| High temperature applications such as turbine engines demand materials that have a variety of properties such as high strength,creep and fatigue resistance,corrosion resistance and toughness under high temperature and aggressive environments.The nickel-based superalloys have many favorable properties at high temperatures,and therefore are widely used to manufacture high-temperature components such as turbine blades.For selected gas turbine engines,the core power output is directly related to the turbine inlet temperature.With the increase of the thrust-weight ratio in aero-engines,the temperature at turbine jet is higher and higher,which needs superalloys with more excellent properties at higher temperatures.To achieve the long-term goal of developing new high-temperature propulsion materials to meet the capability needs of new jet engines,multi-disciplinary basic research will be required.One of the objectives of the basic research is to understand and control microstructures and phases such as eutectics and carbides,which have significant impact on the properties of superalloys.Therefore,the aim of our study is to investigate the fine microstructural characteristics and influencing mechanisms of y/Laves eutectic,y'/y eutectic and MC carbide and M23B6 boride using conventional and aberration-corrected scanning/transmission electron microscopy(TEM)techniques.The main conclusions are listed as follows.1.The microstructural information of y/Laves eutectic in a typical Ni-based superalloy K4169 has been investigated using in-situ and conventional TEM techniques.Although the dissolution behavior of y/Laves eutectic has been studied for many years,we present here new insights about the structural features of y/Laves eutectic constituents.The y/Laves eutectic in Inconel 718 was found to contain three kinds of secondary phases:C14 Laves phase,?-Ni3Nb phase and a new type Ni-Nb phase actually for the first time.The new phase shows higher thermal stability than C14 Laves phase during in-situ TEM heating study.Through EDS,diffraction and atomic-scale HAADF imaging analyses,the new type Ni-Nb phase has been identified as Ni7Nb2 with a monoclinic structure:a=0.473 nm,b=0.794 nm,c=1.21 nm and ?=91.3° with C2/m space group,which is based on the structure of Ni7Zr2.Since Zr,Si and Co display strong partitioning behaviors to Ni7Nb2,it is suggested that Zr,Si and Co promote the formation of Ni7Nb2 phase in superalloys.Moreover,as Ni7Zr2 is considered to be detrimental to the mechanical properties,Ni7Nb2 phase should also have negative influences and should be eliminated through possible approaches.The finding of the Ni7Nb2 phase is beneficial to refine the alloy phase diagram,design and control the microstructures of superalloys.2.Although a lot of attention has been paid so far to carbide decomposition in superalloys,its atomic scale information is new.By detail TEM analyses,the internal decomposition behavior of MC carbide in a crept polycrystalline superalloy after creep is investigated with high spatial resolution for the first time.High density of heavier TM elements segregation bands and M23C6 degeneration products have been confirmed.The mechanism can be explained by climb of dislocations results in Cr,Co,Ni segregation along the dislocation path,and subsequently promote precipitation of M23C6.Significantly,the debonding of M23C6/MC interfaces will make crack initiate in the crept specimen.Thus,the internal decomposition of MC carbides may be helpful for propagation of internal microcracks.3.The microstructure of ?/?' eutectic in a Ni-based superalloy before and after the creep has been investigated.Though the boron usually segregates to grain boundaries,creep induced precipitation of M5B3 in ?/?' eutectic was observed in our case.The M5B3 exhibits a fixed orientation relationship with the ?/?' eutectic,which is[001]?/?'//[001]M5B3,(020)?/?'//(130)M5B3.Furthermore,this precipitation behavior can be explained as high density of dislocations confined within y channel of eutectics provide driving force for M5B3 boride nucleation at elevated temperature.This study is helpful to understand the influence mechanism of boron on the high temperature creep properties of Ni-based superalloys.4.The boride precipitated in ?' phase promoting dislocation channeling is a new finding in this work.The boride is identified as M23B6,which has a cube-on-cube OR with ?' phase.Moreover,the interface between M23B6 and ?' can act as a source of dislocations,especially when the {111}?'//{111}M23B6 interface contains steps.Dislocations emitted from the interface are likely to slip follow the same path and subsequently results in high localized deformation,which is named as dislocation channeling. |
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