Research Development on High Temperature Oxidation Properties of Self-Passivating Tungsten Alloys
Tungsten has been widely applied in the fields of national defense,aerospace and nuclear systems because of its high melting point,high hardness and excellent resistance to ion sputtering.Especially,tungsten was considered to be the primary candidate material for key components in the future nuclear fusion systems,such as divertor and plasma facing materials(PFMs).When the cooling system of fusion system failed due to natural or human factors,the internal temperature would reach around 1200 ℃.At this temperature,tungsten would be oxidized to form volatile WO,,resulting in failure of the device,which severely limited the engineering application of tungsten in the field of nuclear fusion.The oxidation resistance of tungsten could be effectively improved through addition of alloying elements to prepare self-passivating tungsten alloys.There were two kinds of preparation methods including magnetron sputtering and powder metallurgy.The results showed that the oxidation resistance of self-passivating tungsten alloys was mainly affected by three factors.First,microstructure characteristics of the alloy played an important role in the anti-oxidation performance;second,the content of passivation elements played a decisive role in whether protective oxide film could be formed or not;finally,adhesion of the oxide film to the alloy matrix was critical to good oxidation resistance.In recent years,improvement in oxidation resistance of self-passivation tungsten alloys were mainly focused in composition design and composition optimization,where fruitful achievements were obtained.Binary self-passivation tungsten alloys mainly included W-Si and W-Cr.Compared with pure tungsten,the oxidation weight gain per unit area of W-Si alloy decreases by more than 10 times from 600 to 1000 ℃.This was because that Si could form a dense protective SiO2 film on the surface of alloy,which inhibited further oxidation of the alloy matrix.Also,the oxidation rate of W-Si alloy decreased with increasing Si content.However,intermetallic phase such as W5Si3 would be formed with increasing Si content,which would result in deterioration in mechanical properties of the alloy.Therefore,research concerning binary self-passivation tungsten alloys mainly focused on W-Cr alloys.At 800 ℃,the oxidation weight gain per unit area of W-Cr alloy was reduced by more than 4 times,since a protective Cr2O3 film was formed during the oxidation process.On the basis of W-Cr binary self-passivation tungsten alloy,ternary and quaternary self-passivation tungsten alloys were developed through addition of Nb,Ti,Y,Zr and other elements,which further improved the oxidation performance of tungsten.Compared with W-Cr alloy,the oxidation weight gain of W-Cr-Nb alloy was significantly reduced and the oxidation resistance was gradually enhanced with the increasing Cr content due to the refined microstructure.However,the addition of Nb was harmful to the bonding force between the oxide film and the metal matrix.Compared with pure tungsten,the oxidation weight gain per unit area of W-Cr-Ti alloy was reduced by four orders of magnitude,mainly because that Ti would consume diffused oxygen,which further alleviated the oxidation of W.In the case of W-Cr-Y alloy,Y inhibited the formation of pores.When the Cr content was 11.4％and the Y content was 0.6％(mass fraction),W-11.4Cr-0.6Y thin film alloy exhibited the best oxidation resistance.In W-Cr-Zr alloy,Zr accelerated the formation of the initial oxide film,and the ratio of Zr/Cr(％,atom fraction)was an important factor affecting the oxidation behavior of W-Cr-Zr alloy.When the ratio of Zr/Cr were between 5％and 10％,W-Cr-Zr showed the longest passivation time.Quaternary alloy of W-Cr-Y-Zr was prepared by adding Y and Zr at the same time.Finer precipitates were distributed both along grain boundaries and inside grain interior,which could effectively inhibit the grain growth.The fine-grained microstructure provided more channels for the out-diffusion of Cr3+and contributed to the formation of a protective Cr2O3 film.Thus W-Cr-Y-Zr had better oxidation resistance during long-term oxidation.PBR value was an important factor for evaluating whether the metal oxide could be a protective oxide film.Only when the PBR value was between 1 and 3 would it had a protective effect.In addition,PBR value was not a fixed value,which was also affected by environmental factors,especially that the irradiation conditions had a great influence on PBR value.Whether or not a protective oxide film could be formed was also affected by factors such as the melting point of the oxide,the compactness of the oxide film,the stability of the oxide film,the adhesion of the oxide film,the expansion coefficient,and the stress in the oxide film.Therefore,the influence of various factors on the oxide film needed to be integrated.This paper summarized the recent research progress on the oxidation resistance of self-passivating tungsten alloys,and mainly described the oxidation process and mechanism in terms of microstructure evolution of the alloys before and after oxidation,phase analysis,and thermogravimetric analysis.On this basis,some problems faced by self-passivation tungsten alloys were analyzed and their development prospects were prospected.
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