On The Tensile And Low Cycle Fatigue Behaviors Of New Elevated-temperature Structural Materials

In this paper,two nickel-based single crystal(SX)superalloys with different Ru contents(Ru-free and Ru-containing alloys)and high entropy alloy(HEA)were used as experimental materials.The tensile and low cycle fatigue(LCF)behaviors of SX superalloys,as well as the tensile behavior of HEA alloy,were studied.The microstructure and deformation mechanism were mainly characterized and analyzed by scanning electron microscopy(SEM),X-ray diffraction(XRD),electro-probe micro-ananlyzer(EPMA),electron back-scattered diffraction(EBSD)and transmission electron microscopy(TEM).The tensile behavior of two SX superalloys at different temperatures were investigated.The experiment results shows that the difference between the strength and plasticity of two alloys at room temperature and 760? is significant.However,the difference is smaller and smaller with the increasing of temperature and can be negligible at 1000?.The deformation mechanisms of the alloys after fracture are as follows.A large of slip bands(SBs)penetrate through ? matrix and ?' precipitates during deformation process at room temperature.Stacking faults(SFs)generate in ?'precipitates of two alloys and only in ? matrix of Ru-containing alloy.When the testing temperature rises to 760?,slip bands can hardly be observed.The SFs become the main deformation mode and the formation of SFs is similar to that at room temperature.Lots of dislocation networks occur in micro structure of two alloys after 1000? tensile testing.Noteworthily,the regular arrangement of ?' precipitates and denser dislocation network formed at the ?/?' interface in Ru-containing alloy can effectively prevent dislocation cutting into ?' precipitates.In addition,the y channel becomes wider than that of Ru-free alloy.The dislocations blocked in y matrix are not prone to wrapping and entanglement,which weakens the stress concentration arising from dislocation network.The cyclic stress response curves of two SX superalloys at different temperatures reveal that the fatigue life of Ru-containing alloy is longer than that of Ru-free alloy during LCF deformation at room temperature and 600?.The microstructure after fatigue testing shows that SBs cracking leads to the final fracture of specimens.It is to say the difference of fatigue life in two alloys can be attributed to the crack propagation along different direction of SBs.During LCF deformation at 760?,the Ru-containing alloy exhibits better fatigue performance.The SFs generated by dislocation movement is dominant,which is benefit to strengthen alloys.The SFs present in two phases of Ru-containing alloy,including ?matrix and ?'precipitates.While,SFs can only be observed in ?' precipitates of Re-free alloy.Therefore,the formation of SFs is different in two alloys,resulting in the different strengthening effect,which may also leads to the different fatigue behavior.In addition,a simplified model of two alloys at 900? has been established,and the SF energy and anti-phase boundary(APB)energy of the alloys are compared.The effect of the addition of Ru element on the fatigue life may be summarized as follows.The SF and APB energy would change with the increasing of temperature.That is to say the SFs in Ru-containing alloy are more likely to form to strengthen the alloy and finally affect the fatigue performance.The low cycle fatigue curves of two alloys under various total strain during the deformation at 900? indicate that fatigue life decreases with the increasing of strain amplitude from 0.8%to 0.9%.However,the fatigue life begins to increase when strain amplitude reaches up to 1.0%.The deformation mechanisms of two alloys are studied and it have been found that two alloys exhibit the longest fatigue life during the LCF deformation at the strain amplitude of 0.8%.The oxidation damage may mainly affect the fatigue failure of the alloys.Fatigue cracks extend along the growth direction of oxides,and the interface between alloy and oxide becomes the weak link during deformation process.The cross-slip of dislocation is observed in Ru-containing alloy at the strain amplitudes of 0.8%and 0.9%.The difference is that under the strain amplitude of 0.8%,the cross-slip of dislocations generating in y matrix reduces dislocation density and improves fatigue life of the alloy.The dense stacking faults generating in ? matrix can seriously hinder the cross-slip of dislocation at the strain amplitude of 0.9%.The stacking faults and the polyline caused by cross-slip restrict mutually.Dislocation cross-slip and climb occur simultaneously during the LCF of Ru-free at the strain amplitude of 1.0%.The dislocation climb occurs during thermal activation,which may be related to the greater compressive stress of the alloy in each cycle.The tensile behavior of high entropy alloy exhibits excellent strength and plasticity during the tensile at 600? and 700?,which may be related to the effect of cross twinning.During the plastic deformation process,the a/2[110]dislocations react and produce a large number of a/6[112]partial dislocations.When the partial dislocations accumulate at phase boundary,the[112]direction on(111)plane and the[211]direction on(111)plane caused by the larger stress concentration has been activated.Thus,cross twinning is dominant during the tensile deformation.On the one hand,twin boundaries can strengthen the alloy.Especially,the dislocation jogs and kinks effectively hinder the dislocation movement,which plays a important role in strengthening the alloy.On the other hand,elements diffuse obviously during the tensile test.It is well-known that the element diffusion rate at twin boundary is significantly higher than that at crystal boundary,causing a certain deflection of the twin boundaries,which is beneficial to coordinate the deformation.The alloy shows superplasticity during the ?tensile tests and gets the maximum elongation at 1000?.The superplasticity can be interpreted as follows.Two new precipitates appear in the deformed microstructures,including the B2?precipitates in L12 matrix and the L12,? precipitates in B2 matrix.These precipitates seriously impede dislocation movement.Simultaneously,the dynamics recrystallization in L12 matrix reduces the stress concentration arising from dislocation pile-up.Due to the coordinating role of precipitation hardening and recrystallization softening,the high temperature-high entropy alloy exhibits superplasticity.