Oxidation Behavior And Kinetics Of T2 Alloy In Mo-Si-B Ternary System

Mo₅SiB₂(T2) phase is the only ternary compound in Mo-Si-B ternary system.In addition to the highest melting point(about 2200?)and relatively low density(about 8.864g/cm³),its high-temperature strength and creep resistance are significantly better than MoSi₂ and Mo₅Si₃.Therefore,T2 alloy is considered to be a more competitive ultra-high temperature structural material.Due to the high temperature service environment,the research on the oxidation resistance of T2 alloy is essential.In this paper,spark plasma sintering(SPS)technology was used to obtain high-purity and high-density T2 alloys by changing the sintering temperatures.By means of cyclic oxidation experiment,XRD phase analysis,SEM morphology observation and EDS element analysis,the oxidation behaviors of T2 alloys at 500?1200? were systematically studied.The related oxidation mechanism and oxidation kinetics were analyzed.The influences of T2 phase content and relative density on the oxidation behaviors were discussed.The main conclusions are as follows:In SPS sintering,with the increase of sintering temperatures,the content of T2 phase in the alloy increases,while the second phases such as Mo₃Si and Mo decreases,and the relative density appears to increase first and then decrease.Comprehensively considered,the optimal sintering temperature is 1650?.Through the cyclic oxidation tests,the oxidation behaviors of T2 alloys at different temperatures are significantly different,following different oxidation rules,indicating that there are different oxidation mechanisms.In the temperature range?(500-600?),the oxidation of the alloy follows a parabolic law of weight gain.A small amount of MoO₃ is formed on the oxidized surface.Due to the low temperature,the oxidation degree of the alloy is weak,and no obvious oxide layer appears on the cross section.In the temperature range?(about 700?),the oxidation reaction of the alloy is mainly controlled by the rapid formation and strong volatilization of MoO₃,showing a nearly linear law of oxidation weight loss.The borosilicate oxide scale forms very slowly on the alloy surface,showing an uneven and non-dense morphology due to the continuous MoO₃ bubbles bursting out of the surface.Although some MoO₂ instead of MoO₃ is generated at the oxide scale cover because of the reduction of oxygen partial pressure,the cross section still shows a large number of holes and cracks in the oxide scale,indicating that the substrate cannot be completely protected.After 50h,the oxidation weight loss of the alloy is as high as 70.92mg·cm^-2.In the temperature range ?(800-1000?),a parabolic law of oxidation weight loss is observed.With the increase of temperature,the formation rate of the protective borosilicate scale is accelerated,resulting in a gradual decrease in the oxidation rate constant(at 1000?,the oxidation rate constant is only -0.4624mg²·cm^-4·h^-1).At 1000?,the surface of the alloy is covered by a continuous,complete and dense borosilicate oxide scale.With a high viscosity,the oxide scale can effectively block the diffusion of oxygen and the volatilization of MoO₃.From the cross section,there are three structures appearing on the alloy matrix,including the outermost layer of borosilicate,the second outer layer of MoO₂,and the intermediate layer of Mo,indicating that the alloy is effectively protected.In the temperature range ?(1100-1200?),although the alloy still exhibits the oxidation behavior of parabolic weight loss similar to the temperature range ?,the trend of weight loss in the stable stage increases,which is mainly attributed to the influence of thermal stress.After 1h of oxidation at 1200?,the thermal stress in the oxide scale is 609MPa,which is increased compared with that at 1000?(475MPa).As a result,the borosilicate protective scale is cracked and falls off from the substrate.The MoO₂ layer disappears,and the Mo intermediate layer becomes irregular.T2 phase content and relative density have effects on the oxidation behaviors.The alloy samples obtained at different sintering temperatures were subjected to cyclic oxidation experiments.The results show that the differences of these two factors do not change the oxidation kinetics of the alloy in each temperature range,but under the same oxidative conditions,the surface of the sample with high T2 phase content and relative density is more conducive to the formation and coverage of dense borosilicate protective scale,thereby showing the least mass change and the best oxidation resistance.