The Effects of Grain-Boundary Inclusions on High-Temperature Fracture of Metals

本课题旨在研究晶界夹杂物对金属高温变形和断裂行为的影响。本课题实验部分以晶界上分散有Ge <O_2>颗粒的铜多晶为模型体系，对其高温变形和断裂特性进行了详细研究。当温度从室温升高到900 K时，延展性首先降低，在600 K左右达到最小值，然后再次增加。这种中温脆化程度对应变速率和晶界处Ge <O_2>颗粒的尺寸都很敏感。断口表面的显微观察表明，塑性的初始降低与晶界滑动（GBS）的发生同时发生，高温下塑性的恢复是由于广泛的再结晶的开始。由上述实验结果可知，在中温范围内，局部应力集中在滑动晶界处的Ge <O_2>颗粒附近，引起孔洞的形成，孔洞最终长大并聚并导致晶间断裂。在较高温度下，应力集中因再结晶而松弛，从而恢复延性。作为该项目的理论部分，通过考虑包含晶界或空洞的系统的总力学（吉布斯）自由能的变化，研究了扩散控制的GBS和晶界上空洞的生长。发现这些问题可以在不知道局部应力场的情况下进行简单统一的处理，并且可以很容易地推导出GBS速率和空隙生长速率。从这一理论研究中获得的知识被认为对理解高温变形和断裂行为的各个方面非常有用。

This research project is aimed to examine the effects of grain-boundary inclusions on high-temperature deformation and fracture behavior of metals.For the experimental part of the project, copper polycrystals with dispersed Ge <O_2> particles on grain boundaries were used as a model system and high-temperature deformation and fracture characteristics were investigated in detail.As temperature increases from room temperature to 900 K, ductility first decreases with a minimum at around 600 K and then increases again. The degree of such intermediate-temperature embrittlement is sensitive to both strain rate and the size of the Ge <O_2> particles at grain boundaries. Microscopic observation of fracture surfaces has revealed that the initial decrease of ductility occurs concurrently with the occurrence of grain-boundary sliding (GBS) and that the recovery of ductility at higher temperatures is due to the initiation of extensive recrystallization.From the above experimental results, it is concluded that at the intermediate temperature range, local stresses which concentrate near the Ge <O_2> particles at the sliding grain boundary induce the formation of voids which eventually grow and coalesce to cause intergranular fracture. At higher temperatures, the stress concentration is relaxed by the recrystallization, resulting in the recovery of ductility.As the theoretical part of the project, diffusion-controlled GBS and the growth of voids on grain boundaries were examined by considering the change in the overall mechanical (Gibbs) free energy of a system containing a grain boundary or a void. It is found that these problems can be treated in a simple unified manner without knowing local stress fields and that the GBS rate and the void growth rate can be derived very easily. The knowledge derived from this theoretical study is considered to be very useful in understanding various aspects of high-temperature deformation and fracture behavior.