memory signals before and after yielding. The experimental results show that the magnetic memory signal has a one-to-one correspondence with the stress, the external strong magnetic field can cover the magnetic memory signal, but it will not destroy the magnetic memory signal, the lifting value will not affect the detection results in a certain range. When the stress concentration reaches the yield strength of the material, the curve of magnetic memory signal appears inflection point and the slope of the curve changes. The theoretical results of this paper are in good agreement with the experimental results.
【学位授予单位】：沈阳工业大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：O469

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