OXIDATION PERFORMANCE OF Y-DOPED REFRACTORY HIGH ENTROPY ALLOY AL-HF-NB-TI-V SYSTEM AT TEMPERATURE 700-900°C

• Main Author: Muhammad, Fadhli
• Format: Dissertations
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/80500
• Institution: Institut Teknologi Bandung

Over the past two decades, a new generation of materials for high temperature applications has been developed through the development of new materials known as High Entropy Alloys (HEAs,???? ???????????????????????? ???? ????????????????). These materials offer a promising approach to achieving a combination of high temperature strength and corrosion resistance, placing them at the forefront of advanced metallic materials research. High Entropy Refractory Alloys (RHEAs), composed primarily of refractory metals, have attracted particular attention as potentially superior to nickel-based superalloys in high-temperature service. RHEAs are characterized by high melting points and some have excellent thermal stability at high temperatures. Due to these factors, the development of RHEAs is expected to find new applications in extreme environments. However, these alloys exhibit extreme ductility at room temperature. To date, only a few alloy systems such as HfNbTaTiZr, NbTiVZr, HfNbTiZr, and HfNbTiV have been shown to have sufficient properties at room temperature. It is well known that refractory elements and their alloys are limited by poor high temperature oxidation resistance, and RHEA is no exception. To improve oxidation resistance, Al, Cr, and Si are added to form protective oxide scales such as Al2O3, Cr2O3, and SiO2. The addition of these non-refractory metals is a promising method for improving oxidation resistance at high temperatures. However, the addition of Al, Cr, and Si to RHEA has a high probability of forming undesirable intermetallic compounds and causing embrittlement of the material. In addition, the addition of alloying elements such as Al, Cr, and Si to alloys containing Zr or Hf has a limited effect due to the formation of ZrO2 and HfO2, which are thermodynamically favorable compared to Al2O3, Cr2O3, and SiO2. Pesting, the rapid disintegration of metal substrates due to exfoliation of oxide scales, is an unusual process that can occur at moderate temperatures of 600-800 °C. To date, most research efforts have focused on oxidation behavior at very high temperatures, neglecting oxidation resistance at intermediate temperatures between 500 and 800 °C, where pesting is most likely to occur at RHEA. To date, there are only two articles describing detailed cases of pitting in RHEA, which shows the lack of knowledge about pitting in this alloy system. Therefore, this study attempts to identify the temperature range at which pitting occurs in the new RHEA alloy AlxHfNbTiV. The oxidation behavior and oxide layer microstructure of AlxHfNbTiV RHEA, to which the trace element Y is added, where x varies from 0.75 to 1.25, are investigated in this study. The synthesis of novel RHEA materials, AlxHfNbTiVY0.05, has been successfully carried out using the electric arc melting technique. The melted products were cut into coupon-shaped pieces with surface areas ranging from 200 to 250 mm2. Microstructural investigation and X-ray diffraction (XRD) analysis showed the formation of a non-homogenous microstructure with segregation of dendritic and interdendritic regions. The addition of the equimolar ratio of Al resulted in an amorphous phase with a small amount of intermetallic phase formation, although thermodynamic calculations showed a value of ?Hmix = -16.2 kJ.mol-1 and a parameter ? = 5.8%, which met the criteria for single phase solid solution formation. The addition of the minor element Y slightly increases the ?Hmix value, which can promote the formation of intermetallic phase. The model alloys were oxidized under dry air at intermediate temperatures of 700- 900 °C. At 700 °C, all the model alloys exhibited pesting, with two different pesting mechanisms identified. Alloys without the addition of Al and Y suffered pesting caused by the formation of TiNb2O7 oxide from solid-state reactions between Nb2O5 and TiO2 oxides, combined with the evaporation of V2O5 oxide, which accelerated the exfoliation of oxide scales. Meanwhile, the addition of Al and Y to the alloys led to the formation of the intermetallic phase NbAl3 and the Laves phase NbAl2, which initiated the formation of the fast-growing non-protective oxide AlNbO4 controlled by the activity of Nb in the oxides. The formation of AlNbO4 oxide further accelerates the process of exfoliation of the oxide scale that had previously occurred due to the growth of Nb2O5 and TiNb2O7 oxides and the evaporation of volatile V2O5 oxide. However, the higher Al content provides better protection against pests at 700 °C. On the other hand, at 900 °C, increasing the Al content changes the oxidation rate of the alloy from near-linear to near-parabolic. It can also be observed that 800 °C is the transition temperature between pesting and linear or parabolic oxidation mechanisms, which are influenced by the Al content and the addition of the minor element Y.