The effects of grain boundary structure and bulk Cr content on grain boundary degradation behavior of Ni-Cr binary alloys in high temperature CO₂

The effects of grain boundary (GB) structure and bulk Cr content (from 15 wt.% to 30 wt.%) on GB oxidation behavior of Ni-Cr binary alloys were investigated in 650°C/1atm CO₂ gas. The degradation behaviors of GBs, which varies greatly (including preferential intergranular oxidation, general intergranular oxidation and GB passivation), is dictated by the available Cr source which depends on the bulk Cr content and the Cr diffusion flux to the reaction front. The Cr diffusivity along GB is dependent on GB structure and decreases in the order of random high angle GB (RHAB), low angle GB (LAGB), incoherent twin boundary (ITB) and coherent twin boundary (CTB). Interestingly, other coincident site lattice boundary (CSLB (Σ>3)) shows a Cr diffusivity between ITB and CTB. More importantly, the threshold bulk Cr content for achieving GB passivation is also closely related to GB structure. To be specific, LAGB completely passivates when the bulk Cr content is between 20 wt.% and 25 wt.%, while ITB completely passivates at a bulk Cr content of 25 wt.% and CSLB (Σ>3) completely passivates when the bulk Cr content is between 25 wt.% and 30 wt.%. In contrast, CTB is subject to general intergranular oxidation while RHAB shows passivation regardless of the bulk Cr content.