Design Of Ti/Ti-Al Microlaminated Sheets And Its Preparation By EB-PVD Technology

Metal thermal protection system (TPS) is one of the key techniques of the reusable launch vehicle (RLV). Since contacting with the external environment directly, the research on materials of metal TPS sheet is the most important one. To choose the lightweight materials that can take place of the Ni-based superalloys sheet, Ti/Ti-Al microlaminated round sheets with a diameter of 1000mm as well as thickness of 0.1~0.3mm are fabricated successfully by electron beam-physical vapor deposition (EB-PVD) in this paper. Not only the preparation process and characteristics of EB-PVD are discussed but also the microstructure and properties of the alloys are studied by modern analysis and test methods, which establishes the base for the engineeringl application of Ti/Ti-Al microlaminated sheet. The main contents of the study include: the optimum design of the microstructure parameter for Ti/Ti-Al microlaminated sheet, residual stresss analysis of for Ti/Ti-Al microlaminated, the effect of processing parameters on the processs of evaporation and deposition, preparation method of the Ti/Ti-Al microlaminated sheet, as well as the microstructure and phase composition, the evolution and collapse of microstructure under high temperature, and the impact of stresss on the creep deformation at high temperature for the microlaminate.The effect of microstructure parameters on failure performance of microlaminate was studied by simulation of three-point bending test. The results showed that the fracture work increases gradually with the increasing of layer number or thickness ratio. However, the extent of increasement reduced gradually and when the two parameters exceeded a certain value, the fracture work would not be changed.Correlative theory in materiall mechanics and FE method were used to study the residual stress formed during the preparation processs of microlaminate sheet. It was demonstrated that the residual stresss is a function of the locateon along radial direction of microlaminate sheet. It was pointed out that the value of residual stress reached its maximum at the centure and reached its minimum at the edge of microlaminated sheet. Residual stresss in Ti layers and Ti-Al layers was compressive stress and tension stress respectively. With increasing of layer number or thickness ratio, compressive stress would rise gradually and compressive stress would reduce gradually. X ray diffraction was used to determine residual stress distribution of outmost two layers of deposited microlaminated sheet. Results showed that residual stress decreases gradually along the radius from center to edge of sheet metal. The trend of residual stress of actual measurement is similar to that gained with numerical method.The influence and the feasibility of processing parameters on the preparation technology have been discussed. According to the function between target-substrate distance and the collision probability, combined with the processs requirements and utilization ratio of target materials, the optimal source-substrate distance was fixed as 310mm. The effect of saturation vapor pressure on the evaporation process of Ti-Al alloy source was discussed, and the experimental result showed that the addition of Nb into molten pool make it earlier to reach the steady-state compare to that without addition of Nb, and the composition of deposit with Nb addition at steady-state was much closer to that of source materials. From the calculation result of the ratio of re-evaporating capacity with depositing capacity of Al on the substrate, it can be concluded that the effect on deposit by re-evaporation of Al could be neglected.The surface quantity of Ti/Ti-Al microlaminated sheet is well. AFM patterns of outer layer at different scales indicate a fractal characteristic due to growing kinetics. The modulation architecture and inlaid interlaminar interfaces can be seen from the cross-section of microlaminate. The difference of melting points between Ti and Al led to the distinctive structure: the Ti-Al layers were mainly constituted of equiaxed grains and Ti layers were constituted of column grains. The porosity decreased obviously when the as-deposited materials were densificated by hot-pressed technology and the density has been increased from 94.91% to 98.07%.Because of the deviation of saturated vapor pressure between Ti and Al element, the component showed a gradient change periodically along the normal direction of Ti-Al layers and results in several sub-layers. The Ti layers, Ti-Al layers and interfacial layers were constituted ofαphase,γ+α2 phase andα2 phase respectively. The test results of high temperature annealing showed that the ordered phase reduced due to the diffusion of Al into Ti. Based on the Fick's second law, the function between the concentrateion of Al in Ti with diffuseion temperature, time and the thickness of Ti is estabilished from which the concentration distribution of Al in Ti according to different temperature and time. Research results showed that the break down of layered architecture was induced by pore formation, grain growth and the grain boundary grooving.It was found that the nanohardness and elastic modulus of Ti layer and Ti-Al layer are of gradient distribution according to the distance to the interface layer which may be results from the pile-up effect and the stress concentrateion at interface.The addition of Ti ductileity layer can improve the mechanical properties of Ti-Al alloy at room and high temperature and hot-pressed densificated technology is necessary to get a finer performance. The change of interlaminar space and the thickness ratio will affect the mechanical properties by means of deposition characteristic and mixed rule of composite respectively. The cracks will stagger along the interlaminar interface or the layer due to which microlaminate expresses a good characteristic of delayed fracture. The toughening mechanisms are that the crack deflection and micro-bridge connection caused by the toughening layers increases crack propagation resistance.Finally, the impact of vared stresses on the creep property for Ti/Ti-Al microlaminate at 650℃was investigated and the constituteive equations which could be used to describe the slowing creep stage and the steady creep stage was obtained. During the steady creep stage, the creep stresss exponent changed from 1.53 under a stresss of 60～70MPa to 7.66 in the stress range of 70～100MPa, it indicated that with the increasement of stress, the creep mechanism will transmited from the interface slide to the recovery creep controlled by dislocateion climbing.