Heat-treatable Chromium-based alloys for extrem temperatures

本研究的目的是开发一种新的Cr- si - ge合金，该合金在高温下具有完全溶解度。热处理循环可以控制微观结构。所需的微观结构由Cr固溶体基体和金属间a15相析出组成。在微观结构方面，将对析出行为和A15/基体界面以及机械和物理性能（特别是蠕变和抗氧化性）进行研究。为了获得高A15/基体相比和增强的性能，将优化热处理参数。在第二步，将使用两种主要方法评估Cr-Si-Ge体系的合金化潜力。首先是添加替代合金元素（在A15和Cr中），它提供固溶体强化并取代A15相中的铬。钼与纯铬及A15-Cr3Si相均表现出较高的合金化潜力。钼合金铬（含高达9 % Mo）显示完全溶解度在所有温度范围内直至熔点。合金中Mo含量高达0.2%，使纯铬的抗蠕变性能提高了一个数量级，并提高了室温延展性。结果表明，Cr3Si与Mo的合金化温度可达30at。%)通过SiO2的形成改善了保护，从而提高了在宽温度范围内的抗氧化性。此外，关于抗氧化性，Mo是有趣的，因为与Cr相比，Mo在氧化过程中显示出氧化物残留，可以局部增加Si或Ge活性。这是需要调查的。第二种方法是添加一种元素形成A15相，它也可以代替Si和Cr，并且在铬固溶体中表现出高温依赖性的溶解度。Cr-Pt体系表现出类似于由固溶体和A15-Cr3Pt相场组成的Cr-Si和Cr-Ge体系的二元组成。该体系在富铬地区提供了所需的陡峭溶解线。目的是研究A15沉淀的可溶性，以及如何通过添加铂来增加和最终稳定它们的体积分数。因此，有必要研究A15析出相与固溶体基体之间的界面特性。该系统的潜在应用是直接作为优化合金或金属基体，可以通过氧化物分散强化进一步增强，用于涡轮应用以及需要在高温下具有极端耐腐蚀性的化学工业。

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.