Publication: Assessment of the eutectic thorugh and properties of a multiphase alloy in the NiAl-Cr-W system
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2016-09
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2016-09-23
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Abstract
Los compuestos intermetálicos ordenados han ganado atención en la
comunidad científica durante las últimas décadas, debido a la interesante
combinación de propiedades que presentan, siendo candidatos perfectos como
materiales estructurales para aplicaciones de alta temperatura. Especialmente
prometedoras son las propiedades de los aluminuros metálicos, entre las que se
cuentan: un alto punto de fusión, alta conductividad térmica, alto límite elástico,
baja densidad, y una excelente resistencia a la oxidación, debido a la formación
de alúmina en la superficie.
De entre ellos, el NiAl fue señalado como el candidato más prometedor para
mayores temperaturas. Sin embargo, su baja ductilidad y tenacidad a fractura
a temperatura ambiente, así como su baja resistencia y resistencia a fluencia
a alta temperatura, hicieron que se redujese el interés por este material. A
pesar de ello, se continuó investigando para superar estas desventajas, y los
materiales compuestos in situ de eutécticos de NiAl, como son NiAl-Cr, NiAl-W,
etc., han mostrado los mejores resultados. Aún así, las aleaciones eutécticas
binarias tienen una limitación intrínseca, ya que la composición está fijada, y
por ello también lo está la fracción volumétrica de la fase reforzante.
En este trabajo se exploró una manera de abordar esta limitación. A través
de la introducción de un elemento específico, es posible obtener un material
compuesto in situ eutéctico de NiAl, en el que la fracción volumétrica de la fase
reforzante sea ajustable. Esto se debe a que los eutécticos binarios forman
líneas eutécticas en el diagrama de fases ternario, en las que la composición y
la fracción volumétrica de la fase reforzante es variable. Este planteamiento fue
aplicado al sistema NiAl-Cr-W.
Debido a la gran diferecia entre los elementos de dicho sistema, el procesado
de las aleaciones tuvo que ser perfeccionado. Se investigó la producción y
uso de prealeaciones como ruta de procesado, lo cual derivó en resultados
satisfactorios tanto en colada como en solidificación direccional.
Mediante la medida de la composición del microconstituyente eutéctico en las
aleaciones, se evaluó de manera preliminar la línea eutéctica en el diagrama de fases pseudo-ternario. Los resultados indicaron la existencia de una línea
eutéctica discontinua, de la que se determinaron dos secciones en las regiones
de alto y bajo contenido en Cr, y además se confirmó la existencia de un hueco
entre ambas.
Dicha evaluación fue mejorada por medio de un análisis exhaustivo del recorrido
de las aleaciones durante la solidificación, principalmente mediante una mejor
caracterización de su microestructura y la composición de sus fases. Además
se utilizaron cálculos asistidos por ordenador de forma complementaria. Este
estudio demostró que la línea eutéctica discontinua se debe a la existencia
de otra línea de reacción en el sistema pseudo-ternario, que no había sido
detectada hasta el momento. Esta línea de reacción se forma a causa de la
región de inmiscibilidad que presenta el diagrama de fases de Cr-W desde
temperaturas muy altas, y se extiende a través del diagrama pseudo-ternario
como una reacción peritéctica. La interacción de ambas secciones de la línea
eutéctica con esta nueva línea de reacción, es la razón por la cual existe la
discontinuidad, formando un punto de equilibrio de cuatro fases de clase II,
también llamado reacción tipo U, en el sistema pseudo-ternario. Además, se
estudió el efecto de la naturaleza cuaternaria del sistema, respecto al enfoque
pseudo-ternario seguido en la investigación, confirmando la validez del método.
Adicionalmente se estudió el comportamiento mecánico a alta temperatura de la
aleación multifásica NiAl-20Cr-4.5W (en porcentage atómico), y los resultados
indicaron que la aleación presenta unas propiedades a alta temperatura
menores, pero comparables, con aquellas de las aleaciones de NiAl más
avanzadas desarrolladas hasta el momento. Estos resultados animan a
continuar la investigación de éste y otros sistemas de una manera similar, ya
que la aleación estudiada no posee una microestructura optimizada como la de
las avanzadas.
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 initially identified as the most promising candidate for higher temperatures. However, its low ductility and fracture toughness at room temperature, as well as its low strength and creep resistance at high temperatures, reduced the interest in 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, and so on, have brought the best results. All the same, 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 a 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 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 occurs due to the existence of a non-reported reaction line in the pseudo-ternary system. This line forms owing to the miscibility gap present in Cr-W phase diagram, which extends up to high temperatures in the pseudo-ternary diagram, and develops 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. Furthermore, 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 exhibits high-temperature properties which are inferior, but comparable, to those 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 like that of advanced alloys.
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 initially identified as the most promising candidate for higher temperatures. However, its low ductility and fracture toughness at room temperature, as well as its low strength and creep resistance at high temperatures, reduced the interest in 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, and so on, have brought the best results. All the same, 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 a 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 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 occurs due to the existence of a non-reported reaction line in the pseudo-ternary system. This line forms owing to the miscibility gap present in Cr-W phase diagram, which extends up to high temperatures in the pseudo-ternary diagram, and develops 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. Furthermore, 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 exhibits high-temperature properties which are inferior, but comparable, to those 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 like that of advanced alloys.
Description
Mención Internacional en el título de doctor
Keywords
Intermetallic compounds, Metal-aluminides, Eutectic alloys, Characterization and evaluation of materials