| Nickel base single crystal superalloy has widely been applied in turbine blades for advanced aero-engine and gas turbine because of its excellent high-temperature properties.To pursue higher service performance and overcome various problems encountered in the actual service of nickel base single crystal superalloy,the elements of Re and Ru are added in the superalloy to solve a series of problems and the comprehensive performance of the alloys is greatly improved.However,the influences of Ru on the micro structure evolution,the distribution of other alloy elements and the size and distribution of the ?' phase are unclear yet.On the other hand,the influence of Ru on the tensile behavior and its deformation mechanism,the creep behavior and its deformation mechanism and fracture mechanism during the mechanical testing are rarely reported.Moreover,many research results are still controversial.Meanwhile,the relationship between micro structure and mechanical properties during mechanical deformation still need to be further investigated.Based on the above background,two kinds of nickel base single crystal superalloys(2.5 wt.%Ru and 3.5 wt.%Ru)with different Ru content were investigated in this work.The microstructure evolution during long-term aging is studied.The tensile and creep behavior of the alloys and the corresponding internal mechanism are analyzed.The influence of Ru on the micro structure evolution and mechanical behavior are also investigated.This work is conducted to provide experimental support and theoretical guidance for the composition design of the fourth generation Ni-based single crystal superalloys and has important theoretical significance.The experimental achievements are as following:The regular ?/?' structure of the two alloys was obtained after the full heat-treated,and the mismatch degree on the ?/?' interface was more negative due to the increase of Ru content.Therefore,the size of the ?' phase of alloy 3.5 Ru after full heat-treated was smaller than that of alloy 2.5 Ru,which showed higher cubic and more uniform distribution.According to the LSW theory,the coarsening of ?' phase in alloy 2.5 Ru was mainly controlled by interface reaction,while the coarsening of ?' phase in alloy 3.5 Ru was mainly controlled by element diffusion.The increase of Ru content reduced the coarsening rate of ?' phase and promoted the formation of the complete,stable and continuous raft structure.With the increase of Ru content,the TCP phase only observed occasionally in alloy 3.5 Ru,which was caused by the stress and the supersaturation of refractory elements.The TCP phase presented the shapes of block,rod and needle-like,and was determined as ? phase.At the tensile condition of room temperature,some stacking fault appeared in the ?matrix of the two alloys after tensile fracture.In addition,a small amount of stacking faults was also observed in the ?' phase of alloy 3.5 Ru at the same condition.At the tensile condition of 760?,the number of the stacking fault in the ?' phase of alloy 3.5 Ru reached the maximum and was far more than that of alloy 2.5 Ru.At the tensile condition of 1100?,there were some dislocations on the ?/?' interface of the two alloys instead of the stacking faults.It was believed that the formation of stacking fault was due to the decrease of stacking fault energy induced by the increase of Ru content.Meanwhile,the lattice mismatch stress provided the impetus for the interface dislocation reaction.The formation mechanism of the stacking fault of the two alloys at room temperature and 760? was mainly determined by two dislocation reaction mechanisms:a/2<011>?a/6<112>+a/6<121>+SF in ? and a/2<101>?a/3<211>+a/6<121>+SF in ?'.Moreover,it was suggested that the extended dislocations(stacking fault and a/6<112>Shockley dislocation)in the y matrix and the stacking fault in the ?' phase were contributed to the work-hardening behavior of the two alloys at room temperature and 760?.Hence,the extended dislocation and the stacking fault had a great impediment to the movement of the dislocations,which was the cross slip or climbing of the dislocations.Dislocation networks were formed on the ?/?' interface in the two alloys at the tension condition of 1100?,and it became more compact with the increase of Ru content.However,the dislocation networks ineffectively blocked the dislocation movement due to the mechanism of dislocation movement changed from cutting into the ?' phase to bypassing the ?' phase.Meanwhile,the ? matrix channel of alloy 3.5 Ru was much wider than that of alloy 2.5 Ru with the increase of Ru content,which provided convenience for bypassing the ?' phase.Therefore,the tensile property of alloy 3.5 Ru was slightly lower than that of alloy 2.5 Ru.The number of dislocations and stacking faults of the two alloys at room temperature and 760? was mainly affected by the competition results of antiphase boundary energy and stacking fault energy.When the antiphase boundary energy was higher than the stacking fault energy,the number of the dislocation was higher than the stacking fault.On the contrary,when the stacking fault energy was higher than the antiphase boundary energy,the number of the stacking fault dominated in the alloys.The creep curve of alloy 2.5 Ru showed three typical stages.However,alloy 3.5 Ru had one more incubation period than alloy 2.5 Ru,which made the creep strain increase first,then decrease and stabilize at the minimum creep rate,and finally increase.The increase of Ru content hindered the movement of dislocations by solution strengthening the ? matrix,decreasing the size of the initial ?' phase and inhibiting the coarsening of ?'phase.Hence,the primary creep stage of alloy 3.5 Ru was prolonged and the creep performance of the alloy was improved.At the creep condition of 1140?/137MPa,the increase of Ru content could prolong the secondary creep stage and reduce the minimum creep rate of the alloy via promoting the formation of dense square dislocation networks and continuous rafting ?' phase.After creep fracture,the superdislocations in ?'precipitates could be divided into two types:a<010>superdislocations and a<101>superdislocations,in which a<010>superdislocations were mainly long and straight;a<101>superdislocations were short and straight with thick dislocation core,which was mainly composed of two pairs of dislocations with the same Berger vector. |
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