Microstructure evolution during reheating of a Nb-bearing steel Ⅱ. Dislocation-precipitate interaction

Cooled in water after the isothermal relaxation of deformed austenite for different times, a Nb-bearing microalloyed steel always exhibits the synthetic microstructure in which bainitic ferrite dominates. Strain-induced precipitates do not occur in an unrelaxed sample while they distribute outside dislocations in the sample relaxed for long time. Most of the strain induced precipitates distribute along dislocations in the sample relaxed for proper time. After bainitic transformation, the dislocations formed in the deformed austenite remain to be pinned by the precipitates so that the thermostability of the bainitic ferrite is improved. Coarsening of the precipitates accompanied by their distribution density change has caused the first hardness peak of bainite during reheating. The second hardness peak is attributed to the precipitates, which nucleate in bainite. Dislocations inside the laths getting rid of the pinning of precipitates and their polygonization play the precursor to the evolution of microstructures during reheating.

Moiré superlattices arising from growth induced by screw dislocations in layered materials

Moiré superlattices(MSLs) are modulated structures produced from homogeneous or heterogeneous two-dimensional layers stacked with a twist angle and/or lattice mismatch. Enriching the methods for fabricating MSL and realizing the unique emergent properties are key challenges in its investigation. Here we recommend that the spiral dislocation driven growth is another optional method for the preparation of high quality MSL samples. The spiral structure stabilizes the constant out-of-plane lattice distance, causing the variations in electronic and optical properties. Taking SnS_(2) MSL as an example, we find prominent properties including large band gap reduction(~ 0.4 e V) and enhanced optical activity. Firstprinciples calculations reveal that these unusual properties can be ascribed to the locally enhanced interlayer interaction associated with the Moiré potential modulation. We believe that the spiral dislocation driven growth would be a powerful method to expand the MSL family and broaden their scope of application.

Dislocation configuration evolution during extension twinning and its influence on precipitation behavior in AZ80 wrought magnesium alloy

Thermomechanical treatment T10(extension twinning+aging treatment)can largely enhance the precipitation strengthening effect of magnesium alloys.In this study,dislocation structure evolution and precipitation behavior during T10 treatment of an AZ80 extruded bar were analyzed mainly by two-beam diffraction in TEM.At a compressive strain of 1%in the extrusion direction(ED),a typical dislocation configuration,including basal I1 stacking faults(SFs)and<c+a>dislocations,has been established in extension twins.As the strain reaches 7%,the volume fraction of extension twins increases to more than 90%at which point high dense I1 SFs and<c+a>dislocations occur.After aging for 2 h at 150℃for the 7%strained sample,masses of basal I1 SFs and<c+a>dislocations remain in the extension twins and can act as effective nucleation sites and solute fast-diffusion channels for continuous precipitates.Consequently,the precipitates in extension twins become highly dense.

Dislocations-induced precipitates and their effect on mechanical properties of Mg-Gd-Y-Zr alloy

Optical microscope(OM),scanning electron microscope(SEM),transmission electron microscope(TEM)and tensile machine were used to characterize the microstructures and mechanical properties of as-forged and aged Mg-9.5Gd-3.8Y-0.6Zr alloys.The results show that a novel kind of dislocation arrays,comprising parallel arranged dislocations,were obtained in the forged alloy.The arrays tend to extend parallel and are heterogeneously distributed with adjacent distances varying from 0.3μm to 1.4μm.After aging the alloy at 265°C,a large number of preferential-oriented phases were precipitated on the dislocation arrays,forming a structure of"precipitation chains"(PCs),which results in simultaneous increments of strength and ductility.

Interaction between many parallel screw dislocations and a semi-infinite crack in a magnetoelectroelastic solid

Based on the fundamental equations of magnetoelectroelastic material and the analytic theory, and using the Muskhelishvili-introduced well-known elastic techniques combined with the superposition principle, the closed form solution of the generalized stress field of the interaction between many parallel screw dislocations and a semi-infinite crack in an infinite magnetoelectroelastic solid is obtained, on the assumption that the surface of the crack is impermeable electrically and magnetically. Besides, the Peach-Koehler formula of n parallel screw dislocations is given. Numerical examples show that the generalized stress varies with the position of point z and is related to the material constants. The results indicate that the stress concentration occurs at the dislocation core and the tip of the crack. The result of interaction makes the system stay in a lower energy state.

Interaction of two edge dislocations in free-space propagation

This paper studies in detail the interaction of two edge dislocations nested in a Gaussian beam propagating in free space. It shows that in free-space propagation the edge dislocations are unstable and vanish, and two noncanonical vortices with opposite topological charge take place when off-axis distances cl and c2 of two edge dislocations are nonzero, and the condition k2w08＋ 32c1c2（w02- 2C1C2）Z2 〉 0 is fulfilled （k-wave number, w0-waist width）. A noncanonical vortex appears when one off-axis distance is zero. However, one edge dislocation is stable when two edge dislocations are perpendicular and one off-axis distance is zero. Two perpendicular edge dislocations both with zero off-axis distance are also stable. The analytical results are illustrated by numerical examples.

Interaction between infinitely many dislocations and a semi-infinite crack in one-dimensional hexagonal quasicrystal

By means of analytic function theory, the problems of interaction between infinitely many parallel dislocations and a semi-infinite crack in one-dimensional hexagonal quasicrystal are studied. The analytic solutions of stress fields of the interaction between infinitely many parallel dislocations and a semi-infinite crack in one-dimensional hexagonal quasicrystal are obtained. They indicate that the stress concentration occurs at the dislocation source and the tip of the crack, and the value of the stress increases with the number of the dislocations increasing. These results are the development of interaction among the finitely many defects of quasicrystals, which possesses an important reference value for studying the interaction problems of infinitely many defects in fracture mechanics of quasicrystal.

Line-integral representations for extended displacements, stresses,and interaction energy of arbitrary dislocation loops in transversely isotropic magneto-electro-elastic bimaterials

In addition to the hexagonal crystals of class 6 mm, many piezoelectric materials （e.g., BaTiO3）, piezomagnetic materials （e.g., CoFe2O4）, and multiferroic com-posite materials （e.g., BaTiO3-CoFe2O4 composites） also exhibit symmetry of transverse isotropy after poling, with the isotropic plane perpendicular to the poling direction. In this paper, simple and elegant line-integral expressions are derived for extended displace-ments, extended stresses, self-energy, and interaction energy of arbitrarily shaped, three-dimensional （3D） dislocation loops with a constant extended Burgers vector in trans-versely isotropic magneto-electro-elastic （MEE） bimaterials （i.e., joined half-spaces）. The derived solutions can also be simply reduced to those expressions for piezoelectric, piezo-magnetic, or purely elastic materials. Several numerical examples are given to show both the multi-field coupling effect and the interface/surface effect in transversely isotropic MEE materials.

DISLOCATION PINNING IN HIGH TEMPERATURE DEFORMED Ni_3(Al,Ti) SINGLE CRYSTALS CONTAINING DISORDERED γ PRECIPITATES

A transmission electron microscopy （TEM） investigation has been performed on the dislocation pinning in Lie-ordered Ni3（Al, Ti） containing disordered γ precipitates.The morphology of deformation induced dislocations in the γ base alloys containing fine dispersion of disordered γ was investigated by means of weak-beam electron microscopy. The superdislocations are strongly attracted to the disordered particles and dissociate on the （111） plane in the γ particles, while they dissociate on the （010） plane in the γ＇ matrix. The disordered γ precipitates play an important role as a pinning point during the cross-slip of superdislocations from （111） to （010） planes in the γ matrix and restrain the cross-slip of superdislocations. The interaction of superdislocations with disordered particles causes the formation of superkinks, jogs and closed loops.

CYCLIC DEFORMATION OF FACE CENTERED CUBIC CRYSTALS AND ITS DISLOCATION INTERACTION MODEL——Ⅱ.DISLOCATION INTERACTION MODEL OF CYCLIC DEFORMATION

A dislocation interaction model has been proposed for cyclic deformation of fcc crystals.Ac- cording to this model,cyclic stress-strain responses and saturation dislocation structures of a crystal are associated with the modes and intensities of dislocation interactions between slip systems active in the crystal; and,hence,may be predicted by the location of its tensile axis in the crystallographic triangle.This model has successfully explained the different behaviours of double-slip crystals and multi-slip behaviours of some crystals with orientations usually con- sidered as single-slip ones.

Inhomogeneity Dislocation Interaction of Piezoelectric Materials Under Remote Non - uniform Shear & Electric Field

The complex potentials method is used to develop a solution for ascrew dislocation interacting with an elliptical piezoelectricinhomogeneity in piezoelectric materials under remote non-uniformantiplane shear- and non-uniform inplane electric field. Thetheoretical analysis result is formulated via the conformal mappingand Laurent series expansion in the transformed plane by usingcomplex variable method. The general expression of the Complexvariables is dirived explicitly in both the elliptical inhomogeneityand the surrounding matrix.

A CONSTITUTIVE MODEL FOR CREEP-PLASTICITY INTERACTION BASED ON DISLOCATION DYNAMICS

A glide-plus-climb micromechanism of dislocation evolution with the formation of subgrains is pro- posed for modelling of the creep-plasticity interaction (CPI). The long-range internal stress can be divided into the resistance for dislocation climb in subgrain boundaries and that for dislocation glide within grains or subgrains. Their evolution equations are then derived based on dislocation dynamics. Furthermore, a unified constitutive model for CPI is developed from Orowan's formula. Theoretical calculations on the basis of this model show a very good agreement between the model prediction and experimental results of benchmark tests for 2 1/4 Cr -1 Mo steel at 600℃.

Interaction energy of interface dislocation loops in piezoelectric bi-crystals

Interface dislocations may dramatically change the electric properties, such as polarization, of the piezoelectric crystals. In this paper, we study the linear interactions of two interface dislocation loops with arbitrary shape in generally anisotropic piezoelectric bi-crystals. A simple formula for calculating the interaction energy of the interface dislocation loops is derived and given by a double line integral along two closed dislocation curves. Particularly, interactions between two straight segments of the interface dislocations are solved analytically, which can be applied to approximate any curved loop so that an analytical solution can be also achieved. Numerical results show the influence of the bi-crystal interface as well as the material orientation on the interaction of interface dislocation loops.

INTERACTION BETWEEN DISLOCATION AND TWO CIRCULAR CYLINDRICAL INCLUSIONS

The elastic field of the infinite homogeneous medium with two circular cylindrical inclusions under the action of a screw dislocation was investigated. The analytical solution was obtained using the conformal mapping and the theorem of analytical continuation. It has been shown that the elastic field depends on the shear moduli of individual phase, the geometric parameters of the system, and the position and relative slip of the screw dislocation. The specific cases, that two circular cylindrical inclusions are tangent to each other and the screw dislocation is located in inclusions, were considered. The numerical results were illustrated to show the interaction between the dislocation and two circular cylindrical inclusions. (Edited author abstract) 13 Refs.

Mechanism analysis of ions interaction in MgCl_2/LiCl solution and effect of lithium on ammonia precipitation products

Producing magnesium hydroxide is the basic way to utilize magnesium resources of natural brines. However, the effect of lithium on properties of product is always neglected. The interaction between ions in magnesium chloride solution containing lithium was illustrated based on the experimental results, and the effect of lithium on the crystallization of magnesium was clarified. The results of X-ray diffraction（XRD）, scanning election microscope（SEM）, Fourier transform infrared spectroscopy（FTIR）, thermogravimetry analysis（TGA） and laser particle size analysis indicate that the effect of lithium is not obvious on the crystal phase and morphology of the products. But the XRD relative intensity of（001） surface of magnesium hydroxide declines, the specific surface area reduces apparently and the additive mass of lithium affects the heat loss rates of precipitations obviously. Quantum chemical calculations on the interactional systems of Mg（H2O）2＋6 and Li（H2O）＋4 were performed using B3LYP/6-311 G basis set. The results show that when the distance of Mg2＋ and Li＋ is 7-10 , the interaction energy is high and the trend of solvation is strong, which would make hydroxide ions easier to combine with hydrogen ions in ammonia precipitation process. And the absolute value of solvation free energy reduces significantly in MgCl2 solution（1 mol/L） containing lithium ion.

Hardening effects of sheared precipitates on {1121} twinning in magnesium alloys

The interactions between a plate-like precipitate and two twin boundaries(TBs)({1012},{1121}) in magnesium alloys are studied using molecular dynamics(MD) simulations. The precipitate is not sheared by {1012} TB, but sheared by {1121} TB. Shearing on the(110) plane is the predominant deformation mode in the sheared precipitate. Then, the blocking effects of precipitates with different sizes are studied for {1121} twinning. All the precipitates show a blocking effect on {1121} twinning although they are sheared, while the blocking effects of precipitates with different sizes are different. The blocking effect increases significantly with the increasing precipitate length(in-plane size along TB) and thickness, whereas changes weakly as the precipitate width changes. Based on the revealed interaction mechanisms, a critical twin shear is calculated theoretically by the Eshelby solutions to determine which TB is able to shear the precipitate. In addition, an analytical hardening model of sheared precipitates is proposed by analyzing the force equilibrium during TB-precipitate interactions. This model indicates that the blocking effect depends solely on the area fraction of the precipitate cross-section, and shows good agreement with the current MD simulations. Finally, the blocking effects of plate-like precipitates on the {1012} twinning(non-sheared precipitate), {1121} twinning(sheared precipitate) and basal dislocations(non-sheared precipitate) are compared together. Results show that the blocking effect on {1121} twinning is stronger than that on {1012} twinning, while the effect on basal dislocations is weakest. The precipitate-TB interaction mechanisms and precipitation hardening models revealed in this work are of great significance for improving the mechanical property of magnesium alloys by designing microstructure.

Dislocation mechanism of Ni_(47)Co_(53) alloy during rapid solidification

Dislocations and other atomic-level defects play a crucial role in determining the macroscopic properties of crystalline materials,but it is extremely difficult to observe the evolution of dislocations due to the limitations of the most advanced experimental techniques.Therefore,in this work,the rapid solidification processes of Ni_(47)Co_(53) alloy at five cooling rates are studied by molecular dynamics simulation,and the evolutions of their microstructures and dislocations are investigated as well.The results show that face-centered cubic(FCC) structures are formed at the low cooling rate,and the crystalline and amorphous mixture appear at the critical cooling rate,and the amorphous are generated at the high cooling rate.The crystallization temperature and crystallinity decrease with cooling rate increasing.Dislocations are few at the cooling rates of 1×10^(11) K/s,5×10^(12) K/s,and 1×10^(13) K/s,and they are most abundant at the cooling rates of 5×10^(11) K/s and1 × 10^(12) K/s,in which their dislocation line lengths are both almost identical.There appear a large number of dislocation reactions at both cooling rates,in which the interconversion between perfect and partial dislocations is primary.The dislocation reactions are more intense at the cooling rate of 5×10^(11) K/s,and the slip of some dislocations leads to the interconversion between FCC structure and hexagonal close packed(HCP) structure,which causes the twin boundaries(TBs) to disappear.The FCC and HCP are in the same atomic layer,and dislocations are formed at the junction due to the existence of TBs at the cooling rate of 1 ×10^(12) K/s.The present research is important in understanding the dislocation mechanism and its influence on crystal structure at atomic scales.

Interaction between uranium and humic acid (Ⅱ): complexation, precipitation and migration behavior of U(Ⅵ) in the presence of humic substances

The complexation, precipitation, and migration behavior of uranium in the presence of humic acid (HA) or fulvic acid (FA) were investigated by cation exchange, ultrafiltration and dynamic experiment, respectively. The results showed that (i) complex equilibrium between the uranium and humic substances was achieved at approximately 72 h, (ii) the coordination number varied from 1:1 to 1:2 ( U(Ⅵ) : humic acid) as pH increased from 3 to 6; and (iii) , while the complex stability constant decreased when temperature increased, but increased with pH value. We found that the precipitation of uranyl could only be observed in presence of HA, and the precipitation was influenced by conditions, such as pH, uranium concentration, temperature, and the HA concentration. The maximum precipitation proportion up to 60% could be achieved in the condition of 40 mg/L HA solution at pH 6. We further observed that the migration behavior of uranium in soil in the presence of humic acid (HA) or fulvic acid (FA) was different from that in the presence of inorganic colloid, and the effect of humic substances (HS) was limited.

Change in dislocation configuration of deformed Fe-Ni-Nb-Ti-C-B alloy during stress relaxation

Transmission electron microscopy (TEM) was applied to investigate theevolution of dislocation configuration and strain induced precipitation behavior during relaxationprocess after deformation in Fe-Ni-Nb-Ti-C-B alloy. Experimental results indicate that thedislocation density is very high and distribute randornly before relaxation. As the relaxation timeincreasing, dislocation cells will form gradually by polygonization. The strain inducedprecipitation retards the progress. In the final relaxation stage, most dislocations get rid ofpinning of precipitates and the cells have developed into subgrains with large size.

Nanoscale precipitates and comprehensive strengthening mechanism in AISI H13 steel

The effects of heat treatment on the precipitates and strengthening mechanism in AISI H13 steel were investigated. The results showed that the presence of nanoscale precipitates favorably affected grain refinement and improved the yield strength. The volume fraction of precipitates increased from 1.05% to 2.85% during tempering, whereas the average precipitate size first decreased then increased during tempering. Contributions to the yield strength arising from the various mechanisms were calculated quantificationally, and the results demonstrated that grain refinement and dislocation density most strongly influenced the yield strength. In addition, under the interaction of average size and volume fraction, precipitates＇ contribution to the yield strength ranged from 247.9 to 378.5 MPa. Finally, a root-mean-square summation law of σ = σg ＋ σs ＋（σd^2 ＋ σp^2）^1/2, where σg, σs, σd, and σp represent the contributions of fine-grain strengthening, solid-solution strengthening, dislocation strengthening, and precipitation strengthening, respectively, was confirmed as the most applicable for AISI H13 steel, which indicates a strong link between precipitates and dislocations in AISI H13 steel.