Heat-treatable Chromium-based alloys for extrem temperatures

本研究旨在开发在高温下具有完全固溶性能的新型可热处理的铬硅锗合金。热处理循环允许控制微观结构。所需的显微组织由铬固溶体母体和金属间化合物A15相组成。将从微观结构方面研究析出行为和A15/基体界面以及机械和物理性能(特别是蠕变和抗氧化性)。将优化热处理参数，以获得高的A15/基质相比和增强的性能。在第二步中，将使用两种主要方法来评估Cr-Si-Ge系统的合金化潜力。第一种是添加替代合金元素(在A15和Cr中)，它提供固溶强化以及替代A15相中的铬。钼在纯铬和A15-Cr3Si相中表现出很高的合金化电势。钼合金化铬(钼含量高达9at%)在熔点之前的所有温度范围内都表现出完全的溶解性。Mo含量高达0.2at%的合金可将纯铬的蠕变抗力提高一个数量级，并提高室温延展性。此外，在Cr3Si中添加Mo(高达30at.%)可通过SiO_2的形成改善保护作用，从而在较宽的温度范围内提高抗氧化性。此外，关于抗氧化性，钼很有趣，因为与铬相比，钼在氧化过程中表现出氧化剥离，这可以局部增加Si或Ge的活性。这件事将被调查。第二种方法是添加一种形成A15相的元素，它也可以替代Si和Cr，并在铬固溶体中表现出随温度变化的溶解度。铬-铂系统表现出类似于由固溶体和A15-Cr3PT相场组成的铬-硅和铬-锗系统的二元组成。该系统在富铬区提供了所需的陡峭的溶剂线。它的目的是研究A15沉淀物的溶解程度，以及如何通过添加铂来增加并最终稳定它们的体积分数。因此，有必要研究A15析出物与固溶体之间的界面性质。该系统的潜在应用是直接作为优化的合金或作为金属基质，可通过氧化物分散强化进一步增强，并用于涡轮应用以及在高温下要求极高耐腐蚀性的化学工业应用。

This study aims to develop new heat treatable Cr-Si-Ge alloys with Cr > 90 at%, which show complete solubility at high temperatures. The heat treatment cycle allows control of the microstructure. The desired microstructure consists of a Cr solid solution matrix and intermetallic A15-phase precipitates. The precipitation behaviour and A15/Matrix interface as well as mechanical and physical properties (especially creep and oxidation resistance) will be investigated with regard to microstructure. The heat treatment parameters will be optimized in order to obtain a high A15/matrix phase ratio and enhanced properties. In the second step, the alloying potential of the Cr-Si-Ge system will be evaluated using two main approaches. First is the addition of a substitutional alloying element (in both A15 and Cr) which provides solid solution strengthening as well as substituting chromium in the A15 phase. Molybdenum shows a high alloying potential with pure chromium as well as in the A15-Cr3Si phase. Mo-alloyed chromium (with up to 9 at% Mo) shows complete solubility in all temperature ranges up to the melting point. Alloying with up to 0.2 at% Mo improves the creep resistance of pure chromium to an order of magnitude and increases the room temperature ductility. In addition, it is shown that alloying Cr3Si with Mo (up to 30 at.%) enhances the oxidation resistance in a broad temperature range by improved protection through SiO2 formation. Also, concerning the oxidation resistance, Mo is interesting because in comparison to Cr, Mo shows oxide pesting during oxidation which can locally increase the Si or Ge activity. This is to be investigated. The second approach is the addition of an element that forms the A15 phase, which can also substitute Si and Cr, and shows a high temperature dependent solubility in chromium solid solution. The Cr-Pt system shows an analogous binary composition as the Cr-Si and Cr-Ge systems composed of a solid solution and an A15-Cr3Pt phase field. The system provides the required steep solvus line in the Cr-rich region. It is aimed to study how soluble to A15 precipitates are and how their volume fraction can be increased and eventually stabilized through the addition of platinum. Therefore, it is necessary to study the interface properties between the A15 precipitates and the solid solution matrix. Potential applications of this system are directly as an optimized alloy or as a metal matrix, which might be further enhanced via oxide dispersed strengthening and used for turbine applications as well as applications in the chemical industry, where extrem corrosion resistance is required at elevated temperatures.