Phase Field Simulation on γʹ Precipitate Evolution for Novel Cobalt-base Superalloy

Jun Zhang*, Jia Chen, Min Guo, Min Yang, Haijun Su, Lin Liu

State Key Laboratory of Solidification Processing，Northwestern Polytechnical University, Xi’an, 710072, P. R. China

ABSTRACT: The γʹ precipitate strengthened cobalt-base alloys with high melting points, good corrosion resistance, fatigue resistance and weldability, have the potential for anticipated next generation superalloy. The phase field method (PFM) considering the combined effect of multiple energy fields, provides an effective choice for clarifying microstructure evolution processing and mechanism. This paper simulates the γʹ evolution during the aging and creep processes for the novel Co-base superalloy by using PFM with the microstructure evolution model describing the elastic and plastic deformation behaviors. The corresponding coarsening and creep mechanisms are analyzed from the perspective of changes of energies and stress/strain fields.

(1) The γ/γʹ evolution during isothermal aging is simulated by elastic phase field model. The results show that γʹ precipitates evolve from sphere to cube aged at 900℃ while interconnect irregular γʹ precipitates are formed before transition to cube when aging temperature is lowered. And polyhedral precipitates are observed in system with high γʹ volume fraction. These irregular γʹ are caused by the coalescence of adjacent precipitates. Moreover, the coarsening kinetics is analyzed in systems with different γʹ volume fractions. It is found that the coarsening rate of γʹ precipitates shows a positive dependence on γʹ volume fraction, which is the synergistic effect between diffusion potential difference between γ matrix and γʹ precipitate, edge-to-edge distance between γʹ precipitates, and particle size distribution.

(2) The elastoplastic phase field model further coupling with plastic deformation and creep damage is proposed to study γ/γʹ evolution and deformation behavior during the typical creep process. Under tensile creep at 900℃/275 MPa, cubic γʹ precipitates coarsen along the direction parallel to stress axis, resulting in regular P-type rafts during primary creep. Then the wavy rafts form due to the deviation of γ/γʹ interface in the steady stage, and creep pores occur at γ/γʹ interface during tertiary creep. The evolution of stress/strain fields indicates these pores release the concentrated stress at interfaces, which makes the dislocation glide again. The deflection of the γ/γʹ interface provides a continuous slip path for the further movement of the dislocation, and thus the creep rate increase significantly.

(3) Furthermore, the double minimum creep behavior is studied. The dislocation slip in channel intersections and dislocation multiplication in horizontal channel centers lead to the first increase of creep rate. 

Keywords: cobalt-base superalloy; γʹ precipitate; Phase field simulation; Aging; Creep