Potassium in Grain Boundaries of Tungsten

1993 ◽  
Vol 322 ◽  
Author(s):  
Milan R. Vukcevich
Keyword(s):  
Mechanical Properties ◽  
Grain Boundaries ◽  
Recent Work ◽  
Yield Point ◽  
Tungsten Wire ◽  
Recrystallization Process ◽  
Main Attention ◽  
Theory And Experiments ◽  
Good Agreement

AbstractMost of the recent work on the role of potassium in tungsten wire deals with the origin of bubbles and their effect on the recrystallization process. Two very important areas are neglected: the effect of potassium strings on the mechanical properties of wire during manufacturing, and the differences in distribution of bubbles in the grain boundaries vs. the bulk. Although other advances in our understanding of potassium in tungsten are reviewed, the main attention is on these two neglected areas. Derived are expressions for the yield point of the fibrous wire structure, and for the increased concentration of bubbles in the transverse boundaries. The theory and experiments are in good agreement.

scholarly journals On the Role of Grain Boundaries During Sintering

2021 ◽  
Author(s):  
Torsten Staab ◽  
Ricardo Helm ◽  
Andreas Diegeler
Keyword(s):  
Mass Transport ◽  
Grain Boundaries ◽  
Average Distance ◽  
Intermediate Stage ◽  
Sintering Process ◽  
Positron Annihilation Lifetime ◽  
Boundary Signal ◽  
Increasing Temperature ◽  
Good Agreement

We present new results in positron annihilation lifetime spectroscopy (PALS), thermo-optical dilatometry and microscopy, which are indicating a strong correlation between grain-boundaries and mass transport during the sintering process of carbonyl iron powder. In this particular system we were able to show that the change in particle shape and size with increasing temperature yields an anisotropy in shrinkage, which manifests itself in a higher shrinkage perpendicular to the compaction axis. In the intermediate stage of sintering, where the major mass transport occurs, the average distance between two grain boundaries could be determined to (3,73 ± 0,18) μm at T = 744°C. This is in good agreement with previous calculations of positron pathways in defect free particles. Furthermore, due to sintering temperatures far above the annealing temperature of dislocations in iron, the existence of dislocations in the bulk of the particles is very unlikely. These claims are reflected by the collected positron data, which exhibit a clear grain boundary signal of ∼ 250ps while no vacancy or dislocation signal (typically ∼ 160 ps) is evident in the intermediate stage of sintering.

The Critical Heat Flux in Nucleate Boiling Heat Transfer: Part I—The Chemical Actuator Mechanism

2009 ◽  
Author(s):  
M. R. Reda
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Phase Change ◽  
Grain Boundaries ◽  
Recent Work ◽  
Boiling Heat Transfer ◽  
Nucleate Boiling ◽  
Passive Layer ◽  
Nucleate Boiling Heat Transfer ◽  
Good Agreement

Nucleate boiling heat transfer is first introduced and the literature is reviewed. It was concluded that the passive layer and the grain boundaries are responsible for the transfer to the nucleate boiling regime. Based on the recent work of Biener and his collaborators (Nature Material 2008) and the Gibbs rule of thermodynamics, a possible mechanism was outlined. The mechanism assumes that each grain in the passive layer act as a chemical actuator which is driven by microstructure phase change. The new mechanism agrees well with the experimental results, in good agreement with previous models and can explain why and how CHF occurs.

The synergistic effect of dope concentration and jetdrawing on structure development of wet-spun poly(acrylonitrile)

e-Polymers ◽  
2008 ◽  
Vol 8 (1) ◽  
Author(s):  
Shahram Arbab ◽  
Naser Mohammadi ◽  
Parviz Noorpanah
Keyword(s):  
Mechanical Properties ◽  
Synergistic Effect ◽  
Polymer Concentration ◽  
Total Porosity ◽  
Griffith’S Theory ◽  
Acrylic Fibers ◽  
Poly Acrylonitrile ◽  
Remarkable Reduction ◽  
Good Agreement

AbstractAcrylic fibers are the major precursor of carbon fibers. In this work, the synergistic effect of polymer concentration and jet stretching on porosity, morphology and mechanical properties of wet-spun poly(acrylonitrile) fibers were studied and then the role of void size was elucidated. The results showed that the effect of jet stretching on fiber porosity is negligible below a threshold polymer concentration. Increasing polymer concentration up to 20 vol. % reduced the total porosity significantly but increased the nanometric void population slightly. On the other hand increasing polymer concentration and jet stretching simultaneously, improved mechanical properties as a result of a remarkable reduction in porosity. In addition, stretching shifted nanovoids distribution strongly toward smaller sizes. This was attributed to the effect of stretching and increasing polymer concentration on reducing the diffusion coefficient. Young's modulus of fibers also increased with decreasing overall porosity. Finally, strength-diameter correlation showed a good agreement with the Griffith's theory.

A study on the mechanical properties of graphene oxide/epoxy nanocomposites

2021 ◽  
pp. 096739112110111
Author(s):  
Rahim Eqra ◽  
Mohammad Hadi Moghim ◽  
Navid Eqra
Keyword(s):  
Mechanical Properties ◽  
Tensile Strength ◽  
Graphene Oxide ◽  
Solution Casting ◽  
Casting Method ◽  
Epoxy Nanocomposites ◽  
Epoxy Nanocomposite ◽  
Solution Casting Method ◽  
Good Agreement

The aims of this research are to elucidate the role of graphene oxide on the mechanical properties of epoxy and also to obtain an equation for the modeling of its behavior. Accordingly, graphene oxide/epoxy nanocomposite samples are fabricated using the solution casting method. Tensile, flexural, SEM and FTIR tests are conducted on epoxy and the nanocomposite samples afterwards. The obtained results show that the tensile strength of epoxy improves even at low contents of graphene oxide such that 0.3 wt.% of GO yields an improvement of approximately 11.5%. The flexural strength of epoxy is also enhanced by 5.8% with 0.5 wt.% GO. Then, it decreases due to the agglomeration with increasing the GO content. In order to predict the tensile strength of GO/epoxy nanocomposites, a modified Halpin–Tsai equation is obtained with a new introduced correction factor as K = 39.5 Vf 1.135exp(2.9−1644.6 Vf). The obtained equation is in good agreement with the experimental data.

Calculations of Cu L2,3 fine structure at grain boundaries

1996 ◽  
Vol 54 ◽  
pp. 528-529
Author(s):  
P. Rez ◽  
J.M. Maclaren
Keyword(s):  
Mechanical Properties ◽  
Fine Structure ◽  
Energy Loss ◽  
Grain Boundaries ◽  
Recent Work ◽  
Electronic States ◽  
White Line ◽  
Small Probe ◽  
Metal Atoms ◽  
Segregation Of Impurities

The segregation of impurities to grain boundaries in metals and alloys has been known for some time to make changes in ductility. Examples of this effect are the embrittlement of copper by the addition of bismuth and the ductilization of Ni3Al by boron impurities. The mechanism by which these dramatic changes in mechanical properties arise is still largely unknown. It has been suggested that embrittling elements draw charge from neighbouring metal atoms while impurities that enhance ductilty act in the opposite way. Changes in the electronic states can be detected as changes in the energy loss spectrum when a small probe in a FEG STEM is moved across the boundary. Recent work by Muller has shown significant differences between the Ni L3 spectrum from grain boundaries in Ni3Al with and without boron. Bruley has shown that a “white line”, indicative of empty Cu d states, appears in the Cu L3 edge from Cu atoms near boundaries where bismuth has segregated.

Competitive Role of Impurities on the Electrical Activity of as-grown Σ=13, Σ=25 and Deformed Σ=9 Grain boundaries in p-type Silicon Bi-crystals

2012 ◽  
Vol 1474 ◽  
Author(s):  
S. Gaiaschi ◽  
A. El Kouadri Boudjelthia ◽  
G. Regula ◽  
N. Burle ◽  
A. Mesli ◽  
...  
Keyword(s):  
Grain Boundary ◽  
Grain Boundaries ◽  
Before And After ◽  
Electron Beam Induced Current ◽  
Gold Diffusion ◽  
P Type ◽  
Czochralski Process ◽  
Probe Measurements ◽  
Good Agreement

ABSTRACTElectrical properties of grain boundaries grown by Czochralski process were studied by microwave phase shift (μW-PS) and electron beam induced current (EBIC), before and after gold diffusion at 700°C. As-grown samples had similar doping levels determined by four-point probe measurements but somewhat different oxygen concentrations, obtained by Fourier transform infrared spectroscopy (FTIR). It is shown that the increase of the grain boundary activity due to Au gathering at this planar defect can be hindered by native impurities (likely oxygen). EBIC and μW-PS techniques gave respectively electron diffusion lengths and lifetime values, both in good agreement. EBIC images on deformed Σ =9 showed that extrinsic dislocations do not activate the grain boundary at 300K.

Lattice Imaging of Grain Boundaries in Ceramics

1977 ◽  
Vol 35 ◽  
pp. 114-115
Author(s):  
D. R. Clarke ◽  
G. Thomas
Keyword(s):  
Mechanical Properties ◽  
High Temperature ◽  
Silicon Nitride ◽  
Grain Boundaries ◽  
High Temperature Strength ◽  
Current Interest ◽  
Lattice Imaging ◽  
Special Significance ◽  
Structural Applications ◽  
Refractory Ceramics

Grain boundaries have long held a special significance to ceramicists. In part, this has been because it has been impossible until now to actually observe the boundaries themselves. Just as important, however, is the fact that the grain boundaries and their environs have a determing influence on both the mechanisms by which powder compaction occurs during fabrication, and on the overall mechanical properties of the material. One area where the grain boundary plays a particularly important role is in the high temperature strength of hot-pressed ceramics. This is a subject of current interest as extensive efforts are being made to develop ceramics, such as silicon nitride alloys, for high temperature structural applications. In this presentation we describe how the techniques of lattice fringe imaging have made it possible to study the grain boundaries in a number of refractory ceramics, and illustrate some of the findings.

Elucidation of block boundaries as the dissolution channels in hydrated cement

1978 ◽  
Vol 36 (1) ◽  
pp. 484-485
Author(s):  
N.V. Belov ◽  
U.I. Papiashwili ◽  
B.E. Yudovich
Keyword(s):  
Liquid Phase ◽  
Grain Boundaries ◽  
Benzoyl Peroxide ◽  
Phase Contrast ◽  
Active Role ◽  
Point Of View ◽  
Hardened Cement ◽  
Hydrated Cement ◽  
Hardened Cement Paste

It has been almost universally adopted that dissolution of solids proceeds with development of uniform, continuous frontiers of reaction.However this point of view is doubtful / 1 /. E.g. we have proved the active role of the block (grain) boundaries in the main phases of cement, these boundaries being the areas of hydrate phases' nucleation / 2 /. It has brought to the supposition that the dissolution frontier of cement particles in water is discrete. It seems also probable that the dissolution proceeds through the channels, which serve both for the liquid phase movement and for the drainage of the incongruant solution products. These channels can be appeared along the block boundaries.In order to demonsrate it, we have offered the method of phase-contrast impregnation of the hardened cement paste with the solution of methyl metacrylahe and benzoyl peroxide. The viscosity of this solution is equal to that of water.

Surface reconstructions of (001) cleaved GdBa2Cu3O7-δ

1992 ◽  
Vol 50 (1) ◽  
pp. 106-107
Author(s):  
H.W. Zandbergen ◽  
M.R. McCartney
Keyword(s):  
Electron Microscopy ◽  
Copper Oxide ◽  
Grain Boundaries ◽  
Atomic Structure ◽  
Oxygen Deficiency ◽  
Surface Layers ◽  
Layer Sequence ◽  
Surface Reconstructions ◽  
Good Agreement

Very few electron microscopy papers have been published on the atomic structure of the copper oxide based superconductor surfaces. Zandbergen et al. have reported that the surface of YBa2Cu3O7-δ was such that the terminating layer sequence is bulk-Y-CuO2-BaO-CuO-BaO, whereas the interruption at the grain boundaries is bulk-Y-CuO2-BaO-CuO. Bursill et al. reported that HREM images of the termination at the surface are in good agreement with calculated images with the same layer sequence as observed by Zandbergen et al. but with some oxygen deficiency in the two surface layers. In both studies only one or a few surfaces were studied.

Carbide precipitation and chromium depletion on coherent twin boundaries in deformed 304 stainless steels

1992 ◽  
Vol 50 (2) ◽  
pp. 1216-1217
Author(s):  
A.H. Advani ◽  
L.E. Murr ◽  
D.J. Matlock ◽  
W.W. Fisher ◽  
P.M. Tarin ◽  
...  
Keyword(s):  
Grain Boundaries ◽  
Stainless Steels ◽  
Interfacial Free Energy ◽  
Carbide Precipitation ◽  
Twin Boundaries ◽  
Invariant Structure ◽  
Heat Treated ◽  
Constant Structure ◽  
Cr Depletion

Coherent annealing-twin boundaries are constant structure and energy interfaces with an average interfacial free energy of ∼19mJ/m2 versus ∼210 and ∼835mJ/m2 for incoherent twins and “regular” grain boundaries respectively in 304 stainless steels (SS). Due to their low energy, coherent twins form carbides about a factor of 100 slower than grain boundaries, and limited work has also shown differences in Cr-depletion (sensitization) between twin versus grain boundaries. Plastic deformation, may, however, alter the kinetics and thermodynamics of twin-sensitization which is not well understood. The objective of this work was to understand the mechanisms of carbide precipitation and Cr-depletion on coherent twin boundaries in deformed SS. The research is directed toward using this invariant structure and energy interface to understand and model the role of interfacial characteristics on deformation-induced sensitization in SS. Carbides and Cr-depletion were examined on a 20%-strain, 0.051%C-304SS, heat treated to 625°C-4.5h, as described elsewhere.

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