Microstructure And Precipitation Behavior Of Superlattice Strengthening Phase In Ni-Cr-W Based Alloy

Superalloys based on Ni-Cr-W system have great potential in the field of chemical and nuclear industry,due to their superior properties including higher corrosion resistance,superior creep resistance and better tensile properties at high temperature.Most superalloys used in chemical and nuclear industry are utilized at temperatures of500?700?,which is beneficial for the C11_b?D0₂₂and D1a typed precipitation phases in the Ni-based superalloy.These superlattice particles formed in Ni-based superalloys during their service can improve the creep resistance and yield strength of the superalloy significantly,which make the traditional solid solution strengthened alloy which was utilized at the temperatures mentioned above into the age-hardening superalloys.The studies on the superlattice phase were mainly focused on the Ni-Cr-Mo alloys.However,studies on the related basic science problem about the superlattice phase formed in Ni-Cr-W-based alloys have not been performed yet.Furthermore,the temperature at which the traditional Ni-Cr-Mo alloy was utilized was about?700?.Raising the service temperature will be much valuable for energy conservation and emission reduction.Therefore,it is,necessary to study the superlattice phases in the Ni-Cr-W-based alloys.In this paper,the model materials were based on Ni-26at.%Cr-6at.%W alloys with the minor alloy additions of Mo,Ti,Ta and Al elements.The crystallographic characteristic,precipitation behaviors and thermal stability of the superallatice phases,the effect of alloying on the type of the superlattice phase and the effect of the superlattice phases on the mechanical properties of the Ni-Cr-W-based superalloys.In this work,the main research contents and the innovation results are as follows:The as-solutionized Ni-Cr-W-based alloys were aged at 500?800?for short and long time.The types of superlattice phases was determined by TEM.The molecular formula of the superlattice phases and the elements diffusion behavior during the precipitation of superlattice phases were studied by APT.The solution temperature of superlattice phases was studied by DSC and thermal expansion experiments.It was represented that no superlattice phases can be found in the ternary Ni-26Cr-6W alloy.The element Mo can make Ni-Cr-W-based alloy precipitate C11_b and D0₂₂ phases,and the solution temperature of D0₂₂ phase is higher than C11_b.The elements Ti,Ta and Al can make Ni-Cr-W-based alloy precipitate only D0₂₂ phase,the corresponding molecular formulas of which were Ni₃(Cr_0.2W_0.4Ti_0.4),Ni₃(Cr_0.2W_0.4Ta_0.4)and Ni₃(Cr_0.1W_0.2Al_0.7),respectively.The solution temperature of the three D0₂₂ phases are831,813 and 792?,respectively.The subgroup elements Ti and Ta lead to the partition of all elements during the formation of D0₂₂ phase,while the main group element Al results in the partition of only Ni,Cr and Al elements.The SRO phase can be found in the as-solutionized Ni-Cr-W-based alloys in which the LRO phases can be formed after aging treatment.The crystallographic characterization of SRO,C11_b,D0₂₂and D1a phases precipitated in Ni-Cr-W-based alloys were studied by using TEM and HRTEM systematically.The size of SRO phase is?0.5 nm,corresponding to 2?3 unit cells of the Ni-based matrix.Ni and non-Ni atoms arranged in SRO phase in term of N-N-M-M style,which leaded to the appearance of super-diffraction patterns in{1 1/2 0}positions of the reciprocal space.The formations of C11_b,D0₂₂and D1a phases require the non-Ni atoms reoccupy{420}plane of Ni-based matrix,and the d-spacings of the reoccupied planes are triple,fourfold and quintuple{420}lattice distance,respectively.This means that the distance which the non-Ni atoms must travel for the formation of D0₂₂phase than that for the formation of C11_b phase and then cause the greater driving force which needs higher temperature for the formation of D0₂₂ phase.This leads to the result that the precipitation and solution temperatures of C11_b,D0₂₂and D1a phases increase successively.These three LRO superlattice phases result in the appearance of the super-diffraction patterns in 1/3{420}?1/4{420}and 1/5{420}positions of the reciprocal space.The interphases of the three LRO phases and Ni-based matrix are fully coherent ones.The size of the crystal cell of LRO phase is smaller than that of the Ni-based matrix with the same atoms,which leads to the occurrence of the strain in both superlattice phase and matrix in order to maintain the coherent interface.The different variants of all LRO phases can occur equiprobably.The effects of SRO phase and predeformation on the precipitation kinetics of LRO phases in Ni-Cr-W-Mo alloy were studied.It is represented that SRO phase can accelerate the precipitation of the LRO due to the linkage of{420}static concentration waves of SRO and LRO phases.The defects caused by predeformation can be made as the fast channel of atoms diffusion.The precipitation of superlattice phase is diffusive transformation,so the predeformation can accelerate the formation of LRO phases remarkably but also lower the solution temperature of LRO phase.There are two mechanisms of ordering transformation including continuous ordering and nucleation and growth.The characteristics of the LRO superlattice phase formed based on the mechanisms of ordering transformation are as follows:the linkage of super-diffraction patterns between SRO and LRO phase can be found;the interface between LRO phase and matrix is obscure;the growth of LRO phase is so slow;LRO phase can precipitate at the lower temperatures for enough long time;the microhardness of the aged alloys increased with further aging and then remained fairly constant.The characteristics of the LRO superlattice phase formed based on the mechanisms of nucleation and growth is opposite of that based on mechanisms of ordering transformation.The nanoscale D0₂₂ superlattice phases not only can improve the strength of the alloy dramatically but also can change the deformation mechanism of Ni-Cr-W-Ti alloy.The precipitation of superlattice phase makes eight of the{111}<112>twinning systems become energetically favorable and suppresses eight of the{111}<110>slip systems,making the deformation mode change form slip into twinning.A perfect dislocation in the matrix can,when provided with a superlattice phase,dissociate into1/3[1 1 1]Frank sessile dislocation and 1/6[121]Schockley partial dislocation.Deformation twin is nucleated from the dissociation and grows via claiming a Schockley partial dislocation helically to the adjacent slip plane including D0₂₂superlattice phases and matrix.The D0₂₂ superlattice phases are cut through by Schockley partial dislocation to achieve a high required stress during the plastic deformation,leading to the significantly increase in the yield strength of the superalloy.The average size of the D0₂₂ phases formed in the Ni-Cr-W-Ti alloy aged at 800?for 1500 h was about 42 nm,which shows that the D0₂₂ phases have a very high thermal stability.Tungsten controls the growth of D0₂₂ superlattice phase and suppress the coarsening of D0₂₂phase due to its lower diffusivity than other elements in this Ni-Cr-W-Ti alloy,it can improve the thermal stability of the D0₂₂ superlattice phase dramatically.In addition,the coherent interface with extremely low elastic misfit energy between the D0₂₂ particles and matrix provides a good microstructural stability and reduces the driving force for coarsening,and then improves the thermal stability of the Ni₃(Cr,W,Ti)superlatticephases.The lower diffusivity of tungsten and stable interface microstructure stabilize the D0₂₂superlattice phase.