Phase Transformation Behavior And Densification Mechanisms During High-temperature Sintering Of Continuous Alumina Fibers

Continuous alumina fibers,having excellent mechanical properties and high-temperature stability,are widely used in aerospace applications,such as hypersonic vehicles and aero engines.As a key strategic material,continuous alumina fibers are subject to strict product embargoes and technology blockades by western countries.The excellent mechanical properties of continuous alumina fibers depend on the dense microcrystalline structure.However,the evolution rules of the microcrystalline structure and its quantitative relationship with the sintering regime are the key issues that need to be solved urgently.In this paper,the phase transformation,densification,and grain growth of sintered continuous alumina fibers by sol-gel-dry spinning were investigated systematically.The effects of ferric and silica sol additions on the phase transformation behavior of continuous alumina fibers in high-temperature sintering were clarified.Besides,the evolution of fiber structure and properties in high-temperature sintering,as well as the kinetics of densification and grain growth,were revealed.These can provide a theoretical basis and technical support for the preparation of high-performance continuous alumina fibers.The main innovative findings are as follows.（1）The effects of ferric and silica sol additions on the high-temperature phase transition during the preparation of continuous alumina fibers were elucidated.During high-temperature sintering,the phase transformation path of alumina fibers,derived from aluminum carboxylate sol,is amorphous Al₂O₃→amorphous Al₂O₃+γ-Al₂O₃→amorphous Al₂O₃+γ-Al₂O₃+α-Al₂O₃→γ-Al₂O₃+α-Al₂O₃→α-Al₂O₃.The formation temperature ofα-Al₂O₃ is lowered with the addition of ferric sol,and the transformation toα-Al₂O₃ of the fibers is accelerated with the increasing amount of ferric sol.An increase ofγ-Al₂O₃ toα-Al₂O₃transformation temperature by～30°C and that of activation energy in the non-isothermal phase transformation by 87 k J/mol are caused by the introduction of a trace amount of silica sol,besides the addition of ferric sol.But the phase transformation temperature is still lower than that of the additive-free fibers.The phase transformation process of continuous alumina fibers is accelerated by the elevated sintering temperature,while the phase structure evolution rule is not affected,even at temperatures much higher than those of theα-Al₂O₃ phase transformation.The competing mechanism of spontaneous nucleation and epitaxial growth in the phase transformation of the fiber was proposed.（2）The effects of ferric sol additions and the sintering process on the structural evolution of high-temperature sintered alumina fibers were revealed,and the mechanisms controlling densification and grain growth kinetics of the fibers were elucidated.The continuous alumina fibers show a relatively dense structure at the beginning of isothermal sintering at1200-1500°C.With the formation and growth ofα-Al₂O₃,the adjacentα-Al₂O₃ grains are gradually connected and nano-pores are produced between the grains.Densification while grain growth of the fiber results from the elevated temperature or the extended holding time.The increase of the ferric sol content contributes to the improvement of densification and the control of the grain size of the fiber.As the sintering temperature increases,the densification controlling factor of alumina fibers shifts from grain boundary diffusion to lattice diffusion,and the grain growth behavior shifts from surface-diffusion-controlled to lattice-diffusion-controlled.A two-step sintering schedule was designed and proposed based on the kinetic behavior of the fiber microstructure evolution.Alumina fibers,with4.50 wt%ferric sol additive and sintered at 1300°C for 3 min and subsequently 1200°C for 20 min,have a fully dense structure with a grain size of only 100 nm and average tensile strength of 1.73 GPa.（3）The evolution of microstructure and mechanical properties of continuous alumina fibers during high-temperature treatment was revealed,and the influencing factors of thermal stability and high-temperature failure behavior of fibers were investigated.The grain size of continuous alumina fibers grows gradually with the extension of holding time at a high temperature of 1000-1200°C,and reaches～300 nm after holding for 60 h at 1200°C.The tensile strength retention rate of 73%after holding for120 h indicates good thermal stability of the fibers at 1000°C.Grain coarsening of the alumina fiber is aggravated with the elevated holding temperature or the application of water vapor,which results in a significant decrease in the mechanical properties of the fibers.At high temperatures defects,such as grain coarsening and pores among grains,are the main causes of the degradation of mechanical property and the failure of the fibers.