Study On The Interface Reaction Between Niobium Silicon Alloy And Molds

Nb-Si alloy is a promising candidate material of engine blade which isexpected to work at1200¹400℃, because this alloy has high-temperaturestrength, certain toughness at room temperature, high melting point, and lowdensity. But its high activity and high melting point put forward high requirementsto the stability of investment casting mould materials, so it is very meaningful tostudy the interface reaction mechanism between Nb-Si alloy and mold materialsfor the Nb-Si alloy investment casting.Mold materials dissolve thermodynamic model was adopted, from the pointof view of thermodynamics,this paper analyzed the influence law of melttemperature and alloy composition on the casting interfacial reactions betweenNb-Si alloy and ZrO₂, Nb-Si alloy and Y₂O₃, Nb-Si alloy and Y₂O₃-ZrO₂compound mold. Results showed that with temperature arising and silicon contentincreasing the interface reaction between Nb-Si alloy and casting material werestrengthened.The Gibbs free energy of the interface reaction between Y₂O₃andNb-Si alloy was basicly changing in a small negative value range, and the Gibbsfree energy of the interface reaction between ZrO₂and Nb-Si alloy was a biggernegative value.This means the interface reaction between ZrO₂and Nb-Si alloy issevere and the stability of Y₂O₃is higher than ZrO₂. Zr reduced the activity of Nb,lessened the diffusion distance of O in the melt; Y improved the activity of Nb andincreased the diffusion distance of O.The interface reaction experiment between Nb-Si alloy and ZrO₂, Y₂O₃, andY₂O₃-ZrO₂compound mold was carried out with SEM, XRD, and micro hardnesstesting methods et al. It resulted that the interface reaction between Nb-Si alloyand ZrO₂was more severe than that between Nb-Si alloy and Y₂O₃, the thicknessof reaction layer was obviously increased and the diffusion distance of O is shorterthan the latter; the interface reaction between Nb-Si alloy and ZrO₂deviated shellside, and that between Nb-Si alloy and Y₂O₃shell interface reactions biased meltside; the increase of silicon content can make the interface reaction severe; thestability of Nb-Si alloy is the best under the condition of Y₂O₃-ZrO₂compositeshell, for the Y₂O₃-ZrO₂composite shell is the densest, and Zr element reduced thediffusion distance of O in the melt, so as to verify the validity of the thermodyna-mic analysis.Finally, combined with comprehensive analysis of previous research resultsand the Nb-Si alloy phase diagram, the interface reaction mechanism between Nb-Si alloy and three kinds of casting materials were studyed. It resulted that theinterface reaction mechanism between Nb-Si alloy and casting material belongs tothe typical diffusion dissolution mechanism, and influenced by such factors as theinteraction between elements, shell surface microscopic morphology, refractorine-ss,and the thermal conductivity. From the application perspective, ZrO₂shell is notsuitable to be casting material for Nb-Si alloy, Y₂O₃-ZrO₂composite shell isqualified to Nb-Si alloy and can make Nb-Si alloy has the best stability, and ischeaper than Y₂O₃shell.