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Microtwinning is observed as an important mechanism in γ′ precipitate-strengthened Ni-based superalloys during service at intermediate temperatures. Here, the effect of crystal orientation and stress direction on microtwinning was investigated. The result indicates that the formation of microtwins exhibits orientation dependence and tension-compression asymmetry. Based on the analysis of slip and twinning competition behavior under different conditions, it is deemed that the a/2〈110〉 matrix dislocations would dissociate into a/6〈112〉 Shockley partials in the γ matrix and then a Shockley partial sheared γ' phase and formed microtwins. IMPACT STATEMENT: For the first time, this work explains why microtwinning exhibits orientation dependence and tension-compression asymmetry while stacking faults do not, although they both originate from the activity of {111} 〈112〉 slip systems. [ABSTRACT FROM AUTHOR]

Microtwinning is observed as an important mechanism in γ′ precipitate-strengthened Ni-based superalloys during service at intermediate temperatures. Here, the effect of crystal orientation and stress direction on microtwinning was investigated. The result indicates that the formation of microtwins exhibits orientation dependence and tension-compression asymmetry. Based on the analysis of slip and twinning competition behavior under different conditions, it is deemed that the a/2〈110〉 matrix dislocations would dissociate into a/6〈112〉 Shockley partials in the γ matrix and then a Shockley partial sheared γ’ phase and formed microtwins.

Hematite obtained by oxidative thermolysis of iron oxalate dihydrate at temperatures from 320 °C to 680 °C is analyzed by in situ powder X-ray diffraction (XRD) and transmission electron microscopy. XRD patterns of hematite demonstrate the anisotropic broadening of the peaks. The width and difference in the broadening of XRD peaks decrease with increasing annealing temperature. The calculation of XRD patterns based on statistical models of 1D disordered crystals reveals the XRD effects induced by stacking faults (SFs) in anionic and cationic sublattices. It is shown that SFs in the anionic sublattice broaden and shift 012, 104, 024, 018, 214 peaks. In the cationic sublattice, they also broaden the mentioned peaks but do not affect their positions. The width of 110, 113, 116, and 300 peaks does not depend on SFs in the cationic sublattice and is used to determine the particle shape. The anisotropic broadening of XRD peaks is shown to be caused by both particle shape anisotropy and SFs in the cationic sublattice. Average sizes of hematite particles in the [001] direction are approximately 2 times smaller than those in the perpendicular direction. As the temperature increases, the SF concentration gradually decreases from 0.035 °C at 320 °C to 0 °C at 560 °C. [ABSTRACT FROM AUTHOR]

We investigate the tensile deformation behavior and mechanism of nickel-based superalloy CM247LC, which has been subjected to various ageing treatment schemes, but possesses almost the same γ′ particle size, at 600°C. It is found that the transition in the deformation mode from particle shearing plus microtwinning to particle shearing accounts for the decrease in the work-hardening rate and tensile strength with ageing temperature. Our study provides a new strategy to design the heat treatment schedule to achieve a good compromise between the strength and plasticity for superalloys by tunning the operative deformation and fracture mechanisms.

We investigate the tensile deformation behavior and mechanism of nickel-based superalloy CM247LC, which has been subjected to various ageing treatment schemes, but possesses almost the same γ′ particle size, at 600°C. It is found that the transition in the deformation mode from particle shearing plus microtwinning to particle shearing accounts for the decrease in the work-hardening rate and tensile strength with ageing temperature. Our study provides a new strategy to design the heat treatment schedule to achieve a good compromise between the strength and plasticity for superalloys by tunning the operative deformation and fracture mechanisms. The work provides new insights into understanding the influence of ageing heat treatment or microstructural features on the mechanical properties and corresponding deformation and fracture mechanisms of precipitate-hardened superalloys. [ABSTRACT FROM AUTHOR]

Deformation relief at different stages of work hardening of Cu–12 at % Al single crystals is studied by transmission electron microscopy. The relationship between the surface topography and the stages of strain hardening and substructural evolution is analyzed. It is confirmed that stage III of deformation hardening is determined by microtwinning. The surface-topography features associated with the development of microtwinning in the slip planes are found. It is shown that the deformation relief at stage IV of deformation contains short curved slip lines. The condition for the beginning of the microtwinning process is formulated. The dislocation density required for initiation of the microtwinning process is estimated, and the estimates show good agreement with the average dislocation density measured experimentally. [ABSTRACT FROM AUTHOR]

Structural features and catalytic properties of silver and iron binary oxide AgFeO2 in the СО oxidation reaction are investigated. Binary oxide samples are obtained by coprecipitation and hydrothermal synthesis techniques at different temperatures. In the temperature range of 140-180 °С, well crystallized AgFeO2 particles are formed primarily with a 3R polytype structure (~90%), and at T = 60-120 °С highly dispersed particles with a developed defect structure are obtained. It is shown that calculated X-ray diffraction patterns of models of delafossite crystals with microtwinning are well consistent with experimental curves. Binary oxide AgFeO2 samples demonstrate a high activity in the СО oxidation reaction, which increases with a decrease in the synthesis temperature. The highest specific catalytic activity is found for highly dispersed samples with a developed defect structure. During catalytic experiments the effect of the reaction medium modifies the surface composition due to the partial exit of silver from the delafossite structure. [ABSTRACT FROM AUTHOR]

Micro-twinning is the major creep deformation mechanism in Ni-based superalloys at temperatures above 700 °C. Recent experiments suggest that superlattice stacking faults in γ′ phase may serve as precursors for twin formation. Segregation of alloy elements to these precursors may have a significant effect on the formation and extension of micro-twins. Using atomistic modeling, we investigated and explained the effects of Cr on these processes. Our results indicate that depending on the site preference of Cr in Ni 3 Al γ′ phase, two drastically different deformation behaviors can be expected. Occupying Al sites, Cr significantly accelerates deformation twinning. Cr on Ni sites, on the other hand, suppresses twin growth and slows down the high temperature deformation creep. These results help to rationalize the experimentally observed puzzling effects of alloy composition on creep resistance. [Display omitted] [ABSTRACT FROM AUTHOR]

• The simulation methodology was developed to investigate the high temperature deformation creep in Ni superalloys. • First ever atomistic simulation of dislocation-induced, diffusion mediated creep deformation was carried out. • Effect of Nb solutes on the kobe mechanism for microtwinning was demonstrated and explained. As the operating temperature of jet turbine engines increase, creep becomes the life-limiting property for turbine disks and blades. At intermediate temperatures, around 600–800 °C, microtwinning contributes significantly to creep strain in these alloys. Therefore, understanding how microtwins form and grow is critical to improving the creep life of future Ni-base superalloy components. In addition, exploring the effect of different alloying elements, such as Nb and Ta, on the formation of microtwins is critical for future alloy development. Several mechanisms of microtwinning have been proposed among which the Kolbe mechanism, based on thermally activated reordering, is believed to be dominant. In this work we employ atomistic simulation to investigate the effects of Nb solutes on the Kolbe mechanism. The simulation demonstrates that Nb atoms significantly slow down the reordering processes, explaining the experimentally observed improvement in the creep resistance. [ABSTRACT FROM AUTHOR]

FGH96 is a powder metallurgy nickel based superally used for turbine disk of aero-engines. In the present study FGH96 alloy with four different γ′ precipitate microstructures were produced via solution heat-treatment with different cooling rates, and the maximum cooling rate reached 400 ​°C/min which was a super cooling rate for Nickel-based superalloy. The creep tests were conducted for PM FGH96 alloy under the testing condition of 704 ​°C and 690 ​MPa. The relationship between the creep properties and the distribution of γ' precipitate was established. The creep mechanism was analyzed by using TEM and ACTEM, and the dislocation movement was studied at the atomic scale. The creep strain rate was calculated through a physically based crystal slip model established based on crystal plasticity. The calculated results were consistent with the test ones, illustrating the validity of the model. The fracture mechanism was also investigated, and the results showed that the creep cracks generated on the surface due to the oxidation. It was observed that the cracks propagated in different ways depending on the different average diameters of ​γ′ ​precipitate. With the decrease of the average ​γ' ​precipitate size, the critical shearing stress increased and the resistance of the dislocation slipping increased. The fracture mechanism for the primary stage transformed from intragranular to intergranular due to the change of dislocation slipping. [Display omitted] • The relationship between the cooling rate, morphology of γ′ precipitate and creep mechanism has been investigated. • Even the size of γ′ precipitates is smaller than 50 ​nm, the deformation mechanism is still microtwining. • The critical shearing stress increases with the decrease of γ′ size, leading to better creep property of FGH96 superalloy. • With the decrease of size of γ′ precipitates, the fracture mechanism transforms from intragranular to intergranular. [ABSTRACT FROM AUTHOR]

The paper considers the dynamics of changing of misoriented dislocation substructures appearing at a stage of developed plastic deformation. The types of defects, which induce the distortion of the crystal lattice are studied in detail. The quantitative parameters are obtained for the lattice distortion in alloys at a different grain size. It is found that the lattice bending and torsion parameters in the misoriented cellular-network and microtwin substructures depend on the degree of deformation. The lattice distortion is investigated both in the vicinity and far from the microtwins. It is shown that the microtwin formation occurs with increasing deformation at a different grain size. [ABSTRACT FROM AUTHOR]

As best of our knowledge this is the first time to obtain MnO2 nanostructures with different morphologies without any use of surfactants, templates or acidic conditions. Three different crystal shapes(cross, sheet, and star) of nanostructure pure phase of γ -MnO2 have been successfully synthesized by hydrothermal method at same conditions of temperature ( 160ºC), time of reaction( 12 h) and the same mole ratio between the oxidizing agent NaClO3 and the Mn+2 precursor (2:1), the only difference was using different Mn+2 precursor (MnSO4.H2O, MnCl2.4H2O and Mn(OAc)2 .4H2O) separately, the prepared samples had been characterized by using field emission scanning electron microscopy (FE-SEM), x-ray diffraction analysis(XRD) FT-IR spectroscopy, Raman spectroscopy, and Brunauer-Emmett-Teller (BET) surface area analysis. The sample with the highest value of surface area was the sample prepared from MnSO4.H2O (9.163 m2/g ) which had a star likeshape. Also the values of De Wolff and micro-twinning defects have been calculated [ABSTRACT FROM AUTHOR]

Martensite and bainite formed in steel on heat treatment are very complex structures, resisting quantitative analysis. Often such steels are used at high static and dynamic compressive stress. Thorough analysis of their structure after various types of treatment permits their effective use in manufacturing so as to ensure the required physicomechanical properties of the product. The properties of such materials are determined by the solid-solution structure; the presence of secondary-phase nanoparticles; dislocational substructure; the type and position of different types of boundaries; and internal stress fields. Successful control of structure and phase formation and hence of the mechanical properties of the material depends on a quantitative grasp of strain hardening of steels of different structural classes on active plastic deformation. In the present work, transmission diffractional electron microscopy is used to analyze the strain hardening of 38KhN13MFA steel with martensitic structure and 30Kh2N2MFA steel with bainitic structure in active plastic deformation (compression) by up to 26 and 36%, respectively. The contributions of strain hardening associated with intraphase boundaries, dislocational substructure, carbide phases, atoms of alloying elements, and long-range stress fields are considered. The main contributors to the strain hardening of quenched 38KhN13MFA steel are substructural hardening associated with internal long-range stress fields; and solidsolution strengthening associated with carbon atoms. For normalized 30Kh2N2MFA steel, the strain hardening may again be attributed to internal stress fields, the introduction of carbon atoms in the ferrite lattice, and also structural fragmentation when the strain exceeds 26%. The dislocational substructure and carbide particles make relatively small contributions to the hardening of such steels. The loss of strength of bainitic steel at deformation exceeding 15% is due to activation of deformational microtwinning. [ABSTRACT FROM AUTHOR]

The contribution of a microtwinning mechanism to the creep deformation behaviour of single crystal superalloy MD2 is studied. Microtwinning is prevalent for uniaxial loading along 〈 011 〉 at 800 ° C for the stress range 625 to 675 MPa and 825 ° C for 625 MPa. Using quantitative stereology, the twin fraction and twin thickness are estimated; this allows the accumulated creep strain to be recovered, in turn supporting the role of the microtwinning mode in conferring deformation. Atom probe tomography confirms the segregation of Cr and Co at the twin/parent interface, consistent with the lowering of the stacking fault energy needed to support twin lengthening and thickening. A model for diffusion-controlled growth of twins is proposed and it is used to recover the measured creep strain rate. The work provides the basis for a thermo-mechanical constitutive model of deformation consistent with the microtwinning mechanism. [ABSTRACT FROM AUTHOR]

Transmission electron diffraction microscopy is used to analyze strain hardening of steel with a bainitic structure. Contributions to strengthening are considered for lattice friction, interphase boundaries, dislocation substructure, carbide phases, alloying element atoms, and remote stress fields. It is established that substructural and solid solution strengthening make the greatest contributions. The reason for steel loss of strength with large degrees of deformation is connected with activation of deformation microtwinning. [ABSTRACT FROM AUTHOR]

The Ni-based superalloy IN718 is one of the most widely used commercial alloy in the aerospace industry since its development in 1960s. The excellent mechanical properties have been attributed in a great deal to strengthening by coherent γ′ and γ″ precipitates. The deformation mechanisms of these two phases have been well studied individually. Recent experimental characterization has shown coprecipitates of these two phases in a variety of morphologies and it was argued that these coprecipitates may lead to improved strengthening as compared to their monolithic counterparts. However, the deformation mechanisms of these coprecipitates are still not well understood. In this study, we performed microscopic phase field simulations, with generalized-stacking-fault (GSF) energy surfaces from ab initio calculations as inputs, to systematically study the shearing processes of a periodical array of dual-lobed coprecipitates as well as monolithic precipitates. We found that the coupling between the γ′ and γ″ phases in the coprecipitates forces dislocations to take high energy shearing pathways in both phases that would not occur if they were in monolithic forms. The coupling also creates stacking fault configurations in the coprecipitates that require high stress to form. Thus, the presence of coprecipitates in general should increase the resistance to dislocation shearing and lead to higher strength levels. Various fault configurations observed during the shearing process are documented as a reference for future comparison with experimental observations. The link between stacking fault shearing and microtwinning is also discussed. The mechanisms analyzed in this study deepens our understanding of coprecipitation effects on alloy strength and may form a cornerstone for multi-precipitate strengthened alloy design strategies. [Display omitted] [ABSTRACT FROM AUTHOR]

A polycrystalline L1 2 -hardened Co-base superalloy was creep deformed at 750 °C. The investigation of the deformed microstructure in the transmission electron microscope revealed microtwinning to be the prevailing deformation mechanism. The detected twins spanned the entire grain and cut through both, γ and γ′. Detailed high-resolution transmission electron microscopy investigations indicated that twin growth takes place by the slip of single a/6 〈112〉 partial dislocations along the twin boundary. Further analysis of the twin boundaries in the γ′ phase revealed segregation of elements known to decrease the stacking fault energy and a local depletion of γ′ forming elements. We propose that this segregation behavior enables subsequent a/6〈112〉 dislocations to easily slip along the twin boundary and further thicken the twins in the process. [ABSTRACT FROM AUTHOR]

The crystal structure of a natural sulfide CuAgPbBiS ( Сmcm, Z = 4, a = 3.973(1) Å, b = 13.370(2) Å, c = 42.182(7) Å, R = 0.059) is determined. The structure has seven cation positions: two of them (Cu and Ag) are in a tetrahedral environment of sulfur atoms; one (Pb), in a special position ( mm2), has a coordination polyhedron in the form of a bicapped trigonal prism; and the other cation positions are surrounded by sulfur atoms forming distorted octahedra. The mirror symmetry plane perpendicular to the c translation causes microtwinning by cutting a layer of trigonal prisms framed by tetrahedron ribbons. These layers are divided by those composed by edge-linked octahedra with a diagonal ribbon of five octahedra ( N = 5). The cation and anion positions are ordered by individual sublattices with pseudohexagonal subcells on the m planes perpendicular to the a translation, which concentrate the positions of all the atoms. Supposedly, this natural sulfide is the previously described (1885) yet unconfirmed alaskaite mineral from the lillianite-heyrovskyite homological series and may be isostructural to the ourayite mineral. [ABSTRACT FROM AUTHOR]

This paper focusses on the use of the parametrically homogenized crystal plasticity to simulate polycrystalline ensembles of Ni-based superalloys that are governed by the characteristics of the subgrain scale γ – γ ′ precipitate morphology. Critical parameters representing the effect of subgrain-scale morphology are incorporated in the grain-scale crystal plasticity model, which then has the capability of representing mechanisms occurring at three scales, viz. the sub-grain scale of precipitates, the grain scale of single crystals and the scale of polycrystalline aggregates in a unified framework. A dislocation density crystal plasticity constitutive model with APB shearing of γ ′ precipitates is developed for modeling the sub-grain scale representative volume element delineating explicit morphology of the γ – γ ′ microstructure. A framework is developed for a parametrically homogenized activation energy-based crystal plasticity (AE-CP) model at the scale of single crystals by homogenizing the sub-grain model response and incorporating critical morphological parameters of the sub-grain morphology. Nucleation and evolution models for micro-twins are also incorporated for manifesting tension-compression asymmetry. The grain-scale AE-CP model is used to analyze polycrystalline microstructures. [ABSTRACT FROM AUTHOR]

In the present investigation, an attempt was made to understand the cyclic deformation micromechanism of gas turbine alloy Inconel 718 at 600 °C (i) by conducting low cycle fatigue and creep-fatigue interaction tests and (ii) by studying the microstructure evolution in the material during fatigue tests through extensive electron microscopy. Bilinear slope was obtained in the Coffin-Manson plot for all low cycle fatigue tests, and it was confirmed through transmission electron microscopic examination that microtwinning was the predominant mode of deformation at low plastic strain values, whereas slip and shearing of γ″ precipitates were the predominant mode of deformation at higher plastic strain values. Fatigue life was adversely affected when hold time was introduced at peak tensile strain during creep-fatigue interaction tests. Formation of stepped interface at microtwin boundaries and coarsening of niobium carbide precipitates were observed to be the major microsturctural changes during creep-fatigue interaction tests. [ABSTRACT FROM AUTHOR]

A hierarchical crystal plasticity constitutive model, comprising three different scales for polycrystalline microstructures of Ni-based superalloys, is developed in this paper. Three scales, dominant in models of polycrystalline Ni-based superalloys, are: (i) the sub-grain scale of γ - γ ′ microstructure, characterized by γ ′ precipitate size and their spacing; (ii) grain-scale characterized by the size of single crystals; and (iii) the scale of polycrystalline representative volume elements. A homogenized activation energy-based crystal plasticity (AE-CP) FEM model is developed for the grain-scale, accounting for characteristic parameters of the sub-grain scale γ - γ ′ morphology. A significant advantage of this AE-CP model is that its high efficiency enables it to be effectively incorporated in polycrystalline crystal plasticity FE simulations, while retaining the accuracy of detailed sub-grain level representative volume element (SG-RVE) models. The SG-RVE models are created for variable morphology, e.g., volume fraction, precipitate shape and channel-widths. The sub-grain crystal plasticity model incorporates a dislocation density-based crystal plasticity model augmented with mechanisms of anti-phase boundary (APB) shearing of precipitates. The sub-grain model is homogenized for developing parametric functions of morphological variables in evolution laws of the AE-CP model. Micro-twinning initiation and evolution models are incorporated in the single crystal AE-CP finite element models for manifesting tension–compression asymmetry. In the next ascending scale, a polycrystalline microstructure of Ni-based superalloys is simulated using an augmented AE-CP FE model with micro-twinning. Statistically equivalent virtual polycrystals of the alloy CMSX-4 are created for simulations with the homogenized model. The results of simulations at each scale are compared with experimental data with good agreement. [ABSTRACT FROM AUTHOR]

[Display omitted] The tensile creep performance of a polycrystalline Co/Ni-base superalloy with a multimodal γ ′ distribution has been examined at 800 ∘ C and 300 MPa. The rupture life of the alloy is comparable to that of RR1000 tested under similar conditions. Microstructural examination of the alloy after testing revealed the presence of continuous γ ′ precipitates and M 23 C 6 carbides along the grain boundaries. Intragranularly, coarsening of the secondary γ ′ precipitates occurred at the expense of the fine tertiary γ ′. Long planar deformation bands, free of γ ′ , were also observed to traverse individual grains. Examination of the deformation bands confirmed that they were microtwins. Long sections of the microtwins examined were depleted of γ ′ stabilising elements across their entire width, suggesting that certain alloy compositions are susceptible to precipitate dissolution during twin thickening. A mechanism for the dissolution of the precipitates is suggested based on the Kolbe reordering mechanism. [ABSTRACT FROM AUTHOR]

• MTs were introduced into the Ni-Co base superalloy by pre-strain treatment, which enhance the mechanical properties. • After pre-strain treatment, the YS at 23 °C increased by 19.5%, the YS at 760 °C increased by 16.5%. • The creep life is reduced by pre-strain, even if it improved slightly when treatment time increases from 50 h to 100 h. • Impediment of MT to the motion of dislocations is the reason for the better mechanical properties. • Introduction of MT appropriately can be considered as a novel strengthening method in designing advanced superalloy. The effect of pre-strain treatment on mechanical properties of a Ni-Co base disk superalloy was studied. Results show that the pre-strain treatment, creep at 725 °C/630 MPa, improves the yield strength of the alloy at 23 °C and 760 °C. However, the creep life (at 760 °C/480 MPa) of the alloy was reduced by pre-strain treatment. The microstructure indicated that a lot of microtwins and Suzuki segregations were introduced by the pre-strain treatment, besides some unfavorable factors on mechanical properties, i.e., ripening of precipitates. The interactions of microtwin and Suzuki segregation with dislocations were discussed in this study, and it was deemed that the impediment of twin boundary and Suzuki segregation to the motion of dislocation may be responsible for the enhancement of the mechanical properties. Meanwhile, the microtwin and the Suzuki segregation have good stability at service temperature. If the problem of reducing the creep properties caused by pre-strain treatment would be solved, the introduction of microtwin and Suzuki segregation appropriately can be considered as a novel strengthening method in designing advanced superalloys. [ABSTRACT FROM AUTHOR]

A simple hybrid synthesis processing method was developed to synthesize γ-MnO nanocrystalline particles. The polyol method was modified by the addition of nitric acid in order to allow the synthesizing of single-phase MnO in a large scale. In the sequence, the acid digestion technique was used to transform MnO into γ-MnO. Structural and morphological characterization was carried out by X-ray diffractometry, Infrared and Raman spectroscopy, thermogravimetric analysis, nitrogen adsorption isotherm, scanning electron microscopy, and transmission electron microscopy. The electrochemical properties were investigated by cyclic voltammetry and galvanostatic charge-discharge measurements. The synthesized material exhibits a specific capacitance of 125.1 F g at a mass loading of 0.98 mg cm. The relation between structural features and electrochemical activity is discussed by comparing the synthesized material with commercial electrolytic manganese dioxide. [ABSTRACT FROM AUTHOR]

The structures of TmS sulfide, the mineral heyrovskyite PbBiS, and the β-W structure type were examined on the assumption that the crystal structure is created by the ordering of atoms by systems of parallel equidistant planes. An independent atomic ordering in cation and anion matrices was considered for sulfides. Twinning of 'structure moduli' in the heyrovskyite structure was shown to occur within the common cation and anion sublattices owing to vacancies therein. [ABSTRACT FROM AUTHOR]

A pink-purple diamond crystal from the Internatsional'naya kimberlite pipe (Siberia) was studied by single-crystal X-ray diffraction techniques using an area detector. Direct indexing of the diffraction pattern suggested a primitive hexagonal unit cell [ahex = 2.513(4), chex = 6.172(11) Å], instead of the well known face-centred cubic unit cell (acub, ~3.567 Å). Theoretical considerations and diffraction pattern simulation showed that the hexagonal diffraction pattern is the result of the superposition of two diffraction patterns with cubic symmetry due to spinel-law twinning along (111). These data are in good agreement with previous analyses of deformation microtwins in natural pink-purple diamonds using electron paramagnetic resonance spectroscopy and optical microscopy. The results suggest that natural epigenetic plastic deformation of diamonds occurs not only by dislocation slipping but also as a result of mechanical twinning. [ABSTRACT FROM AUTHOR]

Abstract: The evolution of microtwins during high temperature creep deformation in a γ′ strengthened Ni-based superalloy has been investigated through a combination of creep testing, transmission electron microscopy (TEM), theoretical modeling, and computer simulation. Experimentally, microtwin nucleation sources were identified and their evolution was tracked by characterizing the deformation substructure at different stages of creep deformation. Deformation is highly localized around stress concentrators such as carbides, borides and serrated grain boundaries, which act as sources of a/2〈110〉 matrix-type dislocations. Due to fine channels between the γ′ particles, coupled with a low γ matrix stacking fault energy, the a/2〈110〉 matrix dislocations dissociate into a/6〈112〉 Shockley partials, which were commonly observed to be decorrelated from one another, creating extended intrinsic stacking faults in the γ matrix. Microtwins are common and form via Shockley partial dislocations, cooperatively shearing both the γ and γ′ phases on adjacent {111} glide planes. The TEM observations lead directly to an analysis of dislocation–precipitate interactions. The important processes of dislocation dissociation and decorrelation were modeled in detail through phase field simulations and theoretical analyses based on Orowan looping, providing a comprehensive insight into the microstructural features and applied stress conditions that favor the microtwinning deformation mode in γ′ strengthened Ni-based superalloys. [Copyright &y& Elsevier]

The structure of a catalytically active octanuclear mixed-valence cobalt complex [Co(II)Co(III)(μ-O)(μ-OC Ph)( PhCOOH)] ( Ph = phenyl, CH) has been determined using an X-ray diffraction analysis. The presence of 16 ligands containing phenyl rings in each complex gives rise to strong steric stresses and leads to a disordering of terminal groups. The compound crystallizes in the monoclinic crystal system, space group of symmetry P2/ n, with the unit cell parameters a = 18.076(5) Å, b = 26.371(3) Å, c = 22.791(5) Å β = 92.48(1)°, V = 10853(4) Å, Z = 4, and ρ = 1.516 g/cm. The final values of the discrepancy factors are R1 = 0.0441 for 4005 symmetrically nonequivalent reflections with I ≥ 2σ( I), wR2 = 0.1131, and GooF = 0.943. [ABSTRACT FROM AUTHOR]

A series of γ-MnO2 with Mn4+ cation vacancy fraction 1.5 < x < 2.3 was prepared via chemical synthesis. X-ray diffraction (XRD) revealed high degree of microtwinning ( Tw > 70%) and de Wolff disorder (0.32 < Pr < 0.35) in the prepared samples. Thermal analysis showed four-stage decomposition corresponding to the removal of physical water, chemisorbed water, formation of Mn2O3 and Mn3O4, respectively. The chemical composition and physical properties of the prepared samples were expressed in terms of Ruetschi’s cation vacancy model for MnO2. The discharge behavior in 9 M KOH was governed by homogeneous solid-state reduction wherein protons and electrons were simultaneously inserted into MnO2 lattice. Their electrochemical behavior was also studied by performing cyclic voltammetry in 9 M KOH. An attempt was made to correlate obtained energy density as a function of proton transfer rate during the discharge process. [ABSTRACT FROM AUTHOR]

A series of γ-MnO2 with Mn4+ cation vacancy fraction 1.5 < x < 2.3 was prepared via chemical synthesis. X-ray diffraction (XRD) revealed high degree of microtwinning ( Tw > 70%) and de Wolff disorder (0.32 < Pr < 0.35) in the prepared samples. Thermal analysis showed four-stage decomposition corresponding to the removal of physical water, chemisorbed water, formation of Mn2O3 and Mn3O4, respectively. The chemical composition and physical properties of the prepared samples were expressed in terms of Ruetschi’s cation vacancy model for MnO2. The discharge behavior in 9 M KOH was governed by homogeneous solid-state reduction wherein protons and electrons were simultaneously inserted into MnO2 lattice. Their electrochemical behavior was also studied by performing cyclic voltammetry in 9 M KOH. An attempt was made to correlate obtained energy density as a function of proton transfer rate during the discharge process. [ABSTRACT FROM AUTHOR]

Abstract: This paper reviews the presently proposed mechanisms for creep of γ′ strengthened Ni-base superalloys that are typically used for disk applications. Distinct creep strength controlling modes, such as dislocation-coupled antiphase-boundary shearing, shearing configurations involving superlattice stacking faults, Orowan looping, climb by-pass, and microtwinning have been observed. These are strongly influenced by the scale of the γ′ precipitating phase and are operative within specific ranges of temperature and stress. Insight from more recent experimental findings concerning microtwinning and extending stacking fault mechanisms suggest important similarities between these deformation modes. It is suggested that local atomic reordering in the wake of Shockley partials is responsible for the temperature dependence exhibited in this regime. [Copyright &y& Elsevier]

Abstract: It has been recently shown that microtwinning is the principal deformation mechanism under certain creep conditions in several polycrystalline nickel-base superalloys. A creep model based on this mechanism is developed in this paper. Model parameters, supplied by microscopy and by atomistic calculations, are used to compare the model against experiment. [Copyright &y& Elsevier]

Abstract: 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 {111} planes is the dominant deformation process at low applied stress, and changes to shearing by 1/2[110] 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[110] 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〈112〉 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. [Copyright &y& Elsevier]

Abstract: In the literature, the γ-MnO2 structure is considered to be that of ramsdellite (R), in which two types of defects exist. The occurrence of a slab of pyrolusite (r) is named a De Wolff defect and random faults r in R give intergrowths of ramsdellite and pyrolusite, which account well for the global features of many experimental diffraction patterns. The other type of defect results from “microtwinning”, which allows the so-called ɛ-MnO2 to be put with γ-MnO2 in the same classification. This paper discusses the previous models of defect and what could be the features of the “microtwinning”, giving for each possible model the corresponding expected features in reciprocal space. The results of a selected area electron diffraction study of rather well crystallized samples of γ-MnO2 are presented. The splitting of particular diffraction spots and new diffuse intensity are interpreted as the first experimental evidence for “microtwinning”, and a model with orientation variants within microdomains embedded in a “normal” structure is proposed, which is rather different from the previous hypotheses involving parallel twin planes and parallel twin boundaries. [Copyright &y& Elsevier]

De Wolff disorder, microtwinning, and point defects which are characteristic for γ-MnO2 have been studied using molecular modeling. Particular attention was paid to the effects these defects have on the X-ray diffraction (XRD) pattern. Comparisons with observed XRD patterns allow identification of structural features in chemical (CMD) and electrochemical (EMD) manganese dioxide. The major factor determining the structure of γ-MnO2 is de Wolff disorder. CMD materials are characterized by a larger percentage of pyrolusite while EMD materials contain more ramsdellite. Microtwinning occurs to a larger extent in EMD than in CMD materials. EMD materials are also higher in energy. [Copyright &y& Elsevier]

MnO2 was chemically prepared from birnessite (CMD2D) and from spinel-like LiMn2O4 (CMD3D) by acid leaching. X-ray diffractometry, FTIR spectrometry, chemical and thermal analyses were used to determine the structure and chemical disorder. On the basis of cation vacancy model, the structural water was found to be mainly due to Coleman protons in both samples. Both samples were found to be of very close structure belonging to γ-MnO2 type I according to Chabre and Pannetier classification with de Wolff disorder (Pr) of 0.19 and 0.30 and microtwinning (Tw) of 9.9 and 17%, respectively. The activation energy of structural water release calculated from thermal analysis was apparently higher than that of other γ-MnO2 types (150 and 156.6 kJ/mol, respectively).A high reduction capacity in aqueous and non-aqueous batteries was delivered for both samples except for the electro- chemical lithium insertion of CMD2D, which was attributed to the different oxygen packing. [Copyright &y& Elsevier]