Development of elevated temperature structural alloys & intermetallics

有几类合金是为高温应用而设计的。本文主要研究了两类这类合金：单晶(SX)镍基高温合金和TiAl金属间化合物。前者实际上是唯一用于燃气轮机涡轮级叶片的材料。后者正在成为有用的高温材料。拟议中的SX研究代表着长期计划的第三阶段。前两个阶段研究了微观偏析和难熔元素对凝固结构的影响，并开发了创新的热处理方法来控制化学均化、溶解和初始熔化。拟议的第三阶段将通过确定元素相分配、伽马素形态和有害的拓扑紧密堆积相的形成对热和蠕变暴露的响应来研究作为耐火材料含量的函数的微观结构稳定性。这项研究将有助于了解固溶热处理的冶金控制机制及其对后续性能的影响，从而为大幅度缩短固溶热处理并改善后续性能控制提供机会，从而降低这些合金的加工成本，并在延长燃气轮机发动机的使用寿命期间改善性能。对TiAl的研究将利用这种材料可能发生的大规模转变，开发一种直接的淬火和回火工艺，产生具有适合往复式发动机排气门应用的微结构。本研究将考察各种工艺参数的影响，以确定块状转变和回火反应的动力学，并最终表征其热强度、蠕变和疲劳性能。这项研究将从根本上更好地了解TiAl的大规模转变，以及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.