Modeling of the Dislocation Dynamics in Ni3Al and the Flow Stress Anomaly

1996 ◽  
Vol 460 ◽  
Author(s):  
B. Devincre ◽  
P. Veyssiere ◽  
L. Kubin ◽  
G. Saada Lem
Keyword(s):  
Flow Stress ◽  
Single Crystals ◽  
Plastic Flow ◽  
Low Temperatures ◽  
Theoretical Models ◽  
Dislocation Dynamics ◽  
Mesoscopic Scale ◽  
Dislocation Glide ◽  
Cube Plane ◽  
Temperature Regimes

ABSTRACTNi3Al single crystals are known to exhibit a flow stress anomaly between 200 and 800K. The purpose of our work is to examine such an anomaly by means of a simulation of the dislocation dynamics at a mesoscopic scale. The simulation basic rules are: i) the dislocation glide in {111} octahedral planes, ii) the conditions at which screw lines are locked and unlocked by the formation of Kear-Wilsdorf locks, iii) the mobility of jogs in the {100} cube plane. Our results suggest that two different temperature regimes occur in the domain of the anomaly. At low temperatures, the plastic flow is governed by kink bow-out, itself a function of the kink length. At high temperatures, the plastic flow is governed by the unlocking of the weakest Kear-Wilsdorf locks in the microstructure. These outcomes of the simulation are discussed in relation with the existing theoretical models of the flow stress anomaly.

The mechanical properties of pure iron tested in compression over the temperature range 2 to 293 °K

1967 ◽  
Vol 261 (1121) ◽  
pp. 253-287 ◽  
Keyword(s):  
Mechanical Properties ◽  
Flow Stress ◽  
Strain Rate ◽  
Single Crystals ◽  
Yield Stress ◽  
Low Temperatures ◽  
Pure Iron ◽  
Constant Strain Rate ◽  
Frictional Stress ◽  
Strain And Strain Rate

The mechanical properties of pure iron single crystals and of polycrystalline specimens of a zone-refined iron have been measured in compression over the temperature and strain rate ranges 2.2 to 293 °K and 7 x 10 -7 to 7 x 10 -3 s -1 respectively. Various yield stress parameters were determined as functions of both temperature and strain rate, and the reversible changes in flow stress produced by isothermal changes of strain rate or by changes of temperature at constant strain rate were also measured as functions of temperature, strain and strain rate. Both the temperature variation of the flow stress and the strain rate sensitivity of the flow stress were generally identical for the single crystals ( ca. 0.005/M carbon) and the polycrystalline specimens ( ca. 9/M carbon). At low temperatures, the temperature dependence of the yield stress was smaller than that of the flow stress at high strains, probably because of the effects of mechanical twinning, but once again the behaviour of single and polycrystalline specimens was very similar. Below 10 °K, both the flow stress and the extrapolated yield stress were independent of temperature. The results show that macroscopic yielding and flow at low temperatures are both governed by the same deformation mechanism, which is not very impurity sensitive, even in the very low carbon range covered by the experiments. The flow stress near 0 °K is ca. 5.8 x 10 -3 u where [i is the shear modulus. On the basis of a model for thermally activated flow, the activation volume at low temperatures (high stresses) is found to be ca. 5 b 3 . The exponent in the empirical power law for the dislocation velocity against stress relation is ca. 3 near room temperature, but becomes quite large at low temperatures. The results indicate that macroscopic deformation at low temperatures is governed by some kind of lattice frictional stress (Peierls-Nabarro force) acting on dislocations.

The Effect of Prestrain on Deformation of Ni3Al Single Crystals

1988 ◽  
Vol 133 ◽  
Author(s):  
W. E. Dowling ◽  
R. Gibala
Keyword(s):  
Flow Stress ◽  
Plastic Strain ◽  
Yield Strength ◽  
Temperature Dependence ◽  
Single Crystals ◽  
Negative Temperature ◽  
Dislocation Dynamics ◽  
Positive Temperature ◽  
Initial Flow ◽  
Bcc Metals

ABSTRACTPrestrain of bcc metals at temperatures T>0.2Tm decreases the flow stress at lower temperatures (T<O.15Tm) where the yield strength has a large negative temperature dependence. This investigation has examined the influence of prestrain on the flow stress of Ni3A1, for which the yield strength has a large positive temperature dependence above 25°C. Nickel-rich Ni3Al single crystals with axial orientations near <001> or <123> were prestrained in compression up to 20% shear strain at -196°C and subsequently compression tested at 550°C. Specimens near the <123> axial orientation were also prestrained at 550°C and then tested at -196°C. The initial flow stress of samples prestrained at - 196°C and tested 550°C was reduced up to 50% compared to samples solely compression tested at 550°C. The magnitude of the reduced flow stress and its extent as a function of plastic strain were dependent upon the amount of prestrain and orientation. Prestraining at 550°C and subsequent testing at -196°C increased the flow stress by as much as 60% over samples solely tested at -196°C. Dislocation substructures obtained from selected samples coupled with arguments based on dislocation dynamics and obstacle strengthening are used to explain the results.

Dislocation Dynamics Studied by Mechanical Loss in a Ni3(AI, Ta) Single Crystal at Intermediate Temperatures

1998 ◽  
Vol 552 ◽  
Author(s):  
B. L. Cheng ◽  
E. Carreño-Morelli ◽  
N. Baluc ◽  
J. Bonneville ◽  
R. Schaller
Keyword(s):  
Flow Stress ◽  
Single Crystal ◽  
Single Crystals ◽  
Screw Dislocation ◽  
Strain Amplitude ◽  
Annealing Temperature ◽  
Mobile Dislocation ◽  
Dislocation Dynamics ◽  
Mechanical Spectroscopy ◽  
Mechanical Loss

ABSTRACTDislocation dynamics in Ni3AI intermetallic single crystals has been studied by mechanical spectroscopy between 300 and 700 K. It has been found that in the anomaly domain of the flow stress, which is characteristic of this material, the mechanical loss of predeformed specimens is strongly dependent on strain amplitude, predeformation level, annealing temperature and time. The results can be interpreted as a combination of two phenomena which simultaneously occur as temperature is increased from 300 K to about 500 K: exhaustion of the mobile dislocation segments (superkinks) and pinning of the screw dislocation segments via cross-slip from the (111) onto the (010) planes.

scholarly journals Complexity and Anisotropy of Plastic Flow of α-Ti Probed by Acoustic Emission and Local Extensometry

2018 ◽  
Author(s):  
Mikhail Lebyodkin ◽  
Kékéli Amouzou ◽  
Tatiana Lebedkina ◽  
Thiebaud Richeton ◽  
Amandine Roth
Keyword(s):  
Acoustic Emission ◽  
Plastic Flow ◽  
Micromechanical Model ◽  
Local Strain ◽  
Simultaneous Recording ◽  
Distribution Functions ◽  
Crystal Defects ◽  
Dislocation Dynamics ◽  
Dislocation Glide ◽  
Deformation Curves

Current progress in the prediction of mechanical behavior of solids requires understanding of spatiotemporal complexity of plastic flow caused by self-organization of crystal defects. It may be particularly important in hexagonal materials because of their strong anisotropy and combination of different mechanisms of plasticity, such as dislocation glide and twinning. These materials often display complex behavior even on the macroscopic scale of deformation curves, e.g., a peculiar three-stage elastoplastic transition, the origin of which is a matter of debates. The present work is devoted to a multiscale study of plastic flow in &alpha;-Ti, based on simultaneous recording of deformation curves, 1D local strain field, and acoustic emission (AE). It is found that the average AE activity also reveals three-stage behavior, but in a qualitatively different way depending on the crystallographic orientation of the sample axis. On the finer scale, the statistical analysis of AE events and local strain rates testifies to an avalanche-like character of dislocation processes, reflected in power-law probability distribution functions. The results are discussed from the viewpoint of collective dislocation dynamics and are confronted to predictions of a recent micromechanical model of Ti strain hardening.

Slip Systems in Ni3(Al, Ti) At Temperatures Above the Peak in Flow Stress

1988 ◽  
Vol 133 ◽  
Author(s):  
Y. Q. Sun ◽  
P. M. Hazzledine
Keyword(s):  
Flow Stress ◽  
Single Crystals ◽  
Deformation Behaviour ◽  
Slip Systems ◽  
Intermetallic Alloys ◽  
Weak Beam ◽  
Cube Plane ◽  
Ordered Intermetallic ◽  
Beam Technique

ABSTRACTDislocations in single crystals of Ni3(Al, Ti) deformed at temperatures above the peak in flow stress have been studied by the TEM “weak-beam” technique. <110> dislocations on the primary cube plane are mostly of edge character, and they have been observed to transform into “super” Lomer- Cottrell locks. <100> dislocations, by contrast, are principally of 45° character. They are believed also to become immobilized by dissociation on {111} planes. Properties of both dislocations on cube planes are discussed and are related to the deformation behaviour of L12ordered intermetallic alloys.

Dislocation structures around SiO2 Particles in aged Cu-Cr-SiO2

1978 ◽  
Vol 36 (1) ◽  
pp. 594-595
Author(s):  
N.J. Long ◽  
M.H. Loretto ◽  
C.H. Lloyd
Keyword(s):  
Flow Stress ◽  
Single Crystals ◽  
Room Temperature ◽  
Thin Foil ◽  
Age Hardening ◽  
Dispersion Strengthening ◽  
Accurate Picture ◽  
The Matrix ◽  
Very High ◽  
Good Agreement

IntroductionThere have been several t.e.m. studies (1,2,3,4) of the dislocation arrangements in the matrix and around the particles in dispersion strengthened single crystals deformed in single slip. Good agreement has been obtained in general between the observed structures and the various theories for the flow stress and work hardening of this class of alloy. There has been though some difficulty in obtaining an accurate picture of these arrangements in the case when the obstacles are large (of the order of several 1000's Å). This is due to both the physical loss of dislocations from the thin foil in its preparation and to rearrangement of the structure on unloading and standing at room temperature under the influence of the very high localised stresses in the vicinity of the particles (2,3).This contribution presents part of a study of the Cu-Cr-SiO2 system where age hardening from the Cu-Cr and dispersion strengthening from Cu-Sio2 is combined.

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