Development of elevated temperature structural alloys & intermetallics

有几种合金是为高温应用而设计的。本研究的重点是两类合金：单晶（SX）镍基高温合金和钛铝化物（TiAl）金属间化合物。前者实际上是唯一用于燃气涡轮发动机涡轮级叶片的材料。后者正在成为有用的高温材料。拟议的SX研究是一项长期计划的第三阶段。前两个阶段研究了微偏析和耐火元素对凝固组织的影响，并开发了创新的热处理方法来控制化学均质化、溶解和初熔的发生。拟建的第三阶段将研究微观结构稳定性作为耐火材料含量的函数，通过确定元素相分配、γ素形态和有害拓扑紧密堆积相的形成对热和蠕变暴露的响应。这项研究将提供对控制固溶热处理及其对后续性能影响的冶金机制的理解，从而为大幅度缩短固溶热处理和改进后续性能控制提供机会，从而降低这些合金的加工成本，并在燃气涡轮发动机的长期使用期间改善性能。TiAl的研究将利用这种材料可能发生的巨大转变，开发一种直接的淬火和回火工艺，从而产生具有适合往复发动机排气阀应用性能的微观结构。本研究将研究各种工艺参数的影响，以确定大规模转变和回火反应的动力学，并最终表征热强度、蠕变和疲劳性能。这项研究将为TiAl的大规模转变提供更好的基础理解，并开发一种简单的铸造和易于应用的热处理工艺，以降低TiAl阀门的制造成本，帮助它们从实验室转移到道路车辆上。

Several classes of alloys are designed for elevated temperature applications. This research focuses on two classes of these alloys: single crystal (SX) nickel base superalloys and titanium aluminide (TiAl) intermetallics. The former are virtually the only materials used for blades in the turbine stages of gas turbine engines. The latter are emerging as useful high temperature materials. The proposed SX research represents the third phase of a long term program. The first two phases examined microsegregation and the impact of refractory elements on the solidified structure, and developed innovative heat treatments to control chemical homogenization, solutioning and onset of incipient melting. The proposed third phase will study microstructural stability as a function of refractory content by determining the response to thermal and creep exposure in terms of elemental phase partitioning, gamma prime morphology, and formation of harmful topologically close packed phases. This research will provide an understanding of the metallurgical mechanisms controlling solution heat treatments and the influence on subsequent properties and, thereby, offer the opportunity to substantially shorten solution heat treatments and improve subsequent property control, reducing the processing cost of these alloys and improving properties during extended service in gas turbine engines. The research on TiAl will utilize the massive transformation possible in this material to develop a straightforward quench and temper process that produces a microstructure with properties appropriate for application as reciprocating engine exhaust valves. This research will investigate the influence of various process parameters to determine the kinetics of the massive transformation and tempering reactions, and finally characterize the hot strength, creep and fatigue properties. The research will provide a better fundamental understanding of the massive transformation in TiAl, as well as develop a simple cast plus easy-to-apply heat treatment process to reduce the manufacturing cost of TiAl valves helping them move from the laboratory to road vehicles.