Study On Brazing Mechanism Of Ti₂AlC Ceramic And Its Stability

Ti₂AlC is a fascinating member of MAX phases and is renowned for itssuperior properties. It exhibits unusual combinations of properties of ceramic andmetal, i.e., low density, good thermal and electrical conductivity, which make it apromising candidate for use in sliding electrical contacts and high-temperaturestructural applications and as a conducting ceramics in harsh environments.However, large Ti₂AlC ceramics are difficult to synthesize due to their narrowphase range in the Ti-Al-C ternary phase diagram. A common approach to solve thisproblem is to join the ceramics, by which the manufacture of large, complex,multifunctional ceramic components can be fabricated. Therefore, joining of Ti₂AlC is significant in promoting the ceramic’s applications. Considering the electricalconductivity of the joint, Ag and Ag-Cu filler was selected for the joining of Ti₂AlC.Effects of brazing temperature, holding time and content of Cu in Ag-Cu filler onthe microstructure and mechanical properties of the joints were investigated in thisstudy. Nevertheless, during the brazing process, the structure of Ti₂AlC wasprobably changed by its interaction with Ag and Cu element, thus the intrinsicperformance of Ti₂AlC was deteriorated. For this reason, the structural stability wasinvesigated by means of first-principles method.By using first-principles calculations, it can be found that the lattice constantsa and b increase but c decreases with the increase of Al vacancies. The structurealso becomes more unstable for more Al vacancies. On the other hand, Ag and Cucan occupy vacancies in Ti₂AlC, and prefer to occupy the Al vacancies. Owing tothe Ag occupation, the lattice constants a and b remain almost unchanged and cincreases. The bulk modulus increases in a small range. In the case of Cuoccupation, the constants a, b and c decrease in different extent, while the bulkmodulus increases obviously. Both the occupation of Ag and Cu in Ti₂AlC canweaken the Ti-Al bond, and the weakening effect is more effective for Cu. So theformation energy of Al vacancy can become lower in the Cu occupied Ti₂AlC supercell. In addition, the solution of Al into Ag and Cu and the occupying process canboth provide energy for the formation of Al vacancies. Compared with the solutionand occupation behaviors, the formation of Al-Cu intermetallic compound showsmuch stronger effect on the stability of Ti₂AlC. The forming of Al₄Cu₉can furtherdecrease the formation energy of Al vacany to1.9031eV.The representative structure of the joint can be described as the three regions:Ti₂AlC, interaction area in the Ti₂AlC substrate and the brazing layer. In theinteraction area, the filler alloy distributes at the grain boundary. At the interface of filler/Ti₂AlC, Ag dissolves into Ti₂AlC and Ti₂Al_1-xC[Ag] forms. Simutaneously,Al dissolves into Ag, forming Ag based solution. During the cooling process,ordering transition occurs in the Ag based solution, producing Ag₃[Ti,Al]. Theshear strength and electrical conductivity increase and then decrease with theincreasing of brazing temperature. The volatilizing of Ag at high temperature givesrise to the weak bonding between Ag filler and Ti₂AlC. This can be considered asthe primary factor affecting the shear strength of the joint.Well-bonded joints can be obtained by using Ag-Cu filler without any hole orcrack being observed. With increasing the Cu content in Ag-Cu filler, the width ofthe brazing layer decreases, while the width of the interaction increases. Moreover,more Ti₂AlC grains are separated by dissolved filler. When the content of Cu is15wt.%, Al₄Cu₉appears in the joint. The electrical conductivity almost keepsconstant, while the shear strength of the joint increases and then decreases with theincreasing of Cu content. As the brazing temperature increasing and the holdingtime prolonging, the width of the brazing layer also decreases, and the width of theinteraction increases. In addition, more Al₄Cu₉phases form in the joint. The shearstrength of the joint reaches peak value as the brazing temperature rising, and theholding time prolonging. While the Ti₂AlC ceramic is brazed with Ag72Cu28wt.%filler at900℃for10min, the maximum strength （168.1MPa） of joints can beachieved, which is93%of that of Ti₂AlC substrate.Based on the experimental results, the change of lattice constant of Ti₂AlC inthe interaction area can prove the occupation behavior of Ag（Cu） in Ti₂AlC.Compared with the occupation of Ag, the occupation of Cu in Ti₂AlC exhibits lowerdissolution enthalpy, suggesting that Cu has a stronger tendency to occupy the Alvacancy in Ti₂AlC. Moreover, Cu possesses a lower activation energy to transfer inthe Ti₂AlC. Therefore, the EDS result shows that Ti₂AlC in interaction areacontains more Cu than Ag. Besides, Cu shows a wider diffusion depth in Ti₂AlC than that of Ag. Similar to that of Al and Cu, the crystal structure of Al₄Cu₉belongsto the cubic system, which means that the formation of Al₄Cu₉will experiencerelatively less change in crystal structure. Additionally, Al₄Cu₉has the lowestformation energy and binding energy. Thus, Al₄Cu₉forms most easily and keepsstable in various kinds of Cu-Al intermatellic compounds. The experimental resultalso shows that no other Cu-Al intermatellic compounds canbe found exceptAl₄Cu₉.