Formation of aluminides on Ni-based superalloy 690 substrate, their characterization and first-principle Ni(111)/NiAl(110) interface simulations

Ni-based superalloy 690 substrates were pack aluminized in a low Al-containing pack at 1273 K for 4 h in argon atmosphere. Scanning electron microscopy with energy dispersive X-ray analysis along the cross section of aluminized specimen revealed the formation of multilayer. The uppermost layer consisted of NiAl type phase (~ 45 μm), while adjoining one composed of (NiCr)Al and (NiCr)2Al types layer (~ 25 μm) and subsequently Cr-rich layer (~ 35 μm) adjacent to substrate. Al-content was found to increase while moving from Cr-rich layer towards outer layer. Cross-sectional transmission electron microscopy confirmed the formation of NiAl layer on the topmost surface and revealed the formation of nanoparticles of nickel aluminide on the uppermost surface. Microhardness was found to vary from 624 to 157 Vickers hardness number along the cross section of aluminized substrate indicating high, intermediate and low values for NiAl, Cr-rich layer and substrate, respectively. To evaluate the adherence of multilayer, scratch test was performed along the cross section of aluminized substrate at a constant load level of 2 N at ambient temperature. For aluminides, a decrease in friction coefficient with the decrease in Al-content was noticed. Cr-rich layer showed lowest friction coefficient, while its variation was little for substrate. Aluminide layers indicated lower penetration depth than the substrate, whereas no penetration was recorded for Cr-rich layer. Scratched surface did not reveal any peeling off either at the multilayer or layer/substrate interface indicating their good adherence. Aluminized specimens showed good overall oxidation resistance at 1273 K in air due to the formation of α-Al2O3. First-principle spin-polarized calculations on Ni(111)/NiAl(110) interface indicated strong adhesion (Ideal work of adhesion, Wad (ideal) = 3684 mJ/m2; lowest bound value) arising from strong metallic Ni d Ni d interaction and covalent Ni d Al p mixing of states.