Microscopy And Simulation Calculations Of TCP Phases And Dislocations In Nickel-base Single Crystal Superalloys

Nickel-base single crystal superalloys are widely used in the preparation of turbine blades for aero-engines,because of their excellent high-temperature mechanical properties,creep resistance and good microstructure stability.To meet the increasingly stringent design requirements of high-performance aero-engines,a large amount of solid solution strengthening elements have been added to nickel-base single crystal superalloys,which has greatly improved the creep resistance of the alloys.However,excess refractory elements will promote the formation of brittle topologically close packed(TCP)phases.The precipitation of TCP phases will consume a large number of alloying elements,which will affect the solid solution strengthening effect of the matrix.Besides,TCP phases run through both the y and γ’phases,and become the origin of cracks and expansion channels,seriously affecting the high temperature mechanical properties and service life of superalloys.The existence of TCP phases has been restricting the development of high-temperature alloys and even aero-engines.However,a series of issues concerning the mechanism of action of the alloy elements on the TCP phases and the internal characteristics of the TCP phases are still in dispute.At the same time,dislocations permeate throughout the service life of turbine blades made of nickel-based single crystal superalloys.In the early stage of creep,dislocations are first generated in the y phase(primary dislocations).The y phase is a typical fcc structure and contains twelve a/2<110>{111} slip systems.Then,under the combined action of the applied stress and mismatch stress at the γ/γ’ interface,dislocations of different slip systems react to form a y/y’ interface dislocation network.The dislocation networks successfully hinder the dislocations from moving further towards the γ’ phase,thereby increasing the creep performance of the alloy.In the four strengthening methods,the role of dislocation strengthening becomes more prominent.In this study,the issues of TCP phases and dislocations in nickel-base single crystal superalloys are systematically studied by combining electron microscopy with computational materials.(1)The results of comparing the scanning results of 5Cr0Ru and 5Cr3Ru alloys as well as 4Mo0Ru and 4Mo3Ru alloys show that Ru element can effectively inhibit the nucleation of TCP phase,but cannot inhibit the growth of TCP phase.During the TCP phase growth process,alloy elements such as Re,Cr and W are gradually enriched at the front end of the growth,but the content distribution of Co is almost independent of the position.First-principles calculations were used to study the interaction between Re atoms and Cr atoms.The results show that when the distance between Re and Cr is a(the lattice constant of the matrix),the system is the most stable.Furthermore,there is a sequence of diffusion between Cr and Re atoms.It is easy for a Re atom to diffuse towards a Cr atom,resulting in a small,stable group.However,it is difficult for a Cr atom to diffuse into a region containing a Re atom.The effects of three typical refractory elements(rhenium,chromium and zirconium)substituting the molybdenum atom inp phase(Co7Mo6)phase were investigated using first principles calculations,and the results show that when the Re atom replaces the Mo atom in the Co7Mo6,its bonding ability with the matrix atom is enhanced and the system is more stable.(2)Using high-resolution electron microscopy to study the interfacial relationship between a phase and matrix,we found that a phase has a certain orientation relationship with the γ matrix:[001]γ//[112]σ,(110)Y//(110)σ,(110)γ//(111)σ.The interfacial steps between a phase and the γ matrix are made up by(110)γ//(110)σ and(110)γ//(111)σ,respectively.the length of(110)γ//(111)σ is longer than(110)γ//(110)σ,which is caused by that distortion factor of(111)γ//(111)σ is much smaller than that of(110)γ//(110)σ.There are a large number of surface defects on the(001)plane at the growth tip of the μ-phase.The HAADF-STEM results show that these surface defects mainly include two kinds of surface defects.One is twin structure with the atomic plane at the height of 1/2 of Zr4Al3 as the twinning plane,which is generated by shearing the intermediate layer atoms of MgCu2 along the[110]direction.The other one is a mirror-symmetrical stacking fault of two Zr4Al3,which is formed by pulling out a layer of MgCu2.There are also twins in the R phase.The twinning plane is(111).The width of the twin domains is wider than that of μ.The forming reason of the plane defects in the TCP phase is distortion of the coordination and the matrix.The Laves phase C36 can transform to P phase under certain conditions.Each diffraction spot of C36 coincides with the P phase spot,and the two components are completely identical,providing a convenient condition for the internal transformation of the TCP phase.(3)The analysis of high-resolution images of dislocations shows that due to the missing(111)half plane,the atomic arrangement changes.Atoms in the perfect crystal along the[101]direction are arranged in ABAB order,but there will exist AA or BB atoms pair around the dislocation,due to the missing(111)half plane.Additionally,atoms along the[110]direction could no longer be arranged in a straight line,resulting in twists and turns at the half plane locations.The Burgers vector of the dislocation is b=a/2[101]and the projection on the(110)is b,=a/6[112].The EAM potential function proposed by Y.Mishin was used to calculate and analyze the atomic arrangement of the edge type dislocation core structure in Ni3Al and the stress distribution around it.Compared with the perfect crystal,the distribution of atoms around the dislocation core is irregular.The atomic layers on the left and right sides of the dislocation core lean to the core,and due to the absence of the half plane,about eight atoms deviate from their equilibrium positions along the[110]direction.These results show that the dislocations have a tubular influence area and are consistent with the analysis results of the high resolution images.Through stress analysis,it can be found that there is a normal stress and a shear stress component in the edge dislocation stress field.The normal stress is mainly concentrated on both sides of the dislocation line,and the shear stress is several angstroms away from the dislocation line.This work provides an effective theoretical basis for further research on dislocation.(4)The first-principles method was used to study the distribution of alloying elements in the dislocation core.The results show that the introduction of alloying atoms into the dislocation core has a huge impact on the energy and electronic structure of the system.The combined energy results indicate that the Re atom prefers to occupy the A1 site,especially the center of the center-Al system.From the results of the number of Mulliken orbital population,it can be seen that when the Re atom occupies the center position of the center-Al system,the Re atom and its nearest neighbor matrix atom can get more electrons,which means that there are more resonant peaks appear between the Re atom and its nearest neighboring atoms.Therefore,we can conclude that when the Re atom occupies the center position of the center-Al system,the interaction between the Re atom and its nearest neighboring matrix atom is enhanced due to the hybridization of the Re-5d and Ni-3d orbitals,and the system has the highest stability.