Study On Crystal Orientation And Stray Grain Control Of Third-Generation Ni-Based Single Crystal Superalloys

Due to excellent integrated performance under high temperature,Ni-based single crystal(SX)superalloys are widely used in the turbine blades and vanes for aeroengines and land-based gas turbines.Attributing to significant mechanical properties anisotropy of Ni-based SX superalloys,the precise control of crystal orientation has been become one of the important ways to improve the service life of advanced SX blades.Therefore,seeding technique is becoming the preferred method for preparing SX blades.However,the formation of stray grains at melt-back region lead to a low yield rate of SX blades and the single-use of seed,which significantly increases the production cost and becomes a major reason restricting the wide application of the seeding method.At the same time,the misorientation formed during directional solidification also causes the crystal orientation of SX blades deviating from the optimal crystal orientation,resulting in a decrease of the service life.In consequence,how to inhibition the formation and growth of stray grains at melt-back region,the repeated use of seeding,and the misorientation control during directional solidification become important and urgent tasks for preparation of SX blades.In this paper,using a third generation Ni-based SX superalloy DD33,the microstructure,the influencing factors and formation mechanisms of stray grains at melt-back region of seed,and the temperature field evolution during initial solidification stage are investigated.The effect of withdrawal rate and crystal orientation on competitive growth are to be clarified.Two new kinds of SX preparation techniques with the capacity of accurate controlling the crystal orientation are proposed.Finally,the formation of misorientation during directional solidification is explored.The main conclusions can be drawn as follows.(1)Based on the dendritic morphologies,the typical microstructure in seed segment could be divided into four zones: complete melting zone,mushy zone,heataffected zone,and original microstructure zone.An appropriate holding time could melt the broken dendrites at the upper of the mushy zone to improve the yield rate for SX casting.The mushy zone could be completely suppressed under a high temperature gradient during directional solidification.A certain incubation period was required before the temperature field evolution at initial withdrawal stage,and then the migration rate of liquidus isothermal gradually increases from zero to the predetermined withdrawal rate.According to the specific location and formation mechanism,corresponding measures could be taken to suppress the formation of stray grains at melt-back region of seed.For example,the decrease of the clearance between the seed and mold could inhibit the formation of stray grains formed by the backflow of the melt into the cold clearance.The nucleation of stray grain on rough surface of mold,which caused by constitutional undercooling at the initial solidification stage,could be suppressed by reducing the surface roughness of the inner wall of the mold.The stray grains originated from dendrite deformation could be avoided by using seed with small primary dendrite spacing or treated by the solution heat treatment prior to preparation of SX casting.(2)With the increase of withdrawal rate,the favorable oriented(FO)grain gradually dominated on the competitive growth of converging grains.At low withdrawal rate,the unfavorable oriented(UO)grain overgrew the FO grain.Whereas,the FO grains would overgrow the UO grain at high withdrawal rate.And the needed critical withdrawal rate,in which the bi-crystal coexisted during competitive growth,was positively correlated with the secondary dendrite orientation of FO grain.The competitive growth of converging grain occurs in three-dimension space,the phenomenon of dendrite insertion and the corresponding lateral motion could be found near grain boundary and the dendrite growth behavior follows the dendrite spacing adjustment mechanism.The competitive growth of converging grain process was that the dendrite spacing,which deviated from stable range caused by dendrite insertion and lateral motion,was adjusted to a steady state again by the development of new FO dendrites or the elimination of existing FO dendrites.(3)Two methods to precisely control crystal orientation were proposed for preparation of SX casting by seeding method.The first method based on inhabitation of the formation of stray grains at melt-back region of seed by decrease of the surface roughness of mold,the reduction of clearance between the seed and mold,and the original microstructure of seed.According to controlling the competitive grains growth between the epitaxial growth of seed and stray grains,the second mothed realized preparation of SX casting with the crystal orientation of seed via short-sized seed selection technique.After the preparation of SX casting,the short-sized seed had no obvious melting phenomenon.The upper surface of short-sized seed was partially covered with a mixture of Al2O3 and Si O2.When the mold shell was removed,the shortsized seed was naturally separated from the casting.Therefore,the short-sized seed could be repeated used prior to grinding and polishing.(4)The temperature gradient and cooling rate during directional solidification prior to SX cavity showed a non-monotonic change with the increase of the solidification distance.The misorientation formed during the high-order branching growth in spiral selector was larger than that in seed segment.During the steady-state growth of non-<001> oriented crystal,the primary dendrite stem was deformed toward the main heat flow resulting from contraction stress induced by different thermal contraction between metal and mould wall.The accumulated misorientation could be found in directional solidification direction,which was basically independent on the withdrawal rate.