Microstructural stability and creep of elevated temperature alloys

蠕变指的是随时间变化的应变--或在长时间加载期间缓慢的形状变化。蠕变变形和破坏困扰着许多工程材料的高温应用，如燃气轮机发动机部件和许多换热器结构。进行实验室蠕变测试相对简单，但理解控制蠕变的物理冶金因素作为应用温度和载荷的函数则更具挑战性。人们提出了几种解释金属晶体结构蠕变的理论，这些理论依赖于晶体缺陷和晶界的响应来解释蠕变过程中应变的累积。这项研究将系统地研究晶界结构对三种高温合金一次蠕变响应的影响。尤其是晶界结构将由“平面”形态转变为“锯齿状”形态。锯齿形边界通过阻碍沿垂直于主加载方向的边界上蠕变裂纹的扩展来延缓蠕变破坏。然而，关于晶界形态对一次蠕变行为的影响--应变累积速率不断降低的初始阶段--的了解很少。对于许多工程应用，主要蠕变的持续时间决定了部件的使用寿命。这项研究的结果将对控制工程合金蠕变的因素有更好的理解。这一认识可应用于抗蠕变性能提高的合金的微观结构设计。

Creep refers to time dependent strain - or slow shape change during extended periods of loading. Creep deformation and failure afflicts many elevated temperature applications of engineering materials, such as gas turbine engine components and many heat exchanger structures. Undertaking laboratory creep tests is relatively straightforward, but understanding the physical metallurgical factors controlling creep as a function of the applied temperature and load is much more challenging. Several theories to explain the creep of metallic crystalline structures have been developed which rely on the response of crystalline defects and crystal grain boundaries to explain the accummulation of strain during creep. The research proposed will systematically investigate the influence of grain boundary structure on the primary creep response of three alloys designed for use at elevated temperatures. In particular, the grain boundary structure will be modified from 'planar' to 'serrated' morphologies. It is reasonably well established that serrated boundaries delay creep failure by hindering the growth of creep cracks along boundaries that are normal to the main loading direction. However, very little is understood regarding the influence of grain boundary morphology on the primary creep behaviour - the initial period during which the rate of strain accummulation continuously decreases. For many engineering applications the duration of primary creep determines component service life. The result of this research will be an improved understanding of the factors controlling creep of engineering alloys. This understanding can be applied to the microstructural design of alloys with improved creep resistance.