Hexagonal Dislocation Networks Research Articles

High-cycle and low-cycle fatigue characteristics of multilayered dissimilar titanium alloys

Multilayered structures of dissimilar titanium alloys can achieve excellent fracture ductility and strength, while their fatigue characteristics especially dislocation networks and twin formation are rarely reported. Heterogeneous microstructures are observed in the multilayered TC4/TB8 alloys, including fine acicular α grains, continuous α layer at prior β grain boundaries (αGB) and β matrix on the TB8 layer, together with equiaxed α grains on the TC4 layer. High-cycle fatigue (HCF) and low-cycle fatigue (LCF) tests show that the initial fatigue damage appears at the αGB/β matrix interfaces on the TB8 layer instead of the boned TC4/TB8 interfaces. Since the stress concentration induced by dislocation pile-up is prone to micro-void formation and crack propagation at the αGB/β interfaces. For LCF, the αGB/β interfaces can not only act as impenetrable barriers and sources of lattice dislocations, but also allow the dislocations cross boundaries during cyclic tension and compression because of the high boundary energy. The formation characteristic of deformation twins that is beneficial for the plastic deformation of α grains in TC4 layer during cyclic strain is investigated. Furthermore, the hexagonal dislocation networks are also found within the equiaxed α grains of TC4 layer after LCF, and the role between interface barrier and slip direction in the formation mechanism is analyzed.

Anisotropic deformation mechanism of {110} hexagonal dislocation networks in BCC Iron

Molecular dynamics simulation was performed to study the anisotropic mobility behavior of hexagonal dislocation networks (HDNs) in a series of bicrystalline pure Fe samples under a force-control in-plane shear deformation test. It is found that the mobility of HDN is mainly controlled by the characteristic features of <001> dislocations, which formed in the vicinity of ½<111> dislocations lying at the HDN. Furthermore, we elucidate the underlying mechanism of shearing in HDN as the variation of the length and the core energy of <001> dislocation lines with HDN misorientation angle. The energy barrier analysis of kink activities on the HDN plane is used to address the contribution of these geometrical and structural features to the in-plane shearing anisotropy of the HDN.

Molecular dynamics study of hydrogen isotopes at the Be/BeO interface

Molecular dynamics simulations are used to investigate the behaviour of D atoms at two interfaces between beryllium (Be) and beryllium oxide (BeO). After relaxation of the simulation cell, there are (a) localised defects at the interface and (b) a hexagonal misfit dislocation network creating a succession of compressed and expanded area from each side of the interface. The simulations between 750 K and 1500 K for tens to hundreds of nanoseconds show that both interfaces act as trapping sites for D atoms. The simulations also show that D atoms tend to migrate in the material where the hydrogen isotope solubility is the highest as predicted by thermodynamics. However, the simulations also shows that there are additional kinetic barriers (D trapping sites, D2 formation/dissociation in BeO) that slow down the path to equilibrium. These additional kinetic barriers may influence the fuel retention and permeation in Be materials.

Microstructure evolution in Si+ion irradiated and annealed Ti3SiC2MAX phase

AbstractTi3SiC2samples were irradiated by a 6‐MeV Si+ion to a fluence of 2 1016Si+ions/cm2at 300°C followed by annealing at 900°C for 5 h. A transmission electron microscope was used to characterize microstructural evolution. The phase of Ti3SiC2transformed from the hexagonal close‐packed (HCP) to a face‐centered cubic structure after irradiation. Hexagonal screw dislocation networks were identified at the deepest position of the irradiated area, which are the products of dislocations reactions. After annealing, the irradiated region has reverted to the original HCP structure. High‐density cavities and stacking faults were formed along the basal planes. In addition, ripplocations have been observed in the irradiated region in the Ti3SiC2sample after annealing. Our insights into the formation processes and corresponding mechanisms of these defect structures might be helpful in the material design of advanced irradiation tolerance materials.

Dislocation structure analysis in the strain gradient of torsion loading: a comparison between modelling and experiment

Complex stress states due to torsion lead to dislocation structures characteristic for the chosen torsion axis. The formation mechanism of these structures and the link to the overall plastic deformation are unclear. Experiments allow the analysis of cross sections only ex situ or are limited in spacial resolution which prohibits the identification of the substructures which form within the volume. Discrete dislocation dynamics simulations give full access to the dislocation structure and their evolution in time. By combining both approaches and comparing similar measures the dislocation structure formation in torsion loading of micro wires is explained. For the ⟨100⟩ torsion axis, slip traces spanning the entire sample in both simulation and experiment are observed. They are caused by collective motion of dislocations on adjacent slip planes. Thus these slip traces are not atomically sharp. Torsion loading around a ⟨111⟩ axis favors plasticity on the primary slip planes perpendicular to the torsion axis and dislocation storage through cross-slip and subsequent collinear junction formation. Resulting hexagonal dislocation networks patches are small angle grain boundaries. Both, experiments and discrete dislocation simulations show that dislocations cross the neutral fiber. This feature is discussed in light of the limits of continuum descriptions of plasticity.

Micrograin structure evolution associated with grain boundary characteristics in grade 91 steel during long-term creep exposure

Micrograin structure evolution in ASME Grade 91 steel after 233,823 h of creep exposure at 600 °C was characterized after grain boundary classification in terms of misorientation angle (MOA), boundary energy, and lath martensite substructure. Under long-term creep with an applied stress of 5.9 MPa, micrograin coarsening occurs by annihilation of lath boundaries, consisting of hexagonal dislocation networks (HDN), thanks to the “knitting-out” of dislocations from the boundary, and the absorption of interior mobile dislocations into the boundary. This annihilation is expectedly faster when the original lath boundary shows a small MOA, and is decorated by little precipitation (MX carbonitrides and boundary M23C6 carbides). Under long-term creep with an applied stress of 86.7 MPa, micrograin coarsening occurs not only by lath boundary annihilation, but also by the migration of all other boundary types found in the lath martensite microstructure. This migration is described as a strain-induced boundary migration (SIBM), and mainly occurs when boundaries are relatively precipitation-free, and a gradient of stored internal energy is present between adjacent grains. Even within the same class of boundary, the progress of SIBM differs due to the local evolution of precipitates and the actual boundary energy.

Effect of ball milling treatment on the microstructures and properties of Cr2AlC powders and hot pressed bulk ceramics

Ball milling was used on synthesized Cr2AlC powders, and dense Cr2AlC bulk ceramics with almost pure phase were fabricated by hot pressing these ball milled Cr2AlC powders at 1300 °C for 2 h under 30 MPa. The phase compositions and the microstructures of powders and bulk ceramics were characterized. The mechanical properties and dislocation analysis of bulk ceramics were also investigated. Our results indicated that the grain size of powders became uniform and smaller after ball milling, which was also inherited in the bulk ceramics. Moreover, the mechanical properties of hot pressed Cr2AlC ceramics, including flexural strength, fracture toughness, and Vickers hardness, significantly increased by using ball milled powders treated at speed 200 rpm. The organization of hexagonal dislocation networks during hot pressing and reduction of grain size both had a positive effect on improving mechanical properties.

General yield behaviors of the {1 1 0} hexagonal dislocation networks in body centered cubic metal molybdenum

The {110} hexagonal dislocation networks (HDNs) act as important carriers of the low-temperature plastic deformation in the body-centered-cubic (BCC) metals. Erenow, only a handful of studies have been performed on the yield behaviors of this planar networks, resulting in it being perplexing. In this letter, by three types of loading modes, the general yield behaviors, which comprise the critical stresses to move the network along any direction on the (011¯) plane, the partitions on the cooperative motion of two or three sets of dislocations and the conditions under which the steady motion of the HDN takes place, are proposed to disclose the motion mechanism for the HDNs in molybdenum by atomistic simulations. Also, the characteristic that the motion directions of the HDN are changeable, a result of the natural transition to the steady motion, explains why there is no fixed dead band for the (011¯) anomalous slip in these BCC metals.

Evolution of interfacial dislocation networks during long term thermal aging in Ni-based single crystal superalloy DD5

Interfacial dislocations found in single crystal superalloys after long term thermal aging have an important effect on mechanical properties. Long term thermal aging tests for DD5 single crystal superalloy were carried out at 1,100 °C for 20, 100, 200, 500 and 1000 h, and then cooled by air. The effect of long term thermal aging on the dislocation networks at the γ/γ' interfaces was investigated by FE-SEM. Results showed that during the long term thermal aging at 1,100 °C, misfit dislocations formed firstly and then reorientation in the (001) interfacial planes occurred. Different types of square or rectangular dislocation network form by dislocation reaction. Square dislocation networks consisting of four groups of dislocations can transform into octagonal dislocation networks, and then form another square dislocation network by dislocation reaction. Rectangular dislocation networks can also transform into hexagonal dislocation networks. The interfacial dislocation networks promote the γ' phase rafting process. The dislocation networks spacings become smaller and smaller, leading to the effective lattice misfit increasing from −0.10% to −0.32%.

Tensile response of (1 1 0) twist grain boundaries in tungsten: A molecular dynamics study

Molecular dynamics simulations are performed to investigate the tensile behavior of W bicrystals with different twist grain boundaries (TGBs): the low-angle grain boundary (LAGB), the ordinary high-angle grain boundary (HAGB) and the Σ3 TGB. Owing to the initial hexagonal dislocation network (HDN), the LAGB can directly emit dislocations without dislocation nucleation: the 12〈111〉 dislocations in HDN are pinned by bilateral nodes or 〈001〉 dislocations and form Frank-Rend dislocation sources, which continually emit dislocations and remain the structure of HDN under tensile loading. Once the HDN become disordered, Frank-Rend dislocation sources are broken and dislocations in HDN can be directly emitted. In the ordinary HAGB and Σ3 TGB, dislocations are nucleated from interfaces accompanied with apparent stress decrease at nucleation sites. Supplied with enough stress, emitted dislocations can freely pass through adjacent boundaries. By measuring dislocation densities, we find that the Σ3 TGB becomes more active at high temperature and easier to nucleate dislocations, while the LAGB and ordinary HAGB are less affected by temperature. Besides, it is found that 12〈111〉 dislocations play a dominate role in the plastic deformation of W bicrystals regardless of TGB structures and temperature.

Effects of pre-creep on the dislocations of 316LN Austenite stainless steel

The 316LN Austenite stainless steels (316LNASS) were pre-creep treated, the evolution of microstructure were investigated. The samples were pre-creep at 593 K and from 500 to 2000 h at 873 K with a stress in the range of 20 to 150 MPa, Then the evolution of microstructure and precipitation were investigated by optical microscope (OM), and transmission electron microscope (TEM). The results show that the crystal surface slipping resulted in dislocations and original dislocations decomposition during the pre-creep process, and generate quadrilateral or hexagonal dislocation network was obviously. The sub-grain boundary gradually became narrow with the increasing of pre-creep treatment time and temperature. When the pre-creep temperature was 593 K and 873 K, dislocation network gradually disappear with the increasing of pre-creep time and load. When the pre-creep temperature was 873 K under 120 MPa, and the treatment time was 2000 h, the hexagonal dislocation network (HDN) would completely disappeared. When the pre-creep temperature was 593 K under 20 MPa, and the treatment time was 500 h, the quadrilateral dislocation network (QDN) would completely disappeared.

Phase field crystal simulation of grain boundary motion, grain rotation and dislocation reactions in a BCC bicrystal

We investigate grain boundary motion and grain rotation in a body-centered cubic bicrystal composed of a spherical grain embedded in a single crystal matrix by three-dimensional phase-field crystal simulations. Structure and time evolution of dislocation networks formed on the grain boundary during the capillarity-driven grain shrinkage are examined. The results for initially spherical grains rotated about the [110] or [111] axes of the matrix grain reveal the formation of hexagonal dislocation networks (HDNs) on the grain boundary. We demonstrate that the anisotropic distribution of the HDNs is responsible for asymmetric shrinkage of the embedded grain. Through a detailed analysis of the HDNs, we clarify the mechanisms of dislocation reactions during the grain shrinkage in three dimensions, which include dissociation and recombination of a/2<111> and a<100> dislocations. The configuration of the HDNs is strongly affected by the rotation axis of the embedded grain. For large misorientations, the high density of the HDNs accelerates dislocation reactions and leads to very small grain rotations. However, if the misorientation is small and the dislocations are further apart, the lack of dislocation reactions on grain shrinkage results in grain rotation. Even though the rotation axis and the misorientation strongly affect the grain shape and the grain rotation, the kinetics of the grain shrinkage associated with the grain rotation follow the classical theory for grain growth: area of the embedded grain shrinks linearly with time. We also show that the stagnation of the grain rotation slows the shrinkage of the embedded grain.

Core structure of a interfacial superdislocations in a nickel-base superalloy during high-temperature and low-stress creep

Dislocation substructures were investigated in a nickel-base single crystal superalloy subjected to 85h creep at 1100°C and 130MPa using conventional and high resolution transmission electron microscopy. Both hexagonal and square-like interfacial dislocation networks were observed. The square-like networks, which were caused by two perpendicular sets of a/2<110> dislocations forming a[100] superdislocations, were tested. It was concluded that the a[100] interfacial dislocation occurred in the form of a dislocation dipole. The creation and destruction of dislocation dipoles proceeded reversibly via a slip-and-climb mechanism.

On the small angle twist sub-grain boundaries in Ti3AlC2.

Tilt-dominated grain boundaries have been investigated in depth in the deformation of MAX phases. In stark contrast, another important type of grain boundaries, twist grain boundaries, have long been overlooked. Here, we report on the observation of small angle twist sub-grain boundaries in a typical MAX phase Ti3AlC2 compressed at 1200 °C, which comprise hexagonal screw dislocation networks formed by basal dislocation reactions. By first-principles investigations on atomic-scale deformation and general stacking fault energy landscapes, it is unequivocally demonstrated that the twist sub-grain boundaries are most likely located between Al and Ti4f (Ti located at the 4f Wyckoff sites of P63/mmc) layers, with breaking of the weakly bonded Al–Ti4f. The twist angle increases with the increase of deformation and is estimated to be around 0.5° for a deformation of 26%. This work may shed light on sub-grain boundaries of MAX phases, and provide fundamental information for future atomic-scale simulations.

Characterizing twist grain boundaries in BCC Nb by molecular simulation: Structure and shear deformation

Atomic scale modeling was used to study the structure, energy and shear behaviors of (110) twist grain boundaries (TWGBs) in body-centered cubic Nb. The relation between grain boundary energy (GBE) and the twist angle θ agrees well with the Read-Shockley equation in low-angle range. At higher angles, the GBEs show no distinct trend with the variation of the twist angle or the density of coincident lattice sites. All (110) twist boundaries can be classified into two types: low-angle grain boundaries (LAGBs) and high-angle grain boundaries (HAGBs). LAGBs contain a hexagonal dislocation network (HDN) which is composed of 12[111], 12[1¯1¯1] and [001] screw dislocations. HAGBs can be classified into three sub-types further: special boundaries with low Σ, boundaries in the vicinity of special boundaries with similar structures and ordinary HAGBs consisting of periodic patterns. Besides, a dependence of grain boundary shear response vs the twist angle over the entire twist angle range is obtained. Pure sliding behavior is found at all TWGBs. When θ<12°, the flow stress of LAGBs is found to be correlated with the HDNs and decreases with the increasing twist angle. For ordinary HAGBs, the magnitude of flow stress is around 0.8–1.0GPa and the twist angle has little effect on the anisotropy mobility. For special grain boundaries with low Σ, the boundary structures govern the GBEs and shear motion behavior significantly.

Guided Self-Assembly of Nano-Precipitates into Mesocrystals.

We show by a combination of computer simulation and experimental characterization guided self-assembly of coherent nano-precipitates into a mesocrystal having a honeycomb structure in bulk materials. The structure consists of different orientation variants of a product phase precipitated out of the parent phase by heterogeneous nucleation on a hexagonal dislocation network. The predicted honeycomb mesocrystal has been confirmed by experimental observations in an Mg-Y-Nd alloy. The structure and lattice parameters of the mesocrystal and the size of the nano-precipitates are readily tuneable, offering ample opportunities to tailor its properties for a wide range of technological applications.

Insights into High‐Temperature Uniaxial Compression Deformation Behavior of Ti3AlC2

We report on strain‐rate‐dependent compression deformation behavior of Ti3AlC2 at 1000°C–1200°C. At 1000°C and high strain rate (10−2 or 10−3 s−1), Ti3AlC2 deforms in a nonplastic manner. Upon increasing temperature and reducing strain rate, Ti3AlC2 exhibits a limited plasticity. For instance, the true plastic strain at 1200°C and 10−4 s−1 is only 3%, beyond which strain softening following a short hardening regime occurs. The softening results from the formation of localized microvoids and microcracks. Decreasing the strain rate further to 10−5 s−1 at 1200°C, strain hardening instead of softening is identified. Under such conditions, the plastic strain remarkably increases, reaching a value as high as 27%. Postdeformation microstructural analyses of the dislocation configurations explicitly evidence the dislocation reactions, formation of hexagonal dislocation networks and dislocation entanglements. These account for the strain hardening. The extraordinary plasticity at 1200°C and 10−5 s−1 benefits from the initiation of nonbasal slip systems. Finally, a complete high‐temperature deformation scenario for nanolaminated Ti3AlC2 is elaborated.

Quantitative understanding of anomalous slip in Mo

Hexagonal dislocation networks (HDNs) formed by the reaction of <1 1 1>/2 screw dislocations are frequently observed in association with anomalous slip in body-centred cubic (bcc) metals. However, its role assigned in anomalous slip remains obscure due to the absence of quantitative description of its response to uniaxial loading. Here, systematic atomistic simulations are performed in molybdenum (Mo) to study the responses of a typical HDN to different applied loadings. The simulation results are used to develop a quantitative yield criterion for the HDN motion under uniaxial loading. Based on this criterion together with the yield equation that can account for the non-Schmid behaviours of an isolated <1 1 1>/2 screw dislocation, the transition from primary to anomalous slips with the loading direction is predicted to be consistent with the experimental observations in many bcc metals including Mo. This work also sheds light on other experimental results such as the lack of dead-band and the displacement accompanying anomalous slip. In addition, the reason for the absence of anomalous slip in bcc iron (Fe) is found by comparison of the reaction between <1 1 1>/2 screw dislocations in Mo and Fe.

Dislocation cross-slip controlled creep in Zircaloy-4 at high stresses

Abstract Uniaxial creep tests were performed on Zircaloy-4 sheet in the temperature range of 500–600 °C at high stresses (>10−3E), to uncover the rate-controlling mechanism. A stress exponent of 9.3–11 and a stress-dependent activation energy in the range of 220–242 kJ/mol were obtained from the steady state creep data. TEM analyses revealed an extensive presence of hexagonal screw dislocation network on the basal planes indicating recovery of screw dislocations by cross-slip to be the dominant mechanism. The creep data was therefore analyzed in the light of Friedel׳s cross slip model for HCP metals according to which the stress-dependency of the activation energy determined from the creep data was written in the form, U = ( 150 ± 4 ) + ( 2236 ± 124 τ ) kJ / mol The constriction energy of screw dislocations of 150 kJ/mol is in agreement with the values reported in the literature for zirconium and other HCP metals. Further analysis of the yield strength and the activation volume data obtained from stress relaxation tests in the temperature range 500–600 °C favors cross-slip of screw dislocations as the rate controlling mechanism over the test conditions.

X-ray diffraction from hexagonal dislocation networks

We built the honeycomb networks of misfit dislocations by combining angular dislocations. Two different models of the disorder in the dislocation network are explored, the random shifts of nodes and random sources of distortion. The displacement fields of disturbed dislocation networks are used to simulate the X-ray diffraction patterns by Monte Carlo method. We find that substantial disorder is needed for random shifts to be experimentally observable as diffuse scattering. The model of random distortion sources produces comparable diffuse scattering with the dislocation network looking closer to the initial perfect structure. The diffraction patterns due to distorted honeycomb dislocation networks are compared with the ones due to triangular networks of straight dislocations. The disorder parameters of the latter networks can be chosen to produce diffraction patterns similar to the former.