Research On Behavior And Mechanism Of Ageing And Deformation In Co-Cr-W-Ni Alloys

The Co-Cr-W-Ni alloy has better thermal impact resistance and corrosion resistance than nickel-based alloy in high temperature and high-pressure environment,adding its good weldability and workability,it has been widely used in stationary gas turbines and air turbines.Recently,due to its high elastic modulus,good biological compatibility and impenetrability of rays,Co-Cr-W-Ni alloy has been widely used in the biomedical field,especially in cardiovascular stents.The preparation technology of cardiovascular stent is mainly divided into three parts:hot extrusion pressing tube,multiple cold drawing and intermediate annealing.The content of this paper is to study and analyze the material science problems in the three preparation processes(carbide precipitation,deformation-induced phase transition,ε-phase inverse transformation and static recrystallization),so as to provide theoretical guidance for the processing and preparation of Co-Cr-W-Ni alloy.First,the carbide evolution of Co-Cr-W-Ni alloys with two different initial states(hot rolled and solid solution)was studied over a wide time range(10 min-24 h).When the initial state was hot rolled,three spherical carbides were precipitated:M7C3,M23C6 and M6C.M6C carbides,as the main precipitates of alloys aged at 800℃ and 1000℃,preferentially nucleated at grain boundaries.When ageing at 1000℃ for 24 h,M6C carbide began to precipitate at the twin boundaries.When the initial state was solid solution state,two kinds of carbides were precipitated:M23C6 and M6C.The M23C6 carbides had spherical and uniform nucleuses inside the grain,while the M6C carbides needed to be attached to the grain boundaries and had non-uniform nucleuses.On the other hand,the Co-Cr-W-Ni alloy bar with similar microstructure was successfully prepared,on the basis of analyzing the microstructure of the ideal hot extruded bar.Secondly,the effects of microstructure(average grain size,stacking fault and carbide precipitation)on mechanical properties of the alloy were studied and analyzed.The effect of average grain size on yield strength of Co-Cr-W-Ni alloys is in accordance with Hall-Petch relationship.The yield strength of alloy is higher,when the average grain size is finer.The phenomenon of intersection of stacking faults caused by ageing can improve the tensile strength of Co-Cr-W-Ni alloy.Through the construction of the geometric model,the intersection of stacking faults was regarded as a dislocation dipole.The maximum gravity F per unit length is calculated,and the result is 0.78 N/m.The effects of location and morphology of carbides on the plasticity were different.When the solid solution alloy was aged at 800℃ and 1000℃,M6C precipitated along the grain boundary in the shape of long strip,which significantly reduced the plasticity of the alloy,and the fracture mode was intergranular fracture.However,when the hot-rolled alloy was aged at 1000 ℃ for 24 h,M6C began to precipitate at the twin boundary,which prevented deformation-induced phase transition and improved the plasticity of the alloy.Additionally,deformation-induced phase transition occurred in the plastic deformation process of Co-Cr-W-Ni alloys,and y phase transformed into ε phase.The variation selectivity and orientation dependence of phase transition can affect the cold tensile properties of the alloy.In this paper,the tensile and compression processes of Co-Cr-W-Ni alloys at room temperature were studied from the perspective of crystallography,so as to provide theoretical guidance for the control of SIMT process of Co-Cr-W-Ni alloys.The variation of ε phase in the tensile process followed Schmid’s law.The {111}<112>slip system of high Schmid factor preferentially slipped to produce the corresponding ε-phase variant.However,not all the grains with high Schmid factor underwent deformation-induced phase transition,which was related to the mechanical energy when phase transition occurred.According to the theoretical calculation,only the grains with both high Schmid factor and high mechanical energy can initiated the phase transition.During the compression process,the grains gradually rotated,and the substructure of {110}<110>orientation formed inside the grains,which hindered the further growth of the ε phase that had already occurred,and generated stress concentration at the junction of ε phase and γ phase,resulting in the generation of compression crack source.The twin types occurred in the tensile process were {1012} and {1121}.The twin types occurred in the compression process were {1013} and {1122}.Finally,the mechanism of ε phase inverse transformation and static recrystallization during annealing were studied.The nondiffusion-type ε phase inversion occurred between 130℃ and 400℃,and the reversal stopped at 400℃,which was related to the variation of the stacking fault energy of the alloy.When the ageing temperature raised to 1000℃,the inverse transformation of ε phase occurred in diffusion mode.However,when the samples with 40%pre-tensile deformation aged at 1000℃ for 1 h,ε phase was completely redissolved,and static recrystallization process occurred in the alloy.The nucleation site of the static recrystallization process was mainly along the grain boundary of the original large grain.After ageing for 1 h at 1000℃ to the samples with 40%pre-tensile deformation,the static recrystallization process produced bimodal grain size,which increased the compressive strength of the alloy by about 20%without reducing the plasticity.