【作者】 张宁；

【导师】 沈耀；

【作者基本信息】 上海交通大学 ， 材料学， 2010， 硕士

【摘要】 本文采用室温单轴拉伸、纳米压痕以及TEM等试验手段,对冷轧大变形状态下的纯度为99.7 %的工业纯铝,在退火前后力学性能和微观结构的变化,进行了较系统的研究。拉伸试验结果显示,大变形量(轧制量)的冷轧铝材在150℃退火30分钟后表现出了明显的退火强化现象,即退火后铝材的屈服强度比退火前(冷轧态)有显著提高,而延伸率下降。退火后铝材屈服强度的增量随轧制量的增加而增大,而较小的轧制量则可能导致传统的退火软化现象。若随后对退火试样再次进行冷轧变形,则屈服强度下降,延伸率上升。同时,利用循环弛豫试验测量了铝材具有典型意义的变形参数-表观激活体积和物理激活体积。试验结果表明,激活体积随轧制变形量的增加而减小,随退火温度的升高和退火时间的增加而增大。纳米压痕蠕变试验同样证实了退火强化现象的存在,即在相同的加载速率下,退火后的铝材总蠕变量小于未退火的铝材,而在相同的载荷与保载时间下,退火后的铝材压入深度要小于未退火的铝材。基于一系列实验结果,晶界位错源抑制强化被推测为细晶铝材出现退火强化的主要机理。TEM显微组织观察表明,退火后,细晶铝材的晶粒尺度没有明显长大,但可动位错密度显著减少,导致了铝材强度的增加和延伸率的降低。再次冷轧后,铝材中的可动位错密度增加,屈服强度降低。另外,试验测出细晶铝材的激活体积要比粗晶铝材小两个数量级,这表明细晶铝材位错运动的主要的热激活机理不同于粗晶铝材中通常的林位错切割机理。更多还原

【Abstract】 In this paper, the mechanical properties and micro-structural of commercial pure aluminum (99.7%) processed by Cold Rolling before and after the annealing were studied by uniaxial tensile testing, nanoindentation instrument and transmission electron microscopy.From the tensile testing experiments, the large equivalent strain aluminum appears obvious hardening after heat treatment and softening when subsequently deformation, which is in contrast to the typical behavior of coarse aluminum. Micro-structural investigation indicates that slight coarsening occurred during annealing and reduced the dislocation density, leading to an increasing in strength and a reduction in ductility. A subsequent cold rolling deformation may restore the dislocation structural and facilitate the yielding process when the aluminum is stressed. As a result, the strength decreases and the ductility increase. We predict that dislocation source-limited strengthening may play a key role in annealing hardening.A repeated stress relaxation test has been carried out to obtain the strain rate sensitivity, the apparent and physical activation volume of fine grain aluminum. The magnitude observed for these characteristic deformation parameters is very different from those of coarse-grained aluminum. This suggests that the thermally activated process in fine grain aluminum is different from the conventional forest dislocation cutting mechanism. From the very experiments, the heat treatment has impact on activation volumes, which implies the alteration of a microstructure in fine grain aluminum.Constant loading rate/load creep experiments were conducted on fine grain aluminum using nanoindentation instrument. The results indicated that the loading mode has an obvious effect on the nanoindentation creep properties. With increasing loading rate/load, total creep was increased.更多还原