Effects Of Ta And Ti On Creep Behavior Of ?' Strengthened Co-base Single Crystal Superalloys At 900?

The discovery of ?' strengthened Co-base superalloys provides a pathway for the development of new class of Co-base superalloys.However,the research of the novel Co-base superalloys are still focused on the effect of alloying elements on the microstructural stability as well as mechanical properties.The investigation of the deformation mechanism,especially the creep behavior at elevated temperature are limited in the worldwide.Therefore,the deformation mechanism of novel Co-base superalloys at elevated temperature,as well as the effect of alloying elements and sub/micro-structure on the deformation mechanism,is essential for improving the temperature capability of novel Co-base superalloys.The individual and synergetic effects of Ta and Ti additions on the ?'coarsening behavior and creep properties have been investigated in this study.The high temperature creep behavior of Co-Al-W-Ta-Ti single crystal alloy with joint addition of Ta and Ti was investigated through the observation of sub/micro-structural evolution to clarify the creep deformation mechanism of y' strengthened Co-base superalloys.Based on the deformation mechanism of novel Co-base superalloys,the influence of initial sub/micro-structural characteristics on the creep behavior of Co-Al-W-Ta-Ti single crystal alloy were analysed.The investigation of alloying effects on the temperature capability suggests that the coarsening of ?' precipitates in Co-Al-W-base superalloys followed the LSW(Lifshitz-Slyozov-Wagner)theory,and the additions of Ta and Ti could resulted in the increased coarsening rate of y' precipitates due to the increase of the?/?interfacial energy.Moreover,the creep properties of Co-Al-W-base single crystal alloys at 900? and 420 MPa were improved by individual Ta or Ti additions,for the reason that the ?' volume fractions of Co-Al-W-base single crystal alloys were increased with Ta or Ti additions.Moreover,the precipitation of needle-like x phase was promoted in Co-base alloy with Ta adddition,which resulted in the brittle fracture of Co-Al-W-Ta alloy in this study.The creep properties of Co-Al-W-base single crystal alloys were further improved with joint addition of Ta and Ti because of the higher ?' volume fractions.On the other hands,the formation of enclosed y channel as well as the L-C lock in ?' precipitates of Co-Al-W-Ta-Ti-base superalloys led to the improvement of creep properties at elevated temperature.The research of creep deformation mechanism suggests that four regions,including the deceleration region(region ?),the acceleration region A(region ?),the steady-state region(region ?)and the acceleration region B(region IV)were exhibited in Co-Al-W-Ta-Ti single crystal alloy with joint addition of Ta and Ti.The "run and stop" mechanism of the screw dislocations could be the main reason for the decrease of creep rate in the region ?.Moreover,the extension of SFs(stacking faults)over multiple contiguous ?' precipitates as well as the slippage of the dislocations in y channel could lead to the increase of creep rate in the region ?,although the "run and stop" mechanism was still effective.In addition,the extension of SFs on multiple {111} slip planes led to the interaction of SFs and the formation of L-C(Lomer-Cottrell)locks.The formation of L-C locks and enclosed y channel could improve the creep resistance and then resulted in the appearance of the steady-state region.The initiation and propagation of cracks could be the reason for the increase of creep strain rate in the region ?.Moreover,the density of SFs and dislocation networks were also increased in the region ?.The investigation of the influence of initial sub/micro-structural characteristics on the creep behavior of Co-Al-W-Ta-Ti single crystal alloy suggests that the creep curve of Co-AI-W-Ta-Ti single crystal alloy with different initial sub/micro-structure were formed by four regions as mentioned above.The creep life of Co-Al-W-Ta-Ti single crystal alloy at 900? and 420 MPa exhibited a peak as a function of initial y' size in the range of 0.15?0.53?m.On the other hand,the increase of the initial y/y' interfacial dislocation density resulted in the rising of the minimum creep rate.The influence of initial sub/micro-structure on the steady creep rate was negligible.Based on the above results and understandings,the relationship among composition,microstructural evolution and creep properties in Co-Al-W-base superalloys with Ta and Ti additions is established.The understandings could provide the theoretical and experimental basis for the development of Co-Al-W-base superalloys.