Study On Misorientation Defects In The Platform Region Of Ni-based Single Crystal Superalloys Turbine Blade

Ni-based single crystal(SX)superalloys are the key materials of blades in the aero-engine and industrial gas turbine.The fabrication of Ni-base SX superalloy turbine blades is achived through the combination of the grain selection under directional solidification.However,during solidification processing,misorientation defects,such as high-angle boundary(HAB),low-angle boundary(LAB)are easy to appear in the platform region.Those misorientation defects compromise component life and result in non-conformance scrap.The formation mechanism and control method of misorientation defects are thereby the main concerns for the fabrication of SX superalloy turbine blades.So far,the formation mechanism of misorientation defeccts in the platform region are commonly considered as induced by heterogeneous nucleation.However,misorietation defects arising from dendrite deformation are less understood.In fact,during the growth of dendrite arms,its crystallographic orientation will be likely to change,herein,this phenomenon is referred to as dendrite deformation.The issues that whether dendrite deformation could induce high angle boundary(HAB)and lateral sliver defects,have not yet been studied in detail.Additionally,in order to better understand the formation of misorientation defects,three dimensional dendrite growth and associated evolution of dendrite orientation in the platform region under various solidification conditons,such as withdrawal rate,platform dimension,the original crystallographic orientation of the SX blade,etc.have not been well understood.In this study,by using directional solidification experiments and numerical simulation,dendrite growth,microstructure characteristic and the evolution of crystallographic orientation in the platform region under various solidification conditons have been studied.HAB and lateral sliver defects were investigated for clarifying their formation mechanisms and influencing factors.Additionally,through the control of the solid-liquid interface and the original crystallographic orientation of the SX blade,crystallographic orientation of the platform region has been optimized to diminish misorientation angle.The main conclusions can be drawn as follows.(1)The primary dendrite arm spacing(PDAS)and packing pattern of dendrite arms in the platform region have been investigated.PDAS in the platform corner was significantly larger that that in the blade zone,and lateral growth of dendrite arms was found in the platform corner.Additonally,with the incrase of withdrawal rate,PDAS first decreases and then increases in the blade zone,but the platform length has no impact on PDAS.Neither withdrawal rate nor platform length has an impact on the packing patterns of dendrite arms.The dendrite arms always present the hexagonal packing pattern.Moreover,there is difference of PDAS between blade edge zone and blade center zone,and this difference increases with the increase of withdrawal rate or platform length.(2)Dendrite growth in the platform region under various directional solidification conditions have been analysised.Because of the “circuit-like” growth of dendrite arms,the whole platform base could be divided into the blade zone,the zone of secondary dendrite arms(SDZ)and the zone of circuit-like dendrite growth(CZ).The area of CZ increases with the increase of withdrawal rate or the platform length,but the area of CZ is not influenced by the misorientation(?)of secondary dendrite arm to platform edge.In the whole three dimensional platform region,dendrite arms in the blade zone and SDZ grew along the casting axial direction,while in CZ,the growth direction of the dendrite arms was depended on the withdrawal rate and the orientation of secondary dendrite arms: With the increase of withdrawa rate or the decrease of ?,the growth direction of dendrite arms gradually changed from the casting axial direction to the casting lateral direction.(3)Misorientation in the platform region could be diminished through the control of the shape of solid-liquid interface and the the original crystallographic orientation of the SX blade.Misorientation could be diminished from 36.2° to 6.4° by changing the concave interface into near plannar interface,thus the formation of HAB was restrained.Misorientation could be further diminished to 2.1° by regulating the original crystallographic orientation of the SX blade.(4)The formation mechanism and influencing factors of lateral sliver defects are studied.Within the platform base,under the severe concave solid-liquid interface,the liquidus mainly advances from the blade body toward the platform corner,secondary dendrite arms propagate from the blade body toward the platform corner,because of the larger stresses located in the boundary between the blade body and the platform,misorientation between these secondary dendrite arms generate,leading to the occurrence of lateral slivers.The occurrence of lateral slivers is mainly influenced by withdrawal rate,but not by the platform length.After solution heat treatment(SHT),no recrystallization is found,but the gradient of crystallographic orientation along lateral sliver growth direction could reach 4.0°/mm.(5)The HAB caused by dendrite deformation under the severe concave solidliquid interface is investigated.Under the severe concave solid-liquid interface,the liquidus within the platform base mainly advances from platform corner toward the blade body,resulting in the long secondary dendrite arms(LSDAs,which run through the whole platform length)close to the platform edge and the convergent boundary of dendrite arms(CBDA,which run through the whole platform width).Furthermore,the dendrite deformation of LSDA(the gradient of crystallographic orientation along LSDA dendrite growth trace could reach 6.08 °/ mm)induces misorientation between dendrite arms along CBDA(misorientation angle could reach 36.2°),indicating that LSDA dendrite deformation is the formation mechanism of HAB.Moreover,with the increase of platform length,the LSDA become longer,the gradient of crystallographic orientation along LSDA dendrite growth trace become larger,resulting in the larger misorientation angle between dendrite arms across CBDA,and thus the tendency of the formation of HAB on the CBDA is increased.