Investigation to Interface between Nb-based Heterogeneous Nucleus and Al Melts on Micro Scale

Author:Zhuo Zi Ming

Supervisor:zhang bao hong xu hong


Degree Year:2019





As a type of transition metal diboride,Niobium diboride(NbB2)characterized by some specific properties such as high melting point,low density and high conductivity,has been extensively studied and reported.Niobium trialuminide(Al3Nb),has been widely used in high temperature materials because of its low density,good high temperature stability,high strength and excellent oxidation resistance.The common features of the two Nb-based intermetallics,such as the low density,high melting point and low lattice misfit withα-Al,make them strong candidates as a potential heterogeneous nucleus for pure aluminum and aluminium alloy melts.The interfacial microstructure between primary alpha-Al phase and heterogeneous nucleus have great influence on grain refinement.It is of high value to study the interface in order to understand the nucleation process and to induce heterogeneous nucleation.However,it is difficult to use the experimental method to study the interfacial microstructures.In this dissertation,the bonding and electronic properties of the interface were studied by first principles from the microscopic scale,and the mechanism of grain size refinement of melt by adding heterogeneous nucleus was revealed.The main contents are listed as following:(1)Non-stoichiometric NbB2-x、Nb1-xB2 and Al3-xNb和Al3Nb1-x-x phase was studied by the first principle methods,and the influence of vacancy type and concentration on phase and stability are revealed.The formation energy Ecoh and the formation enthalpy with respect to the most competing phases of the four bulk phase with x=0.125、0.25、0.375 and 0.5 were calculation.The results showed that NbB2-x,Nb1-xB2 with x=0.125 are stable(2)The thickness of slabs enough to ensure bulk-like interior for the surfaces of different termination and layers were calculated.Vacancy and solute elements defect type and distribution in doped surface were study.After relaxation,9-layer NbB2(0001)and 5-layer Al3Nb(0001)tend to converge.Defect formation energy of NbB2(0001)with B vacancy and Al3Nb(0001)with Nb vacancy is minimum(-5.35eV,2.49eV),but surface energy of them is larger than that of the undoped surface(1.60 J/m2).The stability of surface with vacancy were reduced.Solute elements M(Fe、Mg、Ti、Si)is easy to substitute for surficial B or Nb,and surface energy increase,stability decrease.(3)In this dissertation,Al/NbB2 and Al/Al3Nb with different stacking sites and terminations were constructed.Interface bonding and electronic structure were investigate to reveal the mechanism of NbB2,Al3N heterogeneous nucleus enhancing the heterogeneous nucleation.For Al(111)/NbB2(0001)-BT-HCP inrerface,the work of adhesion is largest(2.57J/m2)and interfacial energy is lower than that ofα-Al/Al melt,0.15 J/m2.So,α-Al/NbB2,α-Al/Al3N are stable,enhancing the heterogeneous nucleation.(4)The influence of vacancy defect on the interfiacal bonding and electronic structures are investigate to reveal the influence mechanism of vacancy defect on grain refining.Vacancie tend to form at the position of interfacial B atom.Its nearby Al-B covalency has been increased.The diffeerence charge density of interfacial atoms are increased,and interface bonding has been improved.For Al/Al3Nb,vacancy tend to form at the position of interfacial Nb atom.The influence of vacancy defect on the interfiacal bonding and electronic structures have no evident change.Vacancy in Al/NbB2 interface can improve grain refinement.However,vacancy in Al/Al3Nb interface has little effect on grain refinement.(5)The interfiacal bonding and electronic structures M(Fe、Mg、Ti、Si)solute elements doped Al NbB2 and Al/Al3Nb interface are studied and analyzed to reveal the influence mechanism of doping defect on grain refining.Solute elements M tend to is substitute for interfacial B atom.After relaxation,the charge accumulation between M and interfacial Al stronger than Al-B at undoped interface,because of M displacement change and B-B broken near M position.Some other areas of the interface have no evident differences.So,M(Fe、Mg、Ti、Si)solute element doping is beneficial to improve interfacial cohesive,enhancing the heterogeneous nucleation.From the perspective of the work of adhesion,interfacial energy,interfacial electronic structure and bonding,solute element doping have no obvious impact on Al/Al3Nb interface,and grain refinement is not improved significantly.