Yield Point Phenomena and Dislocation Velocities Underneath Indentations Into BCC Crystals

1994 ◽  
Vol 356 ◽  
Author(s):  
W. W. Gerberich ◽  
S. Venkataraman ◽  
J. Nelson ◽  
H. Huang ◽  
E. Lilleodden ◽  
...  
Keyword(s):  
Oxide Film ◽  
Size Effects ◽  
Dislocation Nucleation ◽  
Dislocation Velocity ◽  
Tensile Yield Strength ◽  
Depth Of Penetration ◽  
Organic Contamination ◽  
Yield Phenomena ◽  
Peierls Barrier ◽  
Pile Up

AbstractIn a single material, hardnesses can range from 3σys up to the theoretical strength or approximately E/10 where σys and E are tensile yield strength and modulus. This variation results with decreasing depth of penetration. Such indentation size effects may be associated with surface contamination, passivation films and dislocation phenomena. Even where dislocation nucleation is relatively difficult as in GaAs, the hardness varies from about 1.5 to 15 GPa as the indentation depth decreased from about 100 nm to 10 nm. Similar size effects in BCC metals can give hardnesses which range from about 1 to 30 GPa as the indentations decrease from 1000 nm to 10 nm. Here, there are two types of “yield” phenomena which can be related to an organic contamination film and a dislocation pile-up induced oxide film fracture. As measured in single crystals of Fe, Mo, W, Ta and NiAl, this typically gives lower and upper “yield” points which range from 0.6 to 10 raN. When a dislocation pile-up breaks through the oxide film, velocities can be reasonably large due to the stress at the head of the pile-up. The average dislocation velocity of this avalanche is controlled, to first order, by the Peierls’ energy. A more exhaustive study of NiAl, with a B2/BCC type crystal structure shows that dislocation velocity can be related to the local pile-up stress and a Peierls’ barrier of about 2.2 eV.

A Dislocation Pile-Up Computation for Adiabatic Shear Banding

2014 ◽  
Vol 1650 ◽  
Author(s):  
Stephen D. Antolovich ◽  
Ronald W. Armstrong
Keyword(s):  
Grain Size ◽  
Carbon Steel ◽  
Temperature Increase ◽  
Shear Banding ◽  
Low Carbon Steel ◽  
Adiabatic Shear ◽  
Dislocation Velocity ◽  
Low Carbon ◽  
Adiabatic Shear Banding ◽  
Pile Up

ABSTRACTA model is presented for computing the temperature increase associated with the formation of an adiabatic shear band. The hypothesis is that the heating is supplied by the difference in energy of a pile-up of n dislocations and the energy of n individual dislocations. The heating is assumed to occur within a volume determined by the grain size (i.e. slip band length) and an effective thermal length determined by the dislocation velocity. The model predicts increases in temperature with increasing shear modulus (G), increasing numbers of piled up dislocations (n), increasing Burgers vector (b), increased grain size (d), and increased dislocation velocity (vd). Increasing temperature is also predicted with decreasing heat capacity (c*) and thermal diffusivity (α) as would be expected. The model was applied to low carbon steel for which considerable data are available. Application to low carbon steel gives a temperature increase of about 1400K. The implied result that untempered martensite should be observed after adiabatic shear banding is in agreement with examples cited in the literature. Further investigation into the dynamics of pile-up release and the associated heat transfer mechanisms is discussed.

scholarly journals Dislocation interactions in near-alpha Titanium alloy Ti6242Si under LCF

2020 ◽  
Vol 321 ◽  
pp. 11075
Author(s):  
Sudha Joseph ◽  
Trevor C. Lindley ◽  
David Dye
Keyword(s):  
Titanium Alloy ◽  
Basal Plane ◽  
Crack Nucleation ◽  
Low Cycle Fatigue ◽  
Dislocation Nucleation ◽  
Slip Transfer ◽  
Gauge Section ◽  
Pile Up ◽  
Dislocation Interactions ◽  
Alpha Titanium

Dislocation interactions were investigated in near-alpha titanium alloy Ti6242Si after low cycle fatigue. Samples from the gauge section and the crack initiation site on the fracture surface were studied. Grain pairs with different crystallographic orientations were analysed from the gauge section to understand the dislocation interactions near the boundary. Deformation was primarily caused by planar slip, localized into slip bands in primary alpha (αp) grains. Direct slip transfer was observed within grains having similar orientations. In contrast, slip transfer resulted in a different kind of dislocation nucleation in the neighboring hard grain where there was misorientation between the grains, with the observation of cross-slip. Strain transfer was observed between highly misoriented grain pairs. Crack nucleation occurred on an αp grain by basal plane splitting, due to the large tensile stress developed by a double ended pile-up. This kind of pile-up is suggested to result from the incomplete reversibility of dislocation motion during load reversals. The observation of superjogs on the basal dislocations in the crack nucleated αp grain provides a rationale for why cracks nucleate near, rather than on, the basal plane.

scholarly journals COMPARISON OF FRACTURE BEHAVIOR OF SHARP WITH BLUNT CRACK TIP IN NANOCRYSTALLINE MATERIALS BY MOLECULAR DYNAMICS SIMULATION

2012 ◽  
Vol 05 ◽  
pp. 410-417 ◽  
Author(s):  
MOVAFFAQ KATEB ◽  
KAMRAN DEHGHANI
Keyword(s):  
Molecular Dynamics ◽  
Nanocrystalline Materials ◽  
Fracture Behavior ◽  
Md Simulation ◽  
Crack Tip ◽  
Dislocation Nucleation ◽  
Dynamics Simulation ◽  
Blunt Crack ◽  
Pile Up ◽  
Fracture Behaviors

Molecular Dynamics (MD) simulation was used to figure out the fracture behaviors of nanocrystalline materials (NCM). The simulation was based on more than 13 thousand atoms considered for two systems with sharp and blunt crack tip in NCM. Their atomic level resolution provides novel insights into the fracture behavior of NCM. The results show semi brittle manner for both sharp and blunt tips. Dislocation nucleation and pile up at grain boundary (GB), lead to forming voids at GB. Merging mechanism of voids ahead of crack tip causes crack growth.

Relative mobility of screw versus edge dislocations controls the ductile-to-brittle transition in metals

2021 ◽  
Vol 118 (37) ◽  
pp. e2110596118
Author(s):  
Yan Lu ◽  
Yu-Heng Zhang ◽  
En Ma ◽  
Wei-Zhong Han
Keyword(s):  
Dislocation Nucleation ◽  
Dislocation Velocity ◽  
Relative Mobility ◽  
Dislocation Source ◽  
Cubic Metals ◽  
Brittle Transition ◽  
Ductile To Brittle Transition ◽  
Potential Factors ◽  
Dislocation Sources ◽  
Edge Dislocations

Body-centered cubic metals including steels and refractory metals suffer from an abrupt ductile-to-brittle transition (DBT) at a critical temperature, hampering their performance and applications. Temperature-dependent dislocation mobility and dislocation nucleation have been proposed as the potential factors responsible for the DBT. However, the origin of this sudden switch from toughness to brittleness still remains a mystery. Here, we discover that the ratio of screw dislocation velocity to edge dislocation velocity is a controlling factor responsible for the DBT. A physical model was conceived to correlate the efficiency of Frank–Read dislocation source with the relative mobility of screw versus edge dislocations. A sufficiently high relative mobility is a prerequisite for the coordinated movement of screw and edge segments to sustain dislocation multiplication. Nanoindentation experiments found that DBT in chromium requires a critical mobility ratio of 0.7, above which the dislocation sources transition from disposable to regeneratable ones. The proposed model is also supported by the experimental results of iron, tungsten, and aluminum.

Investigations on indentation size effects using a pile-up corrected hardness

2008 ◽  
Vol 41 (7) ◽  
pp. 074027 ◽  
Author(s):  
Y H Lee ◽  
J H Hahn ◽  
S H Nahm ◽  
J I Jang ◽  
D Kwon
Keyword(s):  
Size Effects ◽  
Indentation Size ◽  
Pile Up

scholarly journals The Correlation of Indentation Size Effect Experiments with Pyramidal and Spherical Indenters

2001 ◽  
Vol 695 ◽  
Author(s):  
J. G. Swadener ◽  
E. P. George ◽  
G. M. Pharr
Keyword(s):  
Size Effect ◽  
Work Hardening ◽  
Size Effects ◽  
Indentation Size Effect ◽  
Experimental Results ◽  
Indentation Size ◽  
Depth Of Penetration ◽  
Wide Range ◽  
Sphere Radius

ABSTRACTExperiments were conducted in annealed iridium using pyramidal and spherical indenters over a wide range of load. For a Berkovich pyramidal indenter, the hardness increased with decreasing depth of penetration. However, for spherical indenters, hardness increased with decreasing sphere radius. Based on the number of geometrically necessary dislocations generated during indentation, a theory that takes into account the work hardening differences between pyramidal and spherical indenters is developed to correlate the indentation size effects measured with the two indenters. The experimental results verify the theoretical correlation.

A RADIOCHEMICAL TECHNIQUE FOR STUDYING RANGE–ENERGY RELATIONSHIPS FOR HEAVY IONS OF KEV ENERGIES IN ALUMINUM

1960 ◽  
Vol 38 (9) ◽  
pp. 1526-1534 ◽  
Author(s):  
J. A. Davies ◽  
J. Friesen ◽  
J. D. McIntyre
Keyword(s):  
Oxide Film ◽  
Heavy Ions ◽  
Aluminum Foil ◽  
Thin Layers ◽  
Ammonium Citrate ◽  
Surface Layers ◽  
Depth Of Penetration ◽  
Energy Relationships ◽  
Anodic Voltage ◽  
Recoil Atoms

A rapid technique has been developed for dissolving successive thin layers of metal from the surface of an aluminum foil: viz. electrochemical oxidation at constant voltage in aqueous ammonium citrate, followed by removal of the oxide film in a phosphoric acid – chromic oxide solution. Due to the highly protective nature of the aluminum oxide film, this two-step process enables very uniform surface layers of metal as thin as 1 μ/cm2 to be removed. The total weight of aluminum dissolved increases with the applied anodic voltage at a rate of 0.30 μg cm−1 volt−1 (approximately 11 Å per volt) over the range 0–150 volts. The technique should be sufficiently sensitive to study the depth of penetration in aluminum of radioactive ions with kinetic energies as low as a few kiloelectron volts.An approximate value for the range of Na24 recoil atoms from the Al27 (n,α) reaction was obtained. A more extensive application to range studies is given in the next paper.

“Barrierless” Misfit Dislocation Nucleation in SiGe/Si Strained Layer Epitaxy

1992 ◽  
Vol 263 ◽  
Author(s):  
D.D. Perovic ◽  
D.C. Houghton
Keyword(s):  
Activation Energy ◽  
Misfit Dislocation ◽  
Strain Relaxation ◽  
Dislocation Nucleation ◽  
Theoretical Treatment ◽  
Misfit Dislocations ◽  
Plan View ◽  
Strained Layer ◽  
Wide Range ◽  
Peierls Barrier

ABSTRACTThe study of the critical thickness/strain phenomenon inherent in metastable, layered heterostructures has led to the development of several models which describe elastic strain relaxation. Hitherto, the nucleation of misfit dislocations required for coherency breakdown is the least well understood aspect of strain relaxation, due to the paucity of experimental data. Moreover, existing theoretical calculations predict relatively large activation energy barriers (>10 eV) for misfit dislocation nucleation in relatively low misfit (<2%) systems. In this work it will be shown that the nucleation of misfit dislocations can occur spontaneously demonstrating a vanishingly small activation energy barrier. Specifically, experimental studies of a wide range of GexSi1−x/Si (x< 0.5) hetero-structures, grown by MBE and CVD techniques, have provided quantitative data from bulk specimens on the observed misfit dislocation nucleation rate and activation energy using large-area diagnostic techniques (eg. chemical etching/Nomarski microscopy). In parallel, the strained layer microstructure was studied in detail using crosssectional and plan-view electron microscopy in order to identify a new dislocation nucleation mechanism, the ‘double half-loop’ source. From the combined macroscopic and microscopic analyses, a theoretical treatment has been developed based on nucleation stress and energy criteria which predicts a “barrierless” nucleation process exists even at low misfits (< 1%). Accordingly, the observed misfit dislocation nucleation event has been found both experimentally and theoretically to be rate-controlled solely by Peierls barrier dependent, glide-activated processes with activation energies of ∼2 eV.

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