The Effects Of Pre-compression Treatment On Microstructure And Creep Properties Of A Single Crystal Nickel-base Superalloy

In this paper, the regularity of the microstructure evolution for a [001] orientation single crystal nickel-base superalloy during the tensile / compression creep is investigated by means of the measurement of creep properties and microstructure observation. The cubicγ' phase in the alloy is transformed into the P-type structure by pre-compression treatment. In the further, by means of the measurement of tensile creep properties and internal friction stress of two kind structure alloys, an investigation has been made into the evolution of the P-type structureγ' phase during tensile creep and the influence of microstructure on the creep behavior of the alloy.The results show that the fineγ' phase is precipitated from the matrix of the alloy by solution treatment. After two times of aged treatment, the cubicγ' phase is regularly arranged along the <100> orientation. The cubicγ' phase is transformed into the N-type structure along the direction vertical to the applied stress axis during tensile creep, and the cubicγ' phase is transformed into the P-type structure along the direction parallel to the applied stress axis during compression creep. During the initial stage of tensile creep, the dissolving or coarsening of theγ' phase occurs in local regions of the P-type structure alloy. As creep goes on, the dissolving ofγ' phase results in the disconnection of the P-type structure, and the coarsening ofγ' phase in the local regions results in the connection ofγ' phase in close regions, so that the N-typeγ' phase structure in alloy is formed. But, compared with full heat treated state alloy, the N-typeγ' rafts structure which is transformed from the P-type structure alloy displays a shorter size feature. The density of dislocation increases with the applied stress, the internal friction stress of dislocation movement is enhanced. But, the internal friction stress decreases with the increase of temperature. Compared with the P-type structure alloy, the alloy with full heat treated exhibits a higher internal friction stress and creep resistance. Under the conditions of high temperature tensile creep, the alloy with full heat treated displays an obvious sensitivity of temperatures, but the P-type structure alloy shows an obvious sensitivity of the applied stressed. In the range of the applied temperatures and stresses, the activation energies of two kinds structure alloys during steady state creep are measured as Q_A=474.6kJ/mol and Q_B=412.5 kJ/mol, respectively. And the stress exponents are measured as n_A=3.8 and n_B=6.4, respectively. Compared with the P-type structure alloy, the superalloy with full heat treated displays a longer creep rupture lifetimes. One of the main reasons for the alloy with P-type structure having a shorter creep lifetime is that the N-typeγ' raft phase which is transformed from in P-type structure alloy possesses a shorter size feature. The dislocation slipping in theγmatrix and shearingγ' phase are considered to be the main deformed mechanism of the alloy, which may obviously decrease the creep resistance of alloy.