The Regularity Of In-situ Growth Of Al₂O₃ Whiskers On FeCrAl Alloy Surface Apply For Automobile Exhaust Gas Purifier

With the?Limits and Measurement Methods of Pollutant Emissions from Light-Duty Vehicles(China Phase Six)?(2016.12.23)and?Limits and Measurements of Pollutants Emissions from Heavy Diesel Vehicles(China Phase Six)?(2018.6.22)announcement,the country's exhaust emission requirements have became more and more stringent.The ceramic carrier is the most widely used,but due to its low mechanical strength,weak vibration resistance,and low thermal conductivity,these shortcomings lead to a short service life,so people turn their attention to metal carriers.The mechanical properties and machinability of the metal carrier are better than those of the ceramic carrier.But the main problem is that its specific surface area is low which can not support more catalyst,and the combination of catalyst and metal carrier is not strong,and the oxidation and corrosion resistance are poor.In order to improve these problems,this article adds La to the Fe-Cr-Al alloy.With pretreatment of the Fe-Cr-Al alloy containing La,whiskers are grown on the surface of the alloy in situ,thereby increasing the specific surface area of the metal carrier.In this paper,Fe-Cr-Al alloy containing La is designed,using Thermo-Calo thermodynamic calculation software,X-ray diffraction(XRD),optical microscope,scanning electron microscope(SEM)and other technologies to systematically study whisker morphology of Fe-Cr-Al alloy with different La contents under different whisker growth atmosphere,temperature,and time.The whisker morphology under different conditions was analyzed and compared,the change process of the whisker morphology and the whisker growth mechanism were summarized,and the whisker growth law of Fe-Cr-Al alloys with different La content was discussed.At the same time,the room temperature mechanical properties of Fe-Cr-Al alloy samples with different La content were studied.The main conclusions are shown as follows:The study found that in the 850?-1050?dry air environment,the more suitable in-situ whisker growth conditions during whisker growth at different temperatures were:whisker growth at 850?for 20 h,at 900?for 15 h,at 950?for 10 h,1000?for 5 h,and 1050?for 1 h.When the temperature does not change,with the increase of the growth time,the whiskers become more dense and uniform,and the size of the whiskers continues to grow;but after a certain growth time,the whiskers gradually become flakes and pyramids,and finally change to Spherical oxide or oxide layer.Among them,the average whisker size of Fe-20Cr-6Al-1.2Co-0.5Nb-0.13Zr-0.08La alloy at 950?for10 hours is the largest,reaching 0.94?m.Under the same whisker growth conditions,when the La content is in the range of0 wt.%-0.11 wt.%,with the La content increase,the whisker size gradually increases.and the whisker density also shows upward trend.However,when the La content reaches 0.15 wt.%,the whisker size begins to decrease,and the phenomenon of whisker shedding is obvious.Among them,the Fe-Cr-Al alloy with La content of 0.08 wt.%-0.11 wt.%has the best growth morphology.La has very low solid solubility in Fe-Cr-Al alloys,and is mainly segregated along grain boundaries.La will preferentially form La₂O₃ particles during the growth of whiskers to promote the growth of Al₂O₃ whiskers.The in-situ growth pattern of Al₂O₃ whiskers in water vapor environment is similar to air environment.As the growth temperature increases,the shape of the whiskers also changes in the same trend,and finally becomes oxidized at higher temperature.needle-like and spindle-like whiskers are more likely to grow in a water vapor environment.Among them,the average whisker size of Fe-20Cr-6Al-1.2Co-0.5Nb-0.13Zr-0.08La alloy at 950?for 10h is the longest,reaching 1.12?m.And the average whisker size increases by 15%-40%in water vapor environment.Gulbransen assuming the whiskers are hollow or contain a screw dislocation at the center.The diffusion of Al³⁺in the vertical direction of the whiskers is carried out along the gradient of the chemical potential,which belongs to surface diffusion;in the horizontal direction of the whiskers,Al³⁺diffuses in the crystal lattice and causes the lateral Grow.Since Al³⁺has a higher diffusion rate D_s on the inner surface of the whisker and a lower activation energy Q_s,the whiskers first grow in a vertical square,but as the growth temperature and growth time of the whiskers increase,the lattice diffusion rate D_l gradually increases high.Macroscopically,as the whisker growth temperature and time increase,the Al₂O₃ whiskers on the alloy surface gradually change from needle-like,fusiform-like whiskers to sheet-like or pyramid-like whiskers,then into spherical oxides,and finally into oxide layers.Through whisker growth experiment,alloy surface whisker morphology analysis and whisker growth mechanism under different conditions,it is shown that Fe-Cr-Al alloys with La content of 0.08 wt.%and 0.11 wt.%were grown in air and water vapor environment The whiskers obtained by growing whiskers at 950?for 10 h are better.