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Two alloys with different Ta and Al contents were applied to study the influence of Ta/Al ratio on the microstructural evolution and creep deformation under high temperature. The increase of Ta/Al ratio made the γ/γ′ lattice misfit more negative and enhanced the volume fraction of γ′ phase, which produced cubic and small γ′ phase in the initial microstructures. These initial tiny γ′ phases impeded the dislocations movement and delayed the course of complete rafted γ′ phase during the origination of creep deformation, which prolonged the time of the primary creep stage. Moreover, the increase of Ta/Al ratio and addition of Ru produced the denser and stable dislocation networks, the high APB energy and better solution strengthening, which hindered the climbing and sliding of dislocations, and restrained the formation of superdislocations in the γ′ precipitate. The second creep stage was extended, and the minimum creep rate was reduced. Hence, the increase of whole creep life of the alloy containing high Ta/Al ratio was attributed to the prolongation of the primary and second creep stages, and the low minimum creep rate. The appearance of the topological inversion phenomenon during the tertiary creep stage was the primary cause for the sudden increase of the creep strain rate of the alloy containing low Ta/Al ratio. However, the high creep strain rate of the alloy containing high Ta/Al ratio during the tertiary creep stage was related to the occurrence and extending of the cracks near the voids. Both alloys would lose efficacy within 20 h–30 h.

The effect of thermal exposure on microstructure and creep properties of a fourth-generation nickel-based single crystal superalloy was investigated. The thermal exposure of samples after the full heat treatment was carried out at 1000 °C, 1100 °C and 1140 °C for 100 h and 200 h. The γ′ coarsening, γ′ rafting and γ channel widening were observed in samples after thermal exposure. When the thermal exposure time was constant, the morphology of γ′ phase in the alloy evolved significantly with increasing aging temperature. The interfacial dislocation networks in aged samples after creep ruptured gradually became irregular and sparse with the increase of exposure temperature. When the higher exposure temperature was used, enlargement of the defect pores was observed in samples, the microcracks were more likely to initiate and propagate at the corner of these pores. After aging at 1000 °C for 100 h, the creep life at 1140 °C/137 MPa was slightly longer than that of heat-treated sample, which could be attributed to the slightly coarsened γ′ phase, homogenization of refractor elements. In contrast, the creep life of sample exposed at 1140 °C for 100 h was greatly decreased. The decrease of creep life was dominated by the rafting of γ′ phase, the irregular interfacial dislocation networks as well as the enlargement of homogenization pores.

Both surface and internal microstructures of a second-generation Ni-based single crystal (SX) superalloy were studied after creep and rejuvenation heat treatment (RHT). It is indicated that the microstructures, such as the dislocation network, the γ phase and the γ' phase, can be recovered to those after the standard heat treatment (SHT). It is found that RHT affected zone (RAZ) formed at the surface is composed of the γ'-free layer, the transition layer and the recrystallization (RX), which are less than 20 μm in depth totally. Such depth of the RAZ doesn’t affect the properties of the superalloy. The morphology of γ' phase at the RAZ is related to the composition of the elements. The average creep life after RHT is close to the average life after SHT. It is concluded that RHT could effectively repair SX parts and increase the total life of the sample after a damage by creep.

Elevated temperature creep behaviors at 1100 °C over a wide stress regime of 120–174 MPa of a third-generation Ni-based single crystal superalloy were studied. With a reduced stress from 174 to 120 MPa, the creep life increased by a factor of 10.5, from 87 h to 907 h, presenting a strong stress dependence. A splitting phenomenon of the close- (about 100 nm) and sparse- (above 120 nm) spaced dislocation networks became more obvious with increasing stress. Simultaneously, a0 superdislocations with low mobilities were frequently observed under a lower stress to pass through γ′ precipitates by a combined slip and climb of two a0 superpartials or pure climb. However, a0 superdislocations with higher mobility were widely found under a higher stress, which directly sheared into γ′ precipitates. Based on the calculated critical resolved shear stresses for various creep mechanisms, the favorable creep mechanism was systematically analyzed. Furthermore, combined with the microstructural evolutions during different creep stages, the dominant creep mechanism changed from the dislocation climbing to Orowan looping and precipitates shearing under a stress regime of 137–174 MPa, while the dislocation climbing mechanism was operative throughout the whole creep stage under a stress of 120 MPa, resulting a superior creep performance.

Scanning speed is a critical parameter for laser process, which can play a key role in the microstructure evolution of laser melting. In the laser melting of single crystal superalloy, the effects of scanning speed were investigated by experimental analysis and computational simulation. The laser was scanning along [ 7 1 ¯ 0 ] direction on (001) surface in different speeds. Solidification microstructures of dendrites growth direction and the primary dendritic spacing were analyzed by metallograph. Besides, a planar interface during solidification was taken into attention. Experiment results indicated that the primary dendritic spacing and thickness of planar interface decrease with the increase of speed. Through simulation, distribution of dendrites growth velocity and thermal gradient along dendrite growth direction were calculated, and the simulation of dendrites growth direction agreed with the experiment results. Additionally, a constant value was acquired which can be used to predict the primary dendritic spacing. Moreover, according to curve-fitting method and inequality relation, a model was proposed to predict the thickness of planar interface.

Precipitation of topologically close-packed phases in a Re-containing Ni-base single crystal superalloy was studied under elevated temperature by in situ transmission electron microscopy. Above 1150 °C, a new intergrowth structure called C phase was found within μ phase with a defined crystallographic orientation relationship. Elements mapping analysis reveals that C phase has a similar element composition as μ phase, but contains a lower level of Cr, W and Re. With increasing temperature, the proportion of C phase increases gradually. At 1250 °C, C phase becomes the dominant precipitate. It is demonstrated that C phase is more thermodynamically stable than μ phase above 1150 °C that leads to an intergrowth pathway from μ phase to C phase.

The effects of a pulsed magnetic field (PMF) on the microsegregation of solute elements during directional solidification of a Ni-based single crystal superalloy were experimentally investigated, and the results show that the PMF significantly affects the microsegregation of Al, Ti, Co, Mo and W elements in the alloy. However, the distribution behavior differs for both positive and negative segregation elements. With the PMF, the microsegregation of negative segregation elements, Co and W, was restrained effectively, while that of positive segregation elements, Al, Ti and Mo, was aggravated. A segregation model was established to reveal the distribution mechanism of the elements with PMF. It is considered that, under the action of PMF, the jumping of solute atoms from the liquid phase to solid phase is hindered, but the jumping of solute atoms from the solid phase into liquid phase is promoted during solidification. As a result, the effective distribution coefficient of the solute atoms is reduced, which leads to the reduction of microsegregation of negative segregation elements and aggravation of microsegregation of positive segregation elements.

The γ/γ' microstructural evolution in a nickel based single crystal superalloy during load-free thermal exposure at 900 °C has been further investigated in this paper. The growth characteristics of γ′ precipitates were discussed in detail. The generation of interfacial dislocations would accelerate the rate of coalescence in the dendrite arms. The average sizes of precipitates were used to compare interface with diffusion controlled growth mechanism and no mechanism seems obviously dominant, although the square rate law gives slightly better fit. The coarsening behavior may be controlled by diffusion through the ragged interface between the γ′ precipitate and the γ matrix.

The orientation dependence of creep rupture lives of a single crystal superalloy at 760 degrees C/760 MPa was investigated. The orientations of the specimens tested were about 30 away from [001]. The results showed that specimens with orientations on the [001]-[011] boundary had the longest rupture life. The deformation of these specimen were controlled by a/2 slip and a few stacking faults with two orientations were observed. On the other hand, specimens with orientations near the [001]-[011] boundary or on the [001]-[111] boundary showed short rupture lives, and stacking faults with single orientation were observed in these specimens. The rupture properties and the deformation mechanisms were discussed based on the dislocation pattern and the calculated Schmid factors for different specimens.

The creep rupture properties of a single crystal superalloy were tested at 975°C/255 MPa as a function of the deviation degrees from [001]. The misorientation of the specimens away from [001] distributed approximately along a line between [001]–[011] and [001]–[111] boundaries in the triangle of the stereographic projection. Creep rupture lifetimes of the specimens were not sensitive to the misorientation until the deviation degree exceeded ∼30 deg. Two steps of lattice rotation were found in all specimens during creep, first towards the [001]–[111] boundary, and then to [001] or [111] along the boundary. Single slip and strong asymmetric deformation were observed during the first stage of lattice rotation in specimens with large misorientation. The rotation mechanism was associated with the activated slip systems according to the calculated Schmid factors. The impact of lattice rotation on the rupture properties was also discussed.