Study On Precipitation Behaviors And Planer Defects Of TCP Phases In Ni Base Single Crystal Superalloys

Due to the remarkable high-temperature mechanical properties,nickel-based single crystal superalloys are thought as the best candidates for materials of turbine blades of aircraft engines.Through adjusting the additions of alloying elements,great progress in improving the strength and creep properties of superalloys has been achieved in previous researches.However,the high content of refractory elements would facilitate the precipitation of topologically close-packed(TCP)phases severely.As the growth of TCP phases consumes lots of solid-solution strengthening elements,mechanical properties would degrade during creep process at high temperatures.Thus,it is very important to make clear the precipitation behaviors of TCP phases,which could give helpful guidance for controlling the size,density,and morphology of TCP phases to achieve the best balance of alloy properties.In addition,during the precipitation process,planar defects such as stacking faults and twins would generate inside the ? phase.Studies have shown that planar defects in ? phase would affect its precipitation behaviors and the transition between TCP phases.Investigating the generation mechanisms of the planar defects will be helpful to explore the growth mechanisms of the ? phase and enrich the structure evolution theory of complex intermetallic compounds.In view of this,nickel-based single crystal superalloys with different compositions were prepared to study the influence of alloy additions on the precipitation of TCP phases.With the help of multi-scale electron microscopy analysis,the TCP phases and their planar defects were characterized,combined with firstprinciples calculations,to explore the physical mechanisms behind the precipitation of TCP phases and the inner planar defects.The specific content is as follows:(1)Analysis of the influence of alloying elements on the precipitation of TCP phases.The addition of Ru can inhibit the nucleation of TCP phases and reduce the density of TCP phases,but did not inhibit the growth of TCP phases.The Mo has a greater influence on P phase;When the Mo is of a high content,the precipitation tendency of the P phase increases.The Cr element promotes the precipitation of the ? phase,and the addition of the Ru to the alloy with a high Cr content can suppress the generation of the ? phase and P phase,and finally increase the precipitation tendency of the ? phase.(2)The precipitation behaviors of P phase were studied.The morphology of the P phase is mainly lath-like.When it forms the interface with the matrix in the orientation relationship of(010)p//(110)?,[201]P//[110]?,the lattice mismatch degree is overall at a lower level,and the interface exhibits the characteristics of the semi-coherent interface.The low interface energy can promote the nucleation of the P phase and become the rapid growth interface.The first principles calculation results show that the distribution of alloying elements in the Cr10Mo24Ni22 P phase is mainly related to the coordination number.Atoms with a large atomic radius tend to occupy high coordinated positions,while atoms with a smaller atomic radius tend to occupy low coordinated positions.(3)The precipitation behavior of ? phase was studied.The orientation relationships between the acicular ? phase and matrix are:(0001)?/(111)?,[0110]?//[110]?.The orientation relationships between the bulk ? phase and matrix are:(0001)?//(110)?,(1100)?/(110)?,[1120]?//[001]?.The atomic scale HAADF images show that it is the specific atomic layers in the ? phase that always match with the matrix to form the interface,so as to achieve the minimum lattice mismatch and the lowest interface energy.For(0001)crystal planes of the needle-like ? phase,the ?c? triplet atomic layers in the Zr4Al3 unit match the {111}? crystal planes to form a coherent or semi-coherent interface.For the(0001)crystal planes of bulk the ? phase,the ?c? triplet atomic layers in the Zr4Al3 structural unit match the {110?? crystal planes to form an interface with higher coherence.The(1100)?/(110)? interface between the bulk ? phase and the matrix has a high-density of structural steps,and the non-flat interface of the stepped structure has a lower degree of lattice mismatch.The specific orientation relationships and the specific atomic layers constituting the interface in the ? phase are all conducive to reduce the interfacial energy and stabilize of the crystal nucleus.The interface with low interfacial energy during the growth process has the highest priority and determines the final morphology of the precipitates.(4)The structural characteristics of the planar defects in ? phase were characterized,and the generation mechanisms of the planar defects are proposed.There are planar defects in(0001),(1102)and(1101)crystal planes of the ? phase in the experimental alloys.The formation mechanism of(0001)planar defects belong to the synchroshear mechanism.There are three main types of(1102)planar defects in the ? phase,which are stacking faults with several atomic layers in thickness,type ? twin boundary(Twin boundary,TB),and type ? twin boundary(Faulted twin boundary,FTB).The formation mechanism of stacking faults belong to the inhomogeneous shear mechanism.Type ?twin boundaries can be regarded as the accumulation of stacking faults,and type ? twin boundaries can be regarded as products after shearing of type ? twin boundaries.