Simulation of Dislocation Dynamics in Ni3Al: A Study of Velocity Autocorrelations

1999 ◽  
Vol 578 ◽  
Author(s):  
C. K. Erdonmez ◽  
D. C. Chrzan
Keyword(s):  
Yield Strength ◽  
Applied Stress ◽  
Critical Stress ◽  
Threshold Stress ◽  
Dislocation Motion ◽  
Cross Slip ◽  
Dislocation Dynamics ◽  
Thermally Activated ◽  
Simple Rules ◽  
Isotropic Elasticity

AbstractThe yield strength anomaly in some L12 compounds has been linked to the thermally assisted cross slip of screw superdislocations. This work continues earlier efforts to understand the yield strength anomaly in L12 alloys using computer simulations of dislocation motion. Dislocations are modelled within isotropic elasticity theory, and simple rules are used to model the cross-slip process in the two dimensional geometry of the simulation. The velocity of a single dislocation in Ni3Al is studied as a function of the applied stress. The observed velocities vary nonlinearly with the applied stress. Further, dislocations are observed to become immobile for small applied loads. At high stresses, the dislocations are observed to advance relatively unhindered by the thermally activated cross slip process. Fluctuations in the velocity of the dislocations are studied, and their autocorrelation function shows an increased correlation time near a threshold stress. This threshold stress is identified with the critical stress proposed in earlier works.

Stochastic Dislocation Dynamics under Creep Conditions in Metals

2002 ◽  
Vol 731 ◽  
Author(s):  
Masato Hiratani ◽  
Hussein M. Zbib
Keyword(s):  
Thermal Fluctuation ◽  
Dislocation Motion ◽  
Cross Slip ◽  
Dislocation Dynamics ◽  
Atomistic Simulations ◽  
Thermal Processes ◽  
New Model ◽  
Slip Mechanism ◽  
Fcc Metals ◽  
Thermally Activated

AbstractA stochastic model is proposed to study dislocation dynamics in metallic single crystals. A Langevin type thermal fluctuation is taken into account for the model to maintain thermal equilibrium. This approach works as Brownian motion of segmental dislocations. Additionally, a new model for implementing the cross slip mechanism in FCC metals is developed based on results obtained by atomistic simulations. This new model is capable of simulating realistic thermal processes such as thermally activated dislocation motion during easy glide or cross slip during cold working of metals.

Thermally Activated Dislocation Motion in an AS21 Alloy and Alloy Reinforced with Short Ceramic Fibres Studied at Elevated Temperatures

2013 ◽  
Vol 592-593 ◽  
pp. 71-74
Author(s):  
Zuzana Zdražilová ◽  
Zuzanka Trojanová ◽  
Kristián Máthis ◽  
Pavel Lukáč
Keyword(s):  
Magnesium Alloy ◽  
Stress Relaxation ◽  
Applied Stress ◽  
Elevated Temperatures ◽  
Activation Volume ◽  
Activation Enthalpy ◽  
Dislocation Motion ◽  
Thermally Activated ◽  
Effective Components ◽  
Thermally Activated Process

AS21 magnesium alloy (2.1Al-1Si-balance Mg in wt.%) and the alloy reinforced with short δ-Al2O3fibres (Saffil®) were deformed in compression at temperatures between 23 and 300 °C. Stress relaxation tests were performed in order to reveal features of the thermally activated dislocation motion. Internal and effective components of the applied stress have been estimated. The activation volume decreases with increasing effective stress. The values of the activation volume and the activation enthalpy indicate that the main thermally activated process in the alloy as well as in the composite is the dislocation motion in non-compact planes.

Dislocation Dynamics Modeling for Yield Strength of Nanoscale Film Heterostructures

Materials ◽  
2005 ◽  
Author(s):  
E. H. Tan ◽  
L. Z. Sun
Keyword(s):  
Thin Films ◽  
Yield Strength ◽  
Mechanical Performance ◽  
Dislocation Motion ◽  
Dislocation Dynamics ◽  
Dislocation Segment ◽  
Dynamics Modeling ◽  
Surface Loop ◽  
Surface And Interface ◽  
Simulation Results

A novel dislocation dynamics framework is developed to simulate dislocation evolutions in thin film heterostructures at nanoscale. It is based on 3-D dislocation motion together with its physical background by adding the solid viscous effect. As the numerical simulation results demonstrate, this new model completely solves a long-standing paradoxical phenomenon with which the simulation results were dependent on dislocation-segment lengths in the classical discrete dislocation dynamics theory. The proposed model is applied to simulate the effect of dislocations on the mechanical performance of thin films. The interactions among the dislocation loop, free surface and interface are rigorously computed by decomposing this complicated problem into two relatively simple sub-problems. This model is allowed to determine the critical thickness of thin films for a surface loop to nucleate and to simulate how a surface loop evolves into two threading dislocations. Furthermore, the relationship between the film thickness and yield strength is constructed and compared with the conventional Hall-Petch relation.

An Analysis of The Slip of Screw Dislocations in Ll2 Alloys

1996 ◽  
Vol 460 ◽  
Author(s):  
Patrick Veyssière ◽  
Lem
Keyword(s):  
Numerical Simulation ◽  
Applied Stress ◽  
External Load ◽  
Elastic Anisotropy ◽  
Cross Slip ◽  
Screw Dislocations ◽  
Thermally Activated ◽  
Cube Plane

ABSTRACTThe annihilation of dislocations by cross-slip is studied by numerical simulation of infinitely long dissociated screw dislocations, allowed to move in an elastically anisotropie crystal. The external load is along [123] and cross-slip is permitted both on the octahedral and on the cube plane. The latter, together with cube slip, is thermally activated. Anisotropie elasticity modifies the properties of cross-slip significantly. Under the conditions of the simulations, the processes of APB jumps (APBJs) and repeated APB jumps (RAPBJs) can be largely promoted by interactions with other dislocations, while it is much less likely to occur at an isolated dislocation submitted to the same applied stress. The encounter of dislocations of opposite signs produces dipoles which may or may not tend to annihilate by cross-slip. APB tubes may form upon annihilation under certain circumstances again largely controlled by elastic anisotropy.

The effects of strain and strain rate on residual microstructures in copper

1986 ◽  
Vol 44 ◽  
pp. 420-421
Author(s):  
M. F. Stevens ◽  
P. S. Follansbee
Keyword(s):  
Strain Rate ◽  
Applied Stress ◽  
Threshold Stress ◽  
Rate Sensitivity ◽  
Viscous Drag ◽  
Strain Rates ◽  
Strain And Strain Rate ◽  
Threshold Strength ◽  
Mechanism Transition ◽  
Intrinsic Strength

The strain rate sensitivity of a variety of materials is known to increase rapidly at strain rates exceeding ∼103 sec-1. This transition has most often in the past been attributed to a transition from thermally activated guide to viscous drag control. An important condition for imposition of dislocation drag effects is that the applied stress, σ, must be on the order of or greater than the threshold stress, which is the flow stress at OK. From Fig. 1, it can be seen for OFE Cu that the ratio of the applied stress to threshold stress remains constant even at strain rates as high as 104 sec-1 suggesting that there is not a mechanism transition but that the intrinsic strength is increasing, since the threshold strength is a mechanical measure of intrinsic strength. These measurements were made at constant strain levels of 0.2, wnich is not a guarantee of constant microstructure. The increase in threshold stress at higher strain rates is a strong indication that the microstructural evolution is a function of strain rate and that the dependence becomes stronger at high strain rates.

Effect of operating parameters on functional fatigue characteristics of an Ni-Ti shape memory alloy on partial thermomechanical cycling

2022 ◽  
pp. 1045389X2110722
Author(s):  
Swaminathan Ganesan ◽  
Sampath Vedamanickam
Keyword(s):  
Yield Strength ◽  
Applied Stress ◽  
Stress Level ◽  
Crack Nucleation ◽  
Maximum Temperature ◽  
Thermomechanical Cycling ◽  
Permanent Strain ◽  
A Value ◽  
The Stability ◽  
Cycle Temperature

In this study, the influence of upper cycle temperature (maximum temperature in a cycle) and the magnitude of applied stress on the functional properties of an SMA during partial thermomechanical cycling has been studied. A near-equiatomic NiTi SMA was chosen and tested under different upper cycle temperatures (between martensite finish (Mf) and austenite finish (Af) temperatures) and stress level (below and above the yield strength of the martensite). The upper cycle temperature was varied by controlling the magnitude of the current supply. The results show that a raise in the upper cycle temperature causes the permanent strain to increase and also lowers the stability. However, decreasing the stress imposed to a value lower than the yield strength of the martensite improves cyclic stability. The upper cycle temperature was found to influence the crack nucleation, whereas the applied stress level the crack propagation during partial thermomechanical cycling of SMAs. Therefore, decreasing the upper cycle temperature as well as the magnitude of stress applied to lower than the yield stress of martensite have been found to be suitable strategies for increasing the lifespan of SMA-based actuators during partial thermomechanical cycling.

scholarly journals Dislocation dynamics modelling of the creep behaviour of particle-strengthened materials

2021 ◽  
Vol 477 (2250) ◽  
pp. 20210083
Author(s):  
F. X. liu ◽  
A. C. F Cocks ◽  
E. Tarleton
Keyword(s):  
Elevated Temperatures ◽  
Creep Behaviour ◽  
Particle Interaction ◽  
Three Dimensional ◽  
Cross Slip ◽  
Dislocation Dynamics ◽  
Dislocation Slip ◽  
Fundamental Particle ◽  
Dislocation Glide ◽  
Time Scale Separation

Plastic deformation in crystalline materials occurs through dislocation slip and strengthening is achieved with obstacles that hinder the motion of dislocations. At relatively low temperatures, dislocations bypass the particles by Orowan looping, particle shearing, cross-slip or a combination of these mechanisms. At elevated temperatures, atomic diffusivity becomes appreciable, so that dislocations can bypass the particles by climb processes. Climb plays a crucial role in the long-term durability or creep resistance of many structural materials, particularly under extreme conditions of load, temperature and radiation. Here we systematically examine dislocation-particle interaction mechanisms. The analysis is based on three-dimensional discrete dislocation dynamics simulations incorporating impenetrable particles, elastic interactions, dislocation self-climb, cross-slip and glide. The core diffusion dominated dislocation self-climb process is modelled based on a variational principle for the evolution of microstructures, and is coupled with dislocation glide and cross-slip by an adaptive time-stepping scheme to bridge the time scale separation. The stress field caused by particles is implemented based on the particle–matrix mismatch. This model is helpful for understanding the fundamental particle bypass mechanisms and clarifying the effects of dislocation glide, climb and cross-slip on creep deformation.

scholarly journals 2 Dimensional Dislocation Dynamics – A New Technique for the Simulation of Deformation Microtextures

1997 ◽  
Vol 28 (3-4) ◽  
pp. 167-179
Author(s):  
F. Roters ◽  
D. Raabe
Keyword(s):  
Cold Deformation ◽  
Dislocation Motion ◽  
Dislocation Dynamics ◽  
New Technique ◽  
Linear Elastic ◽  
Straight Line ◽  
Simulation Step ◽  
Local Stresses ◽  
A New Technique ◽  
Edge Dislocations

A new technique for the simulation of microtexture evolution during cold deformation which is based on 2 dimensional (2D) dislocation dynamics is presented. In the simulation all involved dislocations are regarded as infinite straight line detects which are embedded in an otherwise isotropic linear elastic medium. As the model is 2D only edge dislocations are considered.In the first simulation step the net local stresses are derived and used to calculate the resulting dislocation motion. Dislocation multiplication, annihilation and reactions are taken into account. Thermal activation is included. In the second step the local misorientations arising from the dislocation distribution are calculated.This method shows in microscopic detail how misorientations are generated and distributed within grains during plastic deformation.

A New Dislocation-Dynamics Model and Its Application in Thin Film-Substrate Systems

2005 ◽  
Vol 875 ◽  
Author(s):  
E.H. Tan ◽  
L.Z. Sun
Keyword(s):  
Thin Films ◽  
Mechanical Performance ◽  
Dislocation Motion ◽  
Dislocation Dynamics ◽  
Dislocation Segment ◽  
Dislocation Loops ◽  
Dynamics Model ◽  
Proposed Model ◽  
Simulation Results ◽  
Film Substrate

AbstractBased on the physical background, a new dislocation dynamics model fully incorporating the interaction among differential dislocation segments is developed to simulate 3D dislocation motion in crystals. As the numerical simulation results demonstrate, this new model completely solves the long-standing problem that simulation results are heavily dependent on dislocation-segment lengths in the classical dislocation dynamics theory. The proposed model is applied to simulate the effect of dislocations on the mechanical performance of thin films. The interactions among the dislocation loops, free surface and interfaces are rigorously computed by a decomposition method. This framework can be used to simulate how a surface loop evolves into two threading dislocations and to determine the critical thickness of thin films. Furthermore, the relationship between the film thickness and yield strength is established and compared with the conventional Hall-Petch relation.

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