Oxidation Behavior Of High-entropy Alloys Al_xCoCrFeNi In Supercritical Water

The power industry is the national backbone industries. With the demand for efficiency increase in ultra-supercritical (USC) boiler, a more severe environment for boiler materials is created. Excellent higher temperature mechanical properties and oxidation resistance are necessary for materials worked under such environment. Therefore, the development of new materials that own excellent high-temperature performance is the key to improving the work efficiency of USC boiler. It was reported that high-entropy alloys exhibited excellent air oxidation resistance. However, to the best knowledge of the present authors, no studies have been reported on the oxidation of high-entropy alloys in supercritical water. Therefore, the oxidation behavior of high-entropy alloys AlxCoCrFeNi (x=0.15,0.4,0.6,0.8, denoted as Al0.15, Al0.4, Al0.6, Al0.8 respectively) in supercritical water under 600℃/25 MPa was studied in this paper. Furthermore, the influence of oxidation environment, types and contents of alloying elements in alloy, structure and composition of oxide film on the oxidation resistance of alloys was explored in this paper.The results reveal that all oxide films formed on Alx (x=0.15,0.4,0.6,0.8) in supercritical water under 600℃/25 MPa show a duplex layer structure with a Fe2O3, Fe3O4, FeCr2O4, CoCr2O4 outer layer and a NiCr2O4, CoCr2O4, Fe(Cr, Al)2O4 inner layer. With the increasing of oxidation time, the size of oxide particles and the thickness of oxide films increase, the compactness of films decreases. With the increasing of Al content of Alx, the size of oxide particles and the thickness of oxide films decrease, the Al content of inner oxide films and the compactness of films increase.The oxidation resistance of alloys is related to oxidation environment. All oxide films formed on Alx (x=0.15,0.4) in air at 600℃show a duplex layer structure with a Fe2O3, Fe3O4 CoCr2O4 outer layer and a NiCr2O4, Fe(Cr, Al)2O4 inner layer. Compared to the oxidation behavior of Alx in supercritical water, Alx show a superior oxidation resistance in air.The oxidation resistance of alloys is related to the types and contents of alloying elements in alloys. The oxide films formed on Super304H and P92 in supercritical water under 600℃/25 MPa show a duplex layer and three-layer structure, respectively. The inner oxide film of Super304H and the inner and middle layers of P92 are both composed of Fe3O4 and FeCr2O4, the outer layer of Super304H and P92 are composed of Fe2O3 and Fe3O4. The order of the oxidation resistance of Alx (x=0.15,0.4,0.6,0.8), Super304H and P92 is:Al0.8> Al0.6>Al0.4>Al0.15>Super3O4H>P92.Finally, the oxidation resistance of alloys is also related to the structure and composition of oxide films formed on alloys. The oxidation resistance increases with the increasing thickness and integrity of oxide films, the increasing Al content of inner oxide film, and the decreasing pore numbers in oxide films. In terms of the oxide films formed on Alx (x=0.15, 0.4,0.6,0.8) in supercritical water, the protectiveness of oxide films are influenced by all above factors. And the oxidation protection is dominated by the inner layer. 属性不符...