Oxide Dispersion Strengthens The Oxidation Behavior Of CoNiCrAlY And FeCrAl Superalloys

Oxide dispersion strengthened?ODS?CoNiCrAl Y coatings and FeCrAl base alloy are widely used in high temperature services mainly due to their ability to form protective sale i.e.slow-growing and adherent alumina scales which is produced by mechanical alloying?MA?.The actual scale mechanisms include the scale adherence and growth rate are strongly affected by the reactive elements?RE?yttrium and oxygen active elements such as titanium.However,the effect of yttrium and titanium addition on alumina scale growth mechanism on ODS coating CoNiCrAlY and yttria containing FeCr Al base alloy is still undebated.The effect of test atmosphere composition on the alumina scale growth mechanism on the ODS coating/alloy is not clear.In the present study,an attempt was made to analyses the effect of RE?Y?and Ti during the oxidation,reveal the alumina scale growth mechanism in the different oxygen partial pressure atmospheres,so that it can support to optimize the alloys/coatings composition which was to improve the alloys/coatings lifetime and improve the energy efficiency.In order to investigate the alumina scale growth mechanism,RE?Y?and Ti diffusion and distribution,the present study used the model alloy which produced like the commercial alloys PM2000 and MA956 within similar contents.The free-standing CoNiCrAlY coatings are produced by using conventional powder and ODS powder with Al₂O₃,Y₂O₃,HfO₂.In order to get the different oxygen partial pressure,different atmospheres were introduced in the study like Ar-20%O₂,Ar-4%H₂-7%H₂O,Ar-4%H₂-2%H₂O and Ar-1%CO-1%CO₂.The specimens were oxidized in the mentioned atmospheres for different time at temperature in the range 1100-1200?.The oxidation kinetics and the morphology of the cross section and surface were measured by thermogravimetric analysis?TGA?,scanning electron microscopy?SEM?with energy dispersive X-ray spectroscopy?EDX?and electron backscatter diffraction?EBSD?.The elements distributions were analysed by glow discharge optical emission spectroscopy?GD-OES?and Secondary Neutral Mass Spectrometry?SNMS?for the ¹⁸O oxidation.The distribution of Y and Ti in the alumina scale was measured by transmission electron microscope?TEM?and atom probe tomography?APT?.Compared the CoNiCrAlY coatings with different oxide dispersion?Al₂O₃,Y₂O₃,HfO₂?,the alumina formed on the coating with 2 wt.%Al₂O₃ dispersion present a lower growth rate and an excellent adherence.In order to analyses the ODS coating which has a lower growth rate by using model calculation from aspects of the grain size,internal oxidation and oxygen diffusion in the grain boundaries.The results indicate that oxidation rate of the ODS coating was lower than the conventional coatings not only related to Y/Al oxide pegs formation but also to a faster oxygen transport in the grain boundaries.According to the cyclic oxidation,the ODS coating with 2 wt.%Y₂O₃ dispersion present a high growth rate and due to the bad adherence,the lifetime of coating was decreased as well as the coating with 1wt.%HfO₂+1 wt.%Y₂O₃ dispersion.The ODS alloys with and without Ti oxidized in Ar-20%O₂ showed that during the oxidation,the RE?Y?and Ti diffuse outward along the alumina grain boundaries and form the Ti-rich oxide particles and Ti-Y rich oxide particles on the surface of alumina scale on the ODS alloys.The alumina scale consists of outer equi-axed grains and inner columnar grains.The results indicate that Ti can promote the cation outward diffusion and form the equiaxed grains with the Ti content as low as 0.02 wt.%.The alumina formation on the ODS alloy with Ti addition induced that the Ti can change the growth mechanism by enhancement of outward scale growth.Element Ti diffuses outward along the alumina grain boundaries and changes the vicinal lattice which can promote the Al vacancy V_Al formation.The element Ti was mainly distributed in the outer equi-axed grain boundaries,however,the RE?Y?was distributed in the whole alumina grain boundaries.The ODS alloy FeCrAl with different yttria containing oxidized in different atmospheres.The results present that the growth rate of the alumina scale increases with the increasing yttria containing in the Ar-20%O₂ and Ar-4%H₂-7%H₂O.The reactive element Y can promote the Ti diffusion outward so that enhances the inward growth.The growth rate of the inner columnar grains increases with increasing yttria containing.The effect of the reactive element Y is related to the oxidation atmospheres.1.When the ODS alloy oxidized in the Ar-20%O₂ and Ar-4%H₂-7%H₂O,the yttrium-rich oxide act as a donor so that the growth rate of the alumina scale depend on the oxygen partial pressure of the atmosphere.2.When the ODS alloy oxidized in Ar–1%CO–1%CO₂,the yttrium-rich oxide act as an accepter so that the growth rate of the alumina scale is independent on the oxygen partial pressure.