Interaction Between BNi-2 Filler And Ti₂AlC And Brazing Mechanisms Of Ti₂AlC/Ni Joints

Ti₂AlC is a representative member of MAX phases which refers to a group of nanolaminate ternary carbide or nitride ceramics.It possesses excellent combination of metallic and ceramic properties,for example,with high melting point,large modulus,and excellent machinability.In addition,the Ti₂AlC model is tolerant of Al vacancies,which endows this material with outstanding resistance to oxidation and thermal shock,making it applicable in many high temperature fields.By realizing the bonding between Ti₂AlC phase and metals,it is possible to overcome the shortcomings existing in the fabrication of this material,thereby promoting the application of both two substrates.The present work aims to join the Ti₂AlC ceramics to Ni using brazing technique.Firstly,predict the influence of filler elements on the stability of Ti₂AlC model using DFT method.With the combination of thermodynamic calculation and alloying experiment,analyze the presence and decomposition behaviors of meta-stable phases which might form during the brazing joints;then,perform the brazing experiments and obtain the optimal brazing parameters;finally,reveal the joining mechanisms with a series of characterization techniques.The formation mechanism of intrinsic vacancies in Ti₂AlC model has been simulated using DFT method.It is found the formation of an Al or a carbon vacancy is much easier than that of a Ti vacancy,whose defect formation energies are lower than the latest one.In addition,the influence on structural stability with the introduction of a Ti vacancy is remarkable,while that of a carbon vacancy is insignificant.According to the stacking model,1/3 Al vacancies could be accommandated in the Al planes.In other words,the defective Ti₂Al_0.667C is stable with respect to vibration.Afterwards,the filler element substitutions are introduced.Ni atoms are possible to substitute for Ti and Al atoms.While the later configuration possesses lower defect formation energy,the Ni atoms are most likely to be found on Al sites.However,such a Ni substitution is isolated in the Al plane,and grabs the valence electrons from Ti and carbon layers.In consequence,the bonding strength between Ti and Al layers is weakened,which is harmful to the structural stability.Using the hot pressing technique,the bulk Ti₂AlC with Ni dopant is fabricated.The Ni element is mainly found around Ti₂AlC grains,forming the Ti₂?Al,Ni?C phase with high Ni content,corresponding to the Ti₂(Al_0.875,Ni_0.125)C meta-stable model given by DFT.However,the long-distance diffusion might be difficult to occur,the average solid solution is not achieved in this work.Cr atoms are possible to substitute for Ti atoms,and the achieved(Ti_2-x,Cr_x)AlC model is stable in the whole 0?x?2 range.Si atoms are most likely to be found at Al sites,with a negative defect formation energy,which suggests that the substitution process might be spontanious.The achieved Ti₂(Al_1-x,Si_x)C model is stable in the whole 0?x?1 range.However,the Ti₂SiC part is nonexisting in the Ti-Si-C system,making the Ti₂(Al_1-x,Si_x)C solid solution decompose to its competing phases when the Si content is high.Using spark plasma sinter and hot pressing methods,the bulk Ti₂AlC with Si dopant is fabricated.The Ti₂?Al,Si?C solid solution has been characterized,suggesting that the Ti₂AlC phase with Si substitution might be stable.The BNi-2 filler alloy is adopted in this work,which exhibits an excellent wettability on the Ti₂AlC substrate,whose contacting angle is lower than 30°when the experiment temperature is above 1000?.Afterwards,the Ti₂AlC/Ni joints have been successfully achieved at the temperature range from 1000? to 1100?,together with a holding time from 15 min to 60 min.In the brazing process,the Ni element from filler diffuses into the Ti₂AlC substrate along grain boundary,contributing to the formation of Ti₂Al_1-xC[Ni]phase when the Ni content is low,and inducing the decomposition of Ti₂AlC substrate to TiC and Ni₃?Al,Ti?when the Ni content is high.As a result,the composition of the liquid filler is changed as the same time,and the joint with multi-layer microstructure forms according to the isothermal solidification process:{Ti₂AlC substrate}-{Ti₂AlC+Ti₂Al_1-xC[Ni]}-{Ni₃?Al,Ti?+TiC}-{Ni₃?Al,Ti?+TiB}-{Ni[Si,Ti]_ss+CrB}-{Ni substrate}.The obtained Ti₂AlC/Ni joints possesses excellent mechanical properties,whose maximum shear strength is measured to be 193 MPa,accounts for more than 90%of the ceramic substrate,with the corresponding optimal brazing parameters:1100? holding for 30min.Afterwards,the Ti₂AlC/GH3128 joints have been obtained according to the above brazing parameters,which exhibit excellent high temperature properties:the joint strength remains unchanged when the temperature is lower than 650?,and reduced by32%when the temperature reaches up to 800?.Finally,the interaction mechanisms between filler elements and Ti₂AlC substrate have been further studied using TEM.Ni element is found to be able to diffuse into the Ti₂AlC grains along Al planes,and extract the Al atoms out at the same time.Such an effect induces the decomposition of Ti₂AlC grains,and contributes to the formation TiC and intermetallic lamellas.