| The high-performance aero-engine is the most important device involved in ensuring national strategic security.The turbine working blade is the most stringent core component in the aero-engine,determining the maneuverability and service life of the aero-engine.Ni-based single crystal superalloy is an advanced material for the preparation of turbine working blades.Its stability under thermo-mechanical coupling conditions determines the safety of the aero-engine service.Understanding and mastering the microstructural evolution mechanism of Ni-based single crystal superalloy under service conditions is the basic guarantee for its development with a stable microstructure.During creep at high temperature,three main kinds of microstructural evolution occur in Ni-based single crystal superalloy: the formation and transformation of the dislocation network,the directional coarsening of the ?? shape,and the precipitation and transformation of a topologically close-packed(TCP)phase.Although much research has been carried out on the microstructural evolution of Ni-based single crystal under high-temperature creep conditions,there are still some shortcomings.The research on the formation and transformation of dislocation networks has not yet established the transition relationship between <110>-<100> mixed type and <100>-type dislocation networks.The research on the directional coarsening of the ??-precipitated phase has not revealed the correlation between the five models of rafting.From the aspect of precipitation and transformation of the TCP phase,the reason why the same kinds of TCP phase have different morphology and the orientation relationship with the matrix have not been explained.In view of the present situation,three kinds of microstructural evolution of Ni-based single crystal superalloys have been systematically studied by means of advanced electron microscopy,and the following conclusions have been obtained.(1)The diagonal climbing mechanism of dislocation is proposed for the first time,and the evolution model of the dislocation network is improved based on this mechanism.In addition,the Brooke formula for calculating lattice mismatch is modified to greatly improve its calculation accuracy.Under the guidance of internal stress,dislocations will move towards the {001} ?/?? interface in the form of slip,cross-slip and sweep slip,resulting in the increase of dislocation density at the interface.These accumulated dislocations preferentially form <110>-type dislocation networks with 60° mixed dislocations,and the 60° mixed dislocations will be transformed into edge dislocations in the direction of <100> by diagonal climbing.At the same time the dislocation network will be transformed into <100>-type,with the <110>-<100>-type dislocation network acting as the intermediate stage.The driving force for dislocation motion is to reduce the mismatch stress distributed at the interface;only the projection of the edge component of the dislocation in the {001} plane is effective in reducing the interface mismatch.The existing Brooke formula magnifies the ability of dislocation reduction mismatch,so we propose a modified Brooke formula,which improves the accuracy of the Brooke formula in calculating the degree of mismatch.(2)The influence of various factors on rafting is clarified.The finite element simulation shows that the elastic deformation will not cause anisotropy of the mismatch distribution.Although the elastic deformation will produce anisotropy of the mismatch distribution in mechanical analysis(using approximate conditions),the anisotropy of the mismatch distribution is much lower than the initial mismatch degree of the alloy.That is to say,the mismatch anisotropy caused by elastic deformation is not the main inducement of rafting.During plastic deformation,the shear stress driving dislocation motion in different channels is different,which leads to the difference in the dislocation increment rate.Finally,anisotropy of the dislocation distribution is produced.The difference in the degree of mismatch produced by dislocation distribution anisotropy is close to the initial mismatch degree of the alloy,so it will induce rafting.The stress field produced by dislocation will interact with the stress field produced by elements,which leads to the formation of Cottrell atmospheres on the dislocation core.Because the dislocation is a fast diffusion channel,the ??-forming element will be transmitted to the ?? phase,resulting in only the ?-forming element being left on the dislocation.This process is the embodiment of rafting,not the way of inducing rafting.(3)The minimum mismatch criterion of the precipitated TCP phase and the evolution process of the atomic structure are proposed.P phases with different morphologies were observed during aging at 950°C and 1100°C,including <110> and <112> directional acicular and {110} plate types.The lattice constants of these P phases and the orientation relationship between these P phases and the matrix are different.What they have in common is that the growth direction is the direction of minimum mismatch,and the composition of the monolayer TCP phase is accurately obtained by using a focused ion beam and a large solid angle energy dispersive X-ray spectrum technique.It is found that the reason for the different lattice constants of the TCP phase is that their composition is different.The essence of the composition difference comes from the large cell characteristics of the TCP phase,which makes it difficult to have a fixed chemical ratio alloy composition,and the difference in the lattice constant caused by the different composition of the TCP phase,which leads to the difference between the TCP phase and the matrix arrangement.Under the influence of the minimum mismatch direction,TCP phases with different morphologies are formed.The atomic structure of the TCP phase is characterized,and it is found that the initial atomic structure of the TCP phase(P phase)is similar to that of the ? atomic structure unit,and the formation of this structure can minimize the distortion energy.On the other hand,the atomic structure of P phase will evolve into a theoretical atomic structure,because its structure is the thermodynamically stable state.(4)The influence of low-temperature pre-creep on high-temperature creep properties is revealed.In order to independently study the effect of the dislocation network on creep properties,dislocation distribution structures with different dislocation distribution anisotropy were obtained by pre-creep experiments at 750°C in different loading modes,while ensuring that the shape of the ?? phase remained unchanged.Because the dislocation distribution structure obtained by pre-creep will be rapidly transformed into the dislocation distribution structure induced by the new stress loading mode at high-temperature creep,the dislocation distribution structure obtained by pre-creep will not affect the rafting law during high-temperature creep.The dislocations produced by pre-creep play the role of collecting elements for the precipitation of the TCP phase,so the precipitation of the TCP phase is observed in the pre-creep samples at high-temperature creep,and the dislocations produced by the pre-creep and creep processes will deflect the orientation of the samples.The pre-creep test and creep interruption test will lead to a decrease in creep life.The causes of this phenomenon need to be further studied. |
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