Assessment of the eutectic thorugh and properties of a multiphase alloy in the NiAl-Cr-W system
- Autores: Arcadio Varona Caballero
- Directores de la Tesis: Srdjan Milenkovic (dir. tes.)
- Lectura: En la Universidad Carlos III de Madrid ( España ) en 2016
- Idioma: español
- Tribunal Calificador de la Tesis: Frank Stein (presid.), Mónica Campos Gómez (secret.), Easo George (voc.)
- Programa de doctorado: Programa de Doctorado en Ciencia e Ingeniería de Materiales por la Universidad Carlos III de Madrid
- Materias:
- Enlaces
- Tesis en acceso abierto en: e-Archivo
- Resumen
Ordered intermetallic compounds have gained attention over the past decades in the research community due to their interesting combination of properties, which make them attractive candidates as potential structural materials for high temperature applications. Especially promising are the properties of metal-aluminides, which include high melting point, high thermal conductivity, high yield strength, low density, and excellent oxidation resistance due to the formation of alumina scales.
Among them, NiAl was pointed out at first as the most promising candidate for higher temperatures. However, its low ductility and fracture toughness at room temperature, as well as low strength and creep resistance at high temperatures, reduced the interest for this material. In spite of this, further research has been carried out in order to improve the drawbacks, and in situ composites of NiAl-based eutectic alloys like NiAl-Cr, NiAl-W, etc., have brought the best results. Even so, binary eutectic alloys present an inherent limitation, as the composition is fixed, and thus, the volume fraction of reinforcement phase is fixed as well.
In this work, an approach to overcome this limitation is explored. By the introduction of another specific element, it is possible to obtain a NiAl-based fully-eutectic in situ composite with tuneable reinforcement volume fraction. This is because fixed-composition binary eutectics form eutectic troughs in the ternary phase diagram, in which the composition and volume fraction of the reinforcement phase is variable. This approach was studied in the NiAl-Cr-W system.
Due to the large differences between the elements in the system, the production and processing of the alloys had to be improved. A prealloy production route was investigated, which yielded satisfactory results in casting and directional solidification.
Through the measurement of the composition of the eutectic constituent present in the alloys, the eutectic trough in the pseudo-ternary phase diagram was initially assessed. The results showed a discontinuous eutectic trough, in which two sections in the Cr-rich and Cr-lean regions were assessed, and a gap between both was acknowledged.
The assessment was enhanced by thorough analysis of the path followed during solidification of the alloys, mainly by further characterization of their microstructure and phases composition. Complementary computer-aided calculations were also used. The study revealed that the discontinuous eutectic trough occur due to the existence of a non-reported reaction line in the pseudo-ternary system. This reaction line forms owing to the high temperature miscibility gap present in Cr-W phase diagram, and extends through the pseudo-ternary diagram as a peritectic reaction. The interaction between the eutectic trough sections and this new reaction line is the cause of the discontinuity, forming a point of class II four-phase equilibria, or U-type reaction in the pseudo-ternary system. As well, the effect of the quaternary nature of the system in the pseudo-ternary approach used in the investigation was studied, and the method validity was confirmed.
Additionally, the high temperature behavior of multiphase NiAl-20Cr-4.5W (in at.%) alloy was studied, and the results revealed that the alloy exhibit high temperature properties lower, but comparable, to that of the most advanced NiAl-based alloys developed so far. These results are encouraging for further research in this and other systems with a similar approach, as the tested alloy does not possess an optimized microstructure.
