| Controlling nucleation was an important method to obtain the ideal structure during the solidification process,while the crystal nucleation was mainly determined by nucleation undercooling.At present,the quantitative theoretical description and experimental research between nucleation undercooling and its influencing factors were still lacking,leading to a lack of accurate theoretical guidance in the experimental research related to heterogeneous nucleation undercooling of alloy melts.Therefore,this thesis focused on the factors affecting the nucleation undercooling of superalloys during investment casting,and the effect of alloy composition and interface wetting on the nucleation undercooling and the formation of stray grain(SG)of single-crystal superalloys were studied.A new type of refiner with a low misfit with the alloy was designed,and the role of heterogeneous particles in the heterogeneous nucleation of equiaxed crystal superalloys was studied,aiming to provide theoretical basis and technical guidance for the control of the solidification structure of superalloys.By adjusting the content of Ta,W,Re,and Ru in the superalloys,the effect of the alloy composition on the nucleation undercooling and the formation of variable cross-section platform SG of nickel-based superalloys was studied.The experimental results indicated that the critical nucleation undercooling of the alloy was significantly improved after adding Ta element,and the critical nucleation undercooling of the alloy was reduced after the addition of W,Re,and Ru.In the directional solidification experiment of multi-scale single-crystal variable cross-section platform,Ta element increased the critical size of forming SG in the shadow side platform,indicating that Ta element reduced the tendency of forming SG in the variable cross-section platform,while W,Re,and Ru reduced the critical size of the formation of SG in the shadow side platform,indicating W,Re and Ru enhanced the tendency of forming SG in the variable cross-section platform.Moreover,another type of SG formed by dendrite remelting was found in the variable cross-section platform,and it was proposed that the heat transfer between solid and liquid phases was the main cause of dendrite remelting.However,the heat flow transmission at the solidification front of the undercooled melt would be increased with increasing the critical nucleation undercooling of the alloy,resulting in an increase in the tendency of SG formed by the dendrite remelting.The effect of ceramic substrates on the wettability and formation of SG of nickel-based superalloys was studied.The results indicated that interfacial reactions occurred when the high-temperature alloy melt was in contact with SiO2-based,Al2O3-based.ZrSiO4 and CoAl2O4 ceramic substrates.Therefore,the ceramic substrates in contact with the alloy melt were replaced by reaction products,which were(Al2O3+HfO2),(Al2O3+HfO2),(Al2O3+HfO2+ZrO2),and(Al2O3+HfO2+Co),respectively.Meanwhile,the wettability of the four systems increased successively.This was because that the wettability of the new system mainly depended on the physical properties of the reaction products.The experimental results of single crystal variable cross-section platform samples prepared by Al2O3-based,ZrSiO4,and CoAl2O4 shell indicated that the critical size of variable cross-section platform forming SG corresponding to the three types of ceramic shells reduced successively,and the tendency of forming SG in turn increased.The main reason was that the better the wettability between the alloy and ceramic sunstrate,the lower the interfacial energy when the crystal nuclei are precipitated,and therefore the lower the nucleation undercooling and nucleation work of the undercooled melt.To understand the wettability and interfacial interaction between the alloy melt and ceramics has a good guiding role in the selection of ceramic materials for Nickel-based superalloys during the investment casting.The formation mechanism of variable cross-section platform SG and process controlling methods were analyzed,and proposed that the formation of SG in the variable cross-section platform depended on the relationship between the maximum structural undercooling of the platform and the critical nucleation undercooling of the alloy.The greater the critical nucleation undercooling of the alloy,the less likely it was to form impurity crystals.ProCAST numerical simulation result displayed that when the temperature field of the platforms on both sides of the casting was symmetrically distributed,the probability of undercooled melt at the boundary of the platform was reduced,and the tendency of SG was weakened.When increasing the thickness of the ceramic shell,the undercooling at the corners of the platform was reduced,which effectively suppressed the formation of platform SG.With increasing the withdraw speed,the structural undercooling at the corners of the platform was increased,Therefore,the tendency of SG formation was enhanced.ProCAST numerical simulation was used to analyze the influence of different process conditions on the formation of SG on the variable cross-section platforms,which provided the reference for optimizing the process parameters during directional solidification.A new type of Nickel-based superalloy refiner was developed,and the effects of WC powder and the new refiner on the solidification structure of equiaxed crystal superalloys were studied.The results indicated that when WC powder was added into the alloy melt,the grain size tended to decrease,but the effect was relatively limited.With adding 1wt.%new refiner into the alloy melt,the average grain size of the alloy was reduced from 1.76mm to 0.32mm,and the volume fraction of cross-section equiaxed crystal was increased from 11%to 86%.This was because that the nucleation undercooling required for crystal nucleus precipitation depended on the lattice misfit between the crystal mucleus and the substrate.While the lattice misfit between the new refiner and crystal nuclei was low.Hence,the undercooling and nucleation work required for nucleation were very small. |
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