Investigation On Solidification Microstructure Evolution And Strengthening And Toughening Mechanisms Of Ultrasonic Treated Nb-Si Based Alloys

Nb-Si based superalloys are composed of Nbss matrix and silicide reforcement with excellent high temperature strength,which have good matching of strength and toughness.Compared with the Ni-based superalloys,they have the advantages of lower density,higher service temperature and better high-temperature creep resistance,and they are the most potential candidate for the next generation of structural materials used in the hot-side parts of aircraft.However,the size of the primary silicide is too coarse,which means the room temperature fracture toughness is low,and they can not meet the minimum requirements of hot processing and limits further engineering application.Therefore,it is of vital importantce to research the refinement,strengthen and toughen of silicide.In this paper,the composition design and microstructure control of Nb-Si based alloys are carried out by alloying and ultrasonic treatment to improve the size,morphology and distribution of primary silicide phase,thus achieving the purpose of refining microstructure and improving mechanical properties.In this paper,the effects of ultrasonic treatment on the element distribution,solidification routes,microstructure,mechanical properties,especially fracture toughness of Nb-Si based alloy are systematically studied.The effects of Mo and Cr elements on silicides and Nb Cr₂ precipitates in the ultrasonic field and the effects of Gd and Re rare elements on Nbss matrix in the ultrasonic field are studied,and the microstructure evolution and strengthening and toughening mechanism in the ultrasonic field are explained;Finally,by analyzing the energy distribution and supercooling in ultrasonic field,the refining mechanism is revealed.Research shows that the ultrasonic treatment can significantly improve the flow of Nb-Si melt,and form the more uniform solute and temperature field.Through the characteristics of energy distribution in ultrasonic field and the"mountain"shaped silicide with special morphology in transition zone,it can be estimated that the introduction efficiency of the indirect ultrasonic treatment equipment used in this experiment is about 62.5%,and the maximum flow rate of the melt is 0.26 m/s.It will also produce a feeding force of 0.3 atmospheric pressure,effectively reducing the shrinkage defects and obtaining high densification Nb-Si based alloys ingots after ultrasonic treatment.After ultrasonic treatment,the microstructure is obviously refined,and the silicide with coarse and sharp edges in hypereutectic components disappear and basically change into the fully eutectic.The alloy with developed dendritic Nbss will be formed under the as-cast solidification condition,and the secondary dendrite length will be significantly reduced after ultrasonic treatment,which tends to be equiaxed grain.The effects of ultrasonic treatment on peritectic,eutectic and eutectoid reactions in Nb-Si based alloy change the solidification routes and the phase ratio,so that it gradually approaches the theoretical calculation value under equilibrium solidification.Moreover,the ultrasonic treatment can improve the solid solubility of alloying elements,regulating the formation and dissolution of Nb Cr₂precipitates,and changing the phase composition.After ultrasonic treatment,the mechanical properties of Nb-Si based alloy are significantly improved,including the increase of 3.2%in microhardness,13.4%and21.6%in compression strength and fracture strain at room temperature respectively,and the increase of 20～50%in fracture toughness.The strengthening caused by ultrasonic treatment is the result of the joint action of fine grain,second phase and solid solution strengthening,and the size of the strengthening phase is refined,from the blocky morphology with sharp edges to（101）_Nbss∥（101）_β.The fine eutectic structure co-grown by the semi-coherent relationship reduces the stress concentration and improves the interface bonding strength.However,deflection,bridging and secondary cracks can be observed in the refined eutectic structure during crack propagation,which can prolong the crack propagation path,increasing the surface energy,consuming more strain energy and passivating the stress field at the crack tips,making the cracks are easier to be captured by the ductile phase.Therefore,Nb-Si alloys have obvious strengthening and toughening effect after ultrasonic treatment.The solidification behaviour in ultrasonic field shows that the increase of micro-area pressure in melt caused by cavitation changing the parameters such as the melt distribution coefficient,diffusion coefficient,theoretical crystallization temperature and solidification rate,resulting in the obvious difference between the solidification behavior of Nb-Si based alloys in ultrasonic field and the as-cast solidification:first,the high pressure caused by cavitation effects significantly increases the theoretical crystallization temperature and promotes nucleation.Then,because of the effects of ultrasounic vibration,the tendency of component supercooling is increased,and the growth mode of primary phase has changed.Finally,due to the obvious refinement of the cellular structure in ultrasonic field,the curvature supercooling caused by the micron scale will further increase the supercooling and promote the growth.In the refining mechanism of ultrasonic treatment of Nb-Si based superalloys,the main effect is the ultrasonic cavitation-enhanced nucleation.The cavitation effect produces sufficient undercooling to promote nucleation,and protects the crystal nucleus from the temperature fluctuation and remelting.The auxiliary effect is acoustic streaming,the primary dendrites on the crucible wall will be broken and washed into the melt as the heterogeneous core to promote nucleation and promote the nucleus homogenous redistribution.Moreover,ultrasonic treatment has inhibitory effect on crystal growth,slowing down the growth rate of grains and eutectic clusters to a certain extent.Under the premise of uniformly distributing a large number of crystal nucleus in the melt,each crystal nucleus can grow independently,ultimately obtaining fine grain structure.Therefore,the microstructure refinement by ultrasonic treatment is the combined results of ultrasonic cavitation and acoustic streaming.