Microstructure Of Single Crystal Superalloy Research Articles

Evolution of local misorientations in the γ/γ'-microstructure of single crystal superalloys during creep studied with the rotation vector baseline (RVB) EBSD method.

The present work uses the rotation vector baseline electron back scatter orientation imaging method (RVB-EBSD) to study the evolution of small misorientations between the γ- and γ'-phase in Ni-base single crystal superalloys (SXs) during creep. For this purpose, two material states of the SX ERBO1 (CMSX4 type) were characterized after creep deformation at 850°C and 600 MPa to final strains of 1% and 2%. Obtaining reliable phase boundary misorientation (PBM), kernel average misorientation (KAM) and orientation spread (OS) data represents a challenge for electron backscatter diffraction (EBSD), not only because the method operates at its limits of lateral and angular resolution, but also because it is difficult to differentiate between the two phases merely based on Kikuchi diffraction. The two phases differ in chemical composition which gives rise to different EBSD background intensities. These can be exploited to differentiate between the two phases. In the present work, crystallographic and chemical information are combined to demonstrate that orientation imaging can be used to document the formation of dislocation networks at γ/γ'-interfaces and the filling of γ-channels by dislocations. These findings are in good agreement with reference results from diffraction contrast scanning transmission electron microscopy. It is also shown that misorientations evolve between small groups of equally oriented γ/γ'-neighborhoods, on a size scale above characteristic γ/γ'-dimensions (>0.5 μm) and below distances associated with dendritic mosaicity (<200 μm). The methodological aspects as well as the new material specific results are discussed in the light of previous work published in the literature. RESEARCH HIGHLIGHTS: Microstructure evolution during [001] tensile creep of Ni-based single-crystalline alloy. Application of RVB-EBSD technique, focused on angular misorientations between γ/γ' phases, with accuracy of 0.01°. Separation of γ/γ' phases using experimental post-processing of raw EBSD data.

Study on the homogeneity of tilted dendritic structures in single crystal superalloys

Dendrites play a crucial role in the microstructure of single crystal superalloys, predominantly developing along the [001] orientation. The mechanical performance is greatly influenced by the uniform distribution of dendrites. During the solidification process, we frequently observe instances of dendrites deviating from the [001] orientation, resulting in tilted structures. These deviations give rise to both aligned and misaligned arrays within the transverse section. The study comprehensively examines the homogeneity of tilted dendritic structures. As solidification progresses, new dendrites within the aligned array tend to maintain a hexagonal structure. Simultaneously, the remaining metastable structures gradually transition into hexagonal structures. In contrast, various polygons mutually transform in misaligned array, resulting in a dynamic adjustment of their proportions. Consequently, the aligned array exhibits a higher proportion of hexagonal structures and a more uniform dendrite spacing compared to the misaligned array. Within the transitional region, an increase in heptagonal structures leads to heightened non-uniformity in dendrite spacing. The predominance of hexagonal structures can be attributed to their more uniform solute distribution, facilitated by the characteristics of the aligned array, which promote hexagonal structure formation by adjusting the solute field distribution. On the other hand, due to the random positioning of dendrites in the misaligned array, various stacking structures coexist in dynamic equilibrium. The research reveals an intrinsic relationship between macroscopic array patterns, stacking structures, dendrite spacing, and microscopic solute distribution. These findings provide a theoretical foundation for the production of high-quality single crystal dendritic structures and offer insights into their influence on material properties.

The effect of void defect on the evolution mechanisms of dislocations and mechanical properties in nickel-based superalloys by molecular dynamics simulation of real γ/γ′ structures

It is acknowledged that voids are unavoidable manufacture defects influencing the microstructure evolution and mechanical properties of nickel-based superalloys. The present work investigates the influences of void on the dislocation evolution and mechanical properties of nickel-based superalloys under different thermal conditions and strain rates. The simulation uses an embedded cubic model with voids in which the γ′ phase with L12 structure is surrounded by γ phase with FCC structure to represent the real microstructure of single crystal superalloys. A model without voids is also studied. The existence of voids changes the misfit dislocation network by introducing cylindrical dislocation network around the voids which play a vital role on preventing the void coalescence at low strain. Dislocation extraction algorithm (DXA) analysis finds that new type of dislocation appears in the model with voids and the dislocation length is longer. The elastic modulus and yield strength are decreased by voids, as well as the ductility of the material. This paper demonstrates that the accumulation and reaction between dislocations are the most direct reason for declining the mechanical properties of nickel-based superalloys with void defects.

The nucleation and growth mechanism of Ni-Sn eutectic in a single crystal superalloy

The microstructure of single crystal superalloy with and without tin layer on the surface of as-cast and heat-treatment state was investigated by optical microscope (OM) and scanning electron microscopy (SEM). The composition of different regions on the surface was tested by energy dispersive X-ray (EDS). The reaction intermetallic compound (IMC) formed in the heat treatment process was confirmed by X-ray diffraction (XRD). The orientations of different microstructure in samples as heat treatment state were determined by electron back-scattering diffraction (EBSD) method. The porosity location in the interdendritic region was observed by X-ray computed tomography (XCT).The experiment results showed that the remained Sn on the surface of the superalloy reacted with Ni, and then formed Ni3Sn4 in the as-cast state. Sn enriched by diffusion along the porosity located in the interdendritic region and γ+γ′ (contain a little of Sn) eutectic and Ni3Sn2 formed in single crystal superalloy during heat treatment, and the recalescence behaviors were found. Ni3Sn2 nucleated independently in the cooled liquid at the front of (γ+γ′) (Sn) eutectic. The nucleation and growth mechanism of the eutectic and Ni3Sn2 IMC during heat treatment was discussed in the present paper.

The microstructure of a single crystal superalloy after different aging heat treatments

To study the influence of aging heat treatments on the microstructure of single crystal superalloys with high content of refractory elements and optimal the aging heat treatment regimes, a single crystal superalloy containing 22 wt% refractory elements was investigated. Results show that for the experimental alloy, even the homogenization–solution heat treatment for 25 h cannot homogenize the alloying elements completely. During primary aging heat treatment, γ′ phase grows larger and turns to regular cubes. Higher aging temperature induces larger γ′ cubes. For specimens with primary aging heat treated at 1120 °C, γ′ morphology does not change apparently during secondary aging heat treatment. For specimens with primary aging heat treatment at 1150 °C, γ′ phase in interdendrite grows obviously comparing with that in dendrites. By analyzing the precipitating kinetics of γ′ phase, it is found that owning to the dendrite segregation and different aging heat treatment temperatures, γ′ phase at different regions grows under the control of different factors at different aging heat treatment stages. The two controlling factors that are driving forces of phase transformation and element diffusion rate induce obviously different growth rates of γ′ phase. As a result, the γ′-precipitating behaviors are variable based on different solute concentrations and aging temperatures. For advanced single crystal superalloys that are supposed to be used at relatively high temperatures, the final γ′ size after aging heat treatment is suggested to be close to the crossing point of diffusion controlling curve and driving force controlling curve corresponding to the serving temperature. And then, high-temperature properties can be improved.

Stress relaxation behavior in single crystal superalloys

The objective of this study was to examine the stress relaxation response and corresponding changes in microstructure of single crystal superalloys. The effects of temperature and time on stress relaxation response were examined for single crystal superalloys CMSX-4 and EPM-102. Stress–strain rate data from relaxation tests was in good agreement with constant load tests; it was further refined by testing samples from the same casting, and by eliminating the effects of primary creep in the stress relaxation test. From these tests, it was determined that EPM-102 had higher resistance to stress relaxation than CMSX-4 at and above 982°C. Additionally, time-dependent strain recovery (“viscoelasticity”) was observed after unloading during these tests. The single crystal alloys displayed directional coarsening after stress relaxation testing at high temperatures.

Influence of misfit stresses on dislocation glide in single crystal superalloys: A three-dimensional discrete dislocation dynamics study

In the characteristic γ/γ′ microstructure of single crystal superalloys, misfit stresses occur due to a significant lattice mismatch of those two phases. The magnitude of this lattice mismatch depends on the chemical composition of both phases as well as on temperature. Furthermore, the lattice mismatch of γ and γ′ phases can be either positive or negative in sign. The internal stresses caused by such lattice mismatch play a decisive role for the micromechanical processes that lead to the observed macroscopic athermal deformation behavior of these high-temperature alloys. Three-dimensional discrete dislocation dynamics (DDD) simulations are applied to investigate dislocation glide in γ matrix channels and shearing of γ′ precipitates by superdislocations under externally applied uniaxial stresses, by fully taking into account internal misfit stresses. Misfit stress fields are calculated by the fast Fourier transformation (FFT) method and hybridized with DDD simulations. For external loading along the crystallographic [001] direction of the single crystal, it was found that the different internal stress states for negative and positive lattice mismatch result in non-uniform dislocation movement and different dislocation patterns in horizontal and vertical γ matrix channels. Furthermore, positive lattice mismatch produces a lower deformation rate than negative lattice mismatch under the same tensile loading, but for an increasing magnitude of lattice mismatch, the deformation resistance always diminishes. Hence, the best deformation performance is expected to result from alloys with either small positive, or even better, vanishing lattice mismatch between γ and γ′ phase.

Evaluation of long-term creep-fatigue life of stainless steel weldment based on a microstructure degradation model : Asayama, T. and Hasebe, S. Journal of the Society of Materials Science, Japan (1997) 46(1), 65–69

The objective of this study was to examine the stress relaxation response and corresponding changes in microstructure of single crystal superalloys. The effects of temperature and time on stress relaxation response were examined for single crystal superalloys CMSX-4 and EPM-102. Stress–strain rate data from relaxation tests was in good agreement with constant load tests; it was further refined by testing samples from the same casting, and by eliminating the effects of primary creep in the stress relaxation test. From these tests, it was determined that EPM-102 had higher resistance to stress relaxation than CMSX-4 at and above 982 °C. Additionally, time-dependent strain recovery (“viscoelasticity”) was observed after unloading during these tests. The single crystal alloys displayed directional coarsening after stress relaxation testing at high temperatures.