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Dislocation structures can be directly related to the fatigue properties of metals, such as fatigue strength, including the fatigue limit and saturation stress. We present an indirect dislocation-structure-based method to evaluate the local stresses for an in-depth analysis of sleeper screw failures, as there is little knowledge about the load and local stresses related to these failures. The sleeper screw, fastening baseplates of rails to sleepers, is a small but critical component in the railway. High loads from passing trains are transferred to the screws, leading to cyclic straining. In the present study, three stress-level tension fatigue experiments are designed in the constant stress mode at a stress ratio R = 0 and a testing frequency of 10 Hz. The microstructures in the failed specimens are characterized and compared with those close to the fracture surface of screws that failed in the field. The dislocation structure similarities and differences are analyzed, and the potential of the proposed methodology is discussed. [ABSTRACT FROM AUTHOR]

Dislocation structures can be directly related to the fatigue properties of metals, such as fatigue strength, including the fatigue limit and saturation stress. We present an indirect dislocation-structure-based method to evaluate the local stresses for an in-depth analysis of sleeper screw failures, as there is little knowledge about the load and local stresses related to these failures. The sleeper screw, fastening baseplates of rails to sleepers, is a small but critical component in the railway. High loads from passing trains are transferred to the screws, leading to cyclic straining. In the present study, three stress-level tension fatigue experiments are designed in the constant stress mode at a stress ratio R = 0 and a testing frequency of 10 Hz. The microstructures in the failed specimens are characterized and compared with those close to the fracture surface of screws that failed in the field. The dislocation structure similarities and differences are analyzed, and the potential of the proposed methodology is discussed.

The current work numerically investigates commercial polycrystalline Ag/17vol.%SnO 2 composite tensile deformation behavior with available experimental data. Such composites are useful for electric contacts and have a highly textured initial material status after hot extrusion. Experimentally, the initial sharp fiber texture and the number of Σ 3-twins were reduced due to tensile loading. The local inhomogeneous distribution of hardness and Young's modulus gradually decreased from nanoindentation tests, approaching global homogeneity. Many-scale simulations, including micro-macro simultaneous finite element (FE) and discrete dislocation dynamics (DDD) simulations, were performed. Deformation mechanisms on the microscale are fundamental since they link those on the macro- and nanoscale. This work emphasizes micromechanical deformation behavior. Such FE calculations applied with crystal plasticity can predict local feature evolutions in detail, such as texture, morphology, and stress flow in individual grains. To avoid the negative influence of boundary conditions (BCs) on the result accuracy, BCs are given on the macrostructure, i.e., the microstructure is free of BCs. The particular type of 3D simulation, axisymmetry, is preferred, in which a 2D real microstructural cutout with 513 Ag grains is applied. From FE results, Σ 3-twins strongly rotated to the loading direction (twins disappear), which, possibly, caused other grains to rotate away from the loading direction. The DDD simulation treats the dislocations as discrete lines and can predict the resolved shear stress (RSS) inside one grain with dependence on various features as dislocation density and lattice orientation. The RSS can act as the link between the FE and DDD predictions. [ABSTRACT FROM AUTHOR]

Molecular dynamics simulations were performed to study the evolution of void along different crystallographic orientations of single-crystal copper under shock compression, including [ 1 1 ¯ 0 ] , [ 111 ] and [ 100 ] orientations. For both [ 1 1 ¯ 0 ] and [ 111 ] directions, the void only shrinks and does not collapse, whereas for [ 100 ] direction, the void can gradually shrink until it collapses completely. Dislocations react with each other to form sessile dislocations during the continuous loading of the shock waves, in both [ 1 1 ¯ 0 ] and [111] directions, and almost all the dislocations are found to be a 6 < 110 > stair-rod partial dislocations which are of sessile type. However, for the [100] orientation, sessile dislocations are mainly a 3 < 001 > Hirth partial dislocations. For [ 100 ] direction, the sessile dislocation density is the lowest among the three orientations. Therefore, shock compression along [ 100 ] direction is more conducive to plastic deformation of the void. Dislocation slip is responsible for deformation mechanism of the void, where a 6 < 112 > Shockley partial dislocations are firstly generated on the surface of the void, and then they continue to move and multiply, which shall lay the foundation for the formation of stacking faults. Stacking faults sweep through the crystal plane and consequently the void shrinks. This work gives an atomic-scale observation perspective of the evolution of micro-void defects in single-crystal copper under shock compression and provides a clearer explanation for the understanding of the dislocation evolution mechanism behind the deformation. [ABSTRACT FROM AUTHOR]

We review the recently developed models for load fluctuations in the displacement controlled mode and displacement jumps in the load controlled mode of indentation. To do this, we devise a method for calculating plastic contribution to load drops and displacement jumps by setting-up a system of coupled nonlinear time evolution equations for the mobile and forest dislocation densities by including relevant dislocation mechanisms. These equations are then coupled to the equation defining constant displacement rate or load rate. The model for the displacement controlled mode using a spherical indenter predicts all the generic features of nanoindentation such as the elastic branch followed by several force drops of decreasing magnitudes and residual indentation depth after unloading. The stress corresponding to the elastic force maximum is close to the yield stress of an ideal solid. The predicted numbers for all the quantities match experiments on single crystals of Au using a spherical indenter. We extend the approach to model the load controlled nanoindentation experiments that employ a Berkovich indenter. We first identify the dislocation mechanisms contributing to different regions of the F − z curve as a first step for obtaining a good fit to a given experimental F − z curve. This is done by studying the influence of the parameters associated with various dislocation mechanisms on the model F − z curves. The study also demonstrates that the model predicts all the generic features of nanoindentation such as the existence of an initial elastic branch followed by several displacement jumps of decreasing magnitudes and residual plasticity after unloading for a range of model parameter values. Furthermore, an optimized set of parameter values can be easily determined that give a good fit to the experimental load–displacement curves for Al single crystals of ( 110 ) and ( 133 ) orientations. Our model also predicts the indentation size effect in a region where the displacement jumps disappear. The good agreement of the results of the models with experiments supports our view that the present approach can be used as an alternate method to simulations. The approach also provides insights into several open questions.

Abstract: This paper proposes a new constitutive model to describe the dynamic plasticity of FCC metals using the thermal activation mechanism of dislocation motion. In the model development, the constitutive parameters were directly linked with the characteristics of microstructures of materials. As an example of its application, the model was used to describe the behavior of OFHC copper. To determine the globally optimized parameters of the constitutive model for OFHC copper, an improved multi-variable optimization method of constrained nonlinear programming was used based on the flow stress of the material measured experimentally. A comparison with some models and experimental data in the literature shows that the new model is simple to apply and is much better in terms of its prediction accuracy. It was shown that compared with the MTS model the new constitutive equation is explicit and can be easily embedded into a computational code of material dynamics; while compared with the Zerilli–Armstrong and Johnson–Cook models the new one reflects more precisely experimental observations. It was concluded that the new model is applicable to a wide range of problems with temperature variation from 77K to 1096K, strain rates ranging from 10−3 s−1 to 104 s−1, and strain as high as 1. [Copyright &y& Elsevier]

High temperature deformation properties of Nb–1%Zr alloy were evaluated with a special emphasis on the texture effect. Fully recrystallized Nb–1%Zr samples with different crystallographic textures were prepared by annealing 80% cold-rolled foil at 1500°C for 2h. Subsequent tensile tests at 1100°C showed that the sample oriented in the rolling direction is about 20% stronger than that oriented 30° to the rolling direction. [Copyright &y& Elsevier]

Fuselage skin of the future supersonic civil transport aircraft requires a material resistant to long-term creep at temperatures ranging from 100 to 130 °C. A candidate is the 2650 aluminum alloy which presents such properties at relatively high temperatures (130 °C=0.43Tmelting). From an accelerated creep test conducted at 150 °C under a 280 MPa load and subsequent observations of the dislocation microstructures by transmission electron microscopy, the role of thermal activation on dislocation mechanisms is analyzed. It appears that thermal activation favors cross-slip activity and allows dislocations to glide in non-close-packed planes, namely the {0 0 1} planes. This is the first time that evidence of primary {0 0 1} glide is reported. Associated to a strain-assisted decrease of the precipitate density, thermal activation softens the material and seems to contribute to the acceleration of the strain rate (tertiary stage) by facilitating bypassing of precipitates and the production of mobile dislocations. The different creep stages are explained in terms of individual dislocation mechanisms and not by referring to the evolution of dislocation substructures. [Copyright &y& Elsevier]

In the present work, we investigate the stress and temperature dependence of low-temperature (750 ± 20 °C) and high-stress (300 ± 20 MPa) shear creep of a Ni-base single crystal superalloy. From continuous isothermal experiments and stress and temperature change tests the stress exponent n and the apparent activation energy Q app of the phenomenological Sherby-Dorn equation were determined for the two macroscopic crystallographic shear systems (MCSS) 01 1 ¯ (111) and 11 2 ¯ (111). The activation parameters of creep, the stress exponents and the apparent activation energies were identified as 16 and 620 kJ/mol (MCSS: 01 1 ¯ (111)) and 14 and 460 kJ/mol (MCSS: 11 2 ¯ (111)). We show that during shear creep testing these phenomenological parameters do not change between the early (0.5–1% strain) and later stages of creep (4.5–5% strain), in contrast to what was observed for uniaxial tensile testing. The results are discussed in the light of what is known about stress and temperature dependencies of deformation rates in the creep literature and in view of the recent work by Bürger et al., 2020, who combined shear creep testing with analytical transmission electron microscopy to identify the elementary deformation mechanism, which governs low temperature and high stress creep. [ABSTRACT FROM AUTHOR]

Plastic stress-strain calculations are presented for a substantial range in load versus displacement measurements made with rounded tips of Berkovich nano-indentations on tungsten (111) crystal surfaces. The computed hardness stresses are shown to be comparable to hardness measurements obtained on [111]-axis micro-whiskers; and, the subsequent indentation strain hardening is greater than, but compares favorably with, compression stress measurements made on tungsten [111]-axis micro-pillars. The indentation strain hardening is attributed to the small spacing of nucleated dislocations that proceed to form reacted 〈010〉 dislocation obstacles within the nano-scale deformation zones. The strain hardening is a larger effect than the Indentation Size Effect (ISE) measured for deeper polygonal (or conical) indentations. • Continuous load-displacement curves converted to stress-strain dependence • Exceptional hardness-based strain hardening measurements • Dislocation reaction for enhanced strain hardening • Comparison with other micro-whisker and micro-pillar measurements • Determination of smaller Indentation Size Effect (ISE) measurements [ABSTRACT FROM AUTHOR]

We review the recently developed models for load fluctuations in the displacement controlled mode and displacement jumps in the load controlled mode of indentation. To do this, we devise a method for calculating plastic contribution to load drops and displacement jumps by setting-up a system of coupled nonlinear time evolution equations for the mobile and forest dislocation densities by including relevant dislocation mechanisms. These equations are then coupled to the equation defining constant displacement rate or load rate. The model for the displacement controlled mode using a spherical indenter predicts all the generic features of nanoindentation such as the elastic branch followed by several force drops of decreasing magnitudes and residual indentation depth after unloading. The stress corresponding to the elastic force maximum is close to the yield stress of an ideal solid. The predicted numbers for all the quantities match experiments on single crystals of Au using a spherical indenter. We extend the approach to model the load controlled nanoindentation experiments that employ a Berkovich indenter. We first identify the dislocation mechanisms contributing to different regions of the F − z curve as a first step for obtaining a good fit to a given experimental F − z curve. This is done by studying the influence of the parameters associated with various dislocation mechanisms on the model F − z curves. The study also demonstrates that the model predicts all the generic features of nanoindentation such as the existence of an initial elastic branch followed by several displacement jumps of decreasing magnitudes and residual plasticity after unloading for a range of model parameter values. Furthermore, an optimized set of parameter values can be easily determined that give a good fit to the experimental load–displacement curves for Al single crystals of ( 110 ) and ( 133 ) orientations. Our model also predicts the indentation size effect in a region where the displacement jumps disappear. The good agreement of the results of the models with experiments supports our view that the present approach can be used as an alternate method to simulations. The approach also provides insights into several open questions. [ABSTRACT FROM AUTHOR]