High Strain Rate Deformation and Damage in Ceramic Materials

1993 ◽  
Vol 115 (3) ◽  
pp. 292-299 ◽  
Author(s):  
G. Raiser ◽  
R. J. Clifton
Keyword(s):  
Grain Size ◽  
Grain Boundaries ◽  
High Purity ◽  
Hot Pressing ◽  
Glassy Phase ◽  
Plane Waves ◽  
Weight Percent ◽  
Ultrafine Grain ◽  
Triple Junctions ◽  
Dynamic Tensile Strength

The objective of this investigation is to use a plate impact recovery experiment to identify the dominant failure mechanisms in conventional α-Al2O3 ceramics and thereby gain insight into the most promising, failure-resistant microstructures. A “soft-recovery” configuration is used wherein a star-shaped flyer impacts a square specimen. The impedances, shapes, thicknesses and orientation of all plates are designed to ensure a known history of longitudinal, planar stress waves throughout a central octagonal region of the specimen. The plane waves generated from this experiment are monitored by a laser interferometer system that allows data to be collected at four separate locations. The validity of the approach is demonstrated by a shot in which all plates were stressed within their elastic range. Subsequently, several experiments were conducted at nearly the same stress level with commercially sintered aluminas having different grain size and different glass content. These experiments, taken as a whole, demonstrate that improvement in alumina’s dynamic compressive properties is obtained by reducing the grain size. In compression, a reduction in grain size lowers average residual stresses at triple junctions and grain boundaries and makes the material less susceptible to inelastic deformation and sliding at triple junctions and grain boundaries. A reduction in the weight percent of pre-processing impurities (and therefore the amount of intergranular glassy phase) yields strong improvements in the dynamic tensile strength of the ceramic. A decrease in the amount of glassy phase tends to make tensile damage less likely by improving grain boundary strength. These trends were tested by conducting recovery experiments on a high-purity, small-grain alumina, processed in-house through hot pressing. Both the compressive resistance and, especially, the tensile resistance were superior to those found for all other tested specimens. The overall results suggest that the best failure resistance will be obtained for new, high-purity, ultrafine-grain ceramics that are prepared by hot pressing of nanometer scale powders.

Migration of grain boundaries and triple junctions in high-purity aluminum during annealing after slight cold rolling

2015 ◽  
Vol 107 ◽  
pp. 134-138 ◽  
Author(s):  
Wenhong Yin ◽  
Weiguo Wang ◽  
Xiaoying Fang ◽  
Congxiang Qin ◽  
Xiaoguang Xing
Keyword(s):  
Cold Rolling ◽  
Grain Boundaries ◽  
High Purity ◽  
Triple Junctions ◽  
High Purity Aluminum

High-resolution TEM and analytical study of second-phase particles in Al2O3

1983 ◽  
Vol 41 ◽  
pp. 46-47
Author(s):  
K. J. Morrissey ◽  
Y. Kouh ◽  
C. B. Carter
Keyword(s):  
Grain Size ◽  
High Resolution ◽  
Grain Boundaries ◽  
Hot Pressing ◽  
Analytical Study ◽  
Second Phase ◽  
Initial Powder ◽  
Compact Density ◽  
Second Phase Particles ◽  
High Resolution Tem

The influence of additives such as MgO, NiO, and ZrO2 and impurities such as Na, K, and Ca on the sintering of alumina compacts has been the focus of a considerable amount of research. Since these additives affect compact density and grain size it is of interest to determine the behavior of the elements during processing. That is, it is important to know whether Ca and Mg segregate to grain boundaries or are located in the second-phase particles. Current results suggest that Ca is found uniformly at the grain boundaries and that Mg is accommodated in the second-phase particles.The present investigation is concerned with identifying second-phase particles in different commercially-produced Al2O3 compacts and studying both their structure and composition. Preliminary results have been discussed previously. The investigation has dealt mainly with two different alumina compacts. One compact was prepared from an initial powder containing 0.25% MgO, a small amount of intentionally added Ni, and was prepared by hot pressing.

Atomistic Studies of Plasticity in Nanophase Metals

2000 ◽  
Vol 634 ◽  
Author(s):  
H. Van Swygenhoven ◽  
P. Derlet ◽  
A. Caro ◽  
D. Farkas ◽  
M. Caturla ◽  
...  
Keyword(s):  
Molecular Dynamics ◽  
Computer Simulation ◽  
Grain Size ◽  
Grain Boundaries ◽  
Excess Enthalpy ◽  
Uniaxial Stress ◽  
Polycrystalline Materials ◽  
Triple Junctions ◽  
Grain Sizes ◽  
20 Nm

ABSTRACTMolecular dynamics computer simulation of nanocrystalline Ni and Cu with mean grain sizes ranging from 5 to 20 nm show that grain boundaries in nanocrystalline metals have structures similar to most grain boundaries found in conventional polycrystalline materials. Moreover, the excess enthalpy density in grain boundaries and triple junctions appears to be independent of grain in both, computer generated and experimental measured samples. Simulations of deformation under constant uniaxial stress demonstrate a change in deformation mechanism as function of grain size: at the smallest grain sizes all deformation is accommodated in the grain boundaries, at higher grain sizes, intragrain deformation is observed

Plastic deformation of nanocrystalline copper-antimony alloys

2010 ◽  
Vol 25 (3) ◽  
pp. 411-421 ◽  
Author(s):  
Rahul K. Rajgarhia ◽  
Douglas E. Spearot ◽  
Ashok Saxena
Keyword(s):  
Plastic Deformation ◽  
Grain Size ◽  
Grain Boundaries ◽  
Partial Dislocation ◽  
Dislocation Nucleation ◽  
Uniaxial Tensile ◽  
Triple Junctions ◽  
Dopant Atoms ◽  
Dynamics Simulations ◽  
Antimony Alloys

Molecular dynamics simulations are used to evaluate the influence of Sb dopant atoms at the grain boundaries on plastic deformation of nanocrystalline Cu. Deformation is conducted under uniaxial tensile loading, and Sb atoms are incorporated as substitutional defects at the grain boundaries. The presence of randomly dispersed Sb atoms at the grain boundaries does not appreciably influence the mechanisms associated with dislocation nucleation in nanocrystalline Cu; grain boundary ledges and triple junctions still dominate as partial dislocation sources. However, the magnitude of the tensile stress associated with the partial dislocation nucleation event does increase with increasing Sb concentration and also with increasing grain size. The flow stress of nanocrystalline Cu increases with increasing Sb concentration up to 1.0 at.% Sb, with a maximum observed at a grain size of 15 nm for all Sb concentrations (0.0–2.0 at.% Sb).

Understanding Grain Boundary Junctions: Effect of the Grain Size on Microstructure Evolution

2012 ◽  
Vol 715-716 ◽  
pp. 186-190 ◽  
Author(s):  
Luis A. Barrales Mora ◽  
Lasar S. Shvindlerman ◽  
Günter Gottstein
Keyword(s):  
Grain Size ◽  
Grain Boundary ◽  
Grain Boundaries ◽  
Microstructure Evolution ◽  
Boundary Migration ◽  
Granular System ◽  
Grain Boundary Migration ◽  
Triple Junctions ◽  
Model Simulations ◽  
Grain Sizes

In a previous work [ we introduced the geometry of a granular system that allowed the study of the effect of a finite mobility of the quadruple and triple junctions on grain boundary migration. One of the most important conclusions of this work was that the triple junctions drag more effectively the motion of the grain boundaries than the quadruple junctions. Nevertheless, this conclusion was drawn without consideration of the grain size. For this reason, this conclusion might be contradictory with our understanding of the grain boundary junctions because while the effect of the triple lines is inverse linear with the grain size that of the quadruple junctions is proportional to the inverse square of the grain size and thus, quadruple junctions are expected to drag more effectively, at least, for very small grain sizes. In the present investigation, we studied comprehensively the effect of grain size on the evolution of the granular system under the assumption of a finite mobility of the boundary junctions. For this purpose, several network model simulations were carried out for different grain sizes ranging from nanoto micrometers using a fully periodic grain arrangement. The results seem to corroborate that the triple junctions drag more effectively the motion of the grain boundaries, however, for very low junction mobility and grain sizes the effect appears to be indistinguishable. It was also observed that for very low quadruple junction mobility the geometry of the granular system undergoes a severe transformation which results in the unfulfillment of the equation derived in [.

Modeling of Grain-Boundary Mobility and Nucleation Rate during Discontinuous Dynamic Recrystallization in Ni-Nb Alloys and a High-Purity Alloy Derived from SAE 304L

2016 ◽  
Vol 879 ◽  
pp. 1501-1506 ◽  
Author(s):  
David Piot ◽  
Guillaume Smagghe ◽  
Frank Montheillet
Keyword(s):  
Grain Size ◽  
Steady State ◽  
Grain Boundary ◽  
Dynamic Recrystallization ◽  
Grain Boundaries ◽  
Nucleation Rate ◽  
High Purity ◽  
Boundary Mobility ◽  
Niobium Content ◽  
Grain Boundary Mobility

A simple mesoscale model has been developed for discontinuous dynamic recrystallization. Each grain is considered in turn as an inclusion, embedded in a homogeneous equivalent matrix, the properties of which are obtained by averaging over all the grains. The model includes: (i) a grain-boundary migration-equation driving the evolution of grain size via the mobility of grain boundaries, which is coupled with (ii) a single-internal-variable (dislocation density) constitutive model for strain hardening and dynamic recovery, and (iii) a nucleation equation governing the total number of grains by the nucleation of new grains. All the system variables tend to asymptotic values at large strains, in agreement with the experimentally observed steady-state regime.With some assumptions, both steady-state stress and grain-size are derived in closed forms, allowing immediate identification of the mobility of grain boundaries and the rate of nucleation. An application to Ni–Nb-pure-binary model alloys and high-purity 304L stainless steel with Nb addition is presented. More specifically on one hand, from experimental steady-state stresses and grain sizes, variations of the grain boundary mobility and the nucleation rate with niobium content are addressed in order to quantify the solute-drag effect of niobium in nickel. And on the other hand, the Derby exponents were investigated varying separately the strain rate or the temperature.

Nanograin size effects on deformation mechanisms and mechanical properties of nickel: A molecular dynamics study

2021 ◽  
Vol 11 (11) ◽  
pp. 1841-1855
Author(s):  
Alexandre Melhorance Barboza ◽  
Ivan Napoleão Bastos ◽  
Luis César Rodríguez Aliaga
Keyword(s):  
Mechanical Properties ◽  
Molecular Dynamics ◽  
Grain Size ◽  
Stress Relaxation ◽  
Strain Rate ◽  
Grain Boundaries ◽  
Rate Sensitivity ◽  
Deformation Mechanisms ◽  
Triple Junctions ◽  
Grain Sizes

The grain size refinement of metallic materials to the nanometer scale produces interesting properties compared to the coarse-grained counterparts. Their mechanical behavior, however, cannot be explained by the classical deformation mechanisms. Using molecular dynamics simulations, the present work examines the influence of grain size on the deformation mechanisms and mechanical properties of nanocrystalline nickel. Samples with grain sizes from 3.2 to 24.1 nm were created using the Voronoi tessellation method and simulated in tensile and relaxation tests. The yield and ultimate tensile stresses follow an inverse Hall-Petch relationship for grain sizes below ca. 20 nm. For samples within the conventional Hall-Petch regime, no perfect dislocations were observed. Nonetheless, a few extended dislocations were nucleated from triple junctions, suggesting that the suppression of conventional slip mechanism is not uniquely responsible for the inverse Hall-Petch behavior. For samples respecting the inverse Hall-Petch regime, the high number of triple junctions and grain boundaries allowed grain rotation, grain boundary sliding, and diffusion-like behavior that act as competitive deformation mechanisms. For all samples, the atomic configuration analysis showed that Shockley partial dislocations are nucleated at grain boundaries, crossing the grain before being absorbed in opposite grain boundaries, leaving behind stacking faults. Interestingly, the stress relaxation tests showed that the strain rate sensitivity decreases with grain size for a specific grain size range, whereas for grains below approximately 10 nm, the strain rate sensitivity increases as observed experimentally. Repeated stress relaxation tests were also performed to obtain the effective activation volume parameter. However, the expected linear trend in pertinent plots required to obtain this parameter was not found.

Microstructural and Magnetic Studies of Hddr Magnets from High Boron NdFeB(Zr) Alloys

1999 ◽  
Vol 577 ◽  
Author(s):  
DN Brown ◽  
AJ Williams ◽  
O Gutfleisch ◽  
M Strangwood ◽  
IR Harris
Keyword(s):  
Grain Size ◽  
Magnetic Properties ◽  
Magnetic Property ◽  
Grain Boundaries ◽  
Hot Pressing ◽  
Magnetic Studies ◽  
Rich Phase ◽  
Marked Influence ◽  
High Boron ◽  
Zr Alloys

ABSTRACTSome boron-rich (with respect to the Nd 2Fe,4B composition) NdFeB alloys (with and without small additions of zirconium) have been investigated in an attempt to understand the influence of zirconium in the subsequent magnetic properties of HDDR hot pressed magnets. It was found that zirconium additions had a marked influence on the as-cast microstructures with a general refinement of the grain size and evidence of a zirconium-rich phase (probably ZrB2) at the grain boundaries. The magnetic property measurements of fully dense, isotropic HDDR hot pressed magnets indicated that those containing zirconium maintained their coercivity to significantly higher hot pressing temperatures and this was attributed to the pinning of grain boundaries by the zirconium rich phase.

Characterization of HIP'ed, High Purity Si3N4 Grain Boundaries

1992 ◽  
Vol 287 ◽  
Author(s):  
Ping Lu ◽  
S.C. Danforth ◽  
W.T. Symons
Keyword(s):  
Surface Layer ◽  
Melting Point ◽  
Grain Boundaries ◽  
High Purity ◽  
Glassy Phase ◽  
Oxide Surface ◽  
Sintering Aids ◽  
Oxygen Contamination ◽  
Amorphous Si

ABSTRACTLaser synthesized, ultra-fine, amorphous, Si3N4 powders were densified via HIP'ing without any oxide sintering aids. Exposed samples were made from powder that had been exposed to the atmosphere, thereby picking up an oxide surface layer, and unexposed samples were made from powders processed entirely under glove box conditions, i.e. without oxygen contamination. TEM (and sintering) studies indicate that the exposed samples HIP'ed at temperatures in excess of the melting point of Si02, densified via a solution-reprecipitation mechanism, with a resultant intergranular glassy phase of high purity Si02. In contrast, unexposed samples had to be HIP'ed to 2050°C to achieve a density of ∼70 %p Th. These samples consisted of equiaxed β-Si3N4 grains, with localized high density regions where no inter-granular phase (crystalline or glassy) was detected to within 0.66 nm.

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