Coarsening of a multimodal nickel-base superalloy

The coarsening of γ′-Ni₃Al precipitates in the nickel superalloy Ni115 has been examined and compared to the results of a numerical model based on LSW coarsening theory. Ni115 has a γ′ fraction of around 60%, and at the coarsening temperatures of interest the γ′ distribution is bimodal, with two populations ～5 nm and ～90 nm in radius. It is found that during the initial transient (around 2000 h at 800 °C), the fine γ′ dissolve, leading to a rapid increase in the mean radius followed by a plateau. At long times, the expected steady-state unimodal t^1/3 coarsening is observed. The model reproduces these features in form and approximately in magnitude, a first for LSW model-experiment comparisons in nickel superalloys.     

Lattice strain evolution during creep in single-crystal superalloys

In situ neutron diffraction studies are carried out to characterize the micromechanical deformation occurring during tensile creep of a typical single-crystal nickel-based superalloy, CMSX-4. The loading responses of the matrix γ phase and the precipitate γ′ are distinct. Moreover, the behaviour in the tertiary creep regime (in which the γ′ phase remains intact) is qualitatively different from that in the primary creep regime (when γ′ is sheared). In tertiary creep, initial deformation of the matrix leads to a release of misfit between the phases in the (1 0 0), resulting in elastic compression of the γ in the loading direction. The load state then remains fairly constant during creep. During the initial stages of primary creep, elastic compression of the γ phase is observed until at around 2–4% creep strain this compression stabilizes as the (1 0 0) misfit is released. This is the point at which γ′ shearing is thought to begin. Subsequently, the load in the γ increases by around 200 MPa until a maximum is reached at around 8% creep strain. This load is then suddenly released, which may be due to the release of back-stress.     

Effects of rhenium addition on the temporal evolution of the nanostructure and chemistry of a model Ni–Cr–Al superalloy. I: Experimental observations

The temporal evolution of the nanostructure and chemistry of a model Ni–8.5 at.% Cr–10 at.% Al alloy, with the addition of 2 at.% Re, aged at 1073 K from 0.25 to 264 h, was studied. Transmission electron microscopy and atom-probe tomography were used to measure the number density and mean radius of the γ′ (L1₂ structure)-precipitates and the chemistry of the γ′-precipitates and the γ (face-centered cubic)-matrix, including the partitioning behavior of all alloying elements between the γ- and γ′-phases and the segregation behavior at γ/γ′ interfaces. The precipitates remained spheroidal for an aging time of up to 264 h and, unlike commercial nickel-based superalloys containing Re, there was not confined (nonmonotonic) Re segregation at the γ/γ′ interfaces.     

Nondestructive evaluation of creep and fatigue damages in nickel-base superalloys using a scanning blue laser microscope

A new nondestructive evaluation method for detecting both creep and fatigue damages of Ni-base superalloy used in advanced gas turbine systems has been proposed by applying a scanning blue laser microscope. The change of the microtexture in a grain of the alloy due to creep and fatigue damages can be observed clearly by using this microscope. The reflectance of the γ′ (Ni₃Al) phase of the alloy was found to be less than that of the γ (Ni-base alloy) phase, when a laser beam of wavelength shorter than 500 nm was radiated to the alloy. Since the microtexture of the alloy varies significantly during damage progress, it is possible to evaluate the damages quantitatively and nondestructively by observing the change of the microtexture using the laser beam with wavelength 410 nm.     

A diffusion analysis of transient subsurface γ′-Ni3Al formation during β-NiAl oxidation

Cross-sectional transmission electron microscopy has demonstrated γ′-Ni₃Al formation at the alloy/oxide interface during β-NiAl (1 1 0) oxidation under specific conditions. Diffusion analysis was applied to interpret this subsurface phase formation process. It showed that thermodynamically sufficient conditions for phase formation could be achieved with increasing temperature during the early oxidation stage. To stabilize continued γ′ growth, a kinetic requirement needed to be considered, which led to a thickness criterion. The predicted extent of subsurface γ′ phase formation showed good agreement with the experimental observations.     

Reactively sputtered epitaxial γ′-Fe4N films: Surface morphology,microstructure, magnetic and electrical transport properties

Epitaxial γ′-Fe₄N films with (1 0 0) and (1 1 0) orientations have been fabricated by reactive sputtering; these films were characterized by X-ray θ–2θ and φ scans, pole figures and high-resolution transmission electron microscopy. The film surface is very smooth as the film is less than 58 nm thick. The films exhibit soft ferromagnetism, and the saturation magnetization decreases with an increase in temperature, following Bloch’s spin wave theory. The films also exhibit a metallic conductance mechanism. Below 30 K, magnetoresistance (MR) is positive and increases linearly with the applied field in the high-field range. In the low-field range, MR increases abruptly. Above 30 K, MR is negative, and its value increases linearly with the applied field.     

A neutron diffraction study of lattice distortion,mismatch and misorientation in a single-crystal superalloy after different heat treatments

Two-dimensional neutron diffraction measurements of superlattice and fundamental lattice reflections from a single-crystal superalloy confirm the existence of angular distortion and shear stress in the γ′ phase, and reveal its correlation with the morphology and deformation of the γ′ phase during heat treatment. The tetragonal distortions with c/a > 1 are developed and retained during subsequent heat treatments, whereas the angular distortions are enhanced for the γ′ phase during the aging treatments. The evolution of the lattice mismatch and anisotropy during the subsequent heat treatments are consistent with the transformation of certain macroscopic anisotropic compositional and stress distributions in the as-cast sample to the microscopic level at the γ/γ′-interface upon reprecipitation and the growth of the γ′ from the supersaturated γ-matrix. The high negative γ/γ′ lattice mismatch of the alloy is consistent with the high levels of refractory elements, in particular Cr, in the alloy. Comparison between superlattice and fundamental lattice reflections is revealing. Firstly, the existence of lattice misorientation at the γ/γ′-interface and the discrete misorientations from the splitting of coherently aligned aggregates of γ′-precipitates on the smoothly distributed γ -matrix is shown. Secondly, the measured lattice misorientation correlates with the spacing of dislocations at the γ/γ′-interface and evidenced by the transmission electron microscopy observation. Furthermore, it indicates the anisotropic distribution of dendrites along crystal growth axis and the presence of small misoriented higher-order dendrite arms in variously heat-treated samples.     

Effects of rhenium addition on the temporal evolution of the nanostructure and chemistry of a model Ni–Cr–Al superalloy. II: Analysis of the coarsening behavior

The temporal evolution of the nanostructure and chemistry of a model Ni–8.5 at.% Cr–10 at.% Al alloy with the addition of 2 at.% Re was studied using transmission electron microscopy and atom-probe tomography in order to measure the number density and mean radius of the γ′ (L1₂) precipitates and the chemistry of the γ′-precipitates and the γ (fcc)-matrix. In this article, the coarsening behavior of the γ′-precipitates is discussed in detail and compared with the Umantsev–Olson model for multi-component alloys. In addition, the experimental results are evaluated with PrecipiCalc™ simulations. The results show that the diffusivities of the solute elements play a major role in the coarsening behavior of the γ′-precipitates and that the addition of Re retards the coarsening kinetics and stabilizes the spheroidal morphology of the precipitates by reducing the interfacial energy.     

The effect of annealing on the microstructure and tensile properties of a β/γ′ Ni–Al–Fe alloy

The evolution of the microstructure of a (β/γ ′) Ni–32 at.% Al–5 at.% Fe alloy during annealing has been studied by electron microscopy and X-ray diffraction. Annealing at 800°C and 1100°C causes a reverse martensitic transformation, L1₀→B2 (β), and a B2→L1₂ (γ ′) phase transformation. The lower annealing temperature leads to a higher volume fraction of the γ ′-phase but a smaller size of the γ ′-particles. The kinetic laws of the coarsening and of the increase in the volume fraction of the γ ′-phase are discussed. The orientation relationships between the β and γ ′ phases appeared to be mainly of Nishiyama–Wassermann and Bain types after 800°C annealing, while Kurdjumov–Sachs and Bain orientation relationships were predominant in the alloys annealed at 1100°C. A strong correlation between the volume fraction of the γ ′-phase and the tensile characteristics of the alloy has been established.     

Development of γ phase stacking faults during high temperature creep of Ru-containing single crystal superalloys

An unusual deformation mode involving the formation of intrinsic stacking faults in the γ matrix of experimental Ru-containing γ–γ′ superalloys with high Co and Re contents during high temperature creep at 950 °C/290 MPa has been observed. The morphology, distribution and dependence of these stacking faults on alloy chemistry has been investigated along with their formation mechanism. Additions of Re and Co substantially decrease the stacking fault energy of the γ matrix. The observed stacking faults in the γ matrix form by the dissociation of a/2〈1 1 0〉 matrix dislocations with Burgers vectors perpendicular to the loading direction in the early stages of creep. The dependence of creep properties on elemental additions that influence stacking fault energy is discussed.     

Microstructure and strengthening mechanisms in Cu/Fe multilayers

Nanostructured Cu/Fe multilayers on Si (1 1 0) and Si (1 0 0) substrates were prepared by magnetron sputtering, with individual layer thicknesses h varying from 0.75 to 200 nm. The growth orientation relationships between Cu and Fe at the interfaces were determined to be of the Kurdjumov–Sachs and Nishiyama–Wasserman type. Nanoscale columnar grains in Fe, with an average grain size of 11–23 nm, played a dominant role in the strengthening mechanism when h ? 50 nm. At smaller h the hardness of Cu/Fe multilayers with (1 0 0) texture approached a peak value, followed by softening due to the formation of fully coherent interfaces. However, abundant twins were observed in Cu/Fe films with (1 1 1) texture when h = 0.75 nm, which led to the retention of high hardness in the multilayers.     

The use of cross-linkable interlayers to improve device performances in blue polymer light-emitting diodes

High efficiency blue PLEDs were fabricated by adding a thin interlayer between PEDT:PSS and emitting polymer layers. Two different cross-linkable alkoxysilane-based interlayer materials, X-NPB and X-PDA, were synthesized based on N,N′-bis(4-methylphenyl)-N,N′-diphenyl-1,4-phenylenediamine (PDA) and N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1-biphenyl-4,4-diamine (NPB) which are well-known OLED HTLs. The devices, with configuration of indium tin oxide (ITO)/PEDT:PSS (65 nm)/interlayer (10–20 nm)/emitting polymer layer (70 nm)/BaF₂ (2 nm)/Ca (50 nm)/Al (300 nm), were fabricated by spin coating and thermal evaporation. In this device structure, the cross-linked X-NPB or X-PDA interlayers are more adherent and mechanically robust as well as impervious to spin coating of next emitting polymer layer. In addition, the devices with these interlayers exhibit a higher luminescence and current efficiency than those without interlayers because interlayers have two functions which are blocking electrons and preventing from severe quenching by PEDT:PSS.     

Synthesis,photoluminescent and electroluminescent properties of a novel europium(III) complex involving both hole- and electron-transporting functional groups

A novel europium(III) complex involving a carbazole fragment as hole-transporting group and an oxadiazole fragment as electron-transporting group was synthesized and used as red light-emitting material in organic light-emitting diodes (OLEDs). The complex is amorphous, and exhibits high glass transition temperature (T_g = 157 °C) and high thermal stability with a 5% weight loss temperature of 367 °C. Two devices, device 1: ITO/NPB (40 nm)/Eu(III) complex (30 nm)/Alq₃ (30 nm)/LiF (1 nm)/Al (100 nm) and device 2: ITO/NPB (40 nm)/3% Eu(III) complex: CBP (30 nm)/BCP (10 nm)/Alq₃ (30 nm)/LiF (1 nm)/Al (100 nm), were fabricated, where NPB is N,N′-di(naphthalene-1-yl)-N,N′-diphenyl-benzidine, Alq₃ is tris(8-hydroxyquinoline) Al(III), CBP is 4,4′-bis(carbazole-9-yl)-biphenyl, and BCP is 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline, respectively. In contrast with device 1, owing to less self-quenching and better charge confinement, device 2 shows improved performances: the maximum luminance of device 2 was dramatically increased from 199 to 1845 cd/m², the maximum current efficiency was increased from 0.69 to 2.62 cd/A, the turn-on voltage was decreased from 9.5 to 5.5 V, and higher color purity was attained.     

In situ TEM study of microplasticity and Bauschinger effect in nanocrystalline metals

In situ transmission electron microscopy straining experiments with concurrent macroscopic stress–strain measurements were performed to study the effect of microstructural heterogeneity on the deformation behavior of nanocrystalline metal films. In microstructurally heterogeneous gold films (mean grain size d_m = 70 nm) comprising randomly oriented grains, dislocation activity is confined to relatively larger grains, with smaller grains deforming elastically, even at applied strains approaching 1.2%. This extended microplasticity leads to build-up of internal stresses, inducing a large Bauschinger effect during unloading. Microstructurally heterogeneous aluminum films (d_m = 140 nm) also show similar behavior. In contrast, microstructurally homogeneous aluminum films comprising mainly two grain families, both favorably oriented for dislocation glide, show limited microplastic deformation and minimal Bauschinger effect despite having a comparable mean grain size (d_m = 120 nm). A simple model is proposed to describe these observations. Overall, our results emphasize the need to consider both microstructural size and heterogeneity in modeling the mechanical behavior of nanocrystalline metals.     

Microstructure and mechanical properties of ODS Ni-based superalloy foil produced by EB-PVD

Y₂O₃ dispersion strengthened Ni-based superalloy foil 0.1 mm thick was deposited by EB-PVD technology and followed by hot isostatic pressing (HIP) treatment. The phase composition and microstructure investigations on as-deposited and HIPed superalloy foils were performed by XRD, SEM and TEM. Columnar crystals formed on the evaporation side and equiaxed grains formed on the substrate side. Cross-section observation showed 150–300 nm grains of matrix with 10–25 nm particles of Y₂O₃ homogeneously dispersed in them. After HIP treatment, columnar crystals broke and turned into equiaxed grains. Little growth of oxide dispersoids was observed. The results of room temperature tensile tests indicated that the tensile and plastic properties of the foil after HIP treatment are both promoted with the ultimate tensile strength and elongation percentage reached the values of 1230 MPa and 0.92 in contrast with 725 MPa and 0.49 that of the as-deposited foil, respectively.     

Thermodynamic stability of Co–Al–W L12 γ′

Co-based superalloys in the Co–Al–W system exhibit coherent L1₂ Co₃(Al,W) γ′ precipitates in an face-centered cubic Co γ matrix, analogous to Ni₃Al in Ni-based systems. Unlike Ni₃Al, however, experimental observations of Co₃(Al,W) suggest that it is not a stable phase at 1173 K. Here, we perform an extensive series of density functional theory (DFT) calculations of the γ′ Co₃(Al,W) phase stability, including point defect energetics and finite-temperature contributions. We first confirm and extend previous DFT calculations of the metastability of L1₂ Co₃(Al_0.5W_0.5) γ′ at 0 K with respect to hexagonal close-packed Co, B2 CoAl and D0₁₉ Co₃W using the special quasi-random structure (SQS) approach to describe the Al/W solid solution, employing several exchange/correlation functionals, structures with varying degrees of disorder, and newly developed larger SQSs. We expand the validity of this conclusion by considering the formation of antisite and vacancy point defects, predicting defect formation energies similar in magnitude to Ni₃Al. However, in contrast to the Ni₃Al system, we find that substituting Co on Al sites is thermodynamically favorable at 0 K, consistent with experimental observation of Co excess and Al deficiency in γ′ with respect to the Co₃(Al_0.5W_0.5) composition. Lastly, we consider vibrational, electronic and magnetic contributions to the free energy, finding that they promote the stability of γ′, making the phase thermodynamically competitive with the convex hull at elevated temperature. Surprisingly, this is due to the relatively small finite-temperature contributions of one of the γ′ decomposition products, B2 CoAl, effectively destabilizing the Co, CoAl and Co₃W three-phase mixture, thus stabilizing the γ′ phase.     

Phase equilibria in the Ni-rich portion of the Ni–Ga binary system

The phase equilibria in the composition range from 0 to 60 at% Ga of the Ni–Ga system were determined by electron probe microanalysis (EPMA) using diffusion couples, differential scanning calorimetry (DSC) and X-ray diffraction (XRD). It was found that while the phase equilibria between the α′ (L1₂: Ni₃Ga) and α (Ni-solid solution) or β (B2: NiGa) phases are basically in agreement with the diagram evaluated by Lee and Nash, those between γ (B8₁: Ni₁₃Ga₇), δ (Cmmm: Ni₅Ga₃) and ɛ (C2/m: Ni₁₃Ga₉) are topologically different from that diagram. Three eutectoid reactions (γ → δ + ɛ, β → γ + ɛ, β → α′ + γ) and one peritectoid reaction (α′ + γ → δ) were confirmed and the temperatures and concentrations of those invariant reactions were determined.     

Yellow–green organic light-emitting diode based on tris(2-methyl-8-quinolinolate) scandium

Efficient yellow–green electroluminescence emission at λ_max = 530 nm with CIE coordinates x = 0.3913, y = 0.4947 was obtained with organic light-emitting devices based on tris(2-methyl-8-quinolinolate) scandium (1). The device with the configuration of indium tin oxide/N,N′-bis(3-methylphenyl)-N,N′-diphenylbenzidine/1/Yb exhibits current efficiency of 3.1 cd/A and power efficiency of 1.8 lm/W at a luminance of 100 cd/m². The DFT calculations demonstrate that structural changes of the scandium complex 1 influence the electroluminescence spectrum, the better agreement with experimental data being achieved when monodentate ligands are taken into consideration.     

Trigonal Ir9Al28, a new structure type and approximant to decagonal quasicrystals

During an investigation of the binary system Al–Ir, a new phase with composition Ir₉Al₂₈ was identified. After annealing at 780 °C, its structure was determined by single-crystal X-ray diffraction. Trigonal Ir₉Al₂₈ is the first representative of a new structure type with Pearson symbol hP236-14, a = b = 12.2864(4) Å, c = 27.341(1) Å, γ = 120°, space group P31c, no. 159. The crystal structure can be described likewise as stacking of eight puckered and flat layers with a sequence …P⁰PFpp⁰pⁱFⁱPⁱ… along [0 0 1], as a six-layer stacking sequence along [1 0 0], or as packing of pseudo-Mackay icosahedra. The Ir substructure with pseudosymmetry P6₃/mmc resembles the V₄Al₂₃ structure type. Pentagonal columnar clusters running along [0 0 1] show close resemblance to decagonal quasicrystals with six-layer period along the 10-fold axis.