Element Influence On Oxidation Behavior And Microscopic Mechanism Of HfNbTiZr System High Entropy Alloys

In this paper,the oxidation resistance and oxidation behavior of HfNbTiZr series high-entropy alloys(HEAs)with excellent mechanical properties are studied.By adding the traditional anti-oxidation element Al and replacing the Zr element with Cr and Mo,the effect of element addition on the mechanical properties and oxidation resistance of each alloy system was studied.Scanning electron microscopy(SEM),transmission electron microscopy(TEM),X-ray diffraction(XRD),X-ray energy spectroscopy(EDS)were used to characterize the samples before and after oxidation.Using in-situ environmental transmission electron microscopy(ETEM)and in-situ heating technology,the effects of the addition of different alloying elements on the oxidation resistance and mechanism are studied in detail,and experimental support is provided to improve the balance between the oxidation resistance and mechanical properties of refractory high-entropy alloys(RHEAs).The impact mechanism of specific alloying elements addition is discussed for this high-entropy alloys system.The specific research content is as follows:When the alloying element is Zr with a large atomic radius,Al is added to form an Al_xHfNbTiZr alloy.When x=1,the alloy exhibits excellent mechanical properties and oxidation resistance.The Al Hf Mo Nb Ti alloy has a compressive strength of 1600 MPa and about 20%plastic strain of at room temperature.The 100-hour oxidized weight gain at 800?is 33 mg/cm²,and its overall performance is better than most high-entropy alloys.This group of as-cast alloys has a BCC single-phase structure,but will decompose into a BCC phase and a Zr Al intermetallic phase during a long-term heat treatment.In situ oxidation experiments show that under the action of oxygen,the surface will form an amorphous at 450?,and the same phenomenon exists during the low temperature oxidation of the bulk.The alloy produces dense submicron Al₂O₃islands through the BCC-amorphous-dual phase transition to prevent further oxidation.The unique oxidation mechanism of this alloy is proposed.When the alloy element is a high melting point and high strength Mo element,Al_xHf Mo Nb Ti high entropy alloys with different Al contents are designed.The results show that the addition of Mo will lead to poor oxidation resistance.The additional Al addition can increase the strength,hardness and oxidation resistance of the alloy,but also increases the brittleness of the material.The addition of Al does not form a protective oxide film,but can reduce the overall PBR of the oxide and increase the integrity of the oxide film,thereby reducing the internal diffusion rate of oxygen.In situ experiments show that the oxidation is completely dominated by the internal diffusion of oxygen,and Mo has the fastest diffusion rate.The preferential oxidation of Mo leads to cracking of the oxide layer and increases the passage of oxygen diffusion.When the alloy element is Cr with good oxidation resistance,Al is added to form an Al_xCrHfNbTi high-entropy alloy.Its oxidation gain is lower than that of most refractory high-entropy alloys.The addition of Al will severely embrittle the alloy.When x=0.5,the alloy has the lowest oxidation weight gain,the oxidation performance decreases when x=1.After fitting the weight gain curve,it is found that only Al₁ alloy follows the linear oxidation rule,and other alloys maintain the parabolic law.Analyzing the oxide layer,it is found that Cr₂O₃ does not play a protective role,and XRD can only detect the presence of a small amount of Cr₂O₃.The composite oxide Cr Nb O₄ formed at high temperature may be a protective oxide.In situ experiments show that Cr has the fastest diffusion rate and can form a protective film to a certain extent,which will further react with Nb₂O₅ at high temperature to form a composite oxide.