First Principles Study On Hydrogen Embrittlement Of Ni-Al Intermetallic Compounds

Ni-Al intermetallic compounds are widely used as novel high-temperaturestructural materials, because of its high melting point, good oxidation resistance andthermal performance. Due to the room temperature brittleness, their service life andapplications are severely restricted. In this paper, the first-principles method is usedto study the crystal structure, formation energy, density of states, population andelastic constants of Ni-Al intermetallic compounds, then the electronic structure andmechanical properties are characterized. Secondly, the electronic structure andmechanical properties of H doping NiAl and Ni3Al compounds are calculated, inorder to reveal the mechanism of its room temperature brittleness.The calculated lattice constants of Ni-Al intermetallic compounds are in goodagreement with the experimental and other theoretical results, indicating the highreliability of our calculations. The negative formation energies exhibitthermodynamic stability of Ni-Al intermetallic compounds, the order of stability is:NiAl> Ni2Al3> Ni5Al3> NiAl3> Ni3Al. Based on the analysis of density of states, thedensity of states at the Fermi level for the five compounds are mainly originatedfrom Ni-3d electrons and Al-3p electrons, and the Al-3s and Al-3p orbitals and theNi-3d orbital show orbital hybridization. The values of the density of states at theFermi level of Ni3Al, NiAl, Ni5Al3, NiAl3and Ni2Al3are4.51eV,1.11eV,3.84eV,1.65eV,1.19eV, respectively, which is in good consistent with results of formationenergies; Based on the population analysis, the Al atoms lose electrons while Niatoms get electrons, demonstrating the strong covalent bond between Ni and Alatoms. The five compounds are mechanical stable, according the calculated elasticconstants. The G/B value of NiAl is0.253and has the best ductility. The G/B valueof Ni2Al3is0.645and shows brittleness. The Young’s moduli of Ni5Al3and Ni2Al3are210GPa and209GPa, respectively, which predict their high hardness.For H doping in NiAl and Ni3Al, the preferential site occupation is the Al-rich octahedral interstitial in NiAl and the octahedral interstitial fully composed by Niatoms is firstly occupied in Ni3Al. For all H doping Ni3Al systems, all of its G/B aregreater than0.5, indicating their brittleness. The values of G/B of H doping NiAlcompounds are bigger than that of NiAl and their brittleness are enhanced by Hdoping. In one word, the H doping leads to the brittleness of NiAl and Ni3Al. Basedon the density of states analysis, the H-1s orbital and the Ni-3d orbital have orbitalhybridization. Based on population analysis, the H atoms get electrons, whileelectron accepting ability of the neighboring Ni atoms weakens. The H andneighboring Ni atoms form covalent bonds, and the neighboring Ni atoms bond inanti-bond form. H doping in NiAl and Ni3Al compounds result in the formation ofNi-Ni anti-bonds, therefore Ni-Ni anti-bonds will break even under very low stressconditions and the lattice planes with Ni-Ni anti-bonds become the cleavage planes,that is the brittleness of NiAl and Ni3Al is enhanced by H doping.