Grain Boundary Particles Research Articles

Improved crystalline structure and H2S sensing performance of CuO–Au–SnO2 thin film using SiO2 additive concentration

In situ SiO2-doped SnO2 thin films were successfully prepared by liquid phase deposition. The influence of SiO2 additive as an inhibitor on the surface morphology and the grain size for the thin film has been investigated. These results show that the morphology of SnO2 film changes significantly by increasing the concentration of H2SiF6 solution which decreases the grain size of SnO2. The stoichiometric analysis of Si content in the SnO2 film prepared from various Si/Sn molar ratios has also been estimated. For the sensing performance of H2S gas, the SiO2-doped Cu–Au–SnO2 sensor presents better sensitivity to H2S gas compared with Cu–Au–SnO2 sensor due to the fact that the distribution of SiO2 particles in grain boundaries of nano-crystallines SnO2 inhibited the grain growth (<6nm) and formed a porous film. By increasing the Si/Sn molar ratio, the SiO2-doped Cu–Au–SnO2 gas sensors (Si/Sn=0.5) exhibit a good sensitivity (S=67), a short response time (t90%<3s) and a good gas concentration characteristic (α=0.6074). Consequently, the improvement of the nano-crystalline structures and high sensitivity for sensing films can be achieved by introducing SiO2 additive into the SnO2 film prepared by LPD method.

Tensile strength and creep behaviour of austenitic stainless steel type 18Cr – 12Ni with niobium additions at 700°C

The effect of niobium additions up to 2.36 wt% on the creep behavior of a series of seven extra low carbon 18Cr-12Ni austenitic stainless steels at 700°C has been investigated. Grain size and hardness measurements, hot tensile tests and constant stress creep tests from 90 to 180 MPa were carried out for each alloy, in the solution treated condition at 1050, 1200 and 1300°C followed by quench in water. The mechanical behavior at high temperature was related to the amount of NbC precipitation occurring during the tests. Solid solution and intermetallic compound effects were also considered. Creep data analysis was done to determine the parameters of the creep power-law equation ϵ̇ = A.σn and the Monkman-Grant relation ϵ̇.tmR = K. Niobium-carbide precipitation in these steels reduces the secondary stage dependence of strain rate with applied stress, resulting in n-values which indicate the possibility of operation of various creep mechanisms. The creep strength during the secondary stage is primarily controlled by the amount of NbC available for precipitation. However, the rupture times increase progressively with niobium content, as the amount of undissolved carbide particles in grain boundaries and the Laves phase precipitation increase.

The surface, microstructure and corrosion of magnesium alloy AZ31 sheet

This study investigates the effect of some industrial pre-treatments, including heat-treatment, surface grinding and polishing, acid cleaning and sandblasting, on the corrosion performance of Mg alloy AZ31 sheet. It is found that tempering accelerates the corrosion of AZ31. Surface grinding and acid cleaning dramatically improve its corrosion resistance, while sandblasting severely deteriorates its corrosion performance. Surface analysis indicates that the change in corrosion behavior of AZ31 by these pre-treatments can be associated with the alteration of its impurity contamination, surface state, grain size and intermetallic particles. A corrosion model concerning the dissolution and precipitation of tiny Al–Mn–(Fe) particles in grains and grain boundaries is proposed to explain the effect of mechanical deformation and heat-treatment on the corrosion performance of AZ31.

Influence of MnS particles on the behaviors of grain boundary migration in Fe-3%Si alloys

The precipitation behaviors of MnS particles at 900°C in a hot deformed Fe-3%Si alloy were observed statistically. The ratio of MnS particles on dislocations and in grain boundaries was calculated based on a model concerning the second phase precipitation in supersaturated solid solution. It was indicated that the precipitation of MnS particles on dislocations prevailed. The coarsening process of MnS particles in grain boundaries determined the boundary mobility during the secondary recrystallization. However, the density difference of precipitated MnS particles inside the grains on both sides of a boundary will determine the migration direction of the boundary as well, besides the grain size effect. It was observed that the densities of MnS particles in two neighboring grains were commonly different, and the boundary tended to move towards the area with lower particle density. The factors, e.g. dislocation densities in differently oriented grains will affect the density of precipitated particles, in which the Goss grains with higher particle density could grow more easily.

Nb-Base FS-85 Alloy as a Candidate Structural Material for Space Reactor Applications: Effects of Thermal Aging

The proposed uses of fission reactors for manned or deep space missions have typically relied on the potential use of refractory metal alloys as structural materials. Throughout the history of these programs, a leading candidate has been Nb-1Zr, due to its good fabrication and welding characteristics. However, the less-than-optimal creep resistance of this alloy has encouraged interest in the more complex FS-85 (Nb-28Ta-10W-1Zr) alloy. Despite this interest, only a relatively small database exists for the properties of FS-85. Database gaps include the potential microstructural instabilities that can lead to mechanical property degradation. In this work, changes in the microstructure and mechanical properties of FS-85 were investigated following 1100 hours of thermal aging at 1098, 1248, and 1398 K. The changes in electrical resistivity, hardness, and tensile properties between the as-annealed and aged materials are compared. Evaluation of the microstructural changes was performed through optical microscopy (OM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The development of intragranular and grain-boundary precipitation of Zr-rich compounds as a function of aging temperature was followed. Brittle tensile behavior was measured in the material aged at 1248 K, while ductile behavior occurred in samples aged above and below this temperature. The effect of temperature on the under- and overaging of the grain-boundary particles is believed to have contributed to the mechanical property behavior of the aged materials.

Oxygen Embrittlement and Time-Dependent Grain-Boundary Cracking of ALLVAC 718PLUS

This study focuses on the time-dependent intergranular cracking of a newly developed Ni-base superalloy ALLVAC 718PLUS. Grain-boundary (GB) cracking is due to the penetration of oxygen down grain boundaries and subsequent oxidation of GB particles, i.e., carbides and possibly delta phase present at the grain boundaries. The GB oxidation and subsequent brittle intergranular failure of this material is highly dependent upon moisture level in the testing atmosphere. With increasing moisture level in the test atmosphere, it is demonstrated that hydrogen may become dominant over oxygen as the embrittling species. The degree of susceptibility to time-dependent intergranular cracking is also highly dependent upon microstructure. The behavior exhibited by ALLVAC 718PLUS is common to a wide range of Ni- base superalloys when tested in air or other aggressive environments. A six-step model for the grain-boundary cracking mechanism is presented. The model is presented for hydrogen in high-moisture environments but is equally applicable to oxygen at lower-moisture levels, as well. Structures which provide increased resistance to time-dependent intergranular cracking are also discussed.

Calculation of second phase particle-grain boundary interaction range

On the basis of the grain boundary equation by Hellman and corresponding analysis of Worner, this article deals with the interaction range between the second-phase particle (SPP) and grain boundary (GB) as viewed from the applicability of grain boundary equation. Also, a new expression describing the interaction range has been derived, which solves the problem in theory that the interaction range between SPP and GB can only be qualitatively analyzed previously. It is shown that given the interaction position between SPP and GB, the interaction range can be quantitatively determined by use of this expression.

The Fracture Mechanism of As-Cast and Extruded SiCp/AZ91 Composites Fabricated by Stir Casting

The fracture mechanisms of SiCp/AZ91 composites were investigated by scanning electron microscopy (SEM). For the as-cast composites, the decohesion at SiCp/matrix interface is the main fracture mechanism because of the high stress concentration resulting from the segregation of particles in grain boundaries formed during solidification process. But for the extruded composites, the main fracture mechanism is the particle crack or ductile rupture of the matrix between the particles. So the fracture mechanism of SiCp/AZ91 composites is altered by extrusion because the segregation of particles and defects in the grain boundaries are largely eliminated by extrusion.

Temperature Effects on Oxide Film Properties of Grade-7 Titanium

Abstract The passive corrosion properties of Grade-7 titanium (Ti-7) have been studied at elevated temperatures up to 150°C and as a function of time at a constant temperature of 150°C in 0.27 mol/L sodium chloride (NaCl) solution using a combination of open-circuit potential and electrochemical impedance spectroscopy (EIS) measurements. It was found that the influence of temperature on the passive behavior of Ti-7 depended on the manner in which the temperature was changed. If the temperature was raised slowly in 10°C steps, passivity was maintained up to 150°C, but metastable breakdown events commenced once the temperature was increased to ≥70°C. These events could be associated with the active dissolution of Fe-stabilized β-phase and/or TixFe particles in grain boundaries. Impedance analyses showed that the changes in film resistance and capacitance were observable but minor. If the temperature was raised very rapidly to 150°C, an observable film breakdown occurred, leading to a drop of >150 mV in corr...

粒界粒子追跡による結晶粒変形挙動の4D可視化

粒界粒子追跡による結晶粒変形挙動の4D可視化

Effects of grain-boundary GeO2 particles on intergranular fracture of Cu-GeO2 polycrystals

The Cu-GeO2 alloy polycrystals containing plastically hard GeO2 particles are tensile tested to study intergranular fracture behavior of the alloys at intermediate temperatures. The effects of large GeO2 grain-boundary particles on the intergranular fracture are discussed using the specimens containing the particles of the fixed size (3 μm in diameter) and different area-fractions. The ductility of the Cu-GeO2 alloy polycrystals is larger than that of Cu polycrystals. The grain-boundary GeO2 particles improve the ductility by suppressing grain-boundary sliding. The grain-boundary voids to cause theintergranular fracture preferentially nucleate between the grain-boundary GeO2 particles. The ductility of the Cu-GeO2 alloys increases with increasing the area fraction of the grain-boundary GeO2 particles. The area-fraction dependence of the ductility is explained by considering the amount of GBS as a criterion of the intergranular fracture.

Effect of volume fraction of dispersed SiO2 particles on intermediate-temperature embrittlement in Cu–SiO2 polycrystals

Several kinds of Cu polycrystals with dispersed SiO2 particles of different volume fractions were tensile tested at high temperatures from 473 K to 1023 K. All of the alloys showed clear intermediate-temperature embrittlement (ITE). Although the temperature of minimum elongation was almost the same in all the alloys, temperature dependence of fracture strain depended strongly on the SiO2 volume fraction: (1) At a fixed temperature, the fracture strain tended to first decrease with increase in SiO2 volume fraction, showed minimum in an alloy with certain volume fraction, and increased again with increase in volume fraction. (2) With increase in SiO2 volume fraction, the temperature range of ITE became narrower and sharper. These results were reasonably understood by considering the occurrence of stress concentration at grain-boundary particles induced by grain-boundary sliding (GBS) and occurrence of dynamic recrystallization and stress relaxation by Cu/SiO2 interfacial diffusion.

Intermediate-temperature embrittlement of Cu bicrystals with dispersed Fe particles

The temperature dependence of deformation and fracture behavior of orientation-controlled Cu bicrystals with dispersed Fe precipitate particles was investigated. Deformation and fracture behavior at an intermediatetemperature range showed clear aging-time dependency: With increase in aging time, grain-boundary fracture became more pronounced, although fracture strain remained practically the same. The grain-boundary character and strain rate also affected the mechanical properties: With increase in the misorientation angle between the two crystals and with decrease in strain rate, grain-boundary fracture started to occur more easily at lower temperatures associated with clearer intermediate-temperature embrittlement. These experimental results were analyzed by incorporating various phenomena such as grain-boundary sliding, dynamic precipitation, growth of grain-boundary particles, and stress relaxation induced by interfacial diffusion between grain-boundary particles and the Cu matrix.

Strain-rate effects on intermediate-temperature embrittlement of Cu‒SiO2 bicrystals with [001] twist boundaries

Abstract Cu[sbnd]1.7 vol.%SiO2 bicrystals having [001] twist boundaries with different misorientation angles were tensile tested at various temperatures above 473 K under various strain rates between 4.17 × 10−5 and 4.17 × 10−3s−1. Intermediate-temperature embrittlement was observed in all the strain rates irrespective of the misorientation. As the misorientation became smaller, the intermediate-temperature embrittlement became clearer and sharper. The temperature of the elongation minimum tended to increase with increase in strain rate. These results could be reasonably understood by considering the competition between the stress concentration around the grain-boundary SiO2 particles caused by grain-boundary sliding and the stress relaxation by Cu[sbnd]SiO2 interfacial diffusion.

Microstructure characteristics of NiTi shape memory alloy obtained by explosive compact of elemental nickel and titanium powders

In the present paper the microstructure characteristics of NiTi shape memory alloy manufactured by explosive compaction of elemental and titanium powder and subsequent heat treatment have been investigated in detail. It shows that the NiTi intermetallic compound can be obtained in the whole specimen and a good shape memory effect can be obtained. However there are some differences in microstructure between this explosive compact NiTi shape memory alloy and that of cast/wrought one. By transmission electron microscopy, diffraction and EDX analysis, the crystal structure and the distribution of the phase existed in the sample have been identified. There is a phase of face centered cubic lattice with a lattice constant a = 4.35 A ̊ inside the grain of NiTi intermetallic compound and its composition is about Ti 8Ni. The secondary phase particles in grain boundaries are generally Ti 2Ni. A comparison between this explosive compacted NiTi alloy and the cast/wrought NiTi alloy has also been made.

The presence and consequences of precipitatefree zones in an aluminium-copper-lithium alloy

The addition of lithium to aluminium alloys has the potential for providing a class of high-strength alloys with exceptional properties suitable for aerospace applications. Potential candidates are precipitation hardenable and belong to the Al-Li-Cu family. The intrinsic microstructural features have a pronounced influence on the mechanical response of these alloys. In this work, the mechanisms responsible for the formation of precipitate-free zones along grain boundaries in precipitation-strengthened lithium-containing aluminium alloys were examined. The influence of grain morphology and the nature and type of precipitate coverage at the grain boundary in controlling the formation of these zones was analysed. The presence and influence of these zones along the grain boundaries on mechanical properties was studied for an Al-4.5Cu-1.21 Li alloy. It was found that while strength is comparable with existing high-strength alloys, the ductility decreases due to the presence of precipitate-free zones. The degradation in ductility is attributed to the particular mode of plastic deformation of this alloy, and to the restriction of plastic deformation in narrow planar zones along the grain boundaries. Fracture occurs when a critical local strain is reached in these zones. The overall consequences of precipitate-free zones along grain boundaries on mechanical properties are discussed in the light of competing effects involving the nature of matrix-strengthening precipitates, grain-boundary particles and deformation characteristics.

Effects of grain-boundary segregation and precipitation on the hydrogen susceptibility of nickel

The straining electrode tensile test was employed to study hydrogen embrittlement in three grades of nickel. Aging at intermediate temperatures (400 to 800 °C) after solution annealing caused sulfur segregation to grain boundaries in the materials. This sulfur segregation was found to influence intergranular hydrogen embrittlement, and the effect was most prominent when the surface supply of hydrogen was low and when the grain boundaries were free of graphite precipitates. Aging also induced graphite precipitation at the grain boundaries of one grade of nickel (Nickel 200). The presence of these grain-boundary particles reduced susceptibility to hydrogen embrittlement, but at the same time caused a form of aging embrittlement. The effect of the second-phase precipitate, when present, masked the effect of sulfur segregation on hydrogen susceptibility.

Growth of Ta-oxide on thin tantalum films containing grain boundary particles

The effect of grain-boundary particles on the oxidation process of Ta thin films and on Ta-oxide morphology, has been investigated. Oxidation was performed in air, in a temperature interval between 300 and 700 K. Grain boundary particles mostly of spherical form, of size ∼0.2 μm, dislocations, and dislocation loops have been found by means of SEM. The increased oxidation temperature caused movement of dislocations during the oxidation process. The analysis indicates that a grain boundary dislocation can pass the particles by either climbing over them, or bowing between them. A dislocation passing the particles strongly affects both oxidation rate and morphology of the oxide through the stress concentration at the particles. The result is a roughening of the surface of the Ta-oxide and, in some cases, a highly increased oxidation rate in the stress-affected area.

A percolation model of the conduction threshold in thick-film resistors: segregated structures

The possibility of applying percolation theory to the sheet resistivity of thick-film resistors, noted for the segregation of conducting particles in grain boundaries between sufficiently large particles of glass, is investigated. The experiments, carried out on model resistor films, do not satisfy the models of Malliaris and Turner and R.P. Kusy. A new model, based on the distribution of threshold concentration for two-dimensional systems of finite size, was suggested. The theoretical values of conducting pigment critical concentration in the model films studied, which are in good agreement with the experimental results, are calculated on the basis of this model.

A statistical analysis of cavity nucleation at particles in grain boundaries

Cavity nucleation on grain boundary particles during creep has been examined using a classical thermodynamic method. The particle sizes and spacings are assumed to obey a log-normal distribution. It is found that a threshold shear stress is needed for cavity nucleation to occur. When the resolved shear stress on a grain boundary segment reaches the threshold stress, a critical normal stress for cavity nucleation is produced on the panicle-matrix interface by grain boundary sliding and cavity nucleation occurs. The threshold stress is determined mainly by the concentration and distribution of grain boundary particles and falls in the stress range of engineering applications. A program has been developed to calculate the fraction of particles which can serve as nucleation sites. The model is used to predict the onset of cavitation in the oxide dispersion strengthened alloy Inconel MA 754. Implications for avoiding the nucleation of cavities in engineering alloys are discussed.