【作者】 韩秀丽；

【导师】 孙东立；

【作者基本信息】 哈尔滨工业大学 ， 材料学， 2010， 博士

【摘要】 近年来钛合金氢处理技术和理论的研究得到众多材料工作者的重视,对钛合金的氢处理工艺、氢处理后合金的力学性能、加工性能以及组织结构等方面进行了大量研究,并取得了很多成果。但对于氢致钛合金热塑性机理的研究进展相对缓慢,缺乏从电子、原子尺度上阐述氢在钛及钛合金中的作用机理,以及从物理本质上揭示氢致热塑性机理的理论研究工作。本文采用第一原理平面波赝势方法和分子动力学模拟方法系统研究了Ti-H体系的晶体结构、能量性质、原子扩散性质、弹性性质和力学性质,从电子和原子层次上揭示了氢在钛晶体中的作用机理。氢在钛晶体中的占位是建立合理的钛-氢晶体结构模型进而进行各种性质计算的基础,本文首先采用第一原理方法研究了氢在钛晶体中的占位。结果表明,相同氢含量的α-Ti-H和β-Ti-H晶体中,氢原子位于八面体间隙时的晶体点阵畸变程度比氢原子位于四面体间隙时小,而其溶解热绝对值比氢位于四面体间隙时大。氢原子在α-Ti和β-Ti晶体中均倾向于占据八面体间隙位置。氢原子占据α-Ti晶体的八面体间隙后,其最近邻Ti原子的3p轨道部分电子向氢原子1s轨道转移,H原子与最近邻Ti原子间的键共价性较强。氢原子改变了其最近邻钛原子的电子态密度,降低了Ti-Ti原子间成键的强度,氢在α-Ti中存在弱键效应。氢原子占据β-Ti晶体的八面体间隙后,改变了其周围钛原子的电子态密度,提高了Ti-Ti原子间的成键强度,氢在β-Ti中存在强键效应。原子间的相互作用势是分子动力学模拟的基础和关键。本文在EAM模型的基础上,结合Johnson的分析型EAM模型,建立了Ti-H体系的EAM作用势模型。采用Johson的分析型EAM模型确定了Ti原子的势函数表达式,给出了模型参数;提出了新的H原子的势函数表达式,通过拟合面心立方金属氢的晶格常数、结合能、体积模量的第一原理计算结果以及氢在钛中的溶解热确定了模型参数。基于此模型,采用分子动力学方法模拟计算了Ti-H晶体的结构和能量性质以及氢在α-Ti-H晶体中的溶解体积和溶解热,模拟结果验证了所建模型的合理性和可靠性。基于本文建立的Ti-H体系EAM模型,采用分子动力学方法研究了氢在α-Ti和β-Ti晶体中的扩散机制。氢原子在α-Ti晶体中的扩散存在各向异性,沿c轴方向扩散的激活能要小于沿基平面扩散的激活能,在基平面上的扩散机制为O-T-O扩散。氢在β-Ti晶体中的扩散机制为最近邻八面体间隙之间的直线跳跃。计算了氢在α-Ti和β-Ti晶体中的扩散激活能和频率因子,计算结果与实验结果和其它计算结果吻合较好。基于本文建立的Ti-H体系EAM模型,采用分子动力学方法研究了氢对α-Ti和β-Ti晶体中钛原子自扩散性质的影响。结果表明,氢降低了α-Ti晶体的单空位形成能和钛原子自扩散迁移能及激活能,提高了钛原子自扩散激系数,促进了钛原子自扩散运动。氢提高了β-Ti晶体的单空位形成能和钛原子自扩散迁移能和激活能,降低了钛原子自扩散激系数,阻碍了钛原子自扩散运动。计算了α-Ti-H和β-Ti-H晶体的弹性模量。结果表明,氢提高了α-Ti晶体的体弹性模量,降低了α-Ti晶体的杨氏模量和剪切模量;不同氢含量的β-Ti-H晶体的体弹性模量、杨氏弹性模量和剪切弹性模量的计算值均大于β-Ti晶体的计算值,氢提高了β-Ti晶体的弹性模量。基于本文建立的Ti-H体系EAM模型,采用分子动力学方法模拟了Ti和Ti-H晶体的变形行为。结果表明,α-Ti-H晶体沿c轴断裂的的拉应力临界值和沿{0001}<1210>剪切的切应力临界值随着温度的升高和氢含量的增加而下降。氢降低了α-Ti晶体的理论断裂强度和理论屈服强度。β-Ti和β-Ti-H晶体在单向拉伸应力下发生相转变,不同模拟温度下β-Ti-H晶体发生相转变的拉应力临界值均大于β-Ti,证明氢增加了β相的稳定性。氢对β-Ti-H晶体的理论屈服强度的影响与温度有关,在高温下氢提高了β-Ti晶体的理论屈服强度。更多还原

【Abstract】 Recently a great deal of attention from many material researchers has been given to the studies on thermohydrogen processing (THP) of titanium alloys and its theory. A great lot of investigations on the procedures of THP and mechanical properties, hot workability and microstructure of titanium alloys after hydrogenation have been performed and a large number of research findings devoted to those aspects have been obtained. However, the research work on the mechanisms of hydrogen-induced thermoplasticity of titanium alloys has lagged behind the THP technology, and especially the basic research on the effects of hydrogen in titanium from the electric and atomic scale which can reveal the mechanism of hydrogen-induced thermoplasticity from physical principle is scarce. In the present work, the crystal structures, energy properties, atomic diffusion characteristics, elastic moduli and mechanical properties of Ti-H system were investigated systematically by the first-principles plane-wave pseudopotential method and molecular dynamics (MD) simulations, and the mechanism of effects of hydrogen in titanium crystal is clarified from the electric and atomic scale.The occupation of hydrogen in Ti crystal is the basis of constructing Ti-H crystal model to calculate its properties, so firstly it was investigated by the first-principles method. The results show that the lattice distortion and volume expansion of bothα-Ti-H andβ-Ti-H crystals with hydrogen atoms at the octahedral sites are lower than the ones with hydrogen atoms at the tetrahedral sites, and the absolute values of heat of solution of hydrogen inα-Ti-H andβ-Ti-H crystals with H atoms at the octahedral sites are higher than the ones with hydrogen atoms at the tetrahedral sites, signifying that hydrogen is inclined to occupy the octahedral site in bothα-Ti andβ-Ti crystals. It can be found that in the case of hydrogen occupying the octahedral site inα-Ti, there is a charge transfer from 3p orbit of Ti atoms which are the nearest neighbors of hydrogen to 1s orbit of hydrogen atom, and the bonds between hydrogen atom and its nearest neighboring Ti atoms are covalent. Hydrogen atom changed the density of state of Ti atoms nearest neighboring it, resulting in reduction in the interaction between Ti atoms. In the case of hydrogen occupying the octahedral site inβ-Ti, H atom changed the density of state of Ti atoms around it and enhanced the interaction between Ti atoms. The results indicated that hydrogen weakened the interatomic bonding inα-Ti and enhanced the interatomic bonding ofβ-Ti.Interatomic potentials are the foundation and key element for the molecular dynamics simulation. The EAM (embedded-atom method) model for Ti-H system was established based on original EAM and combined with Johnson’s analytic EAM model. The expressions of potential functions and model parameters for Ti were determined according to the analytic EAM of Johnson. The new potential function expressions for H were provided and the model parameters were determined by fitting to the lattice constant, binding energy and bulk modulus of fcc metal hydrogen and the volume of dissolution and heat of solution of hydrogen inα-Ti.The hydrogen diffusion inα-Ti andβ-Ti were simulated by MD with EAM potentials for Ti-H system. The results show that the hydrogen diffusion inα-Ti is anisotropy and the calculated activation energy of hydrogen diffusion along c axis is smaller than that in the basal plane. The indirect O-T-O mechanism is most favorable for hydrogen diffusion in the basal plane ofα-Ti. The simulated values of activation energy and pre-exponential factor for hydrogen diffusion inα-Ti were in good agreement with the experimental data. The direct O-O mechanism is most favorable for hydrogen diffusion inβ-Ti and the calculated activation energy and pre-exponential factor for hydrogen diffusion inβ-Ti were in agreement with the experimental data.The effect of hydrogen on the Ti self-diffusion characteristics inα-Ti andβ-Ti were simulated by MD with EAM potentials for Ti-H system. The calculated activation energy for Ti self-diffusion inα-Ti-H is lower than that inα-Ti, and the calculated activation energy for Ti self-diffusion inβ-Ti-H is larger than the value inβ-Ti. The results indicated that hydrogen atom decreased and increased potential barriers for Ti self-diffusion inα-Ti andβ-Ti, resulting in the enhancement and weakening in the diffusivity for Ti inα-Ti andβ-Ti, respectively.The elastic moduli of Ti and Ti-H crystals were calculated. The results showed that the addition of hydrogen increased the bulk modulus and decreased the Young’s and shear moduli ofα-Ti. The calculated bulk molulus, Young’s modulus and shear modulus ofβ-Ti-H crystals with different hydrogen were all higher than those ofβ-Ti, that is, the addition of hydrogen increased elastic moduli ofβ-Ti.The deformation behaviors of Ti and Ti-H crystals were simulated by MD method. The results show that the calculated theoretical breaking strength along c axis and the theoretical yield strength ofα-Ti-H crystals with different hydrogen concentration were all lower than the values of pureα-Ti, and moreover, they decreased with increasing hydrogen concentration and temperature. For different simulation temperatures, the critical stress for phase transition inβ-Ti-H is higher thanβ-Ti. The results indicated that hydrogen improve the stability ofβ-Ti. The effect of hydrogen on the theoretical yield strength ofβ-Ti related to the temperatures. Hydrogen enhanced the theoretical yield strength ofβ-Ti at higher temperature.更多还原