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INFLUENCE OF MELTING TEMPERATURE AND TIME ON THE OXYGEN CONTENT IN TiAlNb ALLOYS
Barták, Tomáš ; Zemčík, L. ; Dlouhý, Antonín
The present study investigates a cost-effective melting of TiAlNb intermetallics in a stable yttrium oxide refractory. We identify mechanisms that mainly contribute to the oxygen pick-up during melting. Systematic microstructural and chemical analysis provided evidence that erosion and dissolution of crucible wall are key factors of the process. Experiments were performed using two Ti46Al7Nb alloys with a different initial content of oxygen. The alloys were re-melted in vacuum induction furnace inside U-shaped yttria crucibles. The experiments involved three different temperatures in combination with melting times spanning from 300 to 1800 s. The oxygen content of re-melted and solidified alloys was assessed by means of inert gas fusion technique. Results of the chemical analysis provided clear evidence that the stability of the refractory wall in contact with the molten alloy and its rate of dissolution depend mainly on melting temperature. These results are further supported by microstructural observations that confirmed ever growing content of ceramic phases in the solidified intermetallic with increasing temperature of the melt. The work also contributed to the improved experimental methodology, in particular to temperature measurements of reactive melts.
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Interakce mezi Ti-Al-Nb taveninou a Y2O3 kelímky
Barták, Tomáš ; Dlouhý, Antonín ; Dočekalová, Kateřina ; Zemčík, L.
The present paper focuses on interactions between intermetallic melts (Ti-46Al-7Nb-0.7Cr-0.1Si-0.2Ni alloy (at%)) and ceramic crucibles based on the Y2O3 refractory. Different levels of superheat and exposure time are taken into account. Vacuum induction melting (VIM) experiments were performed using the vacuum furnace BALZERS VSG 02. The controlled argon atmosphere inside the furnace volume was kept at the pressure level of 0.133 bar during the melting experiments. The reactions at the melt - crucible interface were studied by means of scanning electron microscopy (SEM) equipped with the energy dispersive x-ray analyzer (EDX). There has been clear benefit when a common refractory like alumina has been replaced by yttria. However, the results obtained in this study show, that the long - term exposure at melting temperatures above 1650°C lead to notable reactions at the interface. We discuss the stability of the melt - ceramic system using principles of thermodynamic equilibrium.
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Lomová houževnatost a mechanismy porušování intermetalika na bázi TiAl
Dlouhý, Ivo ; Chlup, Zdeněk ; Hadraba, Hynek ; Kozák, Vladislav
The paper is focused on the analysis of the role of microstructure in fracture performance of TiAl intermetalics at room and elevated temperatures. Tensile properties, flexural strength and fracture toughness have been evaluated for Ti-40Al-2Cr-2Nb-1B and Ti-46Al-0,7Cr-0,1Si-7Nb-0,2Ni alloys. Positive Nb effect on fracture resistance was found; fracture toughness of the high Nb alloy was increased at contemporary increase of tensile and flexural strength. Fracture surfaces were evaluated using scanning electron microscopy showing the key role of mechanical twinning in deformation of both alloys. As a governing mechanism of the crack initiation a microcrack nucleation at boundaries between -TiAl grains was confirmed. Based on findings obtained key aspects of the effect of separate microstructural components have been discussed. Summary of toughening effects acting in this group of intermetalics has been provided.
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Fracture toughness and damage mechanisms of intermetalic based on TiAl
Dlouhý, Ivo ; Chlup, Zdeněk ; Hadraba, Hynek ; Kozák, Vladislav
The paper concentrates on the analysis of the role of microstructure in fracture behaviour of TiAl intermetalics at room and elevated temperatures. Tensile properties, flexural strength and fracture toughness have been evaluated for alloys Ti-40Al-2Cr-2Nb-1B and Ti-46Al-0,7Cr-0,1Si-7Nb-0,2Ni. Positive effect of Nb on fracture resistance has been found; fracture toughness of the high Nb alloy increased at contemporary increase of tensile and flexural strength. Fracture surfaces have been evaluated using scanning electron microscopy showing the key role of mechanical twinning in deformation of both alloys followed. As a governing mechanism of the crack nucleation a microcrack formation at boundaries between mechanical twins and gama-TiAl grains has been confirmed. Extensive development of shear ligament toughening was observed supplying quite good values of fracture toughness in high niobium alloy.
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