Annealing Effects on Toughened Intra-Type Nanocomposites

2006 ◽  
Vol 45 ◽  
pp. 1632-1639 ◽  
Author(s):  
Hideo Awaji ◽  
Seong Min Choi
Keyword(s):  
Fracture Toughness ◽  
Elevated Temperatures ◽  
Volume Fraction ◽  
Process Zone ◽  
Nitrate Solution ◽  
Crack Tip ◽  
Hydrogen Atmosphere ◽  
Second Phase ◽  
High Porosity ◽  
The Matrix

Intra-type nanocomposites, in which nanosized second-phase particles are embedded within matrix grains, generate dislocations around the dispersed nanoparticles. The intra-type nanostructure induces a thermal expansion mismatch between the matrix and the dispersed particles, which will yield nanoscale stress distribution around the particles and generate lattice defects, such as dislocations. The dislocations of ceramics can be generated at elevated temperatures, become sessile dislocations at room temperature, and serve as nanocrack nuclei in highly stresses fields, e.g. at a main crack tip. The frontal process zone size ahead of a crack tip is expanded due to creation of nanocracks and hence the fracture toughness is improved. Annealing after sintered nanocomposites is important in controlling the dislocation activities. Appropriate annealing will disperse dislocations into the matrix grains. However, dislocations are sensitive to temperature, and higher temperature or longer annealing time result in dislocation disappearance and cause the reduction of the strength and fracture toughness of nanocomposites. In this study, commercially available γ-alumina agglomerated powder with high porosity was used to create the intra-type nanostructure. Nickel nitrate solution was infiltrated into nanopores of the γ-alumina agglomerates in vacuum. The alumina/nickel composite powder following reduction in hydrogen atmosphere was sintered using a pulse electric current sintering method. The volume fraction of nickel was about 3 vol %. After appropriate annealing, the highest fracture toughness was obtained to be 7.6 MPam1/2, which is two times higher than that of monolithic alumina.

The Room Temperature and Elevated Temperature Fracture Toughness Response of Alloy A-286

1978 ◽  
Vol 100 (2) ◽  
pp. 195-199 ◽  
Author(s):  
W. J. Mills
Keyword(s):  
Fracture Toughness ◽  
Room Temperature ◽  
Elevated Temperatures ◽  
Second Phase ◽  
Surface Micromorphology ◽  
Second Phase Particles ◽  
The Matrix ◽  
Temperature Fracture ◽  
Increasing Temperature ◽  
Base Superalloy

The elastic-plastic fracture toughness (JIc) response of precipitation strengthened Alloy A-286 has been evaluated by the multi-specimen R-curve technique at room temperature, 700 K (800°F) and 811 K (1000°F). The fracture toughness of this iron-base superalloy was found to decrease with increasing temperature. This phenomenon was attributed to a reduction in the materials’s strength and ductility at elevated temperatures. Electron fractographic examination revealed that the overall fracture surface micromorphology, a duplex dimple structure coupled with stringer troughs, was independent of test temperature. In addition, the fracture resistance of Alloy A-286 was found to be weakened by the presence of a nonuniform distribution of second phase particles throughout the matrix.

Microstructures and Mechanical Properties of Fine-Scale Fibrous Alumina / Zirconia Bi-Phase Composite Fabricated by Co-Extrusion Process

2006 ◽  
Vol 317-318 ◽  
pp. 619-622
Author(s):  
Hiroyuki Miyazaki ◽  
Yuichi Yoshizawa ◽  
Kiyoshi Hirao
Keyword(s):  
Fracture Toughness ◽  
Bending Strength ◽  
Volume Fraction ◽  
Fiber Diameter ◽  
Extrusion Process ◽  
Second Phase ◽  
The Matrix ◽  
Zirconia Phase ◽  
The Individual ◽  
Monolithic Alumina

A bundle of feedrod composed of ordinary arranged alumina and zirconia green rods was co-extruded through a 6:1 reduction die. The volume fraction of zirconia phase was varied from 10 to 88 vol%. After the first co-extrusion, the individual pieces were bundled and co-extruded again, reducing the lateral size of each phase and multiplying the number of continuous monofilaments. After a 3rd extrusion step and sintering at 1600oC, crack-free composites with a fiber diameter of ~50 μm were obtained for all compositions. The fracture toughness of the composites was improved by introducing fine second phase filaments into the matrix. The maximum fracture toughness of 6.2 MPam1/2 was attained in the 3rd co-extruded composite which consisted of 53 vol% alumina and 47 vol% zirconia. Bending strength of the composites was almost the same as that of the monolithic alumina regardless of the composition.

Fracture and R-curves in high volume fraction Al2O3/Al composites

2000 ◽  
Vol 15 (5) ◽  
pp. 1131-1144 ◽  
Author(s):  
N. Nagendra ◽  
V. Jayaram
Keyword(s):  
Fracture Toughness ◽  
Volume Fraction ◽  
Porous Alumina ◽  
Crack Tip ◽  
High Volume ◽  
Fracture Mechanisms ◽  
Constraint Factor ◽  
Ductile Phase ◽  
Crack Bridging ◽  
The Matrix

Fracture toughness and fracture mechanisms in Al2O3/Al composites are described. The unique flexibility offered by pressureless infiltration of molten Al alloys into porous alumina preforms was utilized to investigate the effect of microstructural scale and matrix properties on the fracture toughness and the shape of the crack resistance curves (R-curves). The results indicate that the observed increment in toughness is due to crack bridging by intact matrix ligaments behind the crack tip. The deformation behavior of the matrix, which is shown to be dependent on the microstructural constraints, is the key parameter that influences both the steady-state toughness and the shape of the R-curves. Previously proposed models based on crack bridging by intact ductile particles in a ceramic matrix have been modified by the inclusion of an experimentally determined plastic constraint factor (P) that determines the deformation of the ductile phase and are shown to be adequate in predicting the toughness increment in the composites. Micromechanical models to predict the crack tip profile and the bridge lengths (L) correlate well with the observed behavior and indicate that the composites can be classified as (i) short-range toughened and (ii) long-range toughened on the basis of their microstructural characteristics.

scholarly journals Iron-chromium-carbon-vanadium white cast irons: Microstructure and properties

2014 ◽  
Vol 68 (4) ◽  
pp. 413-427 ◽  
Author(s):  
Mirjana Filipovic
Keyword(s):  
Fracture Toughness ◽  
Strain Hardening ◽  
Volume Fraction ◽  
Dynamic Fracture Toughness ◽  
M23c6 Carbide ◽  
Cast Irons ◽  
Repeated Impact ◽  
Secondary Carbides ◽  
The Matrix ◽  
Microstructure Parameters

The as-cast microstructure of Fe-Cr-C-V white irons consists of M7C3 and vanadium rich M6C5 carbides in austenitic matrix. Vanadium changed the microstructure parameters of phase present in the structure of these alloys, including volume fraction, size and morphology. The degree of martensitic transformation also depended on the content of vanadium in the alloy. The volume fraction of the carbide phase, carbide size and distribution has an important influence on the wear resistance of Fe-Cr-C-V white irons under low-stress abrasion conditions. However, the dynamic fracture toughness of Fe-Cr-C-V irons is determined mainly by the properties of the matrix. The austenite is more effective in this respect than martensite. Since the austenite in these alloys contained very fine M23C6 carbide particles, higher fracture toughness was attributed to a strengthening of the austenite during fracture. Besides, the secondary carbides which precipitate in the matrix regions also influence the abrasion behaviour. By increasing the matrix strength through a dispersion hardening effect, the fine secondary carbides can increase the mechanical support of the carbides. Deformation and appropriate strain hardening occur in the retained austenite of Fe-Cr-C-V alloys under repeated impact loading. The particles of precipitated M23C6 secondary carbides disturb dislocations movement and contribute to increase the effects of strain hardening in Fe-Cr-C-V white irons.

Loading Rate Effect on Translaminar Fracture Toughness of Hybrid Composite Laminate

2000 ◽  
Author(s):  
Paul Moy ◽  
Jerome Tzeng
Keyword(s):  
Fracture Toughness ◽  
Composite Laminates ◽  
Glass Matrix ◽  
Small Volume ◽  
Loading Rate ◽  
Volume Fraction ◽  
Rate Effect ◽  
The Matrix ◽  
Small Volume Fraction ◽  
Matrix Property

Abstract Fracture toughness properties of composite laminates were evaluated at a loading rate commonly observed in ordinance applications. The laminates are composed of IM7 graphite and a small volume fraction of S2 glass plies to form a cross-ply laminate. Fracture toughness appears to be very rate sensitive if the crack growth perpendicular to the plane dominated by glass/matrix property. Experimental data shows a 30–40% increase of fracture toughness for various layup as the loading rate was increase by 1000 times. The specimens examined under microscopic indicates the strengthening might due to different failure mechanism in the matrix. In addition, there is no visible rate effect if the crack propagation is perpendicular to the graphite dominant plane.

Brittle-Ductile Transition in Heterogeneous Metallic Materials [PowerPoint Submission]

2006 ◽  
Vol 978 ◽  
Author(s):  
Silvester John Noronha ◽  
Nasr M Ghoniem
Keyword(s):  
Fracture Toughness ◽  
Tensile Stress ◽  
Size Distribution ◽  
Crack Tip ◽  
Transition Curve ◽  
Crack Plane ◽  
Metallic Materials ◽  
Brittle Ductile Transition ◽  
Discrete Dislocation ◽  
The Matrix

AbstractWe present a model for the brittle - ductile transition in heterogenous metallic materials based on two dimensional discrete dislocation simulations of crack-tip plasticity. The sum of elastic fields of the crack and the emitted dislocations defines an elasto-plastic crack field. Effects of crack-tip blunting of the macrocrack are included in the simulations. The plastic zone characteristics are found to be in agreement with continuum models, with the added advantage that the hardening behavior comes out naturally in our model. The present model is composed of a macrocrack with microcracks ahead of its tip. These microcracks represent potential fracture sites at internal inhomogenities, such as brittle precipitates. Dislocations that are emitted from the crack-tip account for plasticity. When the tensile stress at the microcrack situated along the crack plane attains a critical value over a distance fracture is assumed to take place. The brittle-ductile transition curve is obtained by determining the fracture toughness at various temperatures. Factors that contribute to the sharp upturn in fracture toughness with temperature are found to be: the decrease in tensile stress ahead of the crack tip due to increase in blunting, and the increase in dislocation mobility. The inherent scatter in fracture toughness measurements are studied by using a size distribution for microcracks, distributed on the crack plane of the macrocrack. The scatter in fracture toughness measurements is found to be an effect of the size distribution of microcracks rather than their spatial distribution on the matrix ahead of the crack plane. When compared, the obtained results are in agreement with the existing experimental data.

Formation of Cavities From Nondeformable Second-Phase Particles in Low Temperature Ductile Fracture

1976 ◽  
Vol 98 (1) ◽  
pp. 60-68 ◽  
Author(s):  
A. S. Argon
Keyword(s):  
Particle Size ◽  
Stress Concentration ◽  
Size Effect ◽  
Volume Fraction ◽  
Void Formation ◽  
Particle Size Effect ◽  
Interfacial Stress ◽  
Second Phase ◽  
Rigid Particles ◽  
The Matrix

Limiting solutions are discussed for elastic-plastic deformation around rigid particles of both equiaxed and greatly elongated shapes. It is shown that if the matrix can be characterized as a rigid nonhardening continuum the stress concentration at the particle interface and interior is less than two for either equiaxed or elongated particles. In a rapidly strain hardening matrix, however, while the interfacial stress concentration relative to the distant boundary traction remains at a factor of two for the equiaxed particles, it rises nearly linearly with aspect ratio for slender platelets and rods. Interaction between particles can occur when the local volume fraction of particles is high. Such interactions raise the interface tractions for a given state of shear of the matrix and hasten void formation, and are often discerned as a particle size effect. Another particle size effect based on flawed particles is also discussed.

Microstructures and Mechanical Properties of 6061 Al Matrix Smart Composite Containing TiNi Shape Memory Fiber

1996 ◽  
Vol 459 ◽  
Author(s):  
J. H. Lee ◽  
K. Hamada ◽  
K. Miziuuchia ◽  
M. Taya ◽  
K. Inoue
Keyword(s):  
Mechanical Properties ◽  
Shape Memory ◽  
Elevated Temperatures ◽  
Volume Fraction ◽  
Matrix Material ◽  
Matrix Alloy ◽  
Flow Strength ◽  
The Matrix ◽  
Shape Memory Fiber ◽  
Al Matrix

ABSTRACT6061 Al-matrix composite with TiNi shape memory fiber as reinforcement has been fabricated by vacuum hot pressing to investigate the microstructure and mechanical properties. The yield stress of this composite increases with increasing amount of prestrain, and it also depends on the volume fraction of fiber and heat treatment. The smartness of the composite is given due to the shape memory effect of the TiNi fiber which generates compressive residual stresses in the matrix material when heated after being prestrained. Microstructual observations have revealed that interfacial reactions occur between the matrix and fiber, creating two intermetallic layers. The flow strength of the composite at elevated temperatures is significantly higher than that of the matrix alloy without TiNi fiber.

Fracture Toughness of Freshwater Ice—Part II: Analysis and Micrography

1996 ◽  
Vol 118 (2) ◽  
pp. 141-147 ◽  
Author(s):  
L. J. Weber ◽  
W. A. Nixon
Keyword(s):  
Fracture Toughness ◽  
Fracture Surface ◽  
Experimental Technique ◽  
Loading Rate ◽  
Process Zone ◽  
Crack Tip ◽  
Simple Method ◽  
Dislocation Activity ◽  
Freshwater Ice

Experiments have been performed to determine the effect of loading rate and temperature on the fracture toughness of freshwater ice. A first paper (Weber and Nixon, 1996) presents the experimental technique and results, while this paper focuses on discussion of the crack tip process zone and fracture surface micrography. A simple method to estimate the size of the process zone will be presented. Also, fracture surface micrography will be used to interpret the results of the temperature experiments by considering evidence of dislocation activity in preferentially oriented grains.

scholarly journals Effects of HfC Particles on Crack Initiation and Propagation Behavior of WC/Co Composites

2019 ◽  
Vol 37 (3) ◽  
pp. 628-635
Author(s):  
Xinyu Yan ◽  
Shouren Wang ◽  
Daosheng Wen ◽  
Gaoqi Wang ◽  
Wentao Liu
Keyword(s):  
Fracture Toughness ◽  
Crack Propagation ◽  
Bending Strength ◽  
Crack Tip ◽  
Crack Deflection ◽  
Simulation Software ◽  
Homogeneous Distribution ◽  
The Matrix ◽  
Crack Pinning ◽  
Crack Bifurcation

Tungsten carbide composites were prepared by cold-pressing and hot-pressing sintering; fracture toughness and bending strength of the specimens were tested. The microstructures of HfC/WC/Co composites were observed with the SEM. The mathematical models were established to investigate the relationship between stress intensity factors of crack straight-through, crack deflection, and crack bifurcation with crack length, based on the crack propagation energy release rate. The simulation software ABAQUS was used to verify the four crack propagation methods of crack straight-through, crack deflection, crack bifurcation and crack pinning. The simulation results show that adding appropriate amount of HfC can effectively improve the fracture toughness and bending strength of the composites. The homogeneous distribution of HfC and Co in the matrix has a significant effect on the improvement of the strength and toughness of the composites, and the improvement mechanism is to disperse or transfer the stress at the crack tip to HfC by crack deflection, crack bifurcation, crack pinning, transcrystalline fracture, etc. As a result, the stress concentration at the crack tip in the matrix is reduced, and the toughness of the composites is improved.

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