Microstructure And Properties Of Ni-Based Alloy Sheet Prepared By EB-PVD

Metallic thermal protection system (MTPS) is one of the key techniques for developing reusable launch vehicle (RLV). As a new generation key thermal protection component, MTPS has high strength and toughness, high reliability, excellent thermal shock resistance and moisture resistance compared with ceramic tile. At the same time it also has advantages of good processing propoties and producing large size component easily. Therefore, MPTS has become a research focus in this area. In this paper, large-scale Ni-based alloy sheets are fabircatied by electron team physical vapor deposition (EB-PVD), with high strength and toughness, good jointing propoties. Their microstructure and mechanical properties are systematically investigated by means of X-ray diffactometry (XRD), sanning electron microscopy (SEM), atom force microscopy (AFM) and transmission electron microscopy (TEM) as well as mechanical test method. Effection on temperature and rotational speed of the substrate on microstructure and density is studed. Effection of ingots evaporation speed ratio and source-to-substrate distance on sheet thickness distribution and mass evaporation efficiency is studied and the mathematic model of thickness distribution is presented. In addition, high temperature oxidation behaviors of as-deposited and annealed Ni-based alloys are studied.In this paper, the Ni-based alloys were deposited at the substrate temperature 700℃and 450℃. The results showed that the as-deposited sheet with good toughness consisted of equiaxed grains at 700℃and with brittleness cnsisted of columnar microstucture at 450℃. The sheets were fabricated at 700℃with substrate rotational speed 5rpm, 12rpm and 30rpm. The results showed that density of the sheets decredsed with rotational speed increase, but a small decline in the extent.The thickness distribution uniformity is an important target which determines the service performance of Ni-based alloy sheet. According to EB-PVD process feature, a mathematical model of thickness distribution of the sheet deposited on rotary substrate surface is presented by taking the effection of ingots evaporation speed ratio and source-to-substrate distance on sheet thickness distribution uniformity into consideration. The theory of small plate evaporation source under the condition of vacuum evaporation is used in the predicted mathematical model. The predicted model follows cosnθlaw and agrees well with the measured data when n = 5.3. The model shows that the thickness distribution uniformity is worse with decreasing source-to-substrate distance. Thickness distribution can be adjusted through changing ingots evaporation speed ratio according to factual requirement. When source-to-substrate distance is not changed, mass evaporation efficiency increases linearly with increasing evaporaiton speed ratio of the ingot in No.4 crucible. When ingots evaporaiton speed ration is invariable, however, mass evaporation efficiency decreases linearly with decreasing source-to-substrate distance.The study results indicated that as-deposited Ni-based alloys consisted of equiaxis grains. The grain size varies from several nanometers to 1²Î¼m. As-deposited Ni-based alloys sheet consisted of single Ni-based solution and exhibited no texture. The heat treatment experiments were planned to confirm standard heat treatment regime of solution heat treatment at 1020℃for 0.5h, water quenching and aging treatment at 760℃for 48h, air-colled. After standard heat treatment, the grains of alloy sheet grew up to 4⁵μm and fine carbide particles with 20⁵0nm in size separated out on grain boundary. The carbide composition was (Cr,Fe)23C6, whose crystal structure was face-centered cubic (FCC). High temperature tensile strength of the heat treatment samples was improved obviously, such as from 64MPa for as-deposited samples to 275MPa for heat treatment samples at 800℃.The oxidation results show that oxide Cr2O3 is formed on the surface of as-deposited Ni-base alloy sheet firstly at 800℃. Secondly, inside oxide Al2O3 paticals are formed on the interface between matrix and Cr2O3 sacle. Lastly, oxide Al2O3 paticals grow up in landscape orientation and form Quasi continuous inside oxide Al2O3 scale. The oxide Cr2O3 scale is formed when as-deposited Ni-base alloy sheet is oxidezed at 900℃for 96h. There are inside oxide Al2O3 paticals formed on the interface and in matrix, but no Quasi continuous inside oxide Al2O3 scale is formed. TiO₂ and Cr2O3 are formed on the surface of the alloy sheet at 1000℃firstly. The oxide TiO₂ particles grow up shaply with oxidation time increasing. After 100h, the outer oxide TiO₂ forms Quasi continuous oxide scale. The oxide below TiO₂ is Cr2O3 scale. There are inside oxide formed in matrix, whose composition is Al2O3.The oxidation kinetics of as-deposited Ni-based alloy sheet at 800℃follows cubic law. The oxide grains grow up slowly at 800℃, so volume fraction of grain boundary is high. Therefore, the diffusion of elements through grain boundaries plays an important role in the oxidation scale growth on the Ni-based alloy sheet surface. A cubic law is derived and agrees well with the experiment data. The oxidation kinetics of as-deposited Ni-based alloy sheet at 900℃and 1000℃follows a parabolic power law.The oxide Cr2O3 is formed on the surface of aging heat treatment sample at the beginning of oxidation for 800℃. There are inside oxide formed on the interface between Cr2O3 scale and matrix and in matrix. After 100h, Quasi continuous inside oxide Al2O3 scale is not formed. The oxidation kinetics of heat treatment samples at 800℃follows a parabolic power law.