Discrete dislocation plasticity analysis of the high-temperature cyclic response of composites

Microstructural evolution of adiabatic shear bands in pure copper during impact at high strain rates

The present study aimed at strengthening prior austenite grain boundary (PAGB) cohesive energy using carbon segregation and investigated the effect of carbon segregation at PAGB on the microscopic crack propagation behavior of hydrogen-related intergranular fractures in high-strength martensitic steels. At the low hydrogen content (below 0.2 wt. ppm), the fracture initiation toughness (JIC) and tearing modulus (TR), corresponding to crack growth resistance, were significantly improved by carbon segregation. In contrast, JIC and TR did not change by carbon segregation at the high hydrogen content (above 0.5 wt. ppm). Considering the non-linear relationship between the toughness properties and the PAGB cohesive energy, the experimentally evaluated toughness properties (JIC and TR) and the GB cohesive energy previously calculated by first-principles calculations were semi-quantitatively consistent even at the high hydrogen content. The microstructure observation confirmed that the plastic deformation associated with crack propagation, such as the local ductile fracture of uncracked ligaments and the formation of dislocation cell structures / nano-voids, played an important role in the non-linear relationship between the toughness properties and PAGB cohesive energy.

The effect of specimen size on the uniaxial deformation response of planar single crystals and polycrystals is investigated using discrete dislocation plasticity. The dislocations are all of edge character and modelled as line singularities in a linear elastic material. The lattice resistance to dislocation motion, dislocation nucleation, dislocation interaction with obstacles and dislocation annihilation are incorporated through a set of constitutive rules. Grain boundaries are modelled as impenetrable to dislocations. Two types of polycrystalline materials are considered: one that only has grains with a single orientation while the other has a checker-board arrangement of two types of grains which are rotated 90° with respect to each other. The single crystals display a strong size dependence with the flow strength increasing with decreasing specimen size. In sufficiently small single crystal specimens, the nucleation rate of the dislocations is approximately equal to the rate at which the dislocations exit the specimens so that below a critical specimen size the flow strength is set by the strength of the initially present Frank–Read sources. On the other hand, grain boundaries acting as barriers to plastic deformation in polycrystalline specimens of the same size lead to a more diffuse deformation pattern and to a nearly size-independent response.

Cylindrical nano-indentation on metal film/elastic substrate system with discrete dislocation plasticity analysis: A simple model for nano-indentation size effect

With the development of experimental techniques and characterization methods at the microscopic scale, a high concentration of hydrogen-induced vacancies and their clusters have been detected in a large variety of metals and alloys charged with hydrogen, which is supposed to play an important role in hydrogen-induced fracture process. In hydrogen environment, vacancies and their clusters can trap a certain number of H atoms to form vacancy-H complexes or clusters. To investigate the role of hydrogen-induced vacancies and their clusters in brittle-like failure in hydrogen environment, multi-scale modelling was performed in this work, with a particular attention on the interaction between dislocations and vacancy-H complexes. Firstly, the H-enhanced vacancy formation due to H-induced reduction in vacancy formation energy in Al was modelled by first principles calculations, and a hydrogen-influenced vacancy concentration model was proposed. Then, to capture the interaction between gliding edge dislocations and vacancy-H complexes and clusters, atomistic simulations were performed, and a mesoscale model quantitatively describing the pinning effect of vacancy-H complexes and clusters on the gliding edge dislocations was obtained. Finally, these quantitative models were introduced into a newly developed XFEM-based discrete dislocation plasticity scheme. With this scheme, mode-I crack propagation was simulated, where dislocation emission from crack-tip, dislocation evolution and cohesive crack propagation were considered, with a particular attention to the key role of the hydrogen-influenced dislocation evolution in the brittle-like failure of the H-charged metals. Based on these multi-scale simulations, a pinning effect of H-enhanced vacancies and vacancy-H complexes on edge dislocations was identified, which was proposed to be responsible for the brittle-like failure of Al in hydrogen environment. This pinning effect presents a good agreement with the hydrogen-restricted dislocation mobility observed in in situ fracture experiments.

We describe here an electron microscopy study of shear reversion-induced nanograined/ultrafine-grained (NG/UFG) structure and evolution of tensile strained microstructure in metastable type 301 austenitic stainless steel. The NG/UFG structure with grain size in the range of 200 to 500 nm was obtained by severe cold deformation and controlled annealing in the narrow temperature range of 973 to 1073 K (700 to 800 °C). The different stages of annealing involve the following: (a) transformation of strain-induced martensite to highly dislocated lath-type austenite, (b) formation of dislocation-cell structure and transformation to recovered austenite structure with defect-free subgrains, and (c) coalescence of subgrains to form a NG/UFG structure concomitant with a completely recrystallized structure, and consistent with martensitic shear-type phase reversion mechanism. The optimized cold working and annealing treatment resulted in NG/UFG material with a high yield strength (~1000 MPa) and high ductility (~30 pct) combination. Multiple deformation mechanisms were identified from postmortem electron microscopy examination of tensile strained NG/UFG 301 austenitic stainless steel and include dislocation glide and twinning. The evidence of heterogeneous nucleation of overlapping stacking faults and partial dislocations points toward deformation

Single crystals of α-Fe have been fatigued in constant stress amplitude reversed plane bending. TheS-N curve, the plastic strain amplitude variation, and the dislocation substructures developed in the crystals during fatigue have been studied. The formation of a cell structure was found to occur once a critical dislocation density had accumulated, the nature of which was influenced by the crystallography of the specimen and underwent changes with continued testing. The evolution of a continuous cell structure with testing coincided with a large decline in the level of plastic strain amplitude, and it appears that it is these substructures which account for fatigue hardening. From the observations of crack formation and the mode of dislocation movement during saturation, a mechanism for crack initiation is suggested.

Contribution of microstructure and slip system to cyclic softening of 9 wt.%Cr steel

Cyclic deformation behavior in pearlitic eutectoid steel strongly depends on the interlamellar spacing with cyclic softening in fine pearlite, cyclic hardening in coarse pearlite, and both cyclic softening and hardening depending on the strain amplitude in medium pearlite. Dislocations in cyclically softened specimens were uniformly distributed, while dislocation cells were observed with cyclic hardening. The cell size decreased with increasing strain amplitude. Using the cell size to interlamellar spacing ratios, conditions for cell formation were quantified. Based on dislocation structure observations, mechanisms for cyclic softening and hardening were proposed. Both monotonic and cyclic yield stresses follow Hall-Petch type relations when plotted against interlamellar spacing. Surface fatigue microcrack initiation usually occurred in the ferrite matrix associated with extrusions and intrusions. Most microcracks were almost parallel to the cementite lamellae and oriented between 30 and 90 deg with respect to the tensile axis. Little influence of MnS inclusions on microcrack initiation was noticed.

Al-Mg-Sc alloy polycrystals bearing Al3Sc particles with different sizes, i.e. 4, 6 and 11 nm in diameter, have been cyclically deformed at 423 K under constant plastic-strain amplitudes, and the microstructural evolution has been investigated in relation to the stress-strain response. Cyclic softening after initial hardening is found in specimens with small particles of 4 and 6 nm, but no cyclic softening takes place in specimens with larger particles of 11 nm. These features of cyclic deformation behavior are similar to the results previously obtained at room temperature. Transmission electron microscopy observations reveal that dislocations are uniformly distributed under all applied strain amplitudes in the specimens containing large particles of 11 nm, whereas slip bands are formed in the cyclically softened specimens bearing smaller particles. The cyclic softening is explained by a loss of particle strength through particle shearing within strongly strained slip bands. The 6 and 11 nm Al3Sc particles have a stronger retardation effect on the formation of fatigue-induced stable dislocation structure than 4 nm particles at 423 K.

Background. Hydrogels are promising for use in tissue engineering for the restoration and regeneration of various tissues, since they are able to perform the functions of bulk scaffolds, providing the formation of 3D cell structures. Population of such scaffolds with autologous or heterogeneous mesenchymal multipotent stromal cells in vitro makes it possible to localize these cells in the area of target tissues after implantation in a patient. One of the difficult tasks is the choice of the method and mode of sterilization of the hydrogel, which does not change its properties.Aim. Study of the effectiveness of hydrogel sterilization by an accelerated electron beam in various modes, changes in the structure and biocompatibility of the scaffold, to assess the prospects for its use for medical purposes, including as a platform for mesenchymal stromal cells.Materials and methods. We used a hydrogel based on 4 % solutions of sodium alginate and sodium salt of carboxymethyl cellulose, cross-linked with calcium chloride, which was developed, obtained and provided for our research by the team of the Research and Educational Center for Biomedical Engineering of the National University of Science and Technology “MISIS”. Hydrogel samples loaded with Escherichia coli, Lactobacillus acidophilus, Saccharomyces cerevisiae were subjected to electron beam treatment in the range of 5–100 kGy. After electron beam treatment of hydrogel, the presence of living microorganisms and its structure were evaluated by IR-Fourier spectroscopy, as well as the phenotype and formation of 3D structures by mesenchymal multipotent cells.Results. It was found that the treatment of hydrogels with an electron beam at a mode of 25 kGy ensures the death of microorganisms, but does not destroy the structure of the hydrogel and does not inhibit the ability to form capillary-like structures by mesenchymal multipotent cells.Conclusion. Treatment with an accelerated electron beam at a 25 kGy can be used to sterilize hydrogels to obtain bulk scaffolds for cell engineering implants.

Cyclic creep behaviour described in terms of a one-parameter kinetic model

Creep tests were carried out at temperatures from 573 to 773 K on the 6061Al matrix composites reinforced with 11 vol% and 18 vol% alumina short fibers to examine the effect of short fibers reinforcement on the creep rate and the creep curve shape, and their stress and temperature dependences. Furthermore, the density change by creep deformation was measured to examine the formation of cavities during creep of the short fiber reinforced composites.Creep curve shape changed with stress as follows: In the low stress range, the tertiary creep occurred after small extent of the primary stage, so that the creep curve was dominated by the tertiary creep without being related to the occurrence of necking in the specimen. With increasing the stress, the extent of primary and secondary stages became large and occupied a large part of the creep curve in the high stress range at low temperature.The higher the creep temperature and the lower the creep stress, the larger was the density change by creep deformation. It showed that the tertiary creep was associated with the formation of cavities, especially at the fiber/matrix interface and at the grain boundaries at high temperature (773 K).The θ-projection method was useful in describing the creep curve shape of the short fiber reinforced composites and it’s variation with stress and temperature. It was found that rate parameters θ2 and θ4 describing the primary and tertiary creep were about equal, at least at 573 and 673 K, as proposed by Maruyama et al.These results were compared with the creep behaviour of Al matrix composite reinforced with the same short alumina fibers as the present composites and the effect of matrix alloy on creep was discussed.

Creep Properties of Grade 91 Steel Steam Generator Tube at 923K

Fretting fatigue damage formation in carbon steel S45C has been examined by using an interrupted fretting fatigue testing method, which consists of two-stage fatigue loading. The first stage is fatigue loading with fretting under one of the stress ratios R= -1, 0, 0.33 and 0.5, and the second stage is fatigue loading without fretting under R= 0.5. The minimum number of cycles, Ncr, in the first stage which caused a break in the second stage was 7.5 × 104, and was independent of R. The damage layer at Ncr was about one grain size, and was hardened due to the formation of fine cell structures. It was concluded that the fretting fatigue damage is accumulated in the early cycles of fretting, and does not depend on R.