Formation Mechanism Of Sliver Defect And Micro-pore And The Effect On The Mechanical Property Of Ni-based Single Crystal Superalloy

In this dissertation,formation mechanism of sliver defect and micro-pore and the effect on the mechanical property of Ni-based single crystal(SX)superalloy DD413 were systematically investigated.Optical microscopy(OM),scanning electron microscopy(SEM),X-ray computed tomography(XCT),electron backscattered diffraction(EBSD),electro-probe microanalyzer(EPMA),transmission electron microscope(TEM)and ProCAST simulation were used to characterize the microstructure,three dimensional information of micro-pore and the thermal and stress field during directional solidification(DS).Sliver generally initiated from 1-2 tertiary dendrites at the diverging boundary between the mould wall and matrix.All slivers were tilt from matrix along the[001].In most castings,slivers were inclined to DS direction by a smaller angle compared to that of the matrix and formed diverging boundary with matrix.The secondary dendrite arms of sliver were either misaligned or parallel to that of the matrix on the plane perpendicular to the DS direction.The overall misorientation between the sliver and matrix was low,between 3.5°-9.8°in the present study.Formation of sliver was related to the branching and deformation of matrix dendrites.The deformation that induced sliver occurred in the mushy zone and was limited in the lower part of the dendrite.The free growth of the dendrite proceeded after deformation,which led to the strain free dendrite in the upper part of the sliver.Compared to the interdendritic fluid flow,thermal contraction forces played an important role in the formation of sliver.The mechanical deformation that primarily occurred on a diverging boundary can be attributed to the imbalanced force loaded on the dendrite.Besides,the inclusion in SX blade was another possible inducement for sliver formation.Slivers observed in SX castings generally grew with a fixed orientation throughout the DS process.However,in some castings the orientation of the secondary arms of both sliver and matrix could change simultaneously probably as a result of the geometry constraint of the casting.Compared to the contraction stress,thermal convection played an important role in the rotation of secondary arms of sliver and matrix.All slivers were able to overgrow matrix on the longitudinal section along the DS direction.In castings where the misorientation between secondary dendrite arms of sliver and matrix was small,slivers did not extend on the casting surface and the width of them did not change from bottom to top.However,if the misorientation on the cross section was large,sliver extended on the casting surface and into the casting quickly.Such different manners of dendrite branching can be attributed to the interaction of solute field in the interdendritic area.The initiation sites of fatigue crack gradually transferred from micro-pores to MC carbides with increase of stress amplitude in SX superalloy solidified by high rate solidification(HRS)and liquid metal cooling(LMC)at intermediate temperate(760℃).Under the condition of low stress amplitude σa(≤495 MPa in HRS specimens and ≤517.5 MPa in LMC specimens),the fatigue crack initiated at pores with large size and irregular shape and propagated until failure.The carbides on surface cracked due to a combined effect of oxidation and cyclic loading,but the cracks did not penetrate the carbide/matrix interface.At high stress amplitude σα(≥495 MPa in HRS specimens and ≥517.5 MPa in LMC specimens),the cracks in the carbide can penetrate the carbide/matrix interface rapidly under applied stress,and following crack propagation resulted in decisive fatigue fracture.The script carbides with larger size and more irregular shape can induce crack initiation easily compared to blocky carbides.It can be speculated from this work that stress intensity threshold of MC carbide △Kth-MC was greater than 6.2 MPa·m1/2。Under low stress amplitude,fatigue crack initiated from the largest micro-pore in specimen and the process occupied most of the fatigue life(>97%).Once crack initiation finished,the crack propagated rapidly and final failure of specimen would occur.The stage I and stage II crack propagation were both short.The micro-pores with larger size promoted crack propagation,while carbides showed little effect on the propagation.At high stress amplitude,most of the fatigue life(>99%)was associated to the stage I crack propagation.The initiation and stage Ⅱ crack propagation period were both short.The cracked carbides played a significant role in the crack propagation compared to micro-pores.During the crack initiation,the number of overall micro-pores in specimen was attributed to the interaction of two mechanisms.One was the annihilation effect of S/H-pore due to the outward diffusion of vacancies,the other was D-pore formation related to the generation and concentration of vacancies induced by cyclic loading.Under low stress amplitude,annihilation effect of S/H-pore was dominant at the initial stage of fatigue and the number of micro-pores decreased with decrease of S/H-pore.However,lots of D-pore generated after cyclic loading at high stress amplitude,leading to the increasing of micro-pores.During stage Ⅰ and stage Ⅱ crack propagation,the number of micro-pores decreased as fatigue crack penetrated through some micro-pores.It can be deduced in the present work that the number of micro-pores had little effect on the fatigue properties.As fatigue proceeded,the size of the largest micro-pore and D-pore gradually grew,and the size of some smaller S/H-pore decreased firstly and then increased.The micro-pore with large size(S-pore)significantly promoted crack propagation under low stress amplitude,while small micro-pores(H-pore and D-pore)had less influence during propagation.The size of micro-pores affected the fatigue performance.The sharp corner of the micro-pore was the dangerous site for crack initiation.It is believed that the shape of the micro-pore was an important factor that affected the fatigue life.