First-principles Study On Structures And Properties Of NiAl With Rare Earth Element Doping

The intermetallic compound NiAl is one of the most promising engineering materialswith attractive properties including good high-temperature strength, resistance tooxidation and corrosion, relatively low material density and high melting point.However, the application of NiAl is restricted as aerospace materials for its brittlefracture and low tensile ductility at room temperature.Alloying as an effective method is usually used to improve the quality of NiAl. Themost common ternary elements are3d and4d transitional elements like Fe, Cr, V, Co,Mo, Ti, Ga and Mn. Because of its simplicity and effectiveness, one only need to selectthe use of the alloying element or a combination of alloying elements, and then toapplied them to the casting process. Rare earth elements （REEs） are active. Appropriateamount of REE addition can improve the room temperature compressive ductility of thealloy.This paper focus on the effect of single REE（Sc, Y, La, Ce, Nd, Pr, Pm, Sm, Eu）,different amount of Y, combination of REE（La, Ce, Pr） and non-REE（Ti, V, Cr） on thestructures and properties of NiAl. DFT with ultrasoft pseudopotentials is employed inthe CASTEP code, which utilizes plane-wave pseudopotential to performfirst-principles quantum mechanics calculations. Generalized gradient approximation（GGA） is adopted as exchange-correlation functionals for all situations in our models.The lattice parameters, elastic characteristics as well as electronic structures of the NiAlsupercell with additions are investigated.The present study shows that Sc, La, Ce, Pr, Pm, Sm, Eu tend to substitute for Alsite, while Y and Nd tend to substitute for Ni site; Sc, Y, La, Nd, Pr, Pm and Sm are allreduce the hardness of NiAl in varying degrees, where the hardness of Ni₈Al₇La is thelowest and the hardness of Ni₈Al₇Sm is more closer to NiAl. All of the9REE elementsimprove the ductility of NiAl, only the hardness and ductility of Ni₈Al₇Ce and Ni₈Al₇Euare enhanced simultaneously comparing with NiAl. DOS of NiAl with Ce and Eudopping add two sharp peaks which allows for hybridization of s （and p） orbitals of Alwith rare earth element and d orbitals of Ni with rare earth element. The charge densitydifference and bond populations of Ni₈Al₇La, Ni₈Al₇Ce and Ni₈Al₇Eu indicate that theincrease of ionicity and decrease of covalency make Ni₈Al₇La higher ductility and lowerhardness; the ionicity of Ni₈Al₇Ce is stronger than that of Ni₈Al₇Eu, the covalency ofNi₈Al₇Eu is stronger than that of Ni₈Al₇Ce, which make the ductility of Ni₈Al₇Ce higherand the hardness of Ni₈Al₇Eu higher.Models of Ni₃YAl₄, Ni₇YA_l8, Ni₁₁YAl₁₂and Ni₁₅YAl₁₆represent12.500,6.250,4.167 and3.125at.%of Y in NiAl. The hardness of NiAl with the amount of Y as3.125at.%is colsest to NiAl, and the ductility of NiAl with the amount of Y as6.250at.%exhibithigher ductility. The hybridization is stronger by adding6.250at.%of Y into NiAl,while3.125at.%of Y just decreases the hybridization. The covalency of Ni-Ni bondsand Al-Al bonds are stronger in Ni₁₅YAl₁₆than that in Ni₇YA_l8while the covalency ofNi-Al bonds in Ni₇YA_l8are stronger than that in Ni₁₅YAl₁₆. The ionicity of Y-Al bondsin Ni₇YA_l8is stronger than that in Ni₁₅YAl₁₆.In the combination of Ti, V, Cr with La, the complexes of Cr and La possess the besteffect, which improve the ductility and reduce the hardness loss of NiAl. In thecombination of Ti, V, Cr with Ce, the complexes of Cr and Ce possess the best effect,which improve the ductility and hardness of NiAl simultaneously. In the combination ofTi, V, Cr with Pr, the complexes of Ti and Pr possess the best effect, which improve theductility and maintain the hardness of NiAl. In the DOS of Ni₈Al₆CrCe and Ni₇TiAl₇Pr,pseudogap make the covalent bonds more stronger which mainly due to the contributionof4f obitals of Ce and Pr. In the bond populations, the covalency of Ni-Ni bonds, Al-Albonds and Ni-Al bonds in Ni₈Al₆CrCe are all increased. The covalency of Ni-Ni bonds,and Ni-Al bonds in Ni₇TiAl₇Pr are increased, while Ni-Al bonds are not changeobviously. In Ni₈Al₆CrCe, Cr and Ce only bond with Ni, while in Ni₇TiAl₇Pr, Ti and Prboth bond with Ni and Al, so the ionicity of Ni₇TiAl₇Pr is stronger than that ofNi₈Al₆CrCe.NiAl, NiAl-1（containing0.25at.%Ce in NiAl） and NiAl-2（containing1.15at.%Cein NiAl） are prepared by spark plasma sintering and laser remelting. Microstructure andmechanical properties of the three samples show that: in the Laser remelting centralregion, due to the temperature distribution, elements flow rapidly and diffuse fully inthe high temperature region, part of the Ce elements are retained within the grain. Afterlaser remelting, with the increase of the amount of Ce, the hardness, elastic modulus andelastic recovery rate of NiAl-1and NiAl-2are increased to varying degrees. Theseresults are basically consistant with the results in chapter3. The results of single rareearth doping, rare earth and non-rare earth doping are basically consistant with theknown experimental results. Because of the testing methods or elements concentration,the testing results may be better.