Mobile Dislocation Exhaustion Research Articles

Effect of CNTs on Activation Volume and Mobile Dislocation Exhaustion Rate of CNTs/Al under Compression Loading

Effect of CNTs on Activation Volume and Mobile Dislocation Exhaustion Rate of CNTs/Al under Compression Loading

Dislocation-density based crystal plasticity model with hydrogen-enhanced localized plasticity in polycrystalline face-centered cubic metals

Hydrogen-enhanced localized plasticity (HELP) has been recognized as an important mechanism for hydrogen embrittlement. Two recognized explinations have been proposed for HELP. The first one is the elastic shielding theory, in which the elastic stress field of solute hydrogen weakens the interactions between dislocations. Another one is the Gibbs theory of absorption isotherm: the lowered dislocation line energy by segregated hydrogen is considered as the main reason. In this work, a dislocation-density based crystal plasticity framework concerning the explicit incorporation of the dislocation line energy is proposed. The explicit consideration of dislocation line energy leads the way to the modelling of HELP mechanism according to the Gibbs theory of absorption isotherm. It is shown that the experimentally observed hydrogen-reduced activation volume and total activation free energy in thermally activated forest intersection in face-centered cubic (FCC) metals can be easily attributed to the reduction of dislocation line energy in the hydrogen environment. Finite element calculations of tensile tests for polycrystalline Pd-H and Ni-H alloys capture several important hydrogen-affected plasticity behaviors: hydrogen-increased flow stress, hydrogen-enhanced dislocation multiplication, hydrogen-promoted heterogeneity of plastic strain and hydrogen-delayed exhaustion of mobile dislocations, which can be all attributed to hydrogen reduced dislocation line energy in the present model. Besides, the influences of hydrogen-reduced vacancy formation free energy and stacking fault energy on thermal annihilation are also considered. However, the simulation results show that they are less important compared with hydrogen-reduced dislocation line energy. The present work is essential for further physically-based modeling of hydrogen distribution in polycrystals and hydrogen-induced damage and failure.

Size and stress dependences in the tensile stress relaxation of thin copper wires at room temperature

The tensile stress relaxation behavior of thin copper wires with diameters of 20, 35 and 50 μm is experimentally investigated at room temperature. It is revealed that the stress relaxation can occur at low starting stress below the yield strength at room temperature. Significant size and stress dependences in the stress relaxation are observed. That is at a given stress, a smaller-diameter sample exhibits a much faster stress relaxation. At a given diameter, the stress relaxation is accelerated with increasing the starting stress. The value of activation volume increases with increasing the wire diameter, while it increases with decreasing the starting stress. The strain rate elevates as the wire diameter decreases or the starting stress increases. A bilinear relation between the logarithmic strain rate and the logarithmic stress during the relaxation is observed only for the wire sample with a diameter of 20 μm. The intersection of dislocations with grain boundaries is considered to be the main deformation mechanism during the stress relaxation. A rapider exhaustion of mobile dislocations is observed in the wire with smaller diameter in the primary of relaxation. It is attributed to annihilation of dislocations easily escaping from the surface due to larger fraction volume of the near-surface zone. After the primary regime, the higher possibility of dislocations being pinned or immobilized by grain boundaries in a larger-diameter sample is accounted for the size dependence.

On the plasticity mechanisms of lath martensitic steel

The plasticity mechanisms of press hardening steel with a fully lath martensite microstructure were examined experimentally by strain rate sensitivity measurements, repeated relaxation tests and internal friction measurements. The analysis of relaxation tests suggests that the micro-plasticity could be due to the motion of mobile non-screw dislocations, based on mobile dislocation exhaustion observed in the micro-plastic range. In the macro-plastic range, the plasticity is thought to be due to the generation of mobile screw dislocations. The solute carbon-dislocation interaction results in a negative strain rate sensitivity and a Snoek-Köster-Kê peak in the internal friction spectrum of the lath martensitic press hardening steel. The magnitude of the effective activation volume and its stress dependence indicate that plastic deformation is most likely controlled by screw dislocation motion by formation and lateral movement of kink pairs dragging solute carbon atom atmospheres. Both isotropic and kinematical hardening seem to play a role in the strain hardening behavior of lath martensitic steel.

A New Insight in the Plasticity of Ni<sub>3</sub>Al Intermetallic Compounds Using AFM Observations

The positive temperature dependence of flow stress in Ni3Al is examined through fine slip trace analysis performed by atomic force microscopy. Slip traces, which result from dislocation movements, constitute outstanding markers for investigating the elementary dislocation mechanisms that control plasticity. The experiments were performed on two Ni3Al-base alloys, with Ta or Hf as additional elements. The results give evidence that the anomaly domain is accompanied by a drastic exhaustion of mobile dislocations and very short cross-slip distances on the cube cross-slip plane.

A hybrid multiscale computational framework of crystal plasticity at submicron scales

Mechanical behavior for crystals at submicron-to-nanometer scales shows many atypical plastic phenomena. In this paper, a new hybrid multiscale simulation method named DD-FEM is developed by coupling the 3D discrete dislocation dynamics (DDD) simulation with the finite element method (FEM) under elasto-viscoplasticity continuum model. In this methodology, the discrete dislocation plasticity in a finite crystal is solved under a completed continuum mechanics framework. Our approach is competent at dealing with the large and non-linear deformation by communicating deformation information between DDD and FEM. As an application, the deformation of micropillar under uniaxial compression has been predicted. Our results show that the source-truncation hardening and mobile dislocation exhaustion hardening are the main mechanisms for flow intermittency, and stress heterogeneity plays a key role in the observed strain hardening. The effects of contact boundary condition between the indenter and the upper surface of the specimen, as well as the taper of the pillar are also investigated.

Activation volume and density of mobile dislocations in plastically deforming nanocrystalline Ni

We report the analysis of repeated transients to monitor the coupled evolution of dislocation velocity and mobile dislocation density in plastically deforming nanocrystalline Ni. The stress relaxation series allowed the determination of the physical activation volume, indicating a rate-controlling mechanism different from that in coarse-grained Ni. The mobile dislocation exhaustion observed is correlated with the unusually high apparent work-hardening rate during the early stage of straining.

Experimental study of Ni3Al slip traces by atomic force microscopy: an evidence of mobile dislocation exhaustion

Slip markings produced on the surfaces of Ni 3 (Al, Ta) single crystals, plastically deformed at various temperatures in the flow stress anomaly domain, were examined by atomic force microscopy (AFM). A dominant feature is that for all investigated temperatures, the slip traces are rectilinear and correspond to the primary octahedral glide plane. In addition, their lengths drastically decrease when the temperature is raised. This latter result is interpreted as a strong increase of the exhaustion rate of mobile dislocations with increasing temperatures. The consequences of these results in the understanding of Ni 3 Al flow stress anomaly are discussed.

From dislocation cores to strength and work-hardening: a study of binary Ni 3Al

A quantitative model for the peak temperature in work-hardening in L1 2 intermetallics is proposed. It is based on the competition between the exhaustion of mobile dislocations by the Kear Wilsdorf mechanism and the yielding of incomplete locks at high stress. The model is assessed by a set of experimental data measured in binary Ni 3Al polycrystals of three different compositions. These include, in particular, the planar fault energies of the dislocation cores measured by weak-beam electron microscopy, combined with computer image simulations and macroscopic data about flow stress, work-hardening and mobile dislocation exhaustion rates. These parameters are measured as a function of alloy composition. The model also fits successfully data published for other L1 2 compounds.

Dislocation Exhaustion During Plastic Deformation

AbstractRepeated stress-relaxation experiments are used to characterize the deformation parameters in 3 types of single crystals (Cu, Ni3Al and Ge) in which different dislocation mechanisms are known to operate. Mobile dislocation exhaustion rates and the amplitude of the yield point at reloading after stress-relaxation are measured. These two parameters are discussed in terms of the work-hardening coefficient in monotonic tests.

Role of Interfaces in Deformation and Fracture of Ordered Intermetallics

While sub- and grain-boundaries are the primary dislocation sources in Ll{sub 2} alloys, yield and flow stresses are strongly influenced by the multiplication and exhaustion of mobile dislocations from the secondary sources. The concept of enhanced microplasticity at grain boundaries due to chemical disordering is well supported by theoretical modeling, but no conclusive direct evidence exist for Ni{sub 3}Al bicrystals. The strong plastic anisotropy reported in TiAl PST (polysynthetically twinned) crystals is attributed in part to localized slip along lamellar interfaces, thus lowering the yield stress for soft orientations. Calculations of work of adhesion suggest that, intrinsically, interfacial cracking is more likely to initiate on {gamma}/{gamma}-type interfaces than on the {alpha}{sub 2}/{gamma} boundary. 70 refs, 5 tabs, 5 figs.

Effect of Off-Stoichiometry on the Deformation Behavior of Ni3AI Binary Polycrystals

ABSTRACTPolycrystals of binary Ni3Al with different compositions are deformed in compression between 300K and 1300K. They exhibit a strength anomaly with peak temperatures between 800K and 950K. Below the peak, the aluminium rich compounds are the strongest ones while the nickel rich compounds are the weakest ones. As the aluminium content increases, the complex stacking fault energy, measured through weak beam experiments, increases as well. This confirms the cross slip mechanism of mobile dislocation exhaustion through Kear-Wilsdorf lock formation. A ‘ductile-to-brittle transition’ is observed at 1000K. Fracture occurs along grain boundaries, the strength of which decreases as environmental effects are enhanced at high temperatures.

Mechanical strength of the binary compound Ni3Al

Abstract The mechanical properties of binary Ni 76.6 Al 23.4 single crystalline specimens have been studied in compression tests over a wide range of temperatures (293–1100 K). The resolved proof stress ( τ 0.2% ) and the corresponding work-hardening rate ( θ 0.2% ) have been measured as a function of temperature. A technique of repeated stress relaxations has been used to investigate the variation in the density of mobile dislocations that occurs during such transient tests. These experiments have been complemented with strain-rate changes for characterising the strain-rate sensitivity of the flow stress. The high hardening rates and their variation with temperature seem to correlate well with the values of the mobile dislocation exhaustion rates. The strain rate sensitivity of the stress exhibits negative values between 293 and 600 K, below the stress anomaly domain. These mechanical parameters are discussed in terms of the available dislocation mechanisms.

Flow stress anomalies, mobile dislocation exhaustion and strain rate sensitivity

The possible generic origins of flow stress anomalies are discussed and classified into two main categories. Velocity-type anomalies result from an anomalous response of individual dislocation velocities to a temperature variation, whereas exhaustion anomalies involve a variation of the mobile dislocation density. It is shown that the former type usually exhibits a normal strain rate sensitivity, whereas the strain rate sensitivity of the latter type is expected to be zero, after a normal positive transient. This provides a key for the analysis of observed anomalies, which are discussed with some well known examples.

A Statistical Approach for Some Stress Anomalies

AbstractThe yield anomaly of TiAl is modelled in terms of a mechanism of mobile dislocation exhaustion. Several consequences on mechanical properties are derived, that are shown to be applicable to a wide range of anomalies, irrespective of the specific dislocation mechanisms.

Relaxation strain measurements in cellular dislocation structures

The conventional picture of what happens during a stress relaxation usually involves imagining the response of a single dislocation to a steadily decreasing stress. The velocity of this dislocation decreases with decreasing stress in such a way that we can measure the stress dependence of the dislocation velocity. Analysis of the data from a different viewpoint enables us to calculate the apparent activation volume for the motion of the dislocation under the assumption of thermally activated glie. Conventional thinking about stress relaxation, however, does not consider the eventual fate of this dislocation. If the stress relaxes to a low enough level, it is clear that the dislocation must stop. This is consistent with the idea that we can determine the stress dependence of the dislocation velocity from relaxation data only for those cases where the dislocation's velocity is allowed to approach zero asymptotically, in short, for those cases where the dislocation never stops. This conflict poses a dilemma for the experimentalist. In real crystals, however, obstacles impede the dislocation's progress so that those dislocations which are stopped at a given stress will probably never resume motion under the influence of the steadily declining stress present during relaxation. Thus one couldmore » envision stress relaxation as a process of exhaustion of mobile dislocations, rather than a process of decreasing dislocation velocity. Clearly both points of view have merit and in reality both mechanisms contribute to the phenomena.« less

Plastic deformation of tungsten single crystals at low temperatures

The plastic deformation of both strain-anneal grown and melt-grown tungsten single crystals of various orientations and high purity was investigated in the temperature range of 77°K–450°K. It is concluded that: 1. (a) the temperature sensitive yield strength is due to a lattice friction stress, li(b) The very high rates of strain hardening in all orientations can neither be due to the building up of an internal stress, nor due to the accumulation of defects on screw dislocations; it must be due to an insufficiently understood process of exhaustion of mobile dislocations, 2. (c) An anisotropy of the lattice friction stress exists on the {112} planes which produces slip on unexpected slip systems and gives rise to different flow stresses in tension and compression in [001]and [110]oriented crystals.