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The effect of solution treatment on the microstructure and creep properties of forged TiAl–Nb alloys was investigated. The results showed that the microstructure of forged alloy mainly consisted of γ/α2 lamellar colonies and fine equiaxed recrystallized γ/α2 grains. During the solution treatment the microstructure of the alloy transformed into a fully lamellar structure due to the lamellar colonies growth by consuming equiaxed grains. Compared with the forged alloy the creep life of the solution treated alloy at 800 ​°C/220 ​MPa increased from 116 ​h to 339 ​h. The better creep resistance may be attributed to the transform of fine equiaxed γ/α2 grain to the lamellar colonies with serrated grain boundaries due to the solution treatment. The deformation mechanism of the solution treated alloy during creep is considered to be dislocation slipping within the lamellar γ/α2 phases, and the dislocation movement may be hindered by the γ/α2 interface and the formation of dislocation tangles. The interaction of the dislocations with the tangles may increase the resistance of the dislocation motion and hence improve the creep resistance of the alloy. It was found that during the creep of the forged alloys the cracks mainly initiated at the equiaxed grain, and in the solution treated alloy the cracks initiated at the grain boundaries. As creep continued the cracks propagated and connected to each other, leading to the damage and rupture of the forged and solution treated alloys.

By means of creep properties measurement and microstructure observation, the creep and damage behaviours of the Mo-containing single crystal Ni-based superalloys at high temperature are inv...

The deformation and damage features of a 4.5%Re/3.0%Ru-containing single crystal nickel-based superalloy during the creep in the temperature range of 1040–1070 °C and stress range of 137–180 MPa was investigated by means of creep properties measurement and contrast analysis of dislocation configuration. The results showed that the alloy exhibited a better creep resistance in the range of the testing temperatures and stresses, the deformation mechanism of the alloy during steady state creep was dislocations climbing over the rafted γ′ phase. In the latter period of creep, the deformation mechanism of the alloy was dislocations shearing into the rafted γ′ phase. It is believed that the dislocations shearing into γ′ phase may cross-slip from {111} to {100} planes for forming the K-W locks to restrain the slipping and cross-slipping of dislocations on {111} plane. As the creep goes on, the alternate slipping of dislocations results in the twisted of the rafted γ′ phase to promote the initiation and propagation of cracks along the γ/γ′ interfaces up to creep fracture, which is considered to be the damage and fracture feature of alloy during creep at high temperature. : In the later stage of the creep, the deformation mechanism of alloy is dislocations shearing into the rafted γ′ phase, and the dislocations shearing into γ′ phase may cross-slip from {111} to {100} planes to form the K-W locks. Wherein, the deformation mechanism of alloy during steady state creep is dislocations climbing over the rafted γ′ phase, and the strain rate of alloy during steady state creep is expressed as: ε˙=A″(VH)=A·D·h·B3(μ·b)2(1−υ)2H3exp(−Uj+Uvk·T). Keywords: Re/Ru alloying, Single crystal nickel-based superalloy, Creep, Dislocations climbing, K-W locks

A TiAl–Nb alloy was prepared by casting, and its tensile properties and microstructure evolution at different temperatures were investigated. The results show that the brittle-ductile transition temperature (BDTT) of the alloy is 880–920 °C. At low temperatures, deformation mainly occurs in the γ lamellae and coarsening γ phase between the lamellar colonies. As the temperature increase in the transitory stage, a small number of dislocations may pass through the interface from the coarsening γ phase into the γ lamellae. During the brittle stage, the dislocation and dislocation array bow out from the phase interface, and twins are emitted from the phase interface, which may contribute to the accommodation of stress concentration. During the ductile stage, the γ lamellae coarsen due to the thinner and smaller α2 lamellae dissolving and phase boundary transfer. The dislocations interact with the dislocation walls to form subboundaries in the γ lamellae and transform into equiaxed crystals by dynamic recrystallization (DRX), which increases the plasticity of the alloy. With the transition from the brittle model to the ductile model, the fracture mode of the alloy changes from the mixed fracture mode of transgranular fracture and translaminar fracture to intergranular fracture.

By means of creep properties measurement and microstructure observation, the deformation and damage behavior of an as-cast TiAl-Nb alloy during creep at temperature near 750 °C were investigated. The results showed that the microstructure of the alloy consisted of lamellar γ/α2 phase, and the boundaries consisted of γ phase located in between lamellar γ/α2 phases with different orientations. In the latter stage of creep, the dislocation networks appeared in the interfaces of lamellar γ/α2 phases due to the coarsening of them, which made the coherent interface transforming into the semi-coherent one for reducing its adhesive strength. The deformation mechanism of the alloy during creep was twinning and dislocations slipping within lamellar γ/α2 phases. In the later period of creep, significant amount of dislocations plied up in the interfaces of lamellar γ/α2 phases, which may cause the stress concentration to promote the initiation and propagation of the cracks along the lamellar γ/α2 interfaces perpendicular to the stress axis. Wherein, some cracks on the various cross-sections were connected by tearing edge along the direction of maximum shear stress, up to the creep fracture, which is considered to be the damage and fracture mechanism of alloy during creep at 750 °C. Keywords: TiAl-Nb alloy, Creep, Dislocation network, Deformation mechanism, Fracture mechanism

By means of creep property measurement, microstructure observation and atom probe tomography, the elements distribution and deformation features of a 4.5% Re nickel-based single crystal superalloy during creep are investigated. Results show that after fully heat treated, the elements Al and Ta are mainly distributed in γ′ phase of alloy. After the alloy is crept up to fracture at 1100 °C/137 MPa, the change of the elements distributing in γ and γ′ phases occurs, wherein, the concentration decrement of the elements Al and Cr in γ′ and γ phases is attributed to their oxidation which may consume the ones. Moreover, some of the atoms Re, W and Mo dissolving in γ′ phase are repelled into γ matrix to reduce its internal energy. And the peak concentration of the atoms Re and W appears in the γ phase near the transition region, the atoms Re, W in the peak region cause the lattice distortion to increase the resistance of dislocations motion, which may improve the creep resistance of alloy. In the later stage of creep, dislocations shearing into the rafted γ′ phase may cross-slip from {111} to {100} plane to form the configuration of K-W locks, which may restrain the slipping of dislocations to improve the creep resistance of alloy. Wherein, the atom Re with lower diffusion coefficient may delay the dislocations in K-W locks for slipping and cross-slipping, which is thought to be the main reason of the K-W locks keeping in 4.5%Re alloy during creep at 1100 °C.

By means of creep property measurement and microstructure observation, the influence of element Ru on the microstructure and creep behaviour of single crystal nickel-based superalloy is investigate...

In order to understand the influence of Ru and Re on the concentration distributions of elements in γ′ and γ phases, the partitioning behaviors of elements in various alloy are investigated by means of atom probe tomography (APT). Results show that the elements Al, Ta and Ni are enriched in γ′ phase, while the elements Co, Cr, Mo, W, Re and Ru are enriched in γ matrix phase. Adding Ru may decrease the partitioning ratio of elements in γ and γ′ phases. Wherein, when Ru atoms dissolve into γ′ phase to substitute the Al atoms, some Re, Mo and W atoms adhered in the round of Ru atoms, under the hybridization effect of d orbit, may dissolve into γ′ phase, which is thought to be the main reason of Ru decreasing the partitioning ratios of γ′ and γ phases. When adding Re into the alloy, the Re atoms with low diffusion coefficient may hinder the diffusion of other atoms away from γ′ phase during the growth of γ′ phase, which is thought to be the main reason of decreasing the partitioning ratios of elements in γ and γ′ phase. Keywords: Nickel-base single crystal superalloy, Ruthenium, Rhenium, Atom probe

By means of heat treatment at various regimes, creep property measurement, microstructure observation and thermodynamic analysis, the precipitation behaviors of carbides and effect of its configuration on the creep properties of FGH95 alloy are investigated. Results show that after solution treated at 1155 ℃ and fully heat treatment, the microstructure of alloy consists of fine γ' phase, γ matrix, (Nb, Ti)C and M23C6 carbides. The excellent creep life of alloy at 1034 MPa/650 ℃ is attributed to some fine carbide particles precipitating along boundaries to restrain dislocations and boundaries slipping. Moreover, the boundaries appear in the original PPB regions of alloy after solution treated at 1165 ℃, and the carbide films are continuously distributed along boundaries to weaken the bonding strength between grains, which are thought to be the main reason of the alloy displaying a weakest creep resistance. By means of thermodynamic calculation, the precipitation temperatures of NbC and TiC carbides are determined to be 1080 ℃ and 807 ℃, respectively. Compared to (Nb,Ti)C carbide, the reason of no WC carbide precipitated in the alloy is attributed to its smaller driving force of nucleation.

By means of creep property measurement and microstructure observation, combined with finite element analysis (FEA), the creep and damage of a 4.5%Re and 3%Ru single crystal nickel-based alloy at high temperature were investigated. The results show that the creep life of the single crystal alloy at 1100 °C/140 MPa is 476 h. The deformation feature of alloy during steady state creep is dislocation slipping in γ phase and climbing over γ′ rafts. The various stress distributions are displayed in the different regions of sample during creep. As the creep goes on, the stress value in the gauge region of sample may increase to a value being bigger than the yield strength, which may cause the plastic deformation of local region in alloy. In the late creep period, the deformation feature of alloy is dislocations shearing γ′ rafts, and the dislocations of shearing γ′ rafts may cross-slip to form the K–W locks. Moreover, the alternated slipping of dislocations can both twist γ/γ′ rafts and rotate γ′ crystal to form the sub-grain structure, which may reduce the resistance of alloy. Wherein, the twisted γ′ rafts with weaker strength, due to the shearing of dislocations in the late period of creep, is reversely transformed into the block-like configuration, which weakens the resistance of alloy. While the cause of the life being decreased at 1140 °C to a great extent is attributed to the γ′ phase being dissolved to diminish its size and volume fraction.

By means of creep properties measurement, microstructure observation and lattice parameters measurement, the microstructure evolution and creep behaviors of a directionally solidified nickel-based alloy under long-life service condition are investigated. The results show that the creep life of the alloy at 980 °C/90 MPa is measured to be 9714 h. During creep, the various morphologies of γ′ and γ phases display in the different regions of sample. Wherein, the γ′ phase in the region near fracture is firstly transformed into the rafted structure, while the γ′ phase in the stress-free region exhibits the bunch-like structure. The size of the rafted γ′ phase in thickness increases from 0.4 µm to 1.8 µm as the time of creep prolongs to 9714 h, the coarsening regularity of the rafted γ′ phase in thickness obeys the parabolic law. Moreover, the parameters and misfits of γ′/γ phases in the alloy increase with the creep time. When the creeping time is less than 3000 h, the deformation mechanisms of alloy are dislocations slipping in the matrix channels and climbing over the rafted γ′ phase. In the later stage of creep, the deformation mechanisms of alloy are dislocations slipping in the matrix channels and shearing into the rafted γ′ phase, the alternate activation of the main/secondary slipping dislocations promotes the coarsening and twisting of the rafted γ′/γ′ phases. Wherein, the coarsening of the rafted γ′ phase is thought to be a main reason of the alloy having a better creep resistance and longer life.

By means of creep properties measurement and microstructure observation, an investigation has been made into the creep behaviors and deformation features of TiAl–Nb alloy with various states at high temperature. Results show that the microstructure of as-cast alloys consists mainly of lamellar γ/α2 phases with various orientations, the boundaries located in between the ones consist of single γ phase. The bigger plastic deformation of alloy during forging may refine the grains and increase the volume fraction of boundaries which consists of the fine block-like γ/α2 phases with weaker strength. During creep, the bigger volume fraction of boundaries increase the probability of crack initiated and propagated along boundaries, which is thought to be the main reason of the forged alloy displaying a lower creep resistance. Compared to the forged alloy, the as-cast alloy displays a better creep resistance and longer creep life at high temperature. The deformation mechanism of as cast alloy during creep is significant amount of dislocations shearing into the lamellar γ/α2 phases in the form of dislocations rows and rings. Compared with α2-Ti3Al phase, the γ phase possesses a weaker strength, therefore, the crack is easily initiated and propagated along the inclined boundaries. Wherein, the propagation of crack along the direction parallel to the lamellar γ/α2 phases displays the smooth surface of fracture, while the tearing edges appear on the surface of fracture in one side of the inclined boundaries relative to lamellar γ/α2 phases, which is attributed to the α2-Ti3Al phase with better strength hindering the propagation of crack during creep.

By means of creep properties measurement and contrast analysis of dislocation configuration, the deformation features and affecting factors of a Re/Ru-containing nickel-based single crystal alloy during creep is studied. Results show that the creep life of alloy at 1040 °C/160 MPa is measured to be 725 h to display a good creep resistance. During creep, the W, Re atoms in γ′ phase may be excluded for enriching in the γ phase near the interface, which may bring the lattice aberrance to delay the shearing of γ′ phase. The creep feature of alloy during steady state is dislocations climbing over γ′ phase. While the deformation feature of alloy in the latter period of creep is dislocations shearing γ′ phase, and the dislocations in γ′ phase may form the Kear-Wilsdorf locks by cross -gliding. Wherein, the interaction of the Ru and Re, W atoms make some Re, W atoms reserving in γ′ phase to delay the diffusing of elements, which may maintain the K-W locks from being released at high temperature in the later period of creep. This is considered to be one of the reasons for the alloy displaying a better creep resistance.

By means of creep properties measurement and microstructure observation, an investigation has been made into the creep behaviors of a Re/Ru-containing single crystal nickel-based superalloy at high temperatures. Results show that after fully heat treatment, the microstructure of the Re/Ru-containing single crystal nickel-based superalloy consists of the cuboidal γ′ phase about 0.4 μm in size coherent embedded in the γ matrix. The creep life of the superalloy at 137 MPa/1100 °C is measured to be 321 h. And the deformation mechanism of alloy during steady state creep are dislocations slipping in γ matrix and climbing over the rafted γ′ phase. In the latter stage of creep at 1085 °C, the dislocations shearing into γ′ phase may cross-slip form {111} to {100} planes to form the K–W locks, which may restrain the slipping and cross-slipping of dislocations to improve the creep resistance of alloy. While the size of the rafted γ′ phase in thickness diminshes as the creep temperrature rises, which may enhance the climbing rate of dislocations to decrease the creep lifetimes of alloy.

By means of creep property measurement and microstructure observation, an investigation has been made into microstructure and creep behavior of a directional solidification Ni-based superalloy at high temperatures. Results show that after full heat treatment, small cuboidal γ′ precipitates distribute in the dendrite regions, while coarser ones distribute in the inter-dendrite regions. In the primary stage of creep, the γ′ phase in alloy is transformed into the rafted structure along the direction vertical to stress axis, and then the creep of alloy enters the steady state stage. And dislocations slipping in the g matrix and climbing over the rafted γ′ phase are thought to be the deformation mechanism of the alloy during steady creep stage. At the latter stage of creep, the alternate slipping of dislocations may shear and twist the rafted γ′/γ phases, which promotes the initiation and propagation of the micro-cracks along the boundaries near the coarser rafted γ′ phase. And the bigger probability of the creep damage occurs in the grain boundaries along 45° angles relative to the stress axis due to them bearing relatively bigger shearing stress.

By means of heat treatment at different regimes, creep properties measurement and microstructure observation, an investigation has been made into the influence of solution temperatures on the configuration evolution of carbides and creep property of a directionally solidified nickel-based superalloy. Results show that the bulk-like MC type carbide in as-cast superalloy may be decomposed for transforming into the particle-like M23C6 carbides during heat treatment. As the solution temperature enhances and time prolongs, the probability of the decomposition and configuration evolution of the carbide increases. Moreover some fine M23C6 carbide particles are precipitated along the boundaries. Compared to the alloy solution treated at lower temperature, the alloy treated at higher temperature displays a better creep resistance at 780 °C. The fine cubiodal γ′ precipitates are homogeneously distributed in the dendrite/inter-dendrite regions, the fine particle-like carbides are precipitated along boundaries to restrain the boundary sliding, which is thought to be the main reasons of alloy having better creep resistance. At the later stage of creep at 780 °C, the micro-cracks in alloy are firstly initiated and propagated along the boundaries at about 45° angles relative to the stress axis, due to bearing the bigger shearing stress during creep, up to fracture, which is thought to be the fracture mechanism of alloy during creep.

The effect of pre-compressive treatment on the steady-stage creep behavior of a 〈0 1 1〉-oriented single-crystal Ni-based superalloy was studied. Results show that pre-compression at 1040 °C/180 MPa along the [1 0 0] orientation improves the tensile creep strength of the alloy along the [0 1 1] orientation at 850 °C/400 MPa and 1040 °C/137 MPa. The elimination of gable channels, the relatively narrow roof channels and the more activated slip systems of dislocations are responsible for the better creep resistance of the pre-compressed alloy.

By means of the elastic–plastic stress–strain finite element method (FEM), the distribution of the von Mises stress and strain energy density in the regions near the interfaces of the cuboidal γ/γ′ phases is calculated to investigate the rafted behaviors of γ′ phase in a [111] oriented single crystal (SC) nickel-based superalloy. Results show that, after fully heat treated, the microstructure of the superalloy consists of the cuboidal γ′ phase embedded coherently in the γ matrix and arranged regularly along the 〈100〉 orientation. And the parameters and misfits of γ′/γ phases in the alloy increase with the temperature. After crept for 50 h, the γ′ phase in alloy has transformed into the mesh-like rafted structure on (010) plane along [001] and [100] orientations. When the tensile stress is applied along [111] direction, the change of the strain energy on the planes of the cuboidal γ′ phase results in the directional diffusion of the elements. Thereinto, compared with (010) plane, the bigger expanding strain occurs on (100) and (001) planes along the [010], [001] and [010], [100] directions, which may trap the Al and Ti atoms with bigger radius to promote the directional growth of γ′ phase on (010) plane along [100] and [001] directions. This is thought to be the main reason for the γ′ phase directionally growing into the mesh-like rafted structure on (010) plane.

By means of creep properties measurement and microstructure observation, combined with the contrast analysis of dislocation configurations, an investigation has been made into the effects of the element Re on the deformation mechanisms within the γ′ phase of single crystal nickel-based superalloys during creep at elevated temperatures. Results show that, compared to free-Re superalloy, 2% Re alloy at 980 °C displays a lower strain rate and better creep resistance. In the later stage of creep at 980 °C, the deformation feature of the free-Re superalloy is the 〈110〉 super-dislocation shearing into the rafted γ′ phase and slipping on {111} planes, and the Kear–Wilsdorf (KW) locks are formed within the γ′ phase of 2% Re single crystal nickel-based superalloy. And the dislocations in KW locks in Re-containing superalloy under thermal activation may be re-activated for slipping on {111} planes to release the KW locks as the creep temperature further increases to 1070 °C.

By means of direct aging, microstructure observation and creep property measurement, the microstructure and creep behaviors of GH4169G superalloy are investigated. Results show that, after direct aging, the grain size is inhomogeneous in the alloy, and some δ precipitates discontinuously distribute in the grain and along the boundaries, which may improve the bonding strength of the boundaries. Under the experimental conditions, the creep activation energy of the alloy during steady-state creep are calculated to be Q = 594.7 kJ/mol. During creep, the deformation features of the alloy are twinning deformation and dislocations slipping in the matrix. As creep goes on, deformed dislocations pile up near the boundary regions to induce stress concentration for promoting the initiation and propagation of cracks along boundaries.

By means of creep properties measurement and microstructure observation, combined with the finite element method analysis, an investigation has been made into the influence of the cavity with/free cracks on creep behaviors of a single crystal nickel-based superalloy. Results show that during creep, the different evolution features of γ′ phase occur in the regions near the cavity with/free cracks in the single crystal nickel-based superalloy due to the various stress distributions. Therefore, the rafted γ′ phase in the region near the round cavity is transformed into the rafted structure along the direction of 45° angles relative to the stress axis, and the rafted γ′ phase in the region near the cavity-crack is arranged along the direction parallel to the round edge of the cavity in the form of the streamlined model due to the effect of the stress distribution. Compared to the stress value in the region near the round cavity, the bigger von Mises stress value is distributed in the pole b region near the cavity-cracks, and the maximum value of the stress distribution in the pole b region increases as the creep goes on, which may promote the propagation of the cracks along the direction perpendicular to the stress axis. This is thought to be the main reason for the alloy with the cavity-crack having a shorter creep life.

By means of heat treatment, creep properties measurement and microstructure observation, an investigation has been made into the influence of heat treatment on microstructure and creep properties of the high Nb-TiAl alloy. Results show that the microstructure of as-cast high Nb-TiAl alloy consists of lamellar γ/α2 phases with various orientations. The irregular serrated boundaries with single γ phase are located in between the lamellar γ⧸α2 phases with various orientations. After solution and aging treatment, the microstructure of alloy consists of uniform and regular lamellar γ⧸α2 phases, and the regular boundaries are located in between the lamellar γ⧸α2 phases. Under the applied stress of 200 MPa at 800 °C, the creep lifetime of the as-cast high Nb-TiAl alloy is measured to be 147 h, the one of the alloy after heat treatment is measured to be 297 h. In the ranges of the applied temperatures and stresses, the creep activation energy of alloy after heart treatment is measured to be Q=432 kJ/mol. The deformation mechanism of alloy during creep is dislocations slipping in the lamellar γ⧸α2 phases, the creep dislocations may be decomposed to form the configuration of the partials plus stacking faults. The deformation of as-cast alloy during creep occurs mainly in the irregular serrated boundary regions with single γ phase. For the heat treated alloy, the primary/secondary slipping systems of dislocations are alternately activated during creep, which results in the bigger plastic deformation of alloy to contort the lamellar α2/γ phases. In the latter stage of creep, the cracks are firstly initiated along the boundaries parallel to the lamellar γ⧸α2 phases, and propagated along the boundaries vertical or at about 45° angles relative to the stress axis up to the occurrence of creep fracture.

By means of creep propertiesmeasurement, microstructure observation and contrast analysis of dislocation configuration, the creep behaviors and role of dislocation networks in FGH95 powder metallurgy Ni-based superalloy during creep have been investigated. The results show that the microstructure of alloy consists of the fine γ′ phase coherently embedded in the γ matrix, and a few coarser γ′ particles are distributed in the boundary regions. In the ranges of the applied temperatures and stresses, the alloy displays a better creep resistance and longer lifetime. The deformation mechanisms of alloy during creep are the dislocations slipping in the matrix and shearing into the γ′ phase, and the dislocations shearing into the γ′ phase may be decomposed to form the configuration of the partials plus the stacking fault. During creep, two groups of dislocations on different slip planes may knit to form the quadrangular dislocation networks; while two groups of moving dislocations on the same slip plane may encounter and react to form the hexagonal dislocation networks. The strength of boundaries is responsible for the creep resistance of alloy, and the one is related to the deforming behaviors of coarse γ΄ particles distributed in the boundaries. Thereinto, the dislocation networks distributed in the interfaces of coarse γ′/γ phases may release the lattice-misfit stress and relax the stress concentration to delay the dislocation shearing into the γ′ phase, which is beneficial to keep the boundary strength and enhance the creep resistance of the alloy.

By means of solution treatment, microstructure observation and mechanical property measurement, the microstructure and tensile deformation features of tandem hot rolling GH4169 alloy have been investigated. Results shown that, after solution treatment and aging, large numbers of the spherical γ′ phase and the flat-ellipsoidal γ″ phase dispersedly precipitate in the matrix, and acicular or short rod-like δ-phase discontinuously distribute along grain boundaries. As the tensile temperatures increases, the tensile strength and yield strength of the alloy gradually decrease. The tensile deformation features of the alloy are that the twinning and slipping dislocations with double orientations are activated in the matrix. In the later tension, the deformed dislocations pile up near the grain boundary to induce initiation and propagation of cracks due to stress concentration.

Thermomechanical cycling was performed in single crystal superalloy TMS-75 to investigate the detailed microstructural evolution. An interesting phenomenon shows that the area around the micropores tends to create many deformation twins, while the area far from the micropores exhibits few deformation twins. To interpret this feature, finite element analysis of the stress field around micropores proceeded, which gives the results that the stress isoline near the micropores region appears the feature of the acetabuliform distribution and possesses the bigger stress value at 45° angle direction relative to the applied stress axis. In addition, theoretical calculations were further conduced in an effort to clarify the critical conditions for twinning in Ni based single crystal superalloys. The feature that a high density of deformation twins occurs in the area around the micropores in superalloy TMS-75 results from the stress concentration in the region around the micropores.

By means of direct aged treatment, creep property measurement and high resolution TEM microstructure observation, the deformation mechanisms of Tandem Hot Rolled GH4169 superalloy during creep are investigated. Results show that, after direct aging treatment, fine γ″ particles with different sizes and shapes dispersedly precipitate in the alloy, which is one of important factors for the alloy possessing good creep resistance. And the deformation mechanisms of the alloy are that the deformed twinnings with different orientations are activated on {111} plane by pole mechanism, thereinto, the twinning dislocation may continuously slip around a pole axis dislocation on the twinning planes when the applied stress exceeds the critical value, and the twinnings may multiply by the dislocation reactions and mutual indemnification. As creep goes on, the denser dislocations with single or double orientations slip in the different twinnings, which play an important role of coordinating the grain deformation to enhance the creep resistance of the alloy.

By means of creep properties measurement, microstructure and fracture morphology observation, an investigation has been made into the damage and fracture mechanism of a nickel-based single crystal superalloy during creep at intermediate temperature. The results show that the deformation mechanism of the alloy is dislocations slipping in the γ matrix and shearing into γ′ phase, the dislocations shearing into γ′ phase may be decomposed to form the configuration of the partials plus stacking fault and the K–W locking. While in the latter stage of creep, the primary–secondary slipping systems are alternatively activated to shear and twist the cubical γ′/γ phases, which results in the initiation of the micro-crack occurring in the intersection regions of two slip systems. As creep goes on, the micro-cracks are propagated on the (001) plane along 〈110〉 directions. Because the multi-cracks are formed and propagated on the different cross-section of the sample during creep, which may result in the tearing edge or secondary cleavage plane generating at the tip of the cracks along the direction with bigger shearing stress. When the propagating primary cracks on (001) plane intersects with {111} secondary cleavage plane, the propagation of the primary cracks is terminated to form the square-like cleavage plane on the (001) plane. In further, the multi-cracks propagating on the different cross-section are joined by the tearing edge or secondary cleavage plane until the occurrence of creep fracture, this is thought to be the main reason of the creep fracture displaying the uneven and multi-level cleavage features.

By means of the stress–strain finite element method (FEM) in elastic–plastic regime, the influences of the applied stress on the distribution of von Mises stress and evolution of shape regularity of γ′ phase in a [0 1 1] oriented single crystal nickel-based superalloy are investigated. Results show that, after full heat treatment, the microstructure of [0 1 1] oriented single crystal superalloy consists of the cuboidal γ′ phase embedded coherently in the γ matrix phase, and on (1 0 0) plane aligned regularly at angle of 45° relative to the [0 1 1] orientation. During compressive creep, the cuboidal γ′ phase is transformed into the mesh-like lamellar rafted structure on (1 0 0) plane. Therefore, the changes of the lattice strain, the strain energy density and interfacial energy are thought to be the driving forces of the element diffusion and γ′ phase directional growth. During compressive creep, the lattice contraction occurs on (1 0 0) plane of the cuboidal γ′ phase along [0 0 1] and [0 1 0] directions, whose extruding effect may repel Al, Ti atoms with bigger radius. Therefore, the expanding strain of lattices along [1 0 0] direction on (0 1 0) and (0 0 1) planes of the cuboidal γ′ phase may trap Al, Ti atoms to promote the directional growth of γ′ phase on (1 0 0) plane along [0 0 1] and [0 1 0] directions, which is thought to be the main reason for the γ′ phase growing directionally into the mesh-like lamellar structure.