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

本研究旨在研究晶界夹杂物对金属高温变形和断裂行为的影响。在实验部分，以晶界弥散分布的Ge；O_2和Gt；颗粒的铜多晶为模型系统，详细研究了高温变形和断裂特征。这种中温脆性的程度对应变速率和晶界Ge&lt；O2&Gt；粒子的尺寸都很敏感。断口的微观观察表明，塑性的初始下降与晶界滑移(GBS)的发生同时发生，而在较高温度下塑性的恢复是由于广泛的再结晶的开始。根据以上实验结果，得出结论：在中温范围内，滑动晶界Ge&O_2和Gt；颗粒附近的局部应力集中导致空洞的形成，空洞最终长大并聚合，导致沿晶断裂。在较高的温度下，再结晶导致应力集中松弛，导致延性恢复。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体机械(Gibbs)自由能的变化，研究了扩散控制的GBS和晶界上空洞的生长。结果表明，在不知道局部应力场的情况下，可以用简单统一的方法处理这些问题，并且GBS速率和孔洞扩展速率可以很容易地推导出来。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.