Ni-Cr-Al合金择优氧化及其影响机理的第一原理研究

First-principles study on Ni-Cr-Al alloy preferential oxidation and absorption of segregation

采用基于密度泛函理论的赝势平面波第一原理方法,研究了Ni-Cr-Al合金择优氧化及其影响机理。研究结果表明,Al,Cr不在Ni表面发生偏聚,Y却发生偏聚。氧吸附促使含Al,Cr镍合金表面偏聚反转,这对发生择优氧化起到了决定性的作用。合金元素向表面偏聚强弱的顺序为Y>Cr>Al。Cr,Al,Y存在使O于Ni合金表面化学吸附更稳定,尤其是Cr,这可以解释Ni合金氧化皮外层一般是Cr2O3,内层是Al2O3的事实。稀土促进Ni-Cr-Al合金选择性氧化是因为稀土氧化物Y2O3有助于Cr2O3,Al2O3成核。合金元素Al,Cr,Y使Ni合金表面活性增强,有助于表面Al,Cr,Y氧化物形成,产生择优氧化。O与Al,Cr,Y形成较强的共价键,与Ni形成较弱的共价键,因此Ni-Cr-Al合金表面容易生成Al2O3或Cr2O3氧化膜。O吸附使Ni,Cr,Y的d电子转移到氧原子上,因此形成的化学键兼有共价键和离子键的特性。

A first-principles plane-wave pseudo potential method,based on the density functional theory,was used to investigate the selective oxidation and its influence mechanics of Ni-Cr-Al alloy.The study shows that Al and Cr will not segregate on the Ni surface,but Y will.The absorption of oxygen will promote inversion of Al and Cr segregation on the surface,which plays a decisive role on the selective oxidation of Ni-Cr-Al alloy.The order of surface segregation intensity of alloying elements is YCrAl.The alloying elements of Cr,Al and Y make it easier for the chemical absorption of O2 on the surface of Ni,Cr alloy, especially Cr,which will explain the fact that why the outer oxidized scale of Ni based alloy is Cr2O3,while the inner oxidized scale is Al2O3.The mechanics why the doping of rare-earth elements promotes the selective oxidation is that the oxidants Y2O3 are helpful to nucleation of Cr2O3 or Al2O3.Alloying elements Al,Cr and Y will increase the surfactivity of Ni based alloy,which will contribute to the formation of oxidants of Al,Cr and Y on the surface,as a result selective oxidation is brought up.During the process of absorption,the oxygen atom will form relatively strong covalent bonds with Al,Cr and Y,but the covalent effects with Ni are relatively weak,so there is prone to form thin oxidation film of Cr2O3 or Al2O3 on the surface of Ni-Cr-Y. The absorption of O2 will make it possible for electron d of Ni,Cr and Y transferred to the atom of oxygen,so that the chemical bonds are both covalently and electrovalently formed.