Pinning Of Boundaries Research Articles

Hydrogen permeation properties of CrxCy@Cr2O3/Al2O3 composite coating derived from selective oxidation of a Cr[sbnd]C alloy and atomic layer deposition

Metal oxides and carbides are promising tritium permeation barrier coatings for fusion reactors. However, the thermomechanical mismatch between the coating and substrate poses a threat to their interface's integrity during fabrication and operation. To address this issue, a metallic interlayer coating was introduced followed by selective oxidation in which a compact and uniform CrC amorphous alloy coating was successfully deposited on the stainless steel substrate by pulsed electrochemical deposition. A new composite coating of CrxCy@Cr2O3/Al2O3 was formed by subsequent controlled oxidation conversion and atomic layer deposition. The phase, morphology, chemical state and defects of the films were analyzed and compared both before and after hydrogen exposure at 300 °C. The results show that this new kind of composite coating, based on the principles of grain boundary pinning of chromic oxide with carbide and defect healing of alumina, can remarkably improve the hydrogen permeation barrier performance of these materials.

Strengthening effects and thermal stability of the ultrafine grained microstructure of a nickel base superalloy at room and elevated temperatures

An ultrafine grained (UFG) nickel base superalloy doped with 5 vol.%Y2O3 nanoparticles, Alloy 718-5 vol.%Y2O3, was fabricated by a powder metallurgy route which combines high energy mechanical milling of Alloy 718 machining chips, spark plasma sintering, hot extrusion and heat treatment, and its microstructure and mechanical properties at room temperature and 650 °C were studied. The study showed that the Y2O3 nanoparticles reacted with Al from the base alloy and transformed to Y4Al2O9 nanoparticles (average diameter: 12.5 nm) which were stable during heat treatment at 970 °C (0.78Tm, where Tm is the solidus temperature of the alloy in K). As a result of the high thermal stability of Y4Al2O9 nanoparticles and their effective pinning of the grain boundaries, the UFG microstructure (average grain size: 179 nm) of the alloy was stable during heat treatment. Grain boundary and nanoparticle strengthening rendered the heat treated Alloy 718-5 vol.%Y2O3 alloy with a notable room temperature tensile yield strength of 1870 MPa. It was demonstrated that the grain boundary strengthening effect associated with the UFG microstructure was still significant at 650 °C, but clearly decreased from its level at room temperature. The grain boundary strengthening and nanoparticle strengthening effects which are likely to be independent of test temperature sustain a reasonably high tensile yield strength of 800 MPa at 650 °C, despite the absence of γ′ and γ″ precipitates in the UFG microstructure.

Effect of Annealing on Microstructure, Grain Growth and Hardness of Nanocrystalline Cu-Zr Alloy Prepared by Cryogenic Ball Milling

Nanocrystalline Cu-0.75 at.%Zr alloy was synthesized by high energy ball milling under cryogenic temperature. To investigate the influence of 0.75 at.%Zr addition on thermal stabilization of nanocrystalline state of Copper, milled powder was annealed up to T/Tm = 0.79 for 1h in an inert atmosphere. The microstructural changes of both milled and annealed powders were characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Mechanical properties were determined in terms of hardness. It was found that addition of 0.75 at.%Zr can stabilize grain size at higher temperature, i.e., ~ 32 nm at 800oC (T/Tm = 0.79). The hardness of Cu-0.75 at.%Zr at 800oC was found to decrease by only ~ 13% as opposed to a 65% decrease in pure copper from cryomilled condition. The thermal stability of Cu-0.75 at.%Zr system at high temperatures was attributed to the kinetic stabilization, i.e., grain boundary pinning by intermetallic phases. Thermal stability contributions were assessed by thermodynamic models elicits added Zr is not sufficient for stabilization, rather kinetic stabilization (by intermetallic pinning of grain boundary) became active at higher annealing temperature.

Design of non-equiatomic high entropy alloys with heterogeneous lamella structure towards strength-ductility synergy

A non-equiatomic FeNiCoAlCrB (NCACB) high entropy alloy (HEA) with a heterogeneous lamella (HL) structure is fabricated through conventional thermomechanical processing. In the HL microstructure, fine-grain regions result from inhibited grain growth due to Zener pinning of boundaries by NiAl (B2) precipitates, while coarse-grained regions originate from grain growth within large deformation bands in the absence of precipitates. A back-stress strengthening mechanism, unique to deformation of heterogeneous microstructures, is verified through mechanical behavior and electron backscatter diffraction enabled geometrically necessary dislocation density analysis. This mechanism gives rise to the combination of both high strength and high ductility in HL-NCACB-HEA.

High temperature microstructural stability and recrystallization mechanisms in 14YWT alloys

In-situ neutron diffraction experiments were performed on room temperature compressed 14YWT nanostructured ferritic alloys at 1100 °C and 1150 °C to understand their thermally activated static recrystallization mechanisms. Existence of a high density of Y-Ti-O rich nano-oxides (<5 nm) shift the recrystallization temperature up due to Zener pinning of the grain boundaries, making these materials attractive for high temperature applications. This study serves to quantify the texture evolution in-situ and understand the effect of particles on the recrystallization mechanisms in 14YWT alloys. We have shown, both experimentally and theoretically, that there is considerable recovery in the 20% compressed sample after 6.5 h annealing at 1100 °C while recrystallization occurs within an hour of annealing at 1100 °C and 1150 °C in the 60% compressed samples. Moreover, the 60% compressed samples show {112}<110> and {112}<111> texture components during annealing, in contrast to the conventional recrystallization textures in body centered cubic alloys. Furthermore, nano-oxide size, shape, density and distribution are considerably different in unrecrystallized and abnormally grown grains. Transmission electron microscopy analysis shows that oxide particles having a size between 5 and 30 nm play a critical role for recrystallization mechanisms in 14YWT nanostructured ferritic alloys.

The cause of ‘weak-link’ grain boundary behaviour in polycrystalline Bi2Sr2CaCu2O8 and Bi2Sr2Ca2Cu3O10 superconductors

The detrimental effects of grain boundaries have long been considered responsible for the low critical current densities in high temperature superconductors. In this paper, we apply the quantitative approach used to identify the cause of the ‘weak-link’ grain boundary behaviour in YBa2Cu3O7 (Wang et al 2017 Supercond. Sci Technol. 30 104001), to the Bi2Sr2CaCu2O8 and Bi2Sr2Ca2Cu3O10 materials that we have fabricated. Magnetic and transport measurements are used to characterise the grain and grain boundary properties of micro- and nanocrystalline materials. Magnetisation measurements on all nanocrystalline materials show non-Bean-like behaviour and are consistent with surface pinning. Bi2Sr2CaCu2O8: our microcrystalline material has very low grain boundary resistivity which is similar to that of the grains such that (assuming a grain boundary thickness (d) of 1 nm) equivalent to an areal resistivity of The transport values are consistent with well-connected grains and very weak grain boundary pinning. However, unlike low temperature superconductors (LTS) in which decreasing grain size increases the pinning along the grain boundary channels, any increase in pinning produced by making the grains in our Bi2Sr2CaCu2O8 materials nanocrystalline was completely offset by a decrease in the depairing current density of the grain boundaries caused by their high resistivity. We suggest a different approach to increasing from that used in LTS materials, namely incorporating additional strong grain and grain boundary pinning sites in microcrystalline materials to produce high values. Bi2Sr2Ca2Cu3O10: both our micro- and nanocrystalline samples have of at least 103. This causes strong suppression of across the grain boundaries, which explains the low transport values we find experimentally. Our calculations show that low in untextured polycrystalline Bi2Sr2Ca2Cu3O10 material is to be expected and the significant effort in the community in texturing samples and removing grain boundaries altogether is well-founded.

Impression creep properties of hypoeutectic Al-Si alloys with scandium additions

The present work pertains to an improvement of high temperature stability of age hardenable hypoeutectic Al-Si alloy. Addition of scandium (Sc) was attempted to modify the microstructure and to improve the high temperature stability of the hypoeutectic Al-Si alloy. The high temperature stability was determined by impression creep techniques. The tests were performed at temperature 250°C and applied stresses in the range of 160 to 200 MPa for dwell times up to 3000 seconds. The results showed that the creep strength of the hypoeutectic Al-Si alloys was improved. For all loads, hypoeutectic Al-Si alloy with 0.4 wt% Sc had the lowest creep rates and thus the highest creep resistance among all materials tested. This may be attributed to the possible microstructural changes, solute enrichment of the matrix and pinning of the grain boundaries by the fine precipitates.

Phase stability and magnetic performance of nanocrystalline Sm-Co supersaturated solid solution

A new type of Sm-Co supersaturated solid solution has been developed with a stabilized TbCu7-type structure. The high-Co single phases were stabilized in a wide temperature range in the prepared SmCo9.8− x Si x ( x =0–0.9) alloys by concurrent nanostructuring and Si-doping. The significant enhancement of the coercivity and remanence of the new type supersaturated solid solution are attributed to the effective pinning of the ultrafine nanograin boundaries and the high magnetocrystalline anisotropy of the solid solution. It was proposed that the high-temperature magnetic properties of the Sm-Co supersaturated solid solution depend strongly on the phase stability.

Effect of SiC nanoparticles addition on the microstructures and mechanical properties of ECAPed Mg9Al–1Si alloy

Abstract

Synergistic effect of carbon nanotube as sintering aid and toughening agent in spark plasma sintered molybdenum disilicide-hafnium carbide composite

Hafnium carbide (HfC) along with sintering aids was consolidated at a relatively lower temperature i.e. 1600°C (i.e. T=~0.41Tm) under a uniaxial load of 50MPa by spark plasma sintering. Two different sintering aids such as molybdenum disilicide (MoSi2) and carbon nanotube (CNT) were added to enhance the densification and lower the extent of grain growth in the sintered pellets. Density of the sintered pellet increased from 96.0±0.8% in HfC +5wt% MoSi2 (HM) to 99.0±0.5% with the addition of 2wt% CNT in HfC+5wt% MoSi2 (HMC) at sintering temperature of 1600°C. Further, the extent of grain growth drastically reduced from 204% in HM to 50% in HMC. Analysis of linear shrinkage during densification revealed that CNT addition increased densification rate and decreased the time required to reach the density of 99.0±0.5% at 1600°C. Increased densification and lower degree of grain growth could be due to the synergistic effect offered by the CNT, which are as follows: (i) Lubrication effect of CNT, (ii) Lower activation energy for grain boundary diffusion (iii) Reduction in liquid phase sintering temperature and (iv) Grain boundary pinning. Fracture toughness of the sintered HM and HMC composite was obtained using indentation technique. By the addition of 2wt% CNT in HM, drastic increase of 91% in fracture toughness was seen. This significant improvement in fracture toughness was due to the enhanced densification and relatively lower grain size of HMC. Also crack bridging, crack deflection, crack arrest, CNT and graphene sheet pull-out and swording played major role in toughening of HMC pellet.

Reactive spark plasma sintering of MgB2 in nitrogen atmosphere for the enhancement of the high-field critical current density

High density bulks (97%–99%) of MgB2 were prepared by spark plasma sintering (SPS) in nitrogen (N2) atmosphere for different heating rates (10, 20 and 100 °C min−1) and compared with reference samples processed in vacuum and Ar. N2 reacts with MgB2 and forms MgB9N along the MgB2 grain boundaries. The high-field critical current density is enhanced for the sample processed in N2 with a heating rate of 100 °C min−1. At 2–35 K, this sample shows the strongest contribution of the grain boundary pinning (GBP). All samples are in the point pinning (PP) limit and by increasing temperature the GBP contribution decreases.

Synthesis and Sintering of LaCo&lt;sub&gt;1-X&lt;/sub&gt;Fe&lt;sub&gt;x&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt; Ceramics: Microstructure Analysis

This work intends to show the effect of the iron ion doping in LaCoO3 perovskite, both in powders and in sintered samples obtained from combustion reaction route. The phase formation and particle morphology and particle size distribution of the powders were analysed by XRD, SEM and sedimentation techniques, respectively. Relative density, microstructure (secondary phases and grain size) and pore size distribution of LaCo1-xFexO3 sintered ceramics have been investigated by SEM/EDS and Hg porosimetry analysis. Although LaCo1-xFexO3 powders obtained from combustion reaction exhibited smaller grain sizes when sintered at high temperatures, they showed higher volume fraction of secondary phases. The presence of these crystalline phases in addition to the desired perovskite affected the microstructure acting as grain growth inhibitors by grain boundary pinning. It is believed by observing three grain junction pores that LaFeO3 phase has a smaller dihedral angle than LaCoO3. This fact would explain why LaFeO3 presented a smaller driving force for sintering with higher tendency of pore and inclusion coarsening at higher temperatures (1400°C).

Effect of hypoeutectic boron modification on the dynamic properties of Ti-6Al-4V alloy

Abstract

Evolution of microstructure and texture during thermo-mechanical processing of a two phase Al0.5CoCrFeMnNi high entropy alloy

The evolution of microstructure and texture during thermo-mechanical processing of two phase (FCC+BCC) Al0.5CoCrFeMnNi was investigated. For this purpose, the fully recrystallized starting material having 17% of the BCC phase was warm-rolled to 75% reduction in thickness and annealed at temperatures ranging from 1000°C to 1250°C. The volume fractions of the two phases remained unaltered during warm-rolling. Development of a lamellar deformation structure was observed for the FCC phase which also showed presence of typical deformation components. In contrast, the BCC phase showed mechanical fragmentation indicating limited ductility and a deformation texture characterized by RD (//〈110〉) and ND (//〈111〉) fibers. Development of a finer recrystallized microstructure was observed after annealing at 1000°C due to the grain boundary pinning exerted by the BCC phase, thus inhibiting grain growth. The BCC phase fraction decreased consistently with increasing annealing temperature. Considerable grain growth happened after annealing at 1250°C due to the decrease in the BCC fraction, which greatly diminished the grain boundary pinning. The BCC phase also showed stronger ND-fiber in the primary recrystallization texture, which weakened with increasing annealing temperature due to the dissolution of this phase. Retention of the deformation texture components in the recrystallization texture of the FCC phase indicated absence of strong preferential nucleation or growth.

Low-Temperature Solution Synthesis of Few-Layer 1T '-MoTe2 Nanostructures Exhibiting Lattice Compression.

Molybdenum ditelluride, MoTe2 , is emerging as an important transition-metal dichalcogenide (TMD) material because of its favorable properties relative to other TMDs. The 1T ' polymorph of MoTe2 is particularly interesting because it is semimetallic with bands that overlap near the Fermi level, but semiconducting 2H-MoTe2 is more stable and therefore more accessible synthetically. Metastable 1T '-MoTe2 forms directly in solution at 300 °C as uniform colloidal nanostructures that consist of few-layer nanosheets, which appear to exhibit an approx. 1 % lateral lattice compression relative to the bulk analogue. Density functional theory calculations suggest that small grain sizes and polycrystallinity stabilize the 1T ' phase in the MoTe2 nanostructures and suppress its transformation back to the more stable 2H polymorph through grain boundary pinning. Raman spectra of the 1T '-MoTe2 nanostructures exhibit a laser energy dependence, which could be caused by electronic transitions.

Low‐Temperature Solution Synthesis of Few‐Layer 1T ′‐MoTe2 Nanostructures Exhibiting Lattice Compression

AbstractMolybdenum ditelluride, MoTe2, is emerging as an important transition‐metal dichalcogenide (TMD) material because of its favorable properties relative to other TMDs. The 1T ′ polymorph of MoTe2 is particularly interesting because it is semimetallic with bands that overlap near the Fermi level, but semiconducting 2H‐MoTe2 is more stable and therefore more accessible synthetically. Metastable 1T ′‐MoTe2 forms directly in solution at 300 °C as uniform colloidal nanostructures that consist of few‐layer nanosheets, which appear to exhibit an approx. 1 % lateral lattice compression relative to the bulk analogue. Density functional theory calculations suggest that small grain sizes and polycrystallinity stabilize the 1T ′ phase in the MoTe2 nanostructures and suppress its transformation back to the more stable 2H polymorph through grain boundary pinning. Raman spectra of the 1T ′‐MoTe2 nanostructures exhibit a laser energy dependence, which could be caused by electronic transitions.

Effect of intervalence charge transfer on photocatalytic performance of cobalt- and vanadium-codoped TiO2 thin films

Co- and V-codoped TiO2 thin films were prepared by sol–gel, spin coated on glass substrates, and annealed at 450 °C for 2 h. The data suggest substitutional solid solution formation in the anatase films. Macroscopic roughening at the highest dopant levels was attributed to oversaturation, precipitation, and grain boundary pinning. Methylene blue decomposition correlated directly with the grain size and this was attributed to the associated changes in surface area and number of active sites as well as partial amorphisation.X-ray photoelectron spectroscopy data revealed the presence of Ti4+, Co2+, possibly Co3+, V3+, and V4+, where the three clearly detected dopant valences were present as thermodynamically non-equilibrium valences under the annealing conditions used. The present work introduces the concept of intervalence charge transfer to explain the presence of these non-equilibrium species. The present work also considers briefly the potential roles of crystal field stabilization and defect equilibria relative to intervalence charge transfer.

An Experimental Study on Thermal Stability of Age-Hardenable Aluminium Alloys Modified by Scandium Inoculation

The Al-Zn-Mg system is a familiar age-hardenable 7xxx series of aluminium alloy. Aluminium alloys are gaining wide popularity in aeronautical, automotive, and transportation industries. Scandium (Sc) has the ability to refine grain size of cast aluminium structure. It has been possible to achieve an ideal combination of strength, density, and thermal stability because of the unique age-hardening characteristics of Sc. Moreover, low solid solubility of Sc in aluminium is responsible for the improvement of the microstructure and mechanical properties when added in small amounts (≤0.6 wt.%). Further, inoculation is an effective means of grain refinement in liquid state of as-castaluminium alloys. So, density of GP zones formation and early stage of ageing effects assessment main priority in the present work. However, coherent precipitates like ScAl3are finely dispersed to provide thermal stability by increasing recrystallization temperature. Hence, the improvement in the high temperature stability of aluminium alloys (7xxx series) may be attributed to the grain boundary pinning (e.g. Zenerdrag mechanism) by the fine precipitates.In this paper, the relationship between the mechanical behavior and microstructure characteristics of Al-Zn-Mg-Sc based alloys are investigated to understand the thermal stability mechanism of grain refinement and dispersive precipitation.

Thermodynamic evidence for the Bose glass transition in twinnedYBa2Cu3O7−δcrystals

We used a micromechanical torsional oscillator to measure the magnetic response of a twinned YBa$_2$Cu$_3$O$_{7-\delta}$ single crystal disk near the Bose glass transition. We observe an anomaly in the temperature dependence of the magnetization consistent with the appearance of a magnetic shielding perpendicular to the correlated pinning of the twin boundaries. This effect is related to the thermodynamic transition from the vortex liquid phase to a Bose glass state.

GeO2-added MgB2 superconductor obtained by Spark Plasma Sintering

Dense samples (relative density > 93%) of bulk MgB2 with GeO2 additions were obtained by Spark Plasma Sintering. The critical current density Jc of the added samples is improved at high magnetic fields when compared to the pristine sample. The optimum composition is for MgB2(GeO2)0.005. For this sample, a Jc(20 K) = 102 A/cm2 is obtained at 5.1 T versus 3.9 T for the pristine sample. Ge substitution in the crystal lattice of MgB2 can be considered negligible, and Tc,onset and Tc,midpoint from magnetization measurements scatter within 0.2 and 0.6 K, respectively. TEM investigations show some specific details at nano scale: the tendency to form secondary phases (25–100 nm) with sphere-like or irregular shapes is observed and discussed. Samples are composites and the residual strain of MgB2 is constant for pristine and GeO2-added samples. Therefore, pinning enhancement leading to improvement of Jc for the GeO2 added samples is purely a ‘microstructure’ effect due to the presence of secondary phases. The point pinning is determined to be the predominant mechanism. Addition of a higher amount of GeO2 is shifting the pinning mechanism toward a grain boundary pinning.