Dislocation-free Zone Research Articles

Nucleation of nanocracks by a quasicleavage process in a dislocation-free zone

Nucleation of nanocracks by a quasicleavage process in a dislocation-free zone

Near threshold fatigue curve based on the dislocation free zone model of fracture

Dislocation pileup along an inclined direction from a crack tip has been observed in metals during tensile deformation in an electron microscope. Dislocations are emitted from the crack tip, move through a dislocation free zone, and pile up in the plastic zone. The problem of inclined pileup of screw dislocations at a crack tip with a dislocation free zone has been solved by applying the Wiener–Hopf method. The result can be used to calculate the crack tip blunting and the local stress intensity K for the applied stress intensity K 3. According to the fatigue theory of Laird or Neumann, the crack propagation per cycle has the order of magnitude of the blunting. The blunting versus K 3 is then plotted. It has the shape of the near threshold fatigue curve. To plot this curve, the local stress intensity K has been assumed to be the critical value K g for the dislocation emission at the crack tip.

Defect-flow-induced heterogeneous dislocation formation and solute redistribution near a grain boundary in austenitic stainless steel under electron irradiation

The authors investigated radiation-induced segregation (RIS) at a grain boundary in an Fe–Cr–Ni alloy under irradiation taking account of the evolution of faulted dislocation loops and network dislocations. Electron irradiation with a high voltage electron microscope (1000 kV) at Hokkaido University, Japan, was performed to study radiation-induced solute segregation and point defect flow in a typical austenitic Fe–Cr–Ni alloy. The theoretical analysis was conducted by solving the coupled rate equations for solute and defect concentrations near a grain boundary sink. The experimental solute redistribution profiles predicted by the theory were explained qualitatively. The formation of dislocation free zone (DLFZ) in the vicinity of a grain boundary was also observed during in situ observation under electron irradiation. The temperature dependence of the segregation width and that of DLFZ zone width show similar trends. It is thus suggested that RIS and DLFZ formation should be explained by the defect-flow-induced phenomenon under irradiation. The inter-relationship between RIS and such heterogeneous defect clustering near a grain boundary was also clarified substantially.

Liquid metal embrittlement mechanism

Liquid metal embrittlement was studied in the following two aspects. First the first principle and Chen-Nanxian three-dimensional lattice reverse method were employed to obtain the effective potentials for Al-Ga and Ga-Ga. Then with the molecular dynamics simulation, the influence of liquid metal adsorption on dislocation emission was studied. The simulated result shows that after Ga atoms are adsorbed on the crack plane in Al crystal, the critical stress intensity factor decreases, which changes from 0.5 MPam1/2 (without adsorption) to 0.4 MPam1/2 (with adsorption). The reason for the reduction in the critical intensity stress factor is that Ga adsorption reduces the surface energy of the crack plane. Moreover, 7075 Al alloy adsorbing liquid metal (Hg+3atm%Ga) wasin-situ studied in TEM by using a special constant deflection device. The experimental result showed that liquid metal adsorption could facilitate emission, multiplication and motion of dislocations. When this process reached a critical condition, the brittle microcrack was initiated and propagated from the crack tip or in a dislocation free zone.

In situ TEM research of dislocation emission and microcrack nucleation for Ti after adsorption by Hg

Using a special constant deflection device, the change in dislocation configuration ahead of a loaded crack tip for a-Ti, before and after adsorption of Hg atoms, and the initiations of Hg-induced microcracks have been observed in TEM, as well as the in situ extension in TEM without Hg. The results showed that chemisorption of Hg atoms facilitates dislocation emission and motion. When the chemisorption-enhanced dislocation emission and motion develop into a critical situation, a microcrack in LME nucleates from the main crack tip and/or in the dislocation free zone (DFZ), and propagates in cleavage mode. During in situ extension in TEM without liquid metal, when dislocation emission and motion induced by thermal activity and applied stress reach a certain critical condition, microcracks nucleate but do not blunt into a void.

The Behavior of Screw Dislocations Emitted from a Star Crack with a Central Hole

The behavior of screw dislocations emitted from a star crack with a central hole was investigated using discrete dislocation modeling. Cracks are uniformly distributed along the circumference of a circular hole. Dislocations are assumed to be emitted one by one from the crack tips along the radial direction. Each emitted dislocation moves along the radial direction and its velocity is proportional to the third power of the effective shear stress. A dislocation-free zone exists based on the assumption that the crack tip must overcome an energy barrier to emit a dislocation. The effects of the central hole, slit crack, number of cracks and applied stress on the plastic zone, total number of dislocations emitted from all crack tips and the first crack tip, and the dislocation-free zone were studied for a given friction stress.

Dislocation behaviours ahead of crack tip

In this paper, a unified mechanics model for dislocation nucleation, emission and dislocation free zone is proposed based on the Peierls framework. Three regions are identified ahead of the crack tip. The emitted dislocations within the plastic zone in the form of an inverse pile up are treated as discrete elastic edge dislocations. Between that zone and the cohesive zone immediately ahead of the crack tip, there is a dislocation free zone. With the stress field and the dislocation density field in the cohesive zone, respectively, expressed in the first and second Chebyshev polynomial series, and the opening and slip displacements in trigonometric series, a set of nonlinear governing equations are obtained which take into account for the interaction between the emitted dislocations and cohesive zone and the nonlinear interaction between sliding displacement and the opening displacement. After discretization, the governing equations are transformed into a set nonlinear algebraic equations which are solved with Newton-Raphson Method. The results of calculation for pure shearing and combined tension and shear loading after dislocation emission are given in detail. Finally, the process of dislocation nucleation and emission on a pair of symmetric slip planes of angle α with respect to the crack plane under pure mode I load is analysed. The equilibrium positions and the number of emitted dislocation are determined. Several possible competition behaviors of dislocation emission vs cleavage are revealed.

The multifarious plastic zones in front of a crack

Since there is a stress concentration at the tip of a crack, plastic deformation must take place so that there will be a plastic zone whose size depends on the yield stress of the material. As a result the stress concentration disappears and the crack loses its potential to propagate. To show how does the dislocation-free zone develop in the case of Mode II or III, computer simulations were performed and it was shown that the dislocations actually collect at a distance from the crack tip while the stress intensity factor at the crack tip decreases. So the dislocation-free zone is large to begin with and gradually decreases. Only when the stress intensity factor at the crack tip becomes zero does the dislocation-free zone disappear. So the plastic zone can be developed in many different ways even in Mode II or III. For Mode I there are very few studies because of the complexity of the calculations. Their work showed that the plastic zone depends on the availability of slip systems and also on the order of emission of dislocations from the crack tip. The plastic zone can be saturated with dislocations (when no more dislocations can be emitted evenmore » without any critical stress intensity factor for dislocation emission) and yet the stress intensity factor at the crack tip is still not zero. A dislocation-free zone exists if there is a sufficiently large critical stress intensity factor for dislocation emission. The shape of the dislocation-free zone is similar to that of the plastic zone. These and other findings will be discussed here.« less

Size scale effects on brittle to ductile transition

In the present paper the influence of a specific microstructural feature on the brittle to ductile behaviour of semi-brittle materials is emphasised. The dislocation free zone size (formerly introduced to account for the behaviour of these materials) for which incomplete crack tip shielding is expected, is a function of the dislocation sources activity. This is a first scaling factor, it is required but not sufficient. A second factor has to be introduced along the third dimension: the spacing between dislocation sources. For initially dislocation-free crystals a sharp transition is observed when its value is not negligible compared to the sample's thickness. For materials containing bulk sources a soft transition is always noticed: we think that stimulated emission rather than spontaneous nucleation is likely to occur at crack tip, under a low stress level. This soft transition results from the by-passing of the high-energy cost spontaneous emission.

TEM Study of Microcrack Nucleation and Propagation for 310 Stainless Steel

TEM in situ tensile tests of 310 stainless steel show that a dislocation free zone (DFZ) forms if the displacement keeps constant after dislocations are emitted from a crack tip. The elastic DFZ is gradually thinned and the stress in the DFZ will reach the cohesive strength, resulting in nucleation of nanocracks in it and their bluntness into voids. If continuously tensioning, the inhomogeneously thinning ahead of the crack tip, initiat-ing and connecting of microcracks or microvoids will be observed rather than a DFZ, nanocracks' initiation and bluntness into voids. The inverse pile-up ahead of a loaded crack tip can move back to the crack tip when unloading.

Modeling of Segregation and Microstructural Evolution near Grain Boundary in FE-CR-NI Alloy Under Irradiation

AbstractWe have investigated the solute segregation and simultaneous evolution of extended defects in an Fe-Cr-Ni alloy during irradiation by computer simulation. It sheds a light on the accomplishment of performing “the combined total calculation” or “the muliscale modeling” which deals with both radiation-induced segregation and various kinds of internal sink evolution. The formation of dislocation-free zone (DLFZ) was predicted in the vicinity of a grain boundary. It indicated that DLFZ formation is controlled by solute diffusional process via point defects diffusion near the grain boundary and the activation energy obtained by the width of DLFZ corresponds to the half of the value of the radiation-enhanced solute diffusivity.

Chemisorption-facilitated dislocation emission and motion, and induced nucleation of brittle nanocrack

Using a special TEM constant deflection device, the change in dislocation configuration ahead of a loaded crack tip before and after adsorption of Hg atoms and the initiation of liquid metal-induced nanocracks (LMIC) have been observed. The results show that chemisorption of Hg atoms can facilitate dislocation emission, multiplication and motion. Nanocracks will be initiated in the dislocation-free zone (DFZ) or at the crack tip when chemisorption-facilitated dislocation emission, multiplication and motion reach a critical condition. On the basis of the available experimental evidence concerning liquid metal embrittlement (LME), a new mechanism for this phenomenon is considered. This involves the fact that the decrease in surface energy induced by chemisorption of Hg atoms results in a reduction in the critical stress intensity factors for dislocation emission and the resistance for dislocation motion. On the other hand, the plastic work andK IC will decrease with the decrease in the surface energy.

Hydrogen-enhanced dislocation emission, motion and nucleation of hydrogen-induced cracking for steel

The change in dislocation configuration ahead of a loaded crack tip before and after charging with hydrogen wasin situ investigated in TEM using a special constant deflection loading device. The results showed that hydrogen could facilitate dislocation emission, multiplication and motion. The change in displacement field ahead of a loaded notch tip for a bulk specimen before and after charging with hydrogen wasin situ measured by the laser moire interferometer technique. The results showed that hydrogen could enlarge the plastic zone and increase the plastic strain. Thein situ observation in TEM showed that when hydrogen-enhanced dislocation emission and motion reached a critical condition, a nanocrack of hydrogen-induced cracking (HIC) would nucleate in the dislocation-free zone (DFZ) or at the main crack tip. The reasons for hydrogen-enhanced dislocation emission, multiplication and motion, and the mechanisms of nucleation of HIC have been discussed.

Dynamic simulation of dislocation microstructures in Mode III cracking

We have developed a new, self-consistent simulation method for modeling crack growth with dislocation generation and motion in constant-loading-rate, Mode-III fracture. The dislocations emitted from the crack initially self-organize and propagate in very sharply defined lines. These lines undergo bifurcations, forming multiple new branches and shortening the initial line. The growth and bifurcation of these lines occurs repeatedly. Away from the crack, a highly structured plastic zone is formed that is approximately elliptical in shape with a dislocation free zone along its mid-plane. The rate of generation of new dislocations is limited by the rate at which previously generated dislocations move away from the crack tip. This rate is controlled by the crack loading rate K ̇ III and the dislocation mobility. The size of the plastic zone scales as ( K 2 III K ̇ III ) 2 3 . The crack tip stress intensity factor K tip is very much smaller than the applied stress intensity factor. K tip increases sub-linearly with the load and exhibits both jumps and serrations corresponding to instabilities in the dislocation microstructure. K tip increases, however, with increasing loading rate at fixed load and a transition is seen between brittle and ductile behavior with decreasing loading rate. Crack propagation occurs when dislocations cannot be generated at the crack tip at a rate sufficient to counterbalance the increasing loading. This generation rate increases with increasing dislocation mobility. Since dislocation motion is thermally activated, this demonstrates that the brittle-to-ductile transition is ultimately controlled by dislocation migration.

Modelling crack tip plastic zones and brittle-ductile transitions

Plasticity at crack tips may be modelled as self-organising arrays of dislocations emitted from a source near the crack tip. For materials where dislocation motion is slow and any friction stress is low, the modelled arrays are far from equilibrium. If dislocation motion is fast, and there is a substantial dislocation pinning stress, τ f, the arrays are quasi-static and the model predicts behaviour very similar to those of earlier static models. A power-law relation connects the shielded crack tip stress intensity, k, to the pinning stress, τ f, the length of the dislocation array, d and the size of the ‘dislocation free zone’ near the crack tip. Using either of two fracture criteria, (a) k = K Ic, or (b) the stress at some point ahead of the crack tip exceeds a local fracture stress, the variation of stress intensity at fracture K F with temperature is predicted to be controlled by the temperature variation of the yield stress: K F ∝ σ y −0.57.

Corrosion-enhanced dislocation emission and motion resulting in initiation of stress corrosion cracking

A special constant deflection device for TEM has been designed, and then change of dislocation configuration ahead of a crack tip during stress corrosion cracking (SCC) of brass in water and of Ti−24Al−11Nb alloy in methanol and initiation of SCC can be observed in TEM.In situ tensile test in TEM for brass was carried out for comparison. The results show that anodic dissolution during SCC can facilitate dislocation emission, multiplication and motion, and a dislocation free zone (DFZ) is formed. The stress at a particular site in the DFZ, which is an elastic zone and is thinned gradually through corrosion-enhanced dislocation emission and motion, is possibly up to the cohesive strength, resulting in initiating of a nanocrack of SCC in the DFZ or sometimes at the crack tip. Because of the action of the corrosion solution the nanocrack of SCC propagates into a cleavage or intergranular microcrack rather than blunts into a void likein situ tension in TEM.

Dislocation Structures in Single Crystal Nickel- and Cobalt-Based Superalloys under Tensile and Fatigue Loading

Dislocation structures in monocrystalline nickel-based superalloys DD8, ZS26 and a directionally solidified cobalt-based superalloy under tensile and fatigue loading are reviewed. An investigation of dislocation structure in DD8 during fatigue loading at 760 degrees C was conducted. Dislocation bands and three-dimensional networks were found in the matrix, Most of the dislocations observed in the gamma'-phase were in the form of superdislocations. The superimposition of the cyclic component to the process of creep resulted in a minor effect on lifetime of single crystal nickel-based superalloy ZS26. Transmission electron microscopy (TEM) reveals different mechanisms of dislocation-gamma' precipitate interaction under various loading conditions. In addition, TEM in-situ investigation of dislocation behaviour at the crack tip in single crystal nickel-based superalloy DD8 was carried out. Dislocation free zone (DFZ) ahead of the crack tip was frequently observed and affected by the nearby gamma/gamma' interfaces. The defects induced by cyclic straining in the single crystal cobalt-based superalloy are mainly characterized by stacking faults and their intersections.

Dynamic emission of screw dislocations from a propagating crack tip

The dynamic emission of screw dislocations from a propagating crack tip has been investigated. The crack velocity and velocity of each dislocation are assumed to be proportional to the third power of the stress intensity factor and the effective stress, respectively. The stress intensity factor for the ith dislocation emission is derived according to the spontaneous emission criterion. It is found that the stress intensity factor for dislocation emission is independent of crack mobility but the number of dislocations emitted decreases with increasing crack mobility. The number of emitted dislocations and the size of the plastic zone decrease but the maximum stress intensity factor for dislocation emission increases with increasing initial crack length. The size of the dislocation-free zone increases with increasing initial crack length for steady state. The dislocation–crack system reaches the steady state when the dislocations and crack tip move at the same velocity.

In-situ TEM observation of dissolution-enhanced dislocation emission, motion and the nucleation of SCC for Ti24Al11Nb alloy in methanol

Many experiments have showed that anodic polarization can facilitate ambient creep for various metals and alloys. Anodic polarization can decrease the yield strength, and enlarge the plastic zone on the surface ahead of a loaded crack tip. The density of dislocations in a region very close to the fracture surface of SCC was much higher than that far away from the fracture surface. All of these experiments showed that anodic dissolution facilitated the localized plastic deformation. Up to now, however, direct proof of anodic dissolution or corrosion-enhanced dislocation emission, multiplication and motion is lacking. Using a special constant deflection device, the dislocation configuration change ahead of a loaded crack tip before and after anodic dissolution together with the initiation of SCC for Ti-24Al-11Nb alloy in methanol can be in-situ observed in TEM. The results showed that the localized anodic dissolution could facilitate dislocation emission, multiplication and motion and SCC with size of nanometers would initiate in the dislocation-free zone (DFZ) or at the original crack tip after the dissolution-enhanced dislocation emission and motion reached a critical condition.

Theoretical prediction and direct observation of dislocation-free zone formation near a grain boundary in austenitic stainless steel under electron irradiation

We have investigated the radiation-induced segregation (RIS) near a grain boundary in an Fe-Cr-Ni alloy under electron irradiation taking account of the evolution of faulted dislocation loops and network dislocations. First, we predicted, on the basis of the rate theory, the formation dislocation-free zone (DLFZ) in the vicinity (about 30 nm) of a grain boundary and then carried out direct observation of the DLFZ by in-situ irradiation with high-voltage electron microscopy. The inter-relationship between RIS and heterogeneous defect clustering near a grain boundary has been substantially clarified.