铌元素对TiAl合金微观裂纹扩展机理的影响

发布时间：2018-05-28 02:00

本文选题：铌 + γ-TiAl合金　；参考：《兰州理工大学》2017年硕士论文

【摘要】：航空航天领域要求材料具备高强度、低密度、耐高温、抗氧化等综合性能。TiAl合金具备优良的高温性能,广泛应用于航空工业领域,其发展呈现多样化,提高Nb含量,较低Al含量,形成Ti-Al-Nb三元体系是TiAl合金的主要发展趋势。提高TiAl基合金比强度、热变形能力、室温塑性和抗氧化性的有效手段是加入Nb元素,因此向合金中添加铌元素成为目前材料工程领域关注和研究的热点。本文在微观尺度下通过分子动力学理论研究了Nb在TiAl合金中对裂纹扩展的影响。得出的研究结果如下:(1)研究了单晶γ-TiAl合金与含铌γ-TiAl合金对裂纹扩展的影响。Nb对γ-TiAl性能的影响较大,含有3%Nb和不含Nb的γ-TiAl合金中裂纹扩展形式差异很大,加入铌元素后,在相同的应变率条件下,试件的断裂时间延长,能量变化曲线有两个波峰并且出现明显上下波动的现象;随着应变增加,应力先增大后逐渐减小;裂纹扩展缓慢,形成的断面不平滑,而且裂纹扩展的形态发生变化。(2)分析了铌含量对γ-TiAl合金裂纹扩展的影响。研究了铌含量分别在2%,4%,6%,8%的γ-TiAl合金裂纹扩展规律,发现铌元素的加入提高了合金的强度、韧塑性;随着铌元素含量的增加,单晶γ-TiAl合金原子之间的内聚力增加,合金强度增加,说明Nb含量的增加使γ-TiAl合金的延展性增强,合金稳定性变好。(3)研究了温度对含铌γ-TiAl合金裂纹的扩展规律。分析了四种不同温度下裂纹扩展规律及相互影响,研究结果表明:随着温度的升高,裂尖发射位错的频率越高,裂尖的钝化程度也越严重,试件的断裂时间延长;温度的升高导致体系的屈服应力减小,材料越容易发生塑性变形;由于塑性变形的发生阻碍了裂纹向前的扩展,裂纹的扩展越缓慢。(4)分析了裂纹位置对含铌γ-TiAl合金裂纹的扩展规律。研究微观尺度下裂纹相对位置对3%铌含量的单晶γ-TiAl合金裂纹扩展过程的影响,研究结果表明:中心裂纹的γ-TiAl合金在其拉伸初始阶段,受力并不集中,随后由于原子键的断裂形成了孔洞,孔洞部位抑制裂纹的扩展,因此裂纹要继续扩展需要克服更大的阻力。裂纹在中心位置和边界位置对γ-TiAl合金产生的力学影响不同,边界裂纹对材料产生断裂的危害性更大。
[Abstract]:The aerospace field requires the material to have high strength, low density, high temperature resistance, oxidation resistance and other comprehensive properties. Tial alloy has excellent high temperature properties. It is widely used in aviation industry. The formation of Ti-Al-Nb ternary system with lower Al content is the main developing trend of TiAl alloy. The effective means to improve the specific strength, thermal deformation ability, room temperature plasticity and oxidation resistance of TiAl based alloys are to add NB elements into the alloys. Therefore, adding niobium to the alloys has become a hot topic in the field of material engineering. In this paper, the effect of NB on crack growth in TiAl alloy has been studied by molecular dynamics theory at the micro scale. The results obtained are as follows: (1) the effect of single crystal 纬 -TiAl alloy and NB 纬 -TiAl alloy on the crack growth properties of 纬 -TiAl alloy is studied. The effect of NB on the crack growth properties of 纬 -TiAl alloy is larger than that of 纬 -TiAl alloy with or without NB. The crack growth forms of 纬 -TiAl alloy containing 3%Nb and NB are greatly different. Under the same strain rate condition, the fracture time of the specimen is prolonged, and the energy variation curve has two peaks and obviously fluctuates up and down. With the increase of strain, the stress increases first and then decreases gradually, and the crack grows slowly. The effect of niobium content on crack growth of 纬 -TiAl alloy was analyzed. The crack propagation law of 纬 -TiAl alloy with NB content of 2% and 6% is studied. It is found that the addition of niobium improves the strength and ductility of the alloy, and with the increase of niobium content, the cohesion between atoms increases, and the strength of the alloy increases with the increase of niobium content. The results show that the ductility of 纬 -TiAl alloy is enhanced with the increase of NB content, and the stability of 纬 -TiAl alloy is improved. The law of crack propagation and its interaction at four different temperatures are analyzed. The results show that the higher the frequency of the emission dislocation, the more serious the passivation of the crack tip and the longer the fracture time of the specimen with the increase of temperature. The higher the temperature, the smaller the yield stress of the system, and the more likely the plastic deformation of the material is, because the occurrence of plastic deformation hinders the propagation of the crack forward. The slower the crack propagation is, the more the crack growth law of the 纬 -TiAl alloy containing niobium is analyzed. The effect of the relative crack position on the crack propagation process of 3% niobium content single crystal 纬 -TiAl alloy is studied. The results show that the stress of the center cracked 纬 -TiAl alloy is not concentrated during the initial tensile stage. Then because of the fracture of the atom bond the hole is formed and the crack propagation is restrained at the hole position. Therefore the crack growth needs to overcome the greater resistance to continue the crack propagation. The mechanical effect of crack on 纬 -TiAl alloy is different in the center and boundary position, and the boundary crack is more harmful to the fracture of the material.
【学位授予单位】：兰州理工大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TG146.23

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