Microstructural stability and creep of elevated temperature alloys

蠕变是指在长时间加载过程中随时间变化的应变或缓慢的形状变化。蠕变变形和破坏困扰着许多工程材料的高温应用，例如燃气涡轮发动机部件和许多热交换器结构。进行实验室蠕变试验相对简单，但了解控制蠕变的物理冶金因素作为施加温度和载荷的函数则更具挑战性。几种解释金属晶体结构蠕变的理论已经发展起来，它们依赖于晶体缺陷和晶界的响应来解释蠕变过程中应变的积累。提出的研究将系统地研究晶界组织对三种设计用于高温的合金的初次蠕变响应的影响。特别是，晶界结构将从“平面”形态改变为“锯齿”形态。可以很好地确定，锯齿状边界通过阻止蠕变裂纹沿与主加载方向垂直的边界扩展来延缓蠕变破坏。然而，对于晶界形貌对初始蠕变行为（应变累积速率持续下降的初始阶段）的影响，人们知之甚少。在许多工程应用中，主要蠕变的持续时间决定了部件的使用寿命。这项研究的结果将提高对控制工程合金蠕变的因素的认识。这种认识可以应用于提高抗蠕变性能的合金的显微组织设计。

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.