Preparation And Performance Of Rhenium Electrodeposited From Molten Salts

Due to its unique mechanical,physical and chemical properties at elevated temperatures,rhenium?Re?has been considered as one of the most promosing materials in aerospace and military fields.In this dissertation,for the purpose of developing Ir/Re combustion chambers used in the liquid-propellant rocket engine,the preparation and performance of Re electrodeposited in molten salts?ED?were first studied in China.A suit of equipment was first designed and assembled for the preparation of molten salts and electrodeposition of Re coating.Then the effective routine for the preparation of molten salts and the thermal stability of Re ion were studied,respectively.The effect of processing parameters of ED on the microstructure was explored and the optimized parameters for Re coatings with different microstructures were obtained.Further more,thick Re coatings and typical Ir/Re chambers were prepared by electroforming in molten salts.The mechanical properties and the diffusion behavior of Ir/Re were studied,respectively.First,the optimized processing parameters for preparation of K₂Re Cl₆ and effective routine for preparation of mixed molten salts were explored,and the thermal stability of Re ion in mixed molten salts was also studied.The results show that the maximum conversion percent??95%?of K₂ReCl₆ can be obtained at 560?.Disproportionation reaction ourred during the dissolving of K₂ReCl₆,resulting in the formation of Re and volatile ReCl₅.The mixed molten salts can be effectively,steadily and controllably prepared by dissolving K₂ReCl₆ and chlorinating over the background at the some time.The equilibrium of Re???and R e???exists in the mixed molten salts,and the disproportionation reaction of the unstable Re???is incline to occur at the interface.With the increasing of Cl₂ partial pressure in the atmosphere and cationic radius?Na⁺?K⁺?Cs⁺?in mixed molten salts,Re???transform ed to Re???,with the stability of Re???increasing.The effect of processing parameters of ED on the microstructure was systematically studied,the results show that the electrodeposition process of Re coating is mainly controlled by electrochemical reaction process.The rate decision step changes from electrochemical reaction process to diffusion process slowly with the decreasing of temperature and Re ion concentration,and the increasing of cathodic current density,respectively,leading to the transformation of surface morphology and growth orientation of Re coating.Therefore,a wide parameter window is obtained for the preparation of compact Re coating.The optimized parameter for compact Re coating is within the temperature of 800?880?,Re ion concentration of 1?5 wt.%and the cathodic current density of 20?50 mA/cm²,respectively.Black Re coating with spired grain was inclined to develop with the increasing of CsCl content and cathodic current density,and the decreasing of temperature.When the concentration of oxygen-containing impurities in the melt decreased,electrolysis temperature increased,or cathodic current density decreased,the direction of the growth texture changed as follows:<100>?<110>?<002>and the growth model changed from outwards to lateral model.The entire Re coating could be mainly divided into three stages,micro-nanoscale particles near the substrate induced by the disproportionation reaction of the intermediate Re???,the m ixtured stage and the continuous co lumnar grain stage by the sequential electroreduction of Re???,respectively.The effect of processing parameters of electroforming on the surface quality,microstructure and mechanical prpperties of Re were studied,the results show that the CsCl content has great influence on the surface quality of Re,but little influence on the microstructure and mechanical properties of as-electroformed Re.Nubbles will both appear on the surface of Re electroformed in melts with too low or high CsCl content.A fine columnar structure with abundant laminar microstructures and twins within the fractured grains was obtained in the electroformed Re,with the tensile strength higher than 750 MPa and elongation less than 5%.However,the microstructure and mechanical properties of Re electroformed in melts with different CsCl content changed differently with the annealing temperature increasing.For Re electroformed in melts with CsCl content less than 40 wt.%,the grain configuration changed as follows: near-columnar,fiber-like structure?fine equiaxed grains?coarse equiaxed grains ?coarse equiaxed+large columnar grains and the tensile strength decreased slowly.For Re electroformed in melts with CsCl content higher than 60 wt.%,the grain configuration became finer and laminar,the tensile strength increased.The elongation increased from less than 5%to 15?20%after annealing from 1000 to 1600?.The surface quality and tensile strength both improved by decreasing cathodic current density,and the elongation had no obvious change.Based on the surface quality and mechaniacal propertied of Re electroformed at different processing parameters,it can be concluded that the optimized CsCl content in melts is 40?60 wt.%and the cathodic current density is 25 mA/cm².A combined mechanical properties can be obtained by annealing at 1200?1400??tensile strength?800 MPa,elongation?20%?.The purity of electroformed Re was higher than 99.98%,and the density was 99.6%of the theoretical density.Typical Ir/Re chambers with different sizes were prepared by electroforming in molten salts and the diffusion behavior of Ir/Re was studied.The results show that electroforming is suit for the net-shape forming of complex-shaped rotatory component with good thickness uniformity.The diffusion behavior of Ir/Re was studied in the temperature range of 1400?2000?using the sem i-infinite model.The diffusion coefficients and activation energy are obtained.Re diffuses more quickly than Ir at the CVD Re/ED Ir interface,while Ir diffuses more quickly than Re at the ED Re/ED Ir interface.