【作者】 武建；

【导师】 宋申华；

【作者基本信息】 哈尔滨工业大学 ， 材料学， 2010， 博士

【摘要】 溶质元素(杂质或合金元素)在晶界上的偏聚对工程材料的力学行为有着深刻的影响,多年来一直是冶金工作者和材料学工作者感兴趣的问题。溶质原子的晶界偏聚可分为平衡晶界偏聚和非平衡晶界偏聚。对平衡偏聚的研究起步比较早,理论趋于成熟,但对非平衡偏聚的研究目前还存在很多空白和未知的领域,尤其是应力作用引起的非平衡晶界偏聚。以前研究的晶界偏聚行为主要是无应力状态下的,但材料在服役过程中通常要受到应力的作用。因此,研究应力引起的非平衡晶界偏聚对工程实践具有更重要的意义。Cr-Mo低合金结构钢由于其优异的高温力学性能而广泛应用于电力、核能及石油化学工业。然而,当材料在高温高压环境中服役时,P、S、Sn和Sb等杂质元素将在晶界偏聚,使晶界脆化,导致材料的断裂韧性降低,韧-脆转变温度升高。晶界脆化降低了材料的服役性能,使其更容易发生突然的脆性断裂,引起严重的事故。P是钢中一种典型的晶界致脆元素,研究钢中P在高温高压条件下的偏聚行为不仅可以用来指导工程实践,还可以完善晶界偏聚理论。鉴于2.25Cr1Mo钢是最广泛使用的Cr-Mo低合金结构钢之一,本文研究了该钢中P的平衡晶界偏聚和应力引起的非平衡晶界偏聚。对于平衡晶界偏聚,将试样经过980℃淬火和650℃回火后,在480、520和560℃分别时效不同的时间,然后用俄歇电子能谱仪测量P和Mo的晶界浓度。利用Seah模型对不同条件下的P和Mo的晶界浓度进行分析,得到P和Mo的偏聚自由能分别为38和17kJ/mol;P-Mo相互作用系数很小,它们之间的相互作用很弱。探讨了P晶界浓度与韧-脆转变温度之间的关系。将不同时效条件下的试样进行冲击试验并用扫描电子显微镜对断口表面进行观察,通过断口形貌分析得到韧-脆转变温度(断口形貌转变温度)。通过比较不同条件下的韧-脆转变温度与P晶界浓度数据发现,韧-脆转变温度与P晶界浓度之间存在一个线性关系。利用该线性关系,结合平衡晶界偏聚动力学模型,建立了一个温度-时间脆化图。利用该图,可以预测任意温度下经过任意时间后的韧-脆转变温度。探讨了P晶界偏聚对沿晶断裂的影响,发现P晶界偏聚促进沿晶断裂,并且随着冲击试验温度的降低,断口依次呈现韧性断裂、沿晶断裂和解理断裂。对于应力引起的非平衡晶界偏聚,将试样经过980℃淬火和650℃回火后,在520℃无应力时效1000h使晶界浓度达到热力学平衡,然后在40、200和350MPa拉应力下分别时效不同的时间,用俄歇电子能谱仪测量P晶界浓度,得到P偏聚动力学。40MPa拉应力引起的偏聚动力学有一个偏聚峰;350MPa拉应力引起的偏聚动力学有两个偏聚峰;200MPa拉应力引起的偏聚动力学有两个偏聚峰和一个贫化谷。同时测量了这些条件下的蠕变曲线,40MPa拉应力下几乎不发生蠕变,200和350MPa拉应力下发生了蠕变,且350MPa拉应力下的蠕变更加明显。综合分析后认为在应力时效过程中,弹性变形和蠕变变形共同影响P的偏聚动力学。通过考虑空位流和复合体流,建立了一个低应力引起的非平衡晶界偏聚动力学模型。利用该模型,可以预测不同条件(应力、温度、晶粒尺寸、空位迁移能和复合体的迁移能等)下的偏聚动力学。模拟了40 MPa拉应力下的P偏聚动力学,结果与实验符合良好。本工作的完成促进了晶界偏聚和晶界脆化现象的认识,进一步完善了晶界偏聚理论,对工程实践具有重要的指导意义。更多还原

【Abstract】 Solute (impurity or alloying elements) segregation at grain boundaries has an important effect on the mechanical behavior of engineering materials. It has been an interesting topic to metallurgists and material engineers. Solute segregation at grain boundaries may be clarified into equilibrium segregation and non-equilibrium segregation. Studies of equilibrium grain boundary segregation have started long before, and its theory has approached almost perfect. However, there are many things unknown about the non-equilibrium grain boundary segregation, especially the stress-induced non-equilibrium segregation. The segregation behaviors studied before were mainly in a non-stress state. However, the materials in service are usually subject to an applied stress, and thus the research into stress-induced non-equilibrium grain boundary segregation is more important in engineering practice.Due to their excellent high-temperature mechanical properties, Cr-Mo low alloy structural steels are widely used in the power and petrochemical industries. Nevertheless, when the materials serve in a high temperature and pressure environment, impurities, such as phosphorus, sulfur, tin and antimony, would segregate to the grain boundary, making it embrittled. The fracture toughness of the materials is lowered, and the ductile-to-brittle transition temperature (DBTT) is increased. Grain boundary embrittlement deteriorates the service performance, making the materials fracture intergranularly, which may cause disasters. Since phosphorus is a typical grain boundary embrittling element in steel, investigation into the grain boundary segregation of phosphorus in a high temperature and pressure condition can not only guide the engineering practice, but also improve the grain boundary segregation theory. Owing to the fact that 2.25Cr1Mo steel is one of the most widely used Cr-Mo steels, equilibrium and stress-induced non-equilibrium grain boundary segregations of phosphorus in this steel were examined.For equilibrium grain boundary segregation, the samples were quenched at 980oC, tempered at 650oC, and subsequently aged at 480, 520 and 560oC for different times, followed by Auger electron spectroscopy measurements of phosphorus and molybdenum grain boundary concentrations. With the use of Seah’s model, the thermodynamics of phosphorus and molybdenum were analyzed. The free energies of segregation of phosphorus and molybdenum were approximately 38 and 17 kJ/mol, respectively, and the interaction between them was very weak in segregation.The relationship between grain boundary concentration of phosphorus and DBTT was explored. The samples in different ageing conditions were impact fractured and the resulting fracture surfaces were analysed using scanning electron microscopy. The DBTT was obtained, characterized by fracture appearance transition temperature. It was found that there is a linear relationship between DBTT and phosphorus boundary concentration. By use of this relationship in conjunction with the kinetic model of equilibrium grain boundary segregation, a temperature-time embrittlement diagram was established. From the diagram, the DBTT of the sample aged for any time at any temperature can be predicted. The effect of phosphorus boundary segregation on intergranular fracture was explored, indicating that phosphorus boundary segregation facilitated the intergranular fracture and the fracture modes were ductile fracture, intergranular fracture and cleavage fracture when the test temperature goes from high to low levels.For stress-induced non-equilibrium grain boundary segregation, after quenching at 980oC and tempering at 650oC, the samples were aged for 1000h at 520oC without stress to enable the boundary concentration to reach themal equilibrium. Subsequently, the samples were stress aged for different periods of time and the stress levels were 40, 200 and 350 MPa, respectively. The phosphorus boundary concentration was measured using Auger electron spectroscopy so as to obtain the segregation kinetics. For the 40 MPa stress ageing, there was one segregation peak over its equilibrium segregation level. For the 350 MPa stress ageing, there were two segregation peaks over the equilibrium segregation level. For the 200 MPa stress ageing, there were two segregation peaks over the equilibrium level, and between them there was a depletion trough below the equilibrium level. It was proposed that elastic deformation and creep deformation could both affect the phosphorus segregation kinetics during stress ageing.Based on the fluxes of vacancies and complexes, a kinetic model of stress induced non-equilibrium grain boundary segregation was established. The segregation kinetics of phosphorus during stress ageing were predicted under different conditions such as stress, temperature, grain size, and migration energies of vacancies and complexes. The phosphorus segregation kinetics under 40 MPa tensile stress was simulated, and model predictions were well consistent with the experimental results.The outcomes of the present work may promote understanding of grain boundary segregation and embrittlement, and improve the segregation theory. Accodingly, it is of importance to engineering practice.更多还原