Investigation of MAX Phase Reinforced TiAl Matrix Composite and First Principle Calculation of Interfaces

Author:Song Xiao Jie

Supervisor:cui hong zhi


Degree Year:2018





TiAl intermetallic compound is a new type of lightweight intennetallic compound,which is the most potential applications in the field of high temperature structure materials,however,its poor plastic at room temperature has limit its wide application.MAX phase has the excellent properties of both metal and ceramics,and the combination of MAX and TiAl alloy is an effective means to improve the room temperature plasticity and give full play to the excellent properties of the two materials.In this paper,TiAl/Ti2AlC,TiAl/Ti2AlN,TiAl/Ti2Al(C,N)and TiAl-Ti5Si3-Ti2AlC(N)composites were synthesized by vacuum arc melting process.Their synthesis mechanisms,microstructures and mechanical properties are investigated in detail,interfaces of composites between different phases are analyzed.Then the TiB2/TiC,Ti2AlC/TiC and TiAl/Ti2AlC interfaces performance are calculated by first principles calculation based on DFT using VASP package.In vacuum arc melting process,the synthesis mechanism of MAX phase in composite is based on a TiC(N)(s)+TiAl(l)→Ti2AlC(N)(s)peritectic reaction,through the SEM,TEM and EDS analysis of TiC(N)-Ti2AlC(N)"core-shell" structure,the existence of this peritectic reaction is verified,moreover,the amount of residual TiC(N)phase is determined on the reaction of thoroughly or not.MAX phases,Ti5Si3 phase can significantly refine the grain size of TiAl alloy matrix,by a refinement of~200 μm to 20-50 μm.The in situ synthesized MAX phases,tiny Ti5Si3 phase and the rest of the small amount of TiC(N)can significantly improve the microhardness,compressive strength of composite materials without plastic sacrifice.Composite has a highest microhardness(TiAl-20 mol%Ti2AlN,5.92 GPa)with an increase of 72.6%compared with TiAl alloy(3.43 GPa),a highest compressive strength(TiAl-15 mol%Ti2AIC,1878 MPa)with an increase of 56.1%,compared with TiAl alloy(1203 MPa)and a highest engineering strain(TiAl-15 mol%Ti2AIC.26.7%)with an increase of 52.7%compared with TiAl alloy(17.5%),however,the plasiticity of some composites are reduced due to the large size of ceramic phaseThe in-situ synthesized Ti2AlC/TiAl composites with graphite powder,multi-walled carbon nanotubes and TiC as carbon sources show different performances,MWCNTs provide a large number of nucleation of the basement,which lead to the uniform microstructure,and for lack of peritectic reaction heat,the residual amount of TiC phase in composite formed by TiC powder is more,this part of the TiC under the synergy of Ti2AlC has significant effect on material performance.For the first time,a TiAl/Ti2Al(C,N)composite is synthesized by adding g-C3N4 as raw material.the solid solubility of Ti2AI(C,N)solid solution is identified as a chemical formula for Ti2Al(C0.25,N0.75)by composition analysis and first principle calculation.Ti2Al(C,N)solid solution of high value of hardness and good plasticity,can improve the mechanical properties of composites.Si additions to the TiAl/Ti2AlC(N)affects the reaction processes of composites significantly.Due to the introduction of Si,an exothermic reaction of Ti+Si→Ti5S3 was occurred in the system,which formed a conjugate relation with the peritectic reaction of TiAl-xTi5Si3-10 mnol%Ti2AlN system,the endothermic reaction of TiN(s)+TiAl(l)→Ti2AlN(s)inhibits the growth of Ti5Si3 particle,meanwhile,the exothermic reaction of Ti+Si→Ti5Si3 promote the synthesis of Ti2AlN.As a result,the Ti5Si3 particles in composite are with small size and Ti2AlN content is increased,this kind of change can significantly improve the mechanical properties of composites.Through TEM characterization and analysis,three kinds of nearly coherent relationship in TiAl/Ti2Al(C,N),Ti2Al(C,N)/TiC and TiC/Ti2AIC interfaces are discussed,the coherent interface ensures stable interfaces between different phases,the stability of interface play a crucial role on the performance of the composites.According to the interface orientation relationship determined by experiment,TiC(0 10)//TiB2(0 1 1 0),Ti2AIC(0 0 0 1)//TiC(1 1 1)and TiAl(1 1 1)//Ti2AlC(0 0 0 1)interfaces are calculated by first principle calculation using the VASP package.Through the calculation of surface energy,work of adhesive,interface energy,charge density distribution and the electronic density of states,the surface and interface properties of the three configurations are analyzed,the theoretical basis of the interface orientation,and the information of the interface atoms and electronic structure of the specific configuration are discussed.TiC(0 1 0)//TiB2(0 1 1 0)interface models of C-BS1-B1、C-TS-B2 and Ti-BS1-B1 have lower interface energy,which refers structures are stable,C-TS-B2 configuration interface has the lowest interface energy of 1.12 J/m2,In Ti2AlC(0 0 01)//TiC(1 1 1)interface models,all the BS stacking mode interface are refactoring,The interface of HS stacking in the same surface is more stable than that of TS stacking.Ti2-HS1-C,Ti2-HS2-C and Al-HS1-C configuration interface have lower interface energy,in which Ti2-HS2-C is-0.06 J/m2.The coordination number of atoms has important influence on the interaction between atoms.In TiAl(1 1 1)//Ti2AlC(0 0 0 1)interface models,all the BS and TS stacking mode interface are refactoring,C-HS-TiAl,AI-HS-TiAl,Til-HS-TiAl and Ti2-HS-TiAl configurations are stable,the C-HS-TiAl configuration has the lowest interface energy of-0.93 J/m2.Charge density distribution,charge density difference and DOS analysis show that the strong interaction between C and B atoms,C atom and Ti,C atoms and Al atoms is the main reason to form a stable interface,and relatively high interface energy is due to the mutual exclusion of Ti-Ti atoms.