Phase Relation In The Ta₂O₅-WO₃-R_xO_y?R=Si,Ti,Zr,Al? Ternary System And Oxidation Behavior Of Ta-w Base Alloys

The high temperature oxidation resistance of Ta-W alloy is relatively poor,which seriously restricts the application of Ta-W alloy.There are many ways to improve the oxidation resistance of the alloy,the antioxidant surface coating and alloying elements are ways to improve the oxidation resistance of alloy.Whether alloying or surface coating,the knowledge of compatibility and reaction between oxidation products are the key to understand the oxidation of alloy.The compatibility with oxides and the formation of liquid phase can predict the oxidation behavior of the alloys.In the present research,the phase diagrams of Ta₂O₅-WO₃-R_xO_y?R=Si,Ti,Zr,Al?ternary systems have been studied;the results will be used to predict alloy oxidation products according to the oxides relevance and design alloy component.The oxidation resistance and oxidation product composition of alloy also have been studied,the results are as follows:?1?The reacted samples have been heated in different temperature and prolonged the holding time to ensure the phase balance to research the reaction of ternary subsolidus system Ta₂O₅-WO₃-R_xO_y?R=Si,Ti,Zr,Al?.The phase analysis found that the oxide reaction will generate the following compounds: Ta22W4O67,Ta2WO8,Ta16W18O94,TiTa2O7,TaZr2.75O8,Al2W3O12 and AlTaO4,and the area has solid solution.After investigating the compatibility relation,obtained respectively of the Ta₂O₅-WO₃-SiO₂,Ta₂O₅-WO₃-TiO₂,Ta₂O₅-WO₃-ZrO₂,and Ta₂O₅-WO₃-Al2O3 ternary subsolidus system phase diagram.?2?The Ta-W alloys are prepared by electron beam melting,hot pressing sintering and spark plasma sintering.The relative densities of the alloys were all more than 95%.It is found that the tantalum oxide can be formed at the grain boundary in the samples prepared by the powder metallurgy method.The formation of oxide can pin the grain boundary,restrain the grain growth and improve the hardness of alloy.?3?TG was used to analyze the oxidation kinetics of Ta-W alloys.In the nonisothermal oxidation experiment,the initial oxidation temperature of pure Ta was measured at about 580 oC.When tungsten added,initial oxidation temperature increased to 680?.The results of static oxidation showed that in regain of 680 oC 900 oC,the pure Ta followed straight line law,in the meanwhile,the initial stage of Ta-W alloy could be described as parabolic law,with passing of time,the oxidation behavior of the alloy changed to linear law.The oxidation product of Ta-10 W alloy is Ta₂O₅ ss,and the oxidation products of Ta-20 W alloy are Ta22W4O67 and Ta2WO8.Many cracks observed in the oxide film,which made the protective weakened.?4?The Ta-W based alloys were prepared by hot pressing.The densities of the alloys were all achieved to 98%,which showed the alloys had been basically densification.The addition of alloying elements in the alloy can cause the grain size reducing.However,the alloying elements will be gathered in the grain boundaries or the grains.The hardness of Ta-W-Si ternary alloy was improved obviously,but not increased in case of Ta-W-Ti-B quaternary alloy and Ta-W-Zr ternary alloy.?5?The oxidation kinetic of Ta-W based alloy showed that the oxidation weight gain of the alloy increased obviously after adding Si element.The oxidation kinetic of Ta-W-Ti-B quaternary alloy showed that the oxidation kinetic curves were followed by straight line law at the temperature of 700 oC to 900 oC.The oxidation kinetics of Ta-W-Zr ternary alloy showed that the oxidation kinetics curves of the alloy were followed by straight line law at the temperature of 700 oC to 800 oC,followed by parabolic law at 900 oC.