Effect of microstructure and γ′ precipitate from undercooled DD3 superalloy on mechanical properties

The mechanical properties of DD3 superalloy solidified at various undercoolings were measured to investigate the effects of precipitate and microstructure on alloy properties. The influence of melt undercooling on the precipitation is also studied. It is found that not only the size of particle, but its distribution in the as-solidified structure is also drastically controlled by the melt undercooling. The analysis indicates that alloy solidified at a low undercooling is brittle, thus leading to a lower toughness and tensile strength. With increasing undercooling, the toughness and strength of the alloy increased accordingly, which may be attributed to the strengthening effect of precipitate and the reduced micro-segregation.     

Effect of cooling rate on the morphology of γ′ precipitates in a nickel-base superalloy under directional solidification

The morphological evolution of γ′ precipitates in a nickel-based superalloy K5 was studied by zone melting directional solidification under vacuum conditions. The results show that at the lower cooling rate of 12.42 K s⁻¹, γ′ precipitates remand big cuboids. γ′ particles become smaller at the cooling rate ranges from 12.42 to 38.80 K s⁻¹. For a rather fast cooling rate of 50.16 K s⁻¹, γ′ particles retain a spherical shape. The experiments show that big cuboids will become unstable and split into several small ones at the lower cooling rate of 1.1 K s⁻¹. The mechanism of the evolution of the γ′ morphologies is also analyzed by introducing a new parameter-shape factor which classifies the total energy into several energy levels. Based on this, the effect of the cooling rate on the γ′ morphology is discussed.     

Evolution and analysis of γ′ rafting during creep of single crystal nickel base superalloy

Abstract

By means of TEM observation and finite element analysis, an investigation has been made into the directional coarsening of the γ′ phase for a single crystal nickel base superalloy with [001] orientation during creep at 1040°C. The results show that the strain energy change related to the elastic strain is to be the driving force for γ′ rafting. The extruded strain of the lattice in the cuboidal γ′ interfaces results in a supersaturation of the elements Ta and Al of larger atomic radius. The extrusion or expansion strain in the lattice of the cuboidal γ′ planes may repel or trap these atoms to promote the directional growth of the γ′ phase into a needle-like raft structure along the direction parallel to the stress axis under an applied compression stress, or into a meshlike raft structure along the direction perpendicular to stress axis under applied tensile stress. The normal direction of the expanding lattice is supposed to be the one in which the γ′ rafts grow. The rate of γ′ rafting is enhanced by increasing viscoplastic flow in the γ matrix and elastic strain in the γ′ phase. Therefore, there is a smaller rate of growth under compressive than under tensile stress as a result of the smaller expansion strain and viscoplastic flow occurring in the former.     

Effect of very high temperature short exposures on the dissolution of the γ′ phase in single crystal MC2 superalloy

Time-temperature dependence of the γ′ phase volume fraction was investigated for a second generation single crystal nickel-based superalloy exposed to very short high temperature regimes (1,100–1,200 °C). In this temperature range, the dissolution of the strengthening γ′ phase occurs. Evolution of the γ′ volume fraction in transient regimes has been established for each temperature and activation energy of the dissolution phenomenon were determined. They directly attest from the activity of the diffusing species involved during this phenomenon. From these analyses, the volume fraction at equilibrium was established for the entire temperature range where dissolution occurs. A model, based on a time/temperature equivalence, is proposed to quantify the γ′ volume fraction dissolved during short high temperature exposure.     

Effect of cooling rate on the morphology of γ′ precipitates in a nickel–base superalloy under directional solidification

The morphological evolution of γ′ precipitates in a nickel-based superalloy K5 was studied by zone melting directional solidification under vacuum conditions. The results show that at the lower cooling rate of 12.42 K s^–1, γ′ precipitates remand big cuboids, γ′ particles become smaller at the cooling rate ranges from 12.42 to 38.80 K s^–1. For a rather fast cooling rate of 50.16 K s^–1, γ′ particles retain a spherical shape. The experiments show that big cuboids will become unstable and split into several small one at the lower cooling rate of 1.1 K s^–1. The mechanism of the evolution of the γ′ morphologies is also analyzed by introducing a new parameter-shape factor which classifie the total energy into several energy levels. Based on this, the effect of the cooling rate on the γ′ morphology is discussed.     

Influence of local chemical segregation on the γ′ directional coarsening behavior in single crystal superalloy CMSX-4

The influence of local chemical segregation on the directional coarsening behavior of γ′ phase has been investigated in a nickel-base single crystal superalloy during load-free thermal exposure. It is found that the γ′ phase was preferentially oriented with the direction perpendicular to the dendrite arm growth axis. The chemical gradients caused by local chemical heterogeneities were carefully measured on the scale of a dendrite. It showed that the directions of the γ′ rafting and the chemical gradients are consistent and the magnitude of the chemical gradients affects the rafting rates to some extent. It is suggested that the internal chemical gradients can orientate the development of the γ′ rafts by leading to anisotropy in their growth kinetics during Ostwald ripening. Furthermore, influence of the local chemical segregation on the mechanical properties is also discussed.     

Phase field simulation of morphology evolution and coarsening of γ′ intermetallic phase in Ni–Al alloy

Abstract

The morphology evolution and coarsening behaviour of γ′ (Ni₃Al) phase in Ni–Al alloys were investigated by using the Cahn–Hilliard and Ginzberg–Laudan phase field equations, and the composition, order parameter and average particles size evolution coupling with the internal elastic strain were studied. Cubic γ′ phase was presented firstly and then the rectangle particles were formed by means of coarsening; the mechanisms of γ′ phase coarsening are Ostwald ripening or coalescence of neighbouring particles. The average particle size of γ′ phase increases rapidly at the initial stage, and it slows down at the coarsening dominated stage. The average particle size follows a cubic law of ‘Lifshitz–Slyozov’ encounter modified theory in the coarsening regime.     

δ and austenite phases evolution and model in solution treatment of superalloy GH4169

Abstract

The δ phase evolution and its effect on the austenite grain growth in the solution treatment of superalloy GH4169 were investigated at the solution temperatures of 1233, 1253, 1273 and 1293 K and the holding time of 30, 45 and 60 min. The δ phase is dissolved and transformed from short bars to fine bars or particles gradually with the increasing solution temperature. The holding time affects the morphology and the amount of δ phase slightly. The austenite grain grows with the increasing solution temperature and holding time. It grows slowly at the solution temperatures ranging from 1233 to 1373 K, but sharply from 1273 to 1293 K. The phenomenon suggests that the δ phase generates a pinning effect on the austenite grain growth. The kinetic analysis about the austenite grain growth was carried out at the solution temperatures ranging from 1233 to 1373 K. The grain growth exponent n value is calculated to be 1·2 which confirms the pinning effect of δ phase on the austenite grain growth, and activation energy Q value is calculated to be 394·1 kJ mol^?1. Finally, a model with an average difference 9·22% for the austenite grain growth was proposed.     

Controllable current oscillation and pore morphology evolution in the anodic growth of TiO₂ nanotubes

The spatial heterogeneities and temporal instabilities in the anodic growth of TiO? tubes are very important for nanostructure fabrication, but few ordered cases have been reported. In this work, we represent a new current oscillation with pore morphology evolution in the formation of anodic TiO? tubes. Small (less than 8.0% of the minimum value) and fast (period ~10? s) current oscillation was formed under static conditions in a wide range, while significant morphological change such as periodical narrowing, abruption and small pits appeared in the pore with characteristic length scale of 101-102 nm. Surprisingly, the roughness in the pore would be totally eliminated instead of being enhanced by high speed stirring or periodically modulated voltage with the current oscillation still being enhanced, which indicates an important involvement of the ion transport process. It has also been found that the growth rate could be significantly accelerated by tuning the stirring rate or the periodical modulation of the voltage. The mechanism has been described with consideration of the local reactions and the ion transport with a key involvement of the convection process, which can be strongly influenced by the mechanical stirring and the modulated voltage.     

X-ray structure determination and analysis of hydrogen interactions in 3, 3′-dimethoxybiphenyl

The crystal structure of 3, 3′-dimethoxybiphenyl has been determined by X-ray diffraction methods with an aim of describing the hydrogen interaction in biphenyl derivatives. The title compound crystallizes in monoclinic space groupP2₁/c with unit cell dimensions,a = 7.706(1),b = 11.745(2),c = 12.721(2) Å, β = 92.31(1)°,Z = 4 and its structure has been refined up to the reliability index of 3.8%. The average torsion angle about the inter-ring C-C bond is 37.5°. The O1 and Ol′ atoms of the methoxy group are deviated by 0.046(1) Å and 0.234(1) Å from the mean planes of respective rings. The crystal cohesion is pronounced due to three-inter-molecular C-H...O hydrogen bonds.     

Thermal evolution crystal structure and cation valence of Mn in substituted Ba-β-Al2O3 prepared via coprecipitation in aqueous medium

BaMn_xAl_12–xO_19– combustion catalysts with x = 0.5, 1.0, 2.0, 3.0 have been prepared via coprecipitation in aqueous medium. Thermal evolution from 380 up to 1670 K has been followed by XRD and surface area measurements. The crystal structure of the final material and the dominant oxidation state of Mn in the different crystallographic sites, have been investigated by means of XANES spectroscopy and Rietveld refinements of diffraction powder data sets collected in proximity and far from the MnK absorption edge. It was found that, except for the highest Mn content (x = 3), monophasic samples are obtained upon calcination at 1470 K. For the whole compositional range the Ba-_I-Al₂O₃ structure is obtained. A formation mechanism involving Ba ion diffusion within the -Al₂O₃ spinel blocks, similar to that observed for Mn-free samples, is active also in this case. However, the presence of Mn ions favours the formation of the Ba-_I-Al₂O₃ that occurs at lower temperatures. At low Mn loading (up to x = 1), Mn preferentially enters the tetrahedral Al(2) sites of the Ba-_I-Al₂O₃ as Mn²⁺. At higher loading, Mn preferentially enters the octahedral Al(1) sites as Mn³⁺. A charge compensation mechanism, involving the occupancy of Ba sites in the mirror planes, operates to balance the substitution of Al³⁺ with Mn²⁺. The presence of Mn ions also affects the morphological properties of the final material.     

Formation and stability ofγ-iron in high-temperature treated nontronite

Samples of nontronite, exposed to different high-temperature treatments, have been studied by Mössbauer spectroscopy and X-ray diffractometry. Oxidation and subsequent reduction at 900 to 1000° C resulted in formation of metallic iron with about 20 at% of the total iron content as the-phase of iron. Investigations of the phases formed during the oxidation and reduction steps and studies of the stability of the-iron suggests that the-iron formed by this method is stabilized by impurities     

Discovering and analyzing the intellectual structure and its evolution of LIS in China, 1998–2007

The intellectual structure and its evolution of library and information science (LIS) in China are analyzed with time series data from Chinese Social Sciences Citation Index which is the properest database for ACA practice in the field of social science at present. The result indicates that the subfields of Library and Information Science in China kept changing from 1998 to 2007: some subfields have emerged and developed a lot, e.g., webometrics and competitive intelligence; some subfields maintain, e.g., bibliometrics and intellectual property; and some subfields have begun to decline, e.g., cataloging. Through the comparison with the international LIS, it is found that there are some unique subfields in Chinese LIS from 1998 to 2007, such as competitive intelligence and intellectual property. At the same time, I also suggest that Chinese authors in LIS should pay more attention to the applied research in the future.     

Coarsening of γ′-precipitates in single-crystal superalloy SRR 99

Abstract

The effects of isothermal annealing between 800 and 1200°C on the microstructure of single–crystal SRR 99 have been investigated. During aging at either 800 or 900°C, the precipitates adopt an irregular, rounded, and highly interconnected array which suggests that they have undergone coalescence. Aging at higher temperatures was found to produce a regular cuboidal precipitate morphology, although long–term aging promoted the formation of γ′-‘rafts’. The precipitate size distributions were found to be broader than those predicted by the Lifshitz-Slyosov and Wagner theory, and the coarsening kinetics also showed significant deviations from the power law prediction of this theory. The reasons for these deviations are discussed in the light of the available models, and it is suggested that a progressive transition in the dominant coarsening mechanism takes place.

MST/371     

Growth,morphology and slip system of α-Si3N4 single crystal

Under the conditions of growth temperatures 1500 to 1700° C and total gas pressure 10 to 50 Torr, -Si₃N₄ single crystals have been grown by chemical vapour deposition from a mixture of NH₃, SiCl₄ and H₂. The crystals were transparent and brownish-red to colourless. The effects of the growth conditions on the crystal morphology, growth habit and growth direction have been investigated. On the basal and prismatic planes, the variation in Knoop hardness with orientation of the indenter long-axis has been measured at temperatures up to 1500° C; maximum hardness values were obtained along the 1 0 ¯1 0 direction for the basal plane and along the [0 0 0 1] directions for the prismatic planes. Hardness anisotropy analysis suggests that the active slip systems of -Si₃ N₄ are {1 0 ¯1 0} [0 0 0 1] from room temperature to 1500° C.     

Precipitation behaviour of β and γ′ in high Al low thermal expansion superalloy

Abstract

The precipitation behaviour of β and γ′ in a low thermal expansion superalloy IN783 was investigated. It was demonstrated that coarse γ′ precipitates do form during β aging at 845°C. During aging at lower temperatures, not only fine γ′ particles are formed, the coarse γ′ particles also change from spherical to cuboidal. Therefore, a bimodal distribution of γ′ is formed in the matrix after standard aging treatment. The formation of β consumes Al atoms, which constrains the formation of coarse γ′ in the zone around β during aging at 845°C. During γ′ aging, a fine γ′ precipitating zone is formed around β. Prolonging the β aging duration markedly promotes the formation of β at grain boundaries and in the matrix, significantly enlarges the fine γ′ precipitating zone and influences the precipitation of coarse γ′.     

The evolution of structure and properties in GdMn(1−x)TixO3 ceramics

In this work, the effects of Ti doping on the microstructure, dielectric, and magnetic properties of GdMn_(1?x)Ti_xO₃ (x?=?0.00–0.15) ceramic samples synthesized using a solid-state reaction were investigated. All the experimental samples formed a single-phase structure, and no structural transformation occurred within the experimental doping range; however, Ti doping caused lattice shrinkage. Ti doping reduced the grain size, and the microstructure of the synthesized samples appeared more compact in scanning electron microscopy images. The lattice distortion of GdMn_(1?x)Ti_xO₃ caused by Ti substitution at the Mn sites resulted in changes in the Raman vibration modes. X-ray photoelectron spectroscopy results showed that the valence state transition of the Ti and Mn ions occurred and the concentration of Ti⁴⁺, Mn³⁺ ions and oxygen vacancies changed due to the charge compensation induced by Ti doping. Ti doping had a significant influence on the size and concentration of cation vacancies in the GdMn_(1?x)Ti_xO₃ samples. Appropriate Ti doping was shown to reduce the dielectric loss, improve the frequency stability of the dielectric constant, and significantly affect the long-range ordering of Gd³⁺ magnetic moments and clearly reduce magnetization.

     

On the γ′ precipitates of the normal and inverse Portevin-Le Châtelier effect in a wrought Ni-base superalloy

The Portevin-Le Châtelier (PLC) effects in a wrought Ni-base superalloy with different γ? precipitates contents have been investigated. Detailed analysis on the serration type of the tensile curves indicates that the γ? precipitates have a decisive influence on the transformation from normal to inverse PLC behavior, which is rarely proposed in other works. It is considered that the γ? precipitates play the same role in PLC effect as temperature and strain rate for the investigated wrought Ni-base superalloy.