Study On Interfacial Reaction And Properties Of Nb/Re Laminated Composites Deposited By Chemical Vapor Deposition

Rhenium is a high-melting（3180°C）rare metal with a hexagonal structure.It has high comprehensive strength such as high temperature strength,good plasticity,thermal shock resistance and chemical inertness.It has become the preferred structure for the third generation aerospace engine material.The Re-based/Ir-coated engine prepared by chemical vapor deposition（CVD）technology in the United States has successfully implemented its application on satellites.The engine operating temperature exceeds 1800°C.Although Rhenium has excellent high temperature mechanical properties,it has problems such as high price and high density.In this paper,a lightweight and inexpensive refractory metal Niobium is proposed as the matrix,and Rhenium is a strengthening layer.A lightweight and high-strength new Nb/Re layered composite material is designed and constructed.The combination of theoretical simulation calculation and experimental research is used to systematically study the thermodynamics,deposition kinetics,interfacial diffusion and reaction mechanism,microstructure and mechanical properties of the CVD reaction process.The main research conclusions are as follows:Through the global local search and first-principles calculation,it is determined that theχphase formed by the Nb/Re layered composite interface is Nb_0.18Re_0.82,Nb0.25Re0.75,Nb0.29Re0.71,Nb0.31Re0.69 and Nb0.46Re0.54.The crystal structure,thermodynamic stability and mechanical properties of each phase were obtained,and it was confirmed that the Nb_0.25Re_0.75 phase has the best thermal stability and mechanical properties in the predicted mesophase,and its bulk modulus is 738.5GPa,The B_H/G_H value is 1.8 and the anisotropy index is 0.20.The experimental analysis indicates that the Nb/Re interface forms the NbRe₃ mesophase,which is consistent with the first-principles calculation.The effects of deposition temperature and chlorine gas flow rate on the deposition rate of Nb and Re were studied.It is found that within the range of process parameters determined by the paper experiments,the deposition rate increases with the increase of deposition temperature,and the chemical vapor deposition kinetics of Nb and Re in accordance with Arrhenius equation are obtained.In the experimental temperature range,the chemical vapor deposition processes of Nb and Re are dynamic control mechanisms.In order to further optimize the deposition conditions,the molecular dynamics is used to simulate the deposition process,and the optimal process parameters of high quality Nb and Re materials are obtained.The deposition temperature is 1100-1200°C,the atomic incident energy is0.3-0.5 eV,and the incident angle is 0-10°.The atomic diffusion law of Nb/Re interface is studied.An interface diffusion model was established,and the interdiffusion coefficients in different phases of the interface region were obtained by calculation.The atom is rapidly diffused by the grain boundary in the Nb solid solution.The interdiffusion coefficient is1.66×10^-6exp（-198.8/RT）,which accords with the Arrhenius equation.In theχphase,the diffusion mechanisms of the atom above 1600°C and below 1600°C are different.Below 1600°C,the bulk diffusion is dominant,the diffusion coefficient is:2.69×10⁴exp（-584.5/RT）;above 1600°C,the fast channel diffusion along the grain boundary,dislocation and other defects is dominant,and the diffusion coefficient is2.8×10^-10exp（-89.2/RT）.The microstructure and phase structure of the composites during the deposition process were characterized.Nb and Re are formed by fine-grained regions close to the substrate and coarser columnar crystals continuously growing in the deposition direction;low-density dislocations are found in CVDNb,and many different directions of V and W appear in the columnar crystals.Sub-structures such as layered etched grain and growth twins;the recrystallization of Nb and Re grains conforms to the Arrhenius growth equation,and under the same heat treatment conditions,the growth rate of Nb grains is significantly larger than that of Re grains;The diffusion process in the Nb/Re interface region is a large amount of diffusion from Re to Nb,and the interface region is sequentially distributed with Nb solid solution,χphase and a very small amount of Re solid solution.Based on the results of kinetic experiments and molecular dynamics simulations,three series of Nb/Re two-layer composites with Re content fractions of20%,30%and 40%were prepared by optimized CVD technology.The density reached 99.6%and 97.0%and 95.9%of the theoretical density respectively.The tensile strength of the deposited Nb/Re composite increased with the increase of Re volume fraction,but both were lower than the theoretical composite strength.After proper heat treatment,the tensile strength at room temperature reaches 516 MPa,611 MPa and 461 MPa,respectively.The intensity of Nb-30%Re exceeds its theoretical composite strength by 36%,and a strong composite effect appears.The strength value is much higher than the currently widely used Nb-based alloys such as Nb521 and C103,which is close to powder metallurgy Re,but its density is only58%of pure Re,and the material cost is reduced by more than 50%.It is a light,high-strength and low-cost new composite material,which has good application value as aerospace engine nozzle material.Based on the theory of layered composite reinforcement and the study of interfacial diffusion and interfacial reaction phase structure,the composite effect of Nb/Re layered composites was analyzed.It is found that the composite has excellent comprehensive mechanical properties when the thickness of theχphase layer is between 1.5 and 2.0μm and the ratio of Nb solid solution toχphase thickness:α（Nb）:χ=2±0.5.The thickness of theχphase layer and the ratio of the Nb solid solution to the thickness of theχphase are the essential factors determining the strength of the composite effect in the Nb/Re layered composite.