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

On the hierarchy of planar fault energies in TiAl

Ab-Initio Investigation of Planar Defects in <i>Immm</i>-Ni <sub>2</sub>(Cr,Mo,W) Strengthened HAYNES 244 Alloy

The behaviour of superlattice dislocations in polysynthetically twinned (PST) TiAl crystals deformed from room temperature to 850°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 ½〈112] superlattice dislocations dissociate into faulted dipoles and are also sessile. While the dislocation configurations in the crystals deformed at 400°C and 600°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 brittlductile transition of PST TiAl crystals. At 850°C, most 〈011] superlattice dislocations dissociate to form Kear-Wilsdorf locks. The ½〈112] superlattice dislocations are edge in character, dissociated into superpartials bordering a SISF and are glissile. The experimental observations suggest that the behaviour of ½〈112] superlattice dislocations is an important factor controlling the mechanism of brittle-ductile transition of PST TiAl crystals.

MD simulation of asymmetric nucleation and motion of 〈011] superdislocations in TiAl

A method of obtaining the planar fault energies in an L10-type superlattice structure is given as a function of the axial ratio R. Using this method, the anti-phase boundary (APB), stacking fault (SF) and complex stacking fault (CSF) energies are calculated for values of R ranging from 0.65 to 1.5. The increase and decrease in R from the minimum points near unity results in increases in these energies. For values of R, near unity, the APB energies increase in the following order: (1, 0, 0), (1, 1, 0), (1, 0, 1), (1, 1, 1) and (0, 0, 1). The APB energies on the (1, 0, 0) and (1, 1, 1) planes are reversed when R is larger than 14. The symmetry of a lattice structure after translation to form a SF and a CSF is considered. The translation vector b1 =⅙<1, 1, 2> or⅙<2, 1, 1> does not form a symmetrical SF or CSF structure if R is not equal to unity. Therefore the SF or CSF formed by b 1 might be unstable. Translation vectors for planar faults that are symmetrical in the L10 structure when R is not equal to unity are obtained. Such planar faults would be stable. The exchanges of the distances of the nearest-neighbour atom pairs for the second-nearest-neighbour ones, and of the second ones for the third ones occur at values of R of ∼0·7 and ∼ 1·2, respectively.

The planar defects associated with deformation in ordered intermetallic compounds namely, the antiphase boundary, superlattice intrinsic stacking fault, and complex stacking fault are non-equilibrium structures corresponding to a state of disorder within the ordered structure of the lattice and therefore affect both the electronic energy states and the Brillouin zone structure. It is possible that a relationship exists between the antiphase boundary energy, γAPB, and the sum of the number of unfilled outermost d-state electrons in the transition metals on which this class of intermetallics is based. If this hypothesis is taken in conjunction with a set of rules for improving ductility in intermetallics proposed previously, a coherent explanation of recently observed ductilities in transition metal-based intermetallics would seem to be feasible.

The microstructure of ordered (Co 0.78Fe 0.22) 3V alloy

Atomic scale characterization of complex stacking faults and their configurations in cold deformed Fe42Mn38Co10Cr10 high-entropy alloy

Dislocation debris in Ni–23.5Al–0.2B specimens deformed at 77 K is dominated by superlattice intrinsic stacking faults (SISFs) which form from antiphase boundary (APB) coupled dislocations. Based on our transmission electron microscopy (TEM) observations, two new mechanisms are proposed for the formation of “type I” SISF dislocations. SISFs are believed to make a significant contribution to the flow stress of 0.2 at% boron doped Ni3Al at low temperature.

Stacking fault formation and Ag precipitation in Cu-Ag-Sc alloys

Excellent high-temperature mechanical properties of Ni-based single-crystal superalloys (NSCSs) are attributed to the yield strength anomaly of ${\mathrm{Ni}}_{3}\mathrm{Al}$ that is intimately related to generalized stacking fault energies (GSFEs). Therefore, clarifying the effects of alloying elements on the GSFEs is of great significance for alloys design. Here, by means of ab initio density functional theory calculations, we systematically calculated the GSFEs of different slip systems of Ni and ${\mathrm{Ni}}_{3}\mathrm{Al}$ without and with alloying elements using the alias shear method. We obtained that for Ni, except for magnetic elements Mn, Fe, and Co, most of the alloying elements decrease the unstable stacking fault energy (${\ensuremath{\gamma}}_{\text{USF}}$) of the $[01\overline{1}](111)$ and $[11\overline{2}](111)$ slip systems and also decrease the stable stacking fault energy (${\ensuremath{\gamma}}_{\text{SF}}$) of the $[11\overline{2}](111)$ slip system. Interestingly, the reduction effects exhibit a strong correlation with the inverse of atom radii. For ${\mathrm{Ni}}_{3}\mathrm{Al}$, most of the alloying elements in groups IIIB--VIIB show a strong Al site preference. Except for Mn and Fe, the elements in groups VB--VIIB and the first column of group VIII increase the values of ${\ensuremath{\gamma}}_{\text{USF}}$ of different slip systems of ${\mathrm{Ni}}_{3}\mathrm{Al}$, which makes the slip deformation and dislocation emits difficult. On the other hand, the elements in groups IIIB--VIIB also increase the value of ${\ensuremath{\gamma}}_{\text{SF}}$, and thus reduce the stability of the antiphase boundary, complex stacking fault, and superlattice intrinsic stacking fault of ${\mathrm{Ni}}_{3}\mathrm{Al}$. We found that Re is an excellent strengthening alloying element that significantly increases the slip barrier of the tailing slip process for Ni, and also enhances the slip barrier of the leading slip process of three slip systems for ${\mathrm{Ni}}_{3}\mathrm{Al}$. W and Mo exhibit similar effects as Re. We predicted that Os, Ru, and Ir are good strengthening alloying elements as well, since they show the strengthening effects on both the leading and the tailing slip process for Ni and ${\mathrm{Ni}}_{3}\mathrm{Al}$. This work established an exhaustive dictionary of the effects of various alloying elements on the GSFEs of both Ni and ${\mathrm{Ni}}_{3}\mathrm{Al}$ phases, which would help to guide the design of next-generation high-performance NSCSs.

The properties of point defects and planar defects in nickel are investigated using computer simulation techniques. An empirical pair potential consisting of nine cubic splines is derived for nickel. This potential reproduces the experimental vacancy formation and migration energies, the intrinsic stacking fault energy, and the elastic constants. The displacement fields, formation energies, and formation volumes are calculated for the vacancy, the saddle‐point configuration associated with vacancy migration, and six interstitial configurations. In addition, results are presented for the intrinsic and extrinsic stacking faults and the twin boundary. The vacancy results are in agreement with previous calculations but the interstitial formation energies are significantly larger than those calculated previously. The crowdion is found to be the most stable interstitial configuration with a formation energy of 8.77 eV and formation volume of 0.42 atomic volumes. The energies of the twin, intrinsic, and extrinsic stacking faults are found to be 97, 158, and 199 mJm−2, respectively.

Dissociation criteria for superlattice dislocations on {111} planes have been applied to the ordering alloy Cu2NiZn. The calculations showed that the splitting of a superlattice dislocation into two superlattice Shockley partials, with Burgers vector of type 1/3a 0⟨211⟩, bounding a superlattice intrinsic stacking fault. (SISF), is energetically less favourable than the splitting inta two unit dislocations, with Burgers vector of type 1/2a 0⟨110⟩, separated by an antiphase boundary (APB). This theoretical prediction is in agreement with the type of superlattice dislocation observed in the electron microscope. It is rather difficult to predict a priori whether each unit dislocation is split into two Shockley partials separated by a complex stacking fault (CRF) because the cut-off parameters for the dislocation cores are not known with sufficient accuracy. The splitting into two Shockley partials could not be resolved in the electron microscope, which means that either the separation between the Shockley partials is less than about 0.7 nm, or the twofold dissociation is energetically more favourable than the fourfold one. Values for the APB energy have been calculated from the observed separations of the unit dislocations, constituting superlattice dislocations, in Cu2NiZn single crystals quenched from several temperatures. The results are in agreement with theoretical predictions.

Effect of Stacking Fault Segregation and Local Phase Transformations on Creep Strength in Ni-Base Superalloys

The stacking fault behaviors on ilmenite ZnTiO3 were investigated by calculating the generalized stacking fault (GSF) energies using density functional theory (DF T) based on first principles calculations and classical calculations employing effective partial charge interatomic potentials. The results show that stable and unstable stacking fault energies are in qualitative agreement and provide the same sequence of stacking fault energies, although there exist quantitative differences with DFT providing lower stable and unstable stacking fault energy values than those from classical potential calculations. The γ-surfaces of two low energy surfaces, (1 1 0) and (1 0 4), of ZnTiO3 were fully mapped together with ideal shear stress (ISS) calculations. It was found that stacking faults along 〈〉/{1 0 4} are preferred energetically and are most probable to nucleate dislocations due to their significantly lower γsf/ γusf values. The atomic structures of the low energy stacking faults were analyzed and their electronic structures calculated and compared with bulk ZnTiO3 structures. It was found that stacking fault formation led to narrowing of the band gap and creating inter-bandgap states, mainly due to the dangling bonds and bonding defects, as compared to the bulk structures.