The effect of NbC porosity on reaction-layer microstructure in NbC|Si diffusion couples

2000 ◽  
Vol 15 (1) ◽  
pp. 248-252 ◽  
Author(s):  
J. Woodford ◽  
C-Y. Yang ◽  
Y. A. Chang
Keyword(s):  
Reaction Layer ◽  
Powder Compact ◽  
Particle Morphology ◽  
Reactive Diffusion ◽  
High Porosity ◽  
Diffusion Couples ◽  
Two Phase ◽  
Nucleation Sites ◽  
The Matrix ◽  
Sic Particle

Further experimental observations have allowed us to refine and confirm some aspects of our recently proposed mechanism for reactive diffusion between Si single crystal and NbC powder compact, particularly regarding the prediction of Si as the dominant diffusing species and the nature of the dependence of SiC particle morphology on the presence of voids in the NbC end member. In Si|NbC diffusion couples annealed at either 1300 or 1350 °C, a two-phase NbSi2 + SiC reaction layer formed. Although NbSi2 was the matrix in all of the reaction layers, the SiC phase morphology depended upon NbC porosity: when high-porosity NbC was used, SiC was present as discontinuous particles greater than 1-μm-across, while when low-porosity or void-free NbC was used, SiC grew cooperatively with NbSi2 in the form of lamellae less than 0.5 μm thick. We propose that this difference arises from the effect of voids both as nucleation sites for SiC particles and as channels for unrestricted SiC growth. Marker experiments conclusively show that Si is the dominant diffusing species in the reaction layer.

A mechanism for reactive diffusion between Si single crystal and NbC powder compact

1996 ◽  
Vol 11 (4) ◽  
pp. 850-854 ◽  
Author(s):  
C. R. Kao ◽  
J. Woodford ◽  
Y. A. Chang
Keyword(s):  
Single Crystal ◽  
Reaction Layer ◽  
Powder Compact ◽  
Reactive Diffusion ◽  
Particle Sizes ◽  
Diffusion Couples ◽  
Two Phase ◽  
Grain Sizes ◽  
The Matrix ◽  
Sic Particle

Based on our recent experimental observations, a growth mechanism for the reactive diffusion between Si single crystal and NbC powder compact is proposed. In Si–NbC diffusion couples annealed at 1300 °C, a two-phase NbSi2 + SiC reaction layer formed with NbSi2 as the matrix and SiC as discontinuous particles. The NbSi2 grain sizes and SiC particle sizes are both in the μm range. We propose that the SiC particles nucleated at the void surfaces in the NbC powder compact. This proposed nucleation mechanism offers a potential way of controlling the SiC particle size by changing the void size and void density of the NbC powder compact. It is also pointed out that this microstructure requires Si to be the dominant diffusing species. Si must diffuse through the reaction layer, while C only has to undergo local rearrangement, and Nb need not diffuse at all.

Consequences of viscous anisotropy in a deforming, two-phase aggregate. Part 1. Governing equations and linearized analysis

2013 ◽  
Vol 734 ◽  
pp. 424-455 ◽  
Author(s):  
Yasuko Takei ◽  
Richard F. Katz
Keyword(s):  
Shear Stress ◽  
Numerical Solutions ◽  
Shear Plane ◽  
High Porosity ◽  
Governing Equations ◽  
Two Phase ◽  
The Matrix ◽  
Continuum Scale ◽  
Torsional Flow ◽  
Matrix Shear

AbstractIn partially molten regions of Earth, rock and magma coexist as a two-phase aggregate in which the solid grains of rock form a viscously deformable framework or matrix. Liquid magma resides within the permeable network of pores between grains. Deviatoric stress causes the distribution of contact area between solid grains to become anisotropic; in turn, this causes anisotropy of the matrix viscosity at the continuum scale. In this two-paper set, we predict the consequences of viscous anisotropy for flow of two-phase aggregates in three configurations: simple shear, Poiseuille, and torsional flow. Part 1 presents the governing equations and an analysis of their linearized form. Part 2 (Katz & Takei, J. Fluid Mech., vol. 734, 2013, pp. 456–485) presents numerical solutions of the full, nonlinear model. In our theory, the anisotropic viscosity tensor couples shear and volumetric components of the matrix stress/strain rate. This coupling, acting over a gradient in shear stress, causes segregation of liquid and solid. Liquid typically migrates toward higher shear stress, but under specific conditions, the opposite can occur. Furthermore, it is known that in a two-phase aggregate with a porosity-weakening viscosity, matrix shear causes porosity perturbations to grow into a banded or sheeted structure. We show that viscous anisotropy reduces the angle between these emergent high-porosity features and the shear plane. Laboratory experiments produce similar, high-porosity features. We hypothesize that the low angle of porosity bands in such experiments is the result of viscous anisotropy. We therefore predict that experiments incorporating a gradient in shear stress will develop sample-wide liquid–solid segregation due to viscous anisotropy.

Influence of Fabricated Process on Mechanical Properties of Porous SiC-Particle/Si3N4 Composites

2007 ◽  
Vol 336-338 ◽  
pp. 1320-1323 ◽  
Author(s):  
Hong Jie Wang ◽  
Wen Zhang ◽  
Yu Bai ◽  
Guan Jun Qiao ◽  
Ji Qiang Gao ◽  
...  
Keyword(s):  
Mechanical Properties ◽  
Flexural Strength ◽  
Powder Compact ◽  
High Strength ◽  
Uniform Structure ◽  
High Porosity ◽  
Gel Casting ◽  
N2 Atmosphere ◽  
Porous Sic ◽  
Sic Particle

In this paper, the mechanical properties of porous Si3N4/SiC composite prepared by different forming method (uniaxial pressing and gel-casting) were compared. Using gelcasting and two steps sintering technology, the porous SiC-particle/Si3N4 composites with a high strength, uniform structure and a relative high porosity were obtained by adding a little amount of nano carbon in slurry. The flexural strength of optimized material reaches to 100MPa and its porosity is more than 60%. But using uniaxial pressing and sintering at N2 atmosphere, because the green bodies have the high density, and the density distribution of a powder compact was not uniform, the flexural strength is not high, and at the same time, the microstructure is not uniform also.

The transition between solitary wave and diapir emergence from a high porosity disturbance in two-phase flow

2020 ◽  
Author(s):  
Janik Dohmen ◽  
Harro Schmeling
Keyword(s):  
Solitary Wave ◽  
Constant Velocity ◽  
Strong Influence ◽  
Wave Dispersion ◽  
High Porosity ◽  
Initial Radius ◽  
Two Phase ◽  
The Earth ◽  
Porosity Wave ◽  
The Matrix

<p>Many processes in the earth involve the melting of rocks and the percolation of the produced melt through the residuum. These processes have been extensively studied but there is still much left what is not completely understood. In this work we focus on the emergence of solitary porosity waves, which can emerge from disturbances in regions where melt is allowed to percolate relatively to the matrix. These waves are regions of higher melt fractions that ascend with a constant velocity while not changing their shape during this ascending process. The size of these waves depends on the compaction length, which depends on just poorly known parameters such as the permeability and the viscosity of the matrix. As they can vary over several orders of magnitudes it might have a strong influence on porosity waves and their emergence from local disturbances with higher porosities than the background.</p><p>In this work we start with a 2D Gaussian-bell shaped disturbance with a certain porosity amplitude and vary the initial radius which is non-dimensionized by the characteristic compaction length. For some cases this disturbance results in an ascending solitary wave and for others it rises upwards as a diapir. For a few cases a kind of fingering can be observed which  looks like a small emerging porosity wave which is just slightly faster than the following melt of the initial larger disturbance. This leads to a melt ascent with a strongly focused front.</p><p>Comparison of porosity wave dispersion curves with analytical ascent rates of a Stokes sphere helps explaining this transition of diapirs to solitary waves via a melt ascent with a strongly focused front.</p>

Consequences of viscous anisotropy in a deforming, two-phase aggregate. Part 2. Numerical solutions of the full equations

2013 ◽  
Vol 734 ◽  
pp. 456-485 ◽  
Author(s):  
Richard F. Katz ◽  
Yasuko Takei
Keyword(s):  
Simple Shear ◽  
Poiseuille Flow ◽  
Numerical Solutions ◽  
High Porosity ◽  
Two Phase ◽  
Band Formation ◽  
Porosity Evolution ◽  
The Matrix ◽  
Continuum Scale ◽  
Torsional Flow

AbstractIn partially molten regions of Earth, rock and magma coexist as a two-phase aggregate in which the solid grains of rock form a viscously deformable framework or matrix. Liquid magma resides within the permeable network of pores between grains. Deviatoric stress causes the distribution of contact area between solid grains to become anisotropic; this, in turn, causes anisotropy of the matrix viscosity at the continuum scale. In the second of a two-paper set, we use numerical methods to solve the full, nonlinear, time-dependent equations governing this system. We consider porosity evolution in simple shear, Poiseuille and torsional flow. Under viscous anisotropy, there are two modes of porosity evolution: base-state segregation, which modifies the domain-scale porosity distribution, and growth of porosity perturbations into melt-rich bands. Simulation results with fixed anisotropy confirm and extend the linearized analysis of Part 1 (Takei & Katz, J. Fluid Mech., vol. 734, 2013, pp. 424–455). Most importantly, numerical solutions capture the interaction of the two modes: under Poiseuille flow, base-state segregation enhances band formation; under torsional flow, bands are suppressed. Simulations also show that low band angle is maintained by nonlinear processes such as reconnection of high-porosity segments and by back-rotation of the compacted regions between bands. Simulations with dynamic anisotropy modify these results, further lowering the average band angle. The effective viscosity of each flow is controlled by base-state segregation; it does not evolve under simple shear, decreases in Poiseuille flow and increases in torsion. We propose a reinterpretation of experimental results in terms of the consequences of viscous anisotropy.

Transition alpha structure in beta-isomorphous Nb-Hf alloys

1987 ◽  
Vol 45 ◽  
pp. 232-233
Author(s):  
R.W. Carpenter ◽  
Changhai Li ◽  
David J. Smith
Keyword(s):  
Solid Solution ◽  
Solution Phase ◽  
Miscibility Gap ◽  
Large Strains ◽  
Solid Solution Phase ◽  
Two Phase ◽  
Diffuse Intensity ◽  
Metastable Form ◽  
The Matrix ◽  
Equilibrium Morphology

Binary Nb-Hf alloys exhibit a wide bcc solid solution phase field at temperatures above the Hfα→ß transition (2023K) and a two phase bcc+hcp field at lower temperatures. The β solvus exhibits a small slope above about 1500K, suggesting the possible existence of a miscibility gap. An earlier investigation showed that two morphological forms of precipitate occur during the bcc→hcp transformation. The equilibrium morphology is rod-type with axes along <113> bcc. The crystallographic habit of the rod precipitate follows the Burgers relations: {110}||{0001}, <112> || <1010>. The earlier metastable form, transition α, occurs as thin discs with {100} habit. The {100} discs induce large strains in the matrix. Selected area diffraction examination of regions ∼2 microns in diameter containing many disc precipitates showed that, a diffuse intensity distribution whose symmetry resembled the distribution of equilibrium α Bragg spots was associated with the disc precipitate.

Using crystallographic symmetry in diffraction and contrast analysis

1989 ◽  
Vol 47 ◽  
pp. 504-505
Author(s):  
U. Dahmen ◽  
K.H. Westmacott
Keyword(s):  
Electron Microscopy ◽  
High Symmetry ◽  
Two Phase ◽  
Tem Analysis ◽  
Crystallographic Symmetry ◽  
The Matrix ◽  
Contrast Analysis ◽  
Practical Tool ◽  
Convergent Beam ◽  
Beam Diffraction

Despite the increased use of convergent beam diffraction, symmetry concepts in their more general form are not commonly applied as a practical tool in electron microscopy. Crystal symmetry provides an abundance of information that can be used to facilitate and improve the TEM analysis of crystalline solids. This paper draws attention to some aspects of symmetry that can be put to practical use in the analysis of structures and morphologies of two-phase materials.It has been shown that the symmetry of the matrix that relates different variants of a precipitate can be used to determine the axis of needle- or lath-shaped precipitates or the habit plane of plate-shaped precipitates. By tilting to a special high symmetry orientation of the matrix and by measuring angles between symmetry-related variants of the precipitate it is possible to find their habit from a single micrograph.

Fatigue behavior of precipitation strengthened Cu–Ni–Si alloy modified by Cr and Zr addition

2020 ◽  
Vol 11 (6) ◽  
pp. 861-873
Author(s):  
Ş. Hakan Atapek ◽  
Spiros Pantelakis ◽  
Şeyda Polat ◽  
Apostolos Chamos ◽  
Gülşah Aktaş Çelik
Keyword(s):  
Design Methodology ◽  
Fatigue Behavior ◽  
Fatigue Tests ◽  
Lower Stress ◽  
Content Type ◽  
Hardened Alloy ◽  
Zr Addition ◽  
Nucleation Sites ◽  
The Matrix ◽  
Lower Stress Level

Purpose The purpose of this paper is to investigate the fatigue behavior of precipitation-strengthened Cu‒2.55Ni‒0.55Si alloy, modified by the addition of 0.25 Cr and 0.25 Zr (wt%), using mechanical and fractographical studies to reveal the effect of microstructural features on the fracture. Design/methodology/approach For strengthening, cast and hot forged alloy was subjected to solution annealing at 900°C for 60 min, followed by quenching in water and then aging at 490°C for 180 min. Precipitation-hardened alloy was exposed to fatigue tests at R=−1 and different stress levels. All fracture surfaces were examined within the frame of fractographical analysis. Findings Fine Ni-rich silicides responsible for the precipitation strengthening were observed within the matrix and their interactions with the dislocations at lower stress level resulted in localized shearing and fine striations. Although, by the addition of Cr and Zr, the matrix consisted of hard Ni, Zr-rich and Cr-rich silicides, these precipitates adversely affected the fatigue behavior acting as nucleation sites for cracks. Originality/value These findings contribute to the present knowledge by revealing the effect of microstructural features on the mechanical behavior of precipitation-hardened Cu‒Ni‒Si alloy modified by Cr and Zr addition.

Microstructure of Polycrystalline MgO Penetrated by a Silicate Liquid

1996 ◽  
Vol 2 (3) ◽  
pp. 113-128 ◽  
Author(s):  
Sundar Ramamurthy ◽  
Michael P. Mallamaci ◽  
Catherine M. Zimmerman ◽  
C. Barry Carter ◽  
Peter R. Duncombe ◽  
...  
Keyword(s):  
Grain Boundaries ◽  
Grain Growth ◽  
Glassy Phase ◽  
Silicate Liquid ◽  
Two Phase ◽  
The Matrix ◽  
Devitrification Process ◽  
Phase Mixture

Dense, polycrystalline MgO was infiltrated with monticellite (CaMgSiO4) liquid to study the penetration of liquid along the grain boundaries of MgO. Grain growth was found to be restricted with increasing amounts of liquid. The inter-granular regions were generally found to be comprised of a two-phase mixture: crystalline monticellite and a glassy phase rich in the impurities present in the starting MgO material. MgO grains act as seeding agents for the crystallization of monticellite. The location and composition of the glassy phase with respect to the MgO grains emphasizes the role of intergranular liquid during the devitrification process in “snowplowing” impurities present in the matrix.

Failure Mechanisms of Particulate Two-Phase Composites

1998 ◽  
Vol 529 ◽  
Author(s):  
T. Antretter ◽  
E D. Fischer
Keyword(s):  
Residual Stresses ◽  
Crack Growth ◽  
Opening Angle ◽  
Geometrical Parameters ◽  
Two Phase ◽  
Absolute Size ◽  
The Matrix ◽  
Unit Cells ◽  
Angle Criterion ◽  
Probability Of Fracture

AbstractIn many composites consisting of hard and brittle inclusions embedded in a ductile matrix failure can be attributed to particle cleavage followed by ductile crack growth in the matrix. Both mechanisms are significantly sensitive towards the presence of residual stresses.On the one hand particle failure depends on the stress distribution inside the inclusion, which, in turn, is a function of various geometrical parameters such as the aspect ratio and the position relative to adjacent particles as well as the external load. On the other hand it has been observed that the absolute size of each particle plays a role as well and will, therefore, be taken into account in this work by means of the Weibull theory. Unit cells containing a number of quasi-randomly oriented elliptical inclusions serve as the basis for the finite element calculations. The numerical results are then correlated to the geometrical parameters defining the inclusions. The probability of fracture has been evaluated for a large number of inclusions and plotted versus the particle size. The parameters of the fitting curves to the resulting data points depend on the choice of the Weibull parameters.A crack tip opening angle criterion (CTOA) is used to describe crack growth in the matrix emanating from a broken particle. It turns out that the crack resistance of the matrix largely depends on the distance from an adjacent particle. Residual stresses due to quenching of the material tend to reduce the risk of particle cleavage but promote crack propagation in the matrix.

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