【作者】 胡琦；

【导师】 张青川；

【作者基本信息】 中国科学技术大学 ， 固体力学， 2011， 博士

【摘要】 Portevin-Le Chatelier(PLC)效应是指在一定的应变率和温度范围内,许多工程材料中会出现的一种不规则的塑性流动。其表现为连续的应力-时间曲线上的锯齿形起伏和应变-时间曲线上的阶梯状上升。而在空间上,这种不规则的塑性流动导致了应变局域化现象,表现为在试件表面上出现的静止的、跳跃的或连续传播的局部变形带。PLC效应是一种典型的多尺度效应问题,长期得到学术界的重视。通常都认为微观上溶质原子和可动位错之间的相互作用导致了该塑性失稳现象的发生,即动态应变时效。铝合金材料具有的较高的强度重量比使得其非常适合应用于汽车工业中。但铝合金在室温变形时就会发生PLC效应,这导致试件上几何扰动的增加以及颈缩应变的减小,从而会导致材料可成形性下降,制约了其在汽车部件中的使用。所以,开展铝合金PLC效应的研究具有重要的理论及工业应用价值。在溶质原子含量高于极限溶解度的合金材料中,多余的溶质原子将以析出相的形式存在。本文首先研究了在相同退火热处理条件下,不同析出相含量的两种铝镁合金5456和5052在不同加载应变率下的PLC效应。研究发现,析出相的存在增强了对可动位错的阻碍作用,析出相含量越大,对PLC效应产生影响越显著。其次,通过对5456和5052两种铝镁合金在相同的温度下保温时效不同的时间,使得溶质原子含量与析出相的含量同时发生变化。在相同的加载应变率下研究了溶质原子和析出相含量的变化对铝镁合金中的PLC效应所产生的影响,并比较了同样的实验条件下的2024铝铜合金中的溶质原子和析出相含量的变化对PLC效应所产生的影响。结果表明,在铝镁合金中,析出相含量的增加弥补了由于溶质原子的减少所带来的影响,溶质原子和析出相在PLC效应中所起到的作用相等。而在铝铜合金中,PLC效应中起主导作用的是溶质原子,析出相并未能弥补铜溶质原子含量减少所带来的影响。根据动态应变时效理论,通过施加不同的预变形,造成5456铝镁合金中的初始位错密度发生改变,研究了不同的位错密度对PLC效应产生的影响。研究中发现预变形对PLC效应的影响显著,而且随着预变形量的增加,临界应变出现反常的变化。最后在预变形实验的基础上,通过沿着试件的不同的方向进行的拉伸实验,研究了不同的晶粒取向对PLC效应产生的影响。结果表明,除了临界应变之外,各方向拉伸曲线上未见明显的区别,表现出各向同性。理论研究方面,基于动态应变时效过程中的热激活机制,综合考虑了溶质原子与位错的相互作用,位错与位错的相互作用以及伴随PLC效应的试件表面温度变化的影响,建立了一个宏观唯象模型。结合热传导方程,将该模型用于5456铝镁合金中,伴随着PLC效应出现的锯齿形屈服现象以及试件表面出现的温度变化现象的模拟研究中,很好的再现了红外测温实验中所观察到的PLC效应。由于模型中考虑了弹性变形的影响,PLC带出现时带外的弹性收缩现象也得到了很好的再现。模拟结果表明,相关的模型参数是率相关的,并且试件本身的温升对流动应力产生的影响很小。更多还原

【Abstract】 Many engineering materials exhibit irregular plastic flow, which is referred to as Portevin-Le Chatelier(PLC) effect, in limited regimes of strain rate and temperature. The PLC effect manifests itself as temporal continuous stress serrations on stress-time curves or strain staircase behavior on strain-time curves respectively. Furthermore, the irregular plastic flow results in inhomogeneous deformation with various localization bands. These bands can be static, hopping and sometimes propagating along the specimen. The PLC effect is a kind of typical multi-scale effect and has been increasingly attractive to researchers since its discovery. The unstable plastic flow is generally understood as the consequences of solute-dislocation interaction at the microscopic level, namely, dynamic strain ageing (DSA). Aluminum alloy materials are very suitable for application in automobile industry due to the high strength-weight ratio. However, the PLC effect usually occurs at room temperature for aluminum alloy, which leads to the increasingly geometric perturbation in specimen and decreasingly necking strain. Thus, the material formability decreases, restricting its use in the car parts. Thus, the research on the PLC effect in aluminum alloy is of important theoretical and industrial application significance.In the alloy with solute content higher than the limiting solubility, the solute atoms failed to dissolve in matrix will precipitate from the solid solution and form precipitations. At first, the PLC effects in annealed 5456 and 5052 Al-Mg alloys with different precipitation contents are investigated under different applied strain rates respectively. The results indicate that precipitations strengthen the impediment to the motion of dislocations. The greater the contents of precipitations become, the more pronounced the influences on the PLC effect become.Secondly, 5456 and 5052 Al-Mg alloys are aged at the same temperature for different time, making the content of solute atoms and precipitations changing together. Then, tensile experiments under the same applied strain rate are carried out for the two alloys to investigate the effect of the simultaneous changes in the content of solute atoms and precipitations on the PLC effect. Also, this investigation is carried out for the 2024 Al-Cu alloy in the same way and the experimental results are compared with those in the Al-Mg alloys. The findings indicate that the increasing contents of precipitations make up the influences from the decreasing solute atoms. The effects of solute atoms and precipitations on the PLC effect are equivalent in Al-Mg alloy. However, in Al-Cu alloy, the effect of solute atoms is dominant in the PLC effect. The precipitations fail to make up the influences from the decreasing Cu solute atoms.In accordance to the DSA mechanism, the dislocation densities in 5456 Al-Mg alloy are changed by different prestrain magnitudes, and the effect of prestrain on the PLC effect is investigated. The experimental results indicate that the prestrain affects the PLC effect distinctly. The critical strain shows an abnormal evolution as well.Based upon the experiment of prestrain, tensile experiments are carried out along the different lattice directions in 5456 Al-Mg alloy. The findings show that the PLC effect does not present obvious difference with the lattice directions except the critical strain.A macroscopically phenomenological model is presented for the PLC effect based upon the DSA mechanism. The model takes into account the competition between the mobile dislocations and solute atoms, the interaction between dislocations, and the effect of temperature changes. Combined with a heat conduction equation, the model is finally used in the numerical simulations of the stress-strain curves and the temperature evolutions associated with the PLC effect in 5456 Al-Mg alloy. The numerical results reproduce the experimental findings in an infrared pyrometry experiment. The elastic shrinkage outside of deformation band is simulated as well by the consideration of elastic deformation in the model. It shows that the relevant parameters used in the simulation are rate-dependent and the influence on the flow stress from the temperature increment in specimen is minor.更多还原