Superlattice Extrinsic Stacking Faults Research Articles

Deformation mechanism of Ni-based single crystal superalloy under ultrasonic surface rolling and subsequent thermal exposure

In present work, ultrasonic surface rolling (USR) is applied to DD6 Ni-based single crystal (NBSC) superalloys, and the thermal relaxation behavior of the pre-treated alloys are investigated at 650 °C. Combined with mechanical properties, the plastic deformation and recovery mechanisms of the hardened layers are revealed in detail. Results show that compressive residual stress amplitude of 1163 MPa, 75.1 % improvement of surface nano-hardness and 83.02 % reduction on surface roughness are achieved by USR. Both deformed layer thickness and slip density progressively increases with USR cycles, where initial octahedron {111}<110> slips transform to a mixture of octahedron {111}<110> and dodecahedron {111}<112> slips. Meanwhile, shear step length expansions form γ′ segment/γ terminal interface with superlattice extrinsic stacking fault (SESF) on 1¯11, which may have inferior energy threshold for γ dislocation invasions than primary ones through shortening mean free path. Also, this drives γ′/γ solute mingling to induce tortuous interfaces. While dislocation proliferation dominates γ channel hardening, the γ′ hardening derives from planar fault strengthening and net system energy augmentation. During thermal exposure, surface integrities of USRed DD6 are strongly degraded, attributable to slip trace elimination and local γ′ coarsening. Thermal exposure further compels the γ dislocation to migrate towards γ′ segment across γ′ segment/γ terminal interface. Then, they are probably consumed to motivate solutes to phase boundary to reshape the γ′ morphologies through interface rearrangements under the assistance of γ dislocation network. The orientation transformation of SESF on 1¯11 to 010 and the mixture of SESF and superlattice intrinsic stacking fault on 1¯11 are thought to be possible manipulated mechanisms for γ′ coarsening, significant impairing the work hardening of alloy due to interfacial coherency deterioration. This paper could provide guidance for USR strengthening designs of NBSC.

Creep anisotropy of additively manufactured Inconel-738LC: Combined experiments and microstructure-based modeling

The current lack of quantitative knowledge on processing-microstructure–property relationships is one of the major bottlenecks in today’s rapidly expanding field of additive manufacturing. This is centrally rooted in the nature of the processing, leading to complex microstructural features. Experimentally-guided modeling can offer reliable solutions for the safe application of additively manufactured materials. In this work, we combine a set of systematic experiments and modeling to address creep anisotropy and its correlation with microstructural characteristics in laser-based powder bed fusion (PBF-LB/M) additively manufactured Inconel-738LC (IN738LC). Three sample orientations (with the tensile axis parallel, perpendicular, and 45° tilted, relative to the building direction) are crept at 850 °C, accompanied by electron backscatter secondary diffraction (EBSD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) investigations. A crystal plasticity (CP) model for Ni-base superalloys, capable of modeling different types of slip systems, is developed and combined with various polycrystalline representative volume elements (RVEs) built on the experimental measurements. Besides our experiments, we verify our modeling framework on electron beam powder bed fusion (PBF-EB/M) additively manufactured Inconel-738LC. The results of our simulations show that while the crystallographic texture alone cannot explain the observed creep anisotropy, the superlattice extrinsic stacking faults (SESF) and related microtwinning slip systems play major roles as active deformation mechanisms. We confirm this using TEM investigations, revealing evidence of SESFs in crept specimens. We also show that the elongated grain morphology can result in higher creep rates, especially in the specimens with a tilted tensile axis.

Reliable identification of the complex or superlattice nature of intrinsic and extrinsic stacking faults in the L12 phase by high-resolution imaging

Planar defect-based deformation mechanisms are becoming increasingly important for understanding and tuning the mechanical properties of superalloys. Typically, to elucidate such deformation mechanisms, the type of planar defect is determined by conventional transmission electron microscopy or by high-resolution imaging along 〈110〉 crystallographic directions. However, an unambiguous identification of the complex or non-complex nature of intrinsic and extrinsic stacking faults in the L12 phase may not be possible by analyzing only one projection. To overcome this limitation, we present a straightforward approach to identify the superlattice or complex character of intrinsic and extrinsic faults in the L12 structure in high-resolution scanning transmission electron microscopy by tilting to a different zone axis and examining the shift of superlattice planes. Using this approach, two key aspects of the Kolbe mechanism for the formation of superlattice extrinsic stacking faults are verified for the first time: the presence of a complex intrinsic fault segment at the leading end and the occurrence of re-ordering.

Segregation of alloying elements at planar faults during creep in the PM Ni-based superalloy FGH4096

In this work, the segregation of alloying elements at planar faults during creep in the powder metallurgy (PM) Ni-based superalloy FGH4096 was characterized. It was found that Co, Cr, Mo, Ti, and Nb segregated to the superlattice extrinsic stacking fault and microtwin boundaries in the γ′ phase during creep, leading to the disordered local phase transformation in the γ′ phase. This element segregation behavior of FGH4096 was different from other superalloys which had been reported. This study contributed to understanding the interaction between alloying elements and planar faults in FGH4096 during creep.

Enhanced creep performance in a polycrystalline superalloy driven by atomic-scale phase transformation along planar faults

Predicting the mechanical failure of parts in service requires understanding their deformation behavior, and associated dynamic microstructural evolution up to the near-atomic scale. Solutes are known to interact with defects generated by plastic deformation, thereby affecting their displacement throughout the microstructure and hence the material's mechanical response to solicitation. This effect is studied here in a polycrystalline Ni-based superalloy with two different Nb contents that lead to a significant change in their creep lifetime. Creep testing at 750°C and 600 MPa shows that the high-Nb alloy performs better in terms of creep strain rate. Considering the similar initial microstructures, the difference in mechanical behavior is attributed to a phase transformation that occurs along planar faults, controlled by the different types of stacking faults and alloy composition. Electron channeling contrast imaging reveals the presence of stacking faults in both alloys. Microtwinning is observed only in the low-Nb alloy, rationalizing in part the higher creep strain rate. In the high-Nb alloy, atom probe tomography evidences two different types of stacking faults based on their partitioning behavior. Superlattice intrinsic stacking faults were found enriched in Nb, Co, Cr and Mo while only Nb and Co was segregated at superlattice extrinsic stacking faults. Based on their composition, a local phase transformation occurring along the faults is suggested, resulting in slower creep strain rate in the high-Nb alloy. In comparison, mainly superlattice intrinsic stacking faults enriched in Co, Cr, Nb and Mo were found in the low-Nb alloy. Following the results presented here, and those available in the literature, an atomic-scale driven alloy design approach that controls and promotes local phase transformation along planar faults at 750°C is proposed, aiming to design superalloys with enhanced creep resistance.

Doping effects on the stacking fault energies of the γ′ phase in Ni-based superalloys**Project supported by the National Key Research and Development Program of China (Grant No. 2017YFB0701502).

The doping effects on the stacking fault energies (SFEs), including the superlattice intrinsic stacking fault and superlattice extrinsic stacking fault, were studied by first principles calculation of the γ′ phase in the Ni-based superalloys. The formation energy results show that the main alloying elements in Ni-based superalloys, such as Re, Cr, Mo, Ta, and W, prefer to occupy the Al-site in Ni3Al, Co shows a weak tendency to occupy the Ni-site, and Ru shows a weak tendency to occupy the Al-site. The SFE results show that Co and Ru could decrease the SFEs when added to fault planes, while other main elements increase SFEs. The double-packed superlattice intrinsic stacking fault energies are lower than superlattice extrinsic stacking fault energies when elements (except Co) occupy an Al-site. Furthermore, the SFEs show a symmetrical distribution with the location of the elements in the ternary model. A detailed electronic structure analysis of the Ru effects shows that SFEs correlated with not only the symmetry reduction of the charge accumulation but also the changes in structural energy.

Super-X EDS Characterization of Chemical Segregation within a Superlattice Extrinsic Stacking Fault of a Ni- based Superalloy

Super-X EDS Characterization of Chemical Segregation within a Superlattice Extrinsic Stacking Fault of a Ni- based Superalloy

Modeling displacive–diffusional coupled dislocation shearing of γ′ precipitates in Ni-base superalloys

In Ni-base superalloys, superlattice extrinsic stacking fault (SESF) shearing of γ′ precipitates involves coupled dislocation glide and atomic diffusion. A phase-field model is developed to study this process, in which the free energy of the system is formulated as a function of both displacement and long-range order parameter. The free energy surface is fitted to various fault energy data obtained from experiments and ab initio calculations. Three-dimensional simulations at experimentally relevant length scales are carried out to investigate systematically the influence of microstructural features on the critical resolved shear stress. The simulations reveal that the critical resolved shear stress for SESF shearing is determined not only by the SESF energy itself, but also by the complex stacking fault energy and by the shape (interface curvature) and spacing of γ′ precipitates. The effect of reordering kinetics (i.e. temperature effect) is also investigated. It is found that viscous deformation can only occur within certain domain of intermediate temperatures.

A new γ-surface in {111} plane in L12 Ni3Al

The well-known γ-surface concept originally proposed by V.Vitek [1] has been extended to the case of the shift of one part of the crystal with respect to another in two adjacent {111} planes of the stacking fault. The proposed approach has been applied for evaluation of the effective γ-surface in {111} crystallographic plane in L12 Ni3Al. Five stable planar stacking faults has been found, in particular, superlattice intrinsic stacking fault (SISF), superlattice extrinsic stacking fault (SESF), complex stacking fault (CSF), antiphase boundary (APB) and complex extrinsic stacking fault (CESF). The last planar fault configuration has neither been observed experimentally nor predicted analytically in L12 Ni3Al before.

The intermediate temperature deformation of Ni-based superalloys: Importance of reordering

A number of planar deformation mechanisms, such as microtwinning, a[112] dislocation ribbon, and superlattice intrinsic and superlattice extrinsic stacking fault formation, can operate during the intermediate temperature deformation of nickle-based superalloys. The fundamental, rate-limiting processes controlling these deformation mechanisms are not fully understood. It has been recently postulated that reordering of atoms in the wake of the gliding partial dislocations as they shear the γ′precipitates within the γ/γ′microstructure is the limiting process. Experimental evidence that substantiates the validity of the reordering model for the microtwinning mechanism is provided. A conceptual approach to study reordering at the atomic scale using ab-initio calculation methods is also presented. The results of this approach provide a clear conceptualization of the energetics and kinetics of the reordering process, which may be generically important for the aforementioned planar deformation modes.

Investigation of creep deformation mechanisms at intermediate temperatures in René 88 DT

Creep deformation substructures in the superalloy René 88 DT have been investigated after small-strain (0.2–0.5%) creep at 650 °C using conventional and high resolution transmission electron microscopy. Clear differences in creep strength and deformation mechanisms have been observed as a function of applied stress and precipitate microstructure. Both coarse and fine bimodal precipitate microstructures have been tested, produced by relatively slow and fast cooling from the supersolvus solutionizing temperature. The finer γ′ microstructure exhibited significantly lower creep rates. It has been established that microtwinning caused by the passage of Shockley partial dislocations on successive {1 1 1} planes is the dominant deformation process at low applied stress, and changes to shearing by 1/2[1 1 0] dislocations and Orowan looping around the larger secondary precipitates at higher applied stress. In the coarser microstructure, the dominant deformation mode is isolated faulting where 1/2[1 1 0] dislocations shear the matrix while superlattice extrinsic stacking faults are created in the secondary γ′ particles. The detailed mechanisms by which these deformation modes proceed are discussed, leading to the proposition that the thermally activated process for both microtwinning and isolated faulting is similar, involving diffusion-mediated re-ordering within the γ′ particles in the wake of shearing 1/6〈1 1 2〉 Shockley partials. Based on the present evidence, it is proposed that the tertiary γ′ volume fraction is crucial in dictating the transition in mechanism and the creep strength of these alloys.

On the core structure of ⟨112] edge dislocations in γ-TiAl

Weak-beam image simulations have been carried out using the CUFOUR programme of a ] edge dislocation in γ-Ti–50 wt% Al, dissociated into three ⟨112] partial dislocations bounding superlattice intrinsic stacking-fault (SISF) and superlattice extrinsic stacking-fault (SESF) ribbons of equal width. The simulations included one of the two core configurations of the composite Shockley partial dislocation found by Kumar et al. to explain their observed image contrast, and two others that involve dissociation into stair-rods. These latter simulated images cannot be distinguished from those for the model proposed by Kumar et al. for the composite Shockley dislocation and therefore account for the observed image contrast equally well. The claim of these workers that configurations consisting of stair-rod dislocations can be ruled out is not substantiated. The core structures involving sessile stair-rod dislocations explain the locked nature of the edge dislocation. The values found by Kumar et al. for the true separation between the pairs of partial dislocations have been used to calculate (on anisotropic elasticity theory) the SISF and SESF energies to be 123 ± 10 mJ m−2.

On the shearing mechanism of γ′ precipitates by a single ( a /6)⟨112⟩ Shockley partial in Ni-based superalloys

The weak-beam technique of transmission electron microscopy has been used to analyse a new shearing configuration of γ′ precipitates after creep at 700°C of a Ni-based superalloy for gas turbine discs. The shearing configurations are made up of superlattice extrinsic stacking faults, matrix stacking faults and individual (a/6)⟨112⟩ Shockley dislocations. This mechanism is initiated by the decorrelated movement of the two Shockley partials of a single (a/2)⟨110⟩ matrix dislocation. The propagation of the leading partial creates this shearing process. This phenomenon that occurs in small γ channels owing to the flexibility of dislocations can be used to evaluate microstructural evolutions during ageing in the alloy.

Superlattice dislocations in Ti-rich polysynthetically twinned crystals of TiAl

The behaviour of superlattice dislocations in polysynthetically twinned (PST) TiAl crystals deformed from room temperature to 850 degrees C is studied by transmission electron microscopy. The dissociation modes undergo significant changes with increasing deformation temperature. At room temperature, the [011] superlattice dislocations dissociate into superpartials including either an antiphase boundary and a superlattice intrinsic stacking fault (SISF), or superlattice extrinsic stacking faults (SESFs), forming faulted dipoles, and are sessile. The 1/2[112] superlattice dislocations dissociate into faulted dipoles and are also sessile. While the dislocation configurations in the crystals deformed at 400 degrees C and 600 degrees C are identical with those in the crystals deformed at room temperature, one important variation is that the density of faulted dipoles decreases with increasing deformation temperature. In-situ heating observations show that self-annihilation of the SESF-bounding hairpin superpartial occurs within a temperature range that compares with that of the brittle-ductile transition of PST TiAl crystals. At 850 degrees C, most [011] superlattice dislocations dissociate to form Kear-Wilsdorf locks. The 1/2[112] superlattice dislocations are edge in character, dissociated into superpartials bordering a SISF and are glissile. The experimental observations suggest that the behaviour of 1/2[112] superlattice dislocations is an important factor controlling the mechanism of brittle-ductile transition of PST TiAl crystals.

On the hierarchy of planar fault energies in TiAl

The combination of computational and experimental weak-beam transmission electron microscopy (WB-TEM) dislocation imaging is suitable to distinguish between the dislocation core models proposed for the [011]-super dislocation in TiAl on the basis of the energy hierarchies (H1) and (H2). The preliminary occupational results presented here suggest that the hierarchy of planar fault energies in Ti-54at.%Al is E{sub CSF}>E{sub APB}>E{sub SISF}{approx}E{sub SESF}, (H2). The value of the antiphase boundary (APB) energy for TiAl is likely to be larger than E{sub APB}>250 mJm-2, whereas the superlattice intrinsic stacking fault (SISF) energy appears to be E{sub SISF} = 140 mJm{sup {minus}2} which is comparable to the superlattice extrinsic stacking fault (SESF) energy, E{sub SESF} = 142 mJm{sup {minus}2}. CSF stands for complex stacking fault.

Effects of oxygen on the deformation behaviour in single-phase γ-TiAl alloys

Abstract The effect of the interstitial solute element oxygen on the mobility of dislocations in single-phase γ-TiAl alloys has been assessed experimentally. Thus polycrystalline samples of alloys of Ti-52 at.% with oxygen impurity levels of either 750 or 1250wt p.p.m. have been deformed at room temperature. It has been found that, while extensive glide of dislocations with Burgers vectors b given by b = 1/2(110) is observed for higher purity specimens, their activity is very much reduced at increasing levels of oxygen impurity contents. Thus, plastic deformation of samples with an oxygen level of 1250 wt p.p.m. is provided essentially by glide of superdislocations with b = (101), accompanied by superlattice extrinsic stacking fault formation. Possible mechanisms for this effect are discussed.

The mechanisms and temperature dependence of superlattice stacking fault formation in the single-crystal superalloy PWA 1480

Deformation microstructures in PWA 1480 nickel-base superalloy single crystals were studied in the range of 20 °C to 1100 °C. Similar to previous investigations, superlattice stacking faults were observed after slow strain rate deformation at temperatures between 700 °C and 950 °C. Unlike previous studies, a high density of superlattice stacking faults was observed after deformation at 200 °C and below. The mechanisms of fault formation in the two temperature regimes were different. In the range of 700 °C to 950 °C, single isolated superlattice-intrinsic stacking faults (SISFs) were produced by the decomposition of an a/2(110) matrix dislocation in the γ/γ′ interface. The a/3(112) partial shears the particle, while the a/6(112) Shockley remains in the interface. At 200 °C and below, a high density of faults was produced on closely spaced parallel planes. The most common feature after deformation in this range is the faulted loop, which is most often observed to be a superlattice-extrinsic stacking fault (SESF). These low-temperature faults, along with their temperature dependence, were quite similar to those observed in single-phase Ll22 materials. The available evidence suggests that the low-temperature faults were produced by the dissociation of an a unit superdislocation into a pair of a/3 partials. The temperature dependence of the faulting (at low temperatures) was modeled by linear isotropic elasticity, and the results suggest that the SISF energy increases significantly from 20 °C to 400 °C. Multiplanar, overlapping superlattice faults were analyzed with respect to bond violations. This analysis suggested that an antiphase boundary (APB) on top of an SISF has a very high fault energy, similar to that of the complex stacking fault. Therefore, the presence of SISF loops on glide planes promotes further dissociation by the SISF scheme instead of the APB scheme and explains the high density of SESFs and microtwins observed in the deformation structures.

Stair-rod-dislocation-like defects in γ′ precipitates of a deformed two-phase superalloy

Abstract Stair-rod-dislocation-like defects (SRD-like), lying in plastically deformed L12 γ′ precipitates of the CMSX-2 two-phase superalloy, have been observed in transmission electron microscopy using the two-beam technique. For one of these defects a comparison between experimental bright-field images and computed images proves that the defect cannot be a SRD with a Burgers vector 1/3[0 2 0] located at the common edge of two superlattice extrinsic stacking faults, as first expected from an inspection of a fringes. From computed images, it is shown that the total elastic field of the defect is close to that of a dislocation with a Burgers vector 1/3[0 1 0].

The core structure of a frank dislocation in Ni3(Al,Nb)

This paper reports on the core structures of dislocations in L1{sub 2} crystals, in view of understanding the anomalous yield behavior of strength with increasing temperature. Theoretical approaches have been proposed for explaining the dissociations of these dislocations, and experimental evidences have been reported for such dissociations using either the weak beam technique or more recently the HRTEM (High Resolution Transmission Electron Microscopy) technique. Recent HRTEM results show that for Ni{sub 3}A1 the partial dislocations 1/2{l angle}110{r angle} which limit an APB (Antiphase Boundary) lying on (001) are only split over 0.1 - 0.3 nm along (111) planes, producing very high energy CSF's (Complex Stacking Faults) which limit an SISF (superlattice Intrinsic Stacking Fault) extended on a (111) plane. Concerning the 1/2 {l angle}111{r angle} Frank dislocation, which limits an SESF (Superlattice Extrinsic Stacking Fault), there are no reported results yet. Let us note, however, that for aluminum HRTEM images of Frank dislocation loops have been studied. This lack of information in the L1{sub 2} crystals motivated the present HRTEM study.

The effect of covalency on the deformation mechanisms in intermetallic compounds

It has been found that the effects of covalency on the anisotropy of Peierls stresses plays a significant role on the mobilities of dislocations in intermetallic compounds, and therefore influences the ductility exhibited by these materials especially at room temperature. In way of example, brief notes are given for the cases of TiAl and Ti3AL For TiAl, the microstructure of samples deformed at room temperature consists mainly of dislocations of the type b=&lt;011], with the attendant superlattice extrinsic stacking-faults, see Fig.l; also, in some grains twinning has occurred. The density of dislocations with Burgers vectors parallel to &lt;110] is extremely low; their line directions tend to be fairly close to screw orientation. For compression at 600°C, the deformation microstructure tends to be dominated by dislocations with b=1/2&lt;110], as shown in Fig.2(a), and most of the dislocations observed of this type were found to be lying in screw orientation. In agreement with previous work, at the higher temperatures dislocations with b=&lt;101] were observed to be dissociated partly on the {100} planes, the two inclined sets of straight dislocations in Fig.2(b), corresponding to Kear-Wilsdorf locks. These various results, including the line directions of the dislocations, are interpreted on the basis of the effect of covalent bonding on the anisotropy of the Peierls stresses (following) and therefore dislocation mobilities in this compound. The lack of ductility exhibited at room temperature is attributed to a lack of mobility of dislocations with b=1/2&lt;110] caused by covalency, and the formation of sessile configurations by dissociation of dislocations with b=&lt;101]. The increase in ductility of TiAl at higher temperatures is attributed to the operation of the slip systems 1/2&lt;110]{111}, where dislocations with b=1/2&lt;110] become significantly more glissile because of thermal activation, and also to an increased amount of twinning (apparently above 600°C) since the twinning dislocations (with b=1/6&lt;112]) also are expected to be more glissile. When samples of this compound are produced containing 0.4at.%Er, the action of the rare earth addition is to internally getter the interstitial impurities. In this case, the deformation microstructure is dominated by dislocations with Burgers vectors with b=1/2&lt;110], a typical area being shown in Fig.3. This is consistent with a reduced anisotropy of charge density about the Ti atoms, and therefore reduced effects of covalency.