Modeling of healing pores of cylindrical form under the action of shock waves in a crystal subjected to shear deformation

Volumetric defects in crystals worsen operational properties of structural materials; therefore, the problem of reducing discontinuities in solid is one of the most important in modern materials science. In the present work, the results of computer simulation are presented that demonstrate possibility of collapse of pores in a crystal in state of shear deformation under the influence of shock waves. Similar waves can occur in a solid under external high-intensity exposure. For example, in the zone of propagation of displacement cascade, there are regions in which occurs a mismatch between the thermalization times of atomic vibrations and the removal of heat from them. As a result of the expansion of such a region, a shock after cascade wave arises. The simulation was carried out based on molecular dynamics method using the potential calculated by means of mmersed atom method. As a bulk defect, we considered extended pores of cylindrical shape, which can be formed after passing of high-energy ions through a crystal, or, for example, when superheated closed fluid inclusions (mother liquor) reach the surface. The study has shown that such defects are the source of heterogeneous nucleation of dislocation loops, contributing to a decrease in the shear stresses in simulated structure. Dependences of the average dislocation density on the shear angle and temperature of the designed cell were established, and the loop growth rate was estimated. Generated shock waves create additional tangential stresses that contribute to the formation of dislocation loops; therefore, in this case, dislocations are observed even with a small shear strain. If during simulation the thermal effect increases, the pore collapses.

A dislocation density-based model for the work hardening and softening behaviors upon stress reversal

The mechanical behaviours of microalloyed and low-carbon steels under strain reversal were modelled based on the average dislocation density taking into account its allocation between the cell walls and cell interiors. The proposed model reflects the effects of the dislocations displacement, generation of new dislocations and their annihilation during the metal-forming processes. The back stress is assumed as one of the internal variables. The value of the initial dislocation density was calculated using two different computational methods, i.e. the first one based on the dislocation density tensor and the second one based on the strain gradient model. The proposed methods of calculating the dislocation density were subjected to a comparative analysis. For the microstructural analysis, the high-resolution electron backscatter diffraction (EBSD) microscopy was utilized. The calculation results were compared with the results of forward/reverse torsion tests. As a result, good effectiveness of the applied computational methodology was demonstrated. Finally, the analysis of dislocation distributions as an effect of the strain path change was performed.

Structural and hydrogen storage characterization of nanocrystalline magnesium synthesized by ECAP and catalyzed by different nanotube additives

Ball-milled nanocrystalline Mg powders catalyzed by TiO2 powder, titanate nanotubes and carbon nanotubes were subjected to intense plastic deformation by equal-channel angular pressing. Microstructural characteristics of these nanocomposites have been investigated by X-ray diffraction. Microstructural parameters, such as the average crystallite size, the average dislocation density and the average dislocation distance have been determined by the modified Williamson–Hall analysis. Complementary hydrogen desorption and absorption experiments were carried out in a Sieverts’ type apparatus. It was found that the Mg-based composite catalyzed by titanate nanotubes exhibits the best overall H-storage performance, reaching 7.1 wt% capacity. The hydrogenation kinetic curves can be fitted by the contracting volume function for all the investigated materials. From the fitted parameters, it is confirmed that the titanate nanotube additive results in far the best kinetic behavior, including the highest hydride front velocity.

Laser shock processing of AMg6 Al alloy without coating

A microstructure and distribution of residual stresses after the laser shock peening of the AMg6 Al alloy without a protective coating were studied. The X-ray diffraction analysis showed the correlation between the parameters of the crystal structure and the profile of residual stresses of the treated samples. It was found that domain size decreased up to 50 nm, microstrains increased up to 0,0019 and average dislocation density increased up to 4,7·1014 м–2. Laser shock processing generates residual compressive stresses in depth up to 2 mm with a maximum of –120 MPa on the surface of the material. The profile and depth of the residual compressive stresses depend on the power density, overlap coefficient and the number of processing.

Flow-Stress Model of 300M Steel for Multi-Pass Compression

In this work, multi-pass compressions were performed at various strain rates (0.01 s−1, 0.1 s−1, 1 s−1, 10 s−1), temperatures (950 °C, 1050 °C, 1150 °C), inter-pass holding time (1 s, 10 s, 30 s, 120 s, 600 s), interrupt strains (0.3, 0.4, 0.5, 0.6), and total pass numbers (1, 2, 3, 4). The intriguing finding was that the recrystallized fraction, average dislocation density, and plastic cumulative strain were partly eliminated during inter-pass holding, resulting in the early occurrence of recrystallization in subsequent compression. Therefore, a parameter (Θ) to evaluate the overall softening fraction due to recrystallization was proposed, and it was then used to iteratively rectify the average dislocation density and plastic cumulative strain in flow-stress modeling. The flow-stress model parameters of 300M steel for multi-pass compression were identified using an optimization technique based on non-derivative method integrated in MATLAB software. The average deviation of calculated and experimental flow-stress was 0.88 MPa (1.35%), showing good accuracy of the flow-stress model. The microstructure evolution of 300M steel was analyzed by the change of softening fraction during multi-pass compression, which provided a useful reference for the research of stress–microstructure relationships of high-strength steels.

Microstructural Investigation of Nanocrystalline Hydrogen-Storing Mg-Titanate Nanotube Composites Processed by High-Pressure Torsion

A high-energy ball milling and subsequent high-pressure torsion method was applied to synthesize nanocrystalline magnesium samples catalyzed by TiO2 or titanate nanotubes. The microstructure of the as-milled powders and the torqued bulk disks was characterized by X-ray diffraction. The recorded diffractograms have been evaluated by the convolutional multiple whole profile fitting algorithm, which provided microstructural parameters (average crystal size, crystallite size distribution, average dislocation density). The morphology of the nanotube-containing disks has been examined by high-resolution transmission electron microscopy. The effect of the different additives and preparation conditions on the hydrogen absorption behavior was investigated in a Sieverts’-type apparatus. It was found that the ball-milling route has a prominent effect on the dispersion and morphology of the titanate nanotubes, and the absorption capability of the Mg-based composite is highly dependent on these features.

Structure and residual stresses in AMg6 alloy subjected to laser shock processing

The microstructure and distribution of residual stresses in AMr6 alloy after the laser shock processing in the range of power density 1—6 GW / cm2 have been studied. By the X-ray diffraction method it was found that domain size decreased up to 60 nm, microstrains increased up to 0,0018 and average dislocation density increased by a factor of 5,5 in comparing with untreated material (3,7×1014 м–2 vs. 6,6×1013 м–2). The laser shock processing generates residual compressive stresses in depth up to 1 mm, with a maximum of –128 MPa on the surface of the material.

Quantitative Evaluation of Texture and Dislocations during Annealing after Hot Deformation in Austenitic Steel Using Neutron Diffraction

Microstructural change during hot compressive deformation at 700 oC followed by isothermal annealing for a Fe-32Ni austenitic alloy was monitored using in situ neutron diffraction. The evolution of deformation texture with 40% compression and its change to recrystallization texture during isothermal annealing were presented by inverse pole figures for the axial and radial directions. The change in dislocation density was tracked using the convolutional multiple whole profile fitting method. To obtain the fitting results with good accuracies, at least 60 s time-interval for slicing the event-mode recorded data was needed. The average dislocation density in 60 s after hot compression was determined to be 2.8 x 1014 m-2, and it decreased with increasing of annealing time.

Microstructure and Mechanical Properties of 6013 Aluminium Alloy Processed by a Combination of ECAP and Preaging Treatment

Microstructure and mechanical properties of a 6013 Al-Mg-Si-Cu aluminum alloy processed by a combination of equal channel angular pressing (ECAP) and preaging treatment were comparatively investigated using quantitative X-ray diffraction (XRD) measurements, transmission electron microscopy (TEM) and tensile tests. In addition, the precipitation sequences were obtained by thermodynamic calculations using the FactSage software package. Average grain sizes measured by XRD are in the range 211–501 nm while the average dislocation density is in the range 0.35-1.0 × 1014 m-2 in the deformed alloy. TEM analysis reveals that fine needle β′′ precipitates with an average length of 4-10 nm are uniformly dispersed in the preaging ECAPed alloy. The local dislocation density in this sample is as high as 2.2×1017 m-2. The strength is significantly increased in the preaging-ECAPed samples as compared to that of the undeformed counterparts. The highest yield strength among the preaging ECAPed alloys is 322 MPa. This value is about 1.25 times higher than that (258 MPa) of the static peak-aging sample. The high strength in the preaging ECAPed alloy is suggested to be related to grain size strengthening and dislocation strengthening, as well as precipitation strengthening contributed from both preaging treatment and ECAP deformation.

Evaluation of intragranular strain and average dislocation density in single grains of a polycrystal using K-map scanning

Quick scanning X-ray microscopy combined with three-dimensional reciprocal space mapping was applied to characterize intragranular orientation and strain in a single grain of uniaxially deformed Al polycrystal. The strain component perpendicular to the direction of the applied tensile load was found to be very heterogeneous with high compressive and tensile values in the grain interior and near two grain boundaries, respectively. The distribution of the magnitude of diffraction vectors indicates that dislocations are the origin of the strain. The work opens new possibilities for analysing dislocation structures and intragranular residual stress/strain in single grains of polycrystalline materials.