High temperature oxidation behavior of NbZrTiTa high entropy alloy

In recent years, a new design concept of high entropy alloy (HEA) has emerged, which provides a new idea to solve the deficiency of traditional metal materials in comprehensive use. Different from the design concept that traditional alloys take thermodynamic "enthalpy" as the main control parameter, high-entropy alloys realize thermodynamic "entropy" as the main control factor of alloy structure through the proportional supermixing design of polymetallic elements (usually more than 4).On the one hand, the high entropy alloy has the characteristics of high strength, high hardness and corrosion resistance. On the other hand, due to the relatively free choice and ratio of components, high-entropy alloys can have many different properties by selecting components. Among them, refractory high-entropy alloys with high melting point and high activity refractory elements such as Hafnium, Tantalum and tungsten、Molybdenum show good application potential in extreme environmental service fields such as high-temperature structural materials and structural energy-releasing materials due to their characteristics of both high-entropy alloys and refractory elements.

However, the high temperature oxidation performance of materials is an important factor to be considered whether they are used as high temperature structural materials or structural energy-releasing materials. Among them, high temperature structural materials require that the materials have good high temperature oxidation resistance to ensure that they can work for a long time under high temperature oxygen-containing environment. In order to release as much oxidation energy as possible in a very short period of time, energetic materials are required to have a high oxidation rate. However, there are few researches on high temperature oxidation properties of refractory high entropy alloys. Studies have shown that complex oxide regions appear on the surface of this alloy, which shows better oxidation resistance than commercial Nb alloy, NbSiAlTi and NbSiMo. It is found that the addition of Al and Si can significantly reduce the oxidation rate of refractory high-entropy alloy, while the addition of V can increase the oxidation rate at 1300 C. In addition, since the toughening of refractory high entropy alloys has only recently been realized, previous studies have not solved the ductility problem at room temperature on the basis of improving the oxidation resistance of refractory high entropy alloys. For example, NbCrMo.5Ta0.55TiZr can form a protective oxide film layer by oxidation for 100h at 1000°C, but the material lacks room temperature plasticity due to the presence of brittle Laves phase. AICrMoTaTi formed a protective dense alumina layer at 1000°C and 48h after oxidation. Its oxidation weight gain was relatively low and its oxidation kinetics followed the parabolic rate law, but there was no report on its mechanical properties. After adding Si into AlCrMoNbTi and AlCrMoNbTi, the antioxidant property was improved, but its mechanical property was poor and it did not have the ability of plastic deformation at room temperature. On the whole, the current limited researches on refractory high-entropy alloys mainly focus on the influence of the alloy component elements themselves, and lack the connection with the special structure of high-entropy alloys, so the oxidation behavior of high-entropy alloys cannot be revealed in essence, which restricts its application development to a certain extent. Therefore, this study NbZrTiTa refractory with good mechanical properties of high temperature oxidation behavior of high entropy alloys were studied, combine the unique structure of high entropy alloys and in the oxidation process of alloy structure evolution law of NbZrTiTa refractory high entropy alloys oxidation mechanism research in detail, in order from the structure interpretation can be two aspects of material and high temperature structural materials to further explore the potential applications of refractory high entropy alloys.

The high temperature oxidation behavior of NbZrTiTa high entropy alloy prepared by arc melting was studied by cyclic oxidation test and XRD, SEM and other techniques. The results showed that the oxidation rate constant and oxidative activation energy of NbZrTa high entropy alloy reached 1.1x10^-7-1.0x10^-6g^-2·cm^-4·s^-1 and 97 kJ/mol, respectively, in the temperature range of 1000~1400°C.The oxidation reaction type of NbZrTiTa high entropy alloy is internal oxidation. In the process of high temperature oxidation, oxygen element diffuses along the TiZr rich region with strong lattice distortion and a large number of dislocation, and reacts with metal components to generate unsaturated oxide and saturated oxide in turn. NbZrTiTa high entropy alloy has high oxidation activity under the action of high activity of components and severe lattice distortion, and shows strong application potential as structural energy-releasing materials.